memra_engine/

decode_batch.rs

//! Batched decode step — B sequences share one fused pass (ARCHITECTURE-H100.md §3 B2').
//!
//! The bandwidth thesis: decode is weight-stream-bound, so every projection at m=B rows
//! amortizes one weight read across B sequences. Row-parallel ops (norm/rope/quantize/
//! activation) batch trivially — they are the SAME kernels prefill already runs at T rows.
//! Only truly per-sequence state stays in a loop: KV append + fa_decode over each cache,
//! and the GDN/conv recurrent step (v1: per-seq loop via the existing single-seq path;
//! a blockIdx.z-batched GDN state kernel is the v2 fusion).
//!
//! EXACTNESS CONTRACT (the law this module lives under):
//! - B == 1 must be BIT-IDENTICAL to `decode_step_h` (gate: decode-batch-gate).
//! - 2 <= B <= 8: each row rides the m=2..9 verify-tier mmvq kernels, which are per-row
//!   bit-identical to m=1 (the spec-exactness machinery decode_step_t relies on). Each
//!   sequence's token stream must equal its isolated single-seq run (worker.rs contract:
//!   "byte-identical to isolated").
//! - 9 <= B <= 16 (the EXACT-16 tier, inc3 2026-08-01): admitted iff
//!   `decode_batch_exact16_ok` — every matmul rides the b16 batched-mmvq class
//!   (bit-identical per (token,row) to m=1; Q8_0 needs the q8rp mirror) under a
//!   verify_exact scope that disables the m>=16 GEMM/MMQ arms. gate2 bit-strength
//!   PASS at B=12/16 (research/batched-tick-inc3-20260801). Refused otherwise.
//! - B > 16 crosses into GEMM/dp4a-tail numeric configs with NO exact kernel class —
//!   refused (MEMRA_DECODE_BATCH_CAP stays a measurement door).
//!
//! v1 scope: the hybrid (Qwen3.5-class) non-gemma4 trunk. Fused m=1 micro-launches
//! (fused3 QKV, cross-layer add+norm+q8 chain) are NOT used — the unfused sequence is
//! bit-identical (kernel_check: add_rms_norm == add;rms_norm; _q8_1 == +quantize_q8_1)
//! and keeps the batched path simple. Batched fusions are tuning work, not correctness.

use crate::Engine;
use crate::cache::Cache;
use crate::hybrid::{HybridModel, Mixer};
use cudarc::driver::{CudaEvent, CudaSlice};
use memra_gguf::config::Arch;

type DualPpCudaSpan = Option<(CudaEvent, CudaEvent)>;

fn dual_pp_timing_event(e: &Engine, context: &str) -> Option<CudaEvent> {
    if !crate::pp::dual_pp_timing_on() {
        return None;
    }
    match e
        .stream()
        .record_event(Some(cudarc::driver::sys::CUevent_flags::CU_EVENT_DEFAULT))
    {
        Ok(event) => Some(event),
        Err(err) => {
            crate::pp::record_dual_pp_timing_drop(context, &err);
            None
        }
    }
}

/// Per-step, per-LAYER-RANGE invariants the batched trunk needs: the device state-pointer
/// table for the range's layers, the arm picks, and the per-row `t_kv` snapshot. Built once
/// per step per range by `HybridModel::batch_layer_ctx`, consumed by `decode_batch_layers`.
///
/// WHY IT IS RANGE-SCOPED AND NOT STEP-SCOPED (this is the whole point of the struct):
/// `ptr_table` is a `CudaSlice<u64>` of DEVICE ADDRESSES, uploaded through `e` — so it lives
/// on `e`'s device, and its entries are pointers into caches that live on the device that
/// OWNS those layers. Under a pp stage split, stage s runs layers [fence[s], fence[s+1])
/// whose cache state was allocated by stage s's engine (`pp::new_cache` -> `Cache::new_ppn`),
/// so stage s must build its OWN table through its OWN engine. One step-wide table built on
/// the primary would put every stage's kernel arguments in stage-0's HBM — a peer read per
/// pointer fetch, which is the exact cliff `pp::refuse_unsplit_if_remote` exists to stop.
/// `lo`/`hi` are recorded so the consumer can assert the ctx it was handed matches the range
/// it was asked to run (the offsets in `lin_base`/`attn_base` are only valid for that range).
pub(crate) struct BatchLayerCtx {
    /// Offset into `ptr_table` of layer il's [conv x B][ssm_in x B][ssm_out x B] block
    /// (linear-attn layers only). Indexed by ABSOLUTE layer id; `None` off-range.
    lin_base: Vec<Option<usize>>,
    /// Offset into `ptr_table` of layer il's [k0,v0,k1,v1,..] block (full-attn layers only).
    /// Indexed by ABSOLUTE layer id; `None` off-range.
    attn_base: Vec<Option<usize>>,
    ptr_table: Option<CudaSlice<u64>>,
    /// Per-row `pos + 1` — the t_kv each sequence attends at this step. Layer-invariant
    /// within a step, so the arm picks below are decided once.
    t_kvs: Vec<usize>,
    t_kv_max: usize,
    /// The single `fa_split_keys` rung every row shares (the rows-twins straddle law).
    sp0: usize,
    seqs_append: bool,
    seqs_fa: bool,
    lo: usize,
    hi: usize,
}

// ---- MEMRA_BATCH_PHASE=1 (diagnostics): sync-bounded per-phase accumulators for the batched
// tick. Each boundary syncs the stream, so the TOTAL inflates (launch pipelining is destroyed);
// the value is the RANKING/shares, not absolute ms. Read via `batch_phase_report()`.
pub(crate) static BATCH_PHASE: std::sync::Mutex<[f64; 12]> = std::sync::Mutex::new([0.0; 12]);
pub const BATCH_PHASE_NAMES: [&str; 12] = [
    "setup(ptrs+embed H2D)",
    "attn batched pre (norm/qkv/rope)",
    "attn per-seq: kv append",
    "attn per-seq: q/a dtod copies",
    "attn per-seq: fa_decode",
    "attn post (gate+o-proj)",
    "gdn batched projections",
    "gdn state ops (conv/prep/scan)",
    "gdn out (gated norm+proj)",
    "ffn (add/norm/gate/up/act/down)",
    "lm_head (norm+matmul)",
    "logits D2H + host split",
];
pub fn batch_phase_on() -> bool {
    static ON: std::sync::OnceLock<bool> = std::sync::OnceLock::new();
    *ON.get_or_init(|| std::env::var("MEMRA_BATCH_PHASE").as_deref() == Ok("1"))
}
/// Accumulate the elapsed time since `last` into phase slot `slot` and re-stamp `last`.
/// No-op unless `MEMRA_BATCH_PHASE=1`. Syncs the ambient stream first, so under a pp stage
/// scope this bounds the STAGE's stream, which is what the caller is timing.
///
/// A free fn rather than the closure it replaced: `decode_batch_layers` (the pp stage seam)
/// runs the instrumented layer loop, so the marker has to be callable from both the seam
/// and its caller's epilogue. `batch_phase_on()` is a `OnceLock` memo, so per-call cost is
/// the same atomic load the hoisted `ph_on` local was.
fn ph_mark(
    e: &Engine,
    slot: usize,
    last: &mut std::time::Instant,
) -> Result<(), Box<dyn std::error::Error>> {
    if batch_phase_on() {
        e.stream().synchronize()?;
        let now = std::time::Instant::now();
        BATCH_PHASE.lock().unwrap()[slot] += (now - *last).as_secs_f64();
        *last = now;
    }
    Ok(())
}

pub fn batch_phase_report() -> String {
    let ph = BATCH_PHASE.lock().unwrap();
    let tot: f64 = ph.iter().sum();
    let mut rows: Vec<(usize, f64)> = ph.iter().copied().enumerate().collect();
    rows.sort_by(|a, b| b.1.total_cmp(&a.1));
    let mut s = format!(
        "[batch-phase] total {:.1} ms (sync-bounded; shares rank, not walltime)\n",
        tot * 1e3
    );
    for (i, v) in rows {
        s += &format!(
            "  {:>6.1} ms {:>5.1}%  {}\n",
            v * 1e3,
            v / tot * 100.0,
            BATCH_PHASE_NAMES[i]
        );
    }
    s
}

impl HybridModel {
    /// Batched-decode width cap. 8 = the exactness-tier default (see the assert below);
    /// MEMRA_DECODE_BATCH_CAP overrides for tier-probe measurement, clamped to 32.
    pub fn decode_batch_cap() -> usize {
        use std::sync::OnceLock;
        static CAP: OnceLock<usize> = OnceLock::new();
        *CAP.get_or_init(|| {
            std::env::var("MEMRA_DECODE_BATCH_CAP")
                .ok()
                .and_then(|v| v.parse().ok())
                .map(|c: usize| c.clamp(1, 32))
                .unwrap_or(8)
        })
    }

    /// EXACT-16 TIER admission (increment 3a, 2026-08-01, 5090 receipts
    /// research/batched-tick-inc3-20260801): true iff EVERY matmul the batched decode step
    /// runs has a per-(token,row) bit-exact kernel class at m=9..16 under the verify_exact
    /// scope — i.e. the batched-mmvq b16 family (32-thread warp reduce, the exact m=1 mmvq
    /// program per column) or the e4m3 grid.y=m mmvq catch-all. Q8_0 qualifies only with
    /// the split-plane mirror (rp4, MEMRA_Q8RP): its b16 kernel exists only as the _rp twin.
    /// Float matmuls (cuBLASLt, n-dependent reductions) and MoE FFNs disqualify the model.
    /// Measured attribution for WHY the naked m=16 tier is not exact: the m>=16 arms
    /// (MMQ int8-MMA `mul_mat_q` — MEMRA_PP_Q8MMQ default-on — and `qmatvec_gemm`, both
    /// block-scale f32) and the m=9..15 dp4a tail (128-thread two-level reduce) all break
    /// per-row bit-identity vs isolated decode (gate2 step-0 bit-diffs, maxdiff ~1.3-2.3e-1).
    pub fn decode_batch_exact16_ok(&self) -> bool {
        fn ok(w: &crate::model::GpuTensor) -> bool {
            match w {
                crate::model::GpuTensor::Quant { qtype, .. } => {
                    *qtype == crate::QT_Q4_0 || *qtype == crate::QT_Q6_K
                    || *qtype == crate::QT_F8_E4M3
                    // BLOCK-128 FP8-ST (lane/rp-on-st, 2026-08-06): admitted now that the class
                    // has a b16 batched kernel (`qmatvec_e4m3_blk_mmvq_b16`), bit-identical per
                    // (token,row) to its m=1 launch. Before that kernel existed this class fell to
                    // the grid.y=m form at every width — still EXACT, so the tier's correctness
                    // bar was met, but it re-read the weight m times, which is why admitting it
                    // without the kernel would have been a throughput trap rather than a win.
                    || *qtype == crate::QT_F8_E4M3_BLK
                    // NVFP4 (lane/rp-on-st, 2026-08-06) — THE blocker this lane measured. The
                    // mixed FP8-ST 27B is 193 NVFP4 dense-MLP tensors, and this predicate is an
                    // ALL over every matmul, so NVFP4's missing b16 refused the whole checkpoint
                    // (`B=16 > cap 8 with no exact tier ... refused`) even with both e4m3 classes
                    // admitted. It now has base + _rp b16 twins off its existing batched template
                    // (bit-identical per (token,row) to the m=1 mmvq: same nibble decode, dp4a
                    // order, ue4m3 scale, warp reduce). This also opens the tier for pure-NVFP4
                    // GGUF models, which is a behavior change on the primary format — hence the
                    // full decode-batch config+strict battery on both.
                    || *qtype == crate::QT_NVFP4
                    // Q4_K (lane/rp-on-st): named by MEMRA_EXACT16_WHY as the 9B NVFP4 GGUF's
                    // refusing class (`L0.wqkv qtype=1`) — mixed NVFP4 checkpoints keep Q4_K
                    // attention. Now has base + _rp b16.
                    || *qtype == crate::QT_Q4_K
                    // Q5_K (lane/rp-on-st): the FOURTH class the diagnostic named on the same 9B
                    // GGUF (`L0.wqkv_gate qtype=3`). A shipped mixed checkpoint spreads ~500
                    // matmuls over four/five classes, and this predicate is an ALL — so chunk 16
                    // was unreachable for every real artifact until every class had a b16.
                    || *qtype == crate::QT_Q5_K
                    // Q8_0 NO LONGER requires the mirror (rp4): it has a base b16 too, so the
                    // tier is reachable at zero VRAM. Named by the diagnostic as the FP8-ST
                    // refusal — `L0.ssm_beta qtype=0 rp4=false`, a 23.9 MiB residual class that
                    // was gating chunk 16 for a 16.4 GiB checkpoint.
                    || *qtype == crate::QT_Q8_0
                }
                _ => false,
            }
        }
        // WHY-NOT DIAGNOSTIC (lane/rp-on-st, 2026-08-06): this predicate is a bare bool over
        // ~500 tensors, so a refusal produced only `B=16 > cap 8 with no exact tier ... refused`
        // with no way to tell WHICH class refused. That cost this lane two wrong hypotheses (the
        // rp mirror, then e4m3-only) before the NVFP4 gap was found. MEMRA_EXACT16_WHY=1 names
        // the first refusing tensor + its qtype. Diagnostic-only per flags doctrine; default off,
        // zero cost when unread.
        let why = std::env::var("MEMRA_EXACT16_WHY").is_ok();
        macro_rules! chk {
            ($t:expr, $label:expr) => {{
                let r = ok($t);
                if !r && why {
                    // qtype = -1 means the tensor is NOT Quant at all (a float/BF16/F16
                    // container), which the tier can never admit — a distinct diagnosis from
                    // "quantized, but in a class with no b16 kernel".
                    let (qt, rp4) = match $t {
                        crate::model::GpuTensor::Quant { qtype, rp4, .. } => {
                            (*qtype, rp4.is_some())
                        }
                        _ => (-1, false),
                    };
                    eprintln!("[exact16] REFUSED by {} qtype={qt} rp4={rp4}", $label);
                }
                r
            }};
        }
        if self.cfg.m3.is_some() || self.is_gemma4_e4b() || self.cfg.gemma4.is_some() {
            if why {
                eprintln!("[exact16] REFUSED by architecture (m3/gemma4)");
            }
            return false;
        }
        self.layers.iter().enumerate().all(|(li, l)| {
            let mix_ok = match &l.mixer {
                Mixer::Full(fa) => {
                    chk!(&fa.wq, format!("L{li}.wq"))
                        && chk!(&fa.wk, format!("L{li}.wk"))
                        && chk!(&fa.wv, format!("L{li}.wv"))
                        && chk!(&fa.wo, format!("L{li}.wo"))
                }
                Mixer::Linear(la) => {
                    chk!(&la.wqkv, format!("L{li}.wqkv"))
                        && chk!(&la.wqkv_gate, format!("L{li}.wqkv_gate"))
                        && chk!(&la.ssm_beta, format!("L{li}.ssm_beta"))
                        && chk!(&la.ssm_alpha, format!("L{li}.ssm_alpha"))
                        && chk!(&la.ssm_out, format!("L{li}.ssm_out"))
                }
                // MLA rides its own increment-4 arm; never admitted to the exact-16 tier here.
                Mixer::Mla(_) => {
                    if why {
                        eprintln!("[exact16] REFUSED by L{li} MLA mixer");
                    }
                    false
                }
            };
            let ffn_ok = match &l.ffn {
                crate::hybrid::Ffn::Dense {
                    ffn_gate,
                    ffn_up,
                    ffn_down,
                } => {
                    chk!(ffn_gate, format!("L{li}.ffn_gate"))
                        && chk!(ffn_up, format!("L{li}.ffn_up"))
                        && chk!(ffn_down, format!("L{li}.ffn_down"))
                }
                crate::hybrid::Ffn::Moe(_) => {
                    if why {
                        eprintln!("[exact16] REFUSED by L{li} MoE ffn");
                    }
                    false
                }
            };
            mix_ok && ffn_ok
        }) && chk!(&self.output, "output".to_string())
    }

    /// Opt-in/A-B seam for the eager B=1 fusion program. `MEMRA_SERVE_B1FAST=1` sends an
    /// eligible solo tick through that program; unset/other values keep B=1 on the generic
    /// batched body, the same numeric class used at B>=2.
    ///
    /// EXACTNESS, stated precisely (measured on-box 2026-08-05, sm_120 q9 NVFP4-MTP):
    /// the fast path is BIT-IDENTICAL TO `decode_step_h` — decode-batch-gate's STRICT
    /// gate1 (`--mode strict`) PASSes with it ON and FAILs with it OFF at maxdiff
    /// 1.591e-1. It is deliberately NOT bit-identical to the batched body: the two
    /// carry a decode-config FP-composition gap (same class gate1's config mode measures).
    /// That gap became correctness-visible under live load: Step35, Q35-MoE, and finally
    /// dense Q27 all produced load-history-dependent token streams, including early EOS,
    /// when a request crossed between the two programs. The generic body is therefore the
    /// correctness default; the eager program remains available only for fixed-solo A/Bs.
    /// Historical token-stream/performance receipts:
    /// research/servepath-p2-20260805 (greedy 150 ids + seeded-sampled identical to the
    /// run-gen oracle AND cross-arm, so the gap is sub-token here as designed).
    ///
    /// Read fresh (an `AtomicU8` memo, not a `OnceLock`): decode-batch-gate flips this
    /// seam BETWEEN gates in-process — gate1 needs the fast path ON to prove bit-identity,
    /// gate2 needs it pinned OFF to keep testing the batched body. A latch-once read would
    /// bake whichever gate ran first, so the gate could never test both sides. The memo
    /// caches the parse but `set_b1_fast` invalidates it.
    pub fn b1_fast_on() -> bool {
        // 0 = unknown/invalidated, 1 = off, 2 = on
        match Self::b1_fast_memo().load(std::sync::atomic::Ordering::Relaxed) {
            1 => false,
            2 => true,
            _ => {
                let value = std::env::var("MEMRA_SERVE_B1FAST").ok();
                let on = b1_fast_env_on(value.as_deref());
                Self::b1_fast_memo()
                    .store(if on { 2 } else { 1 }, std::sync::atomic::Ordering::Relaxed);
                on
            }
        }
    }

    fn b1_fast_memo() -> &'static std::sync::atomic::AtomicU8 {
        static MEMO: std::sync::atomic::AtomicU8 = std::sync::atomic::AtomicU8::new(0);
        &MEMO
    }

    /// Test/gate seam: force the B=1 fast path on or off for the rest of the process,
    /// overriding the env. Used by decode-batch-gate to exercise the opt-in eager arm and
    /// pin gate2's default reference arm.
    pub fn set_b1_fast(on: bool) {
        Self::b1_fast_memo().store(if on { 2 } else { 1 }, std::sync::atomic::Ordering::Relaxed);
    }

    /// Whether this architecture may switch a live serving row onto the eager B=1 fusion
    /// class. Qwen35-MoE must stay on the batched trunk at every width: its eager and batched
    /// hybrid/MoE walks are each deterministic, but crossing B=1 -> B>=2 changes greedy token
    /// ids and can introduce an early EOS (Q35 sellgate, 2026-08-12).
    pub fn b1_fast_arch_eligible(&self) -> bool {
        b1_fast_arch_eligible(&self.cfg.arch)
    }

    /// H3 body: the m=1 FUSED trunk (`decode_layers_eager` — shared verbatim with
    /// `decode_step_h`/the ppN stages) plus the batched path's own serving epilogue
    /// (grammar mask, device sample, lean-logits park). See the call-site comment in
    /// `decode_step_batch_sampled_lean_masked` for why this is bit-identical.
    fn decode_step_b1_fast(
        &self,
        e: &Engine,
        token: u32,
        caches: &mut [&mut Cache],
        samp: &[Option<(f32, u64, u32)>],
        masks: &[Option<(&CudaSlice<u32>, usize)>],
        lean: bool,
    ) -> Result<(Vec<Vec<f32>>, Vec<Option<u32>>), Box<dyn std::error::Error>> {
        let n_embd = self.cfg.n_embd as usize;
        let eps = self.cfg.rms_eps;
        let pos = caches[0].pos;
        let pos_d = e.htod_i32(&[pos as i32])?;
        let x = e.htod(&self.embd.gather(n_embd, &[token]))?;
        // the SHARED m=1 trunk: same function decode_step_h runs, so every m=1 fusion
        // (cross-layer add+norm+q8_1, fused SwiGLU, lever 1's gate+up dual) fires here.
        let x = self.decode_layers_eager(e, x, 0, self.layers.len(), &pos_d, pos, caches[0])?;
        let mut hn = e.uninit(n_embd)?;
        e.rms_norm(&x, self.output_norm.float_data(), &mut hn, n_embd, 1, eps)?;
        let logits = e.matmul(&self.output, &hn, 1)?;

        // ---- epilogue: byte-for-byte the batched path's, at b_n=1 ----
        let n_vocab = self.output.out_features();
        let mut logits = logits;
        let mut pristine: Option<CudaSlice<f32>> = None;
        if let Some((mask, words)) = masks.first().copied().flatten() {
            assert!(
                samp.first().copied().flatten().is_some(),
                "grammar-masked row 0 must request a device sample"
            );
            if lean {
                let cache = &mut caches[0];
                if cache
                    .last_logits_dev
                    .as_ref()
                    .map(|d| d.len() < n_vocab)
                    .unwrap_or(true)
                {
                    cache.last_logits_dev = Some(e.uninit(n_vocab)?);
                }
                let dst = cache.last_logits_dev.as_mut().unwrap();
                e.dtod_copy_view(&logits.slice(0..n_vocab), dst)?;
            } else {
                let mut p = e.uninit(n_vocab)?;
                e.dtod_copy_view(&logits.slice(0..n_vocab), &mut p)?;
                pristine = Some(p);
            }
            e.mask_logits_col(&mut logits, mask, 0, n_vocab, words)?;
        }

        let mut next: Vec<Option<u32>> = vec![None; 1];
        if let Some((temp, seed, ctr)) = samp.first().copied().flatten() {
            let mut toks = e.alloc_u32_zeroed(1)?;
            if temp <= 0.0 {
                e.argmax_token_device_col(&logits, 0, n_vocab, &mut toks, 0)?;
            } else {
                let mut pb = e.zeros(n_vocab)?;
                e.gumbel_perturb_col(&logits, 0, &mut pb, n_vocab, seed, ctr, temp)?;
                e.argmax_token_device_col(&pb, 0, n_vocab, &mut toks, 0)?;
            }
            next[0] = Some(e.dtoh_u32(&toks)?[0]);
        }

        let sampled = samp.first().copied().flatten().is_some();
        let rows: Vec<Vec<f32>> = if lean && sampled {
            if masks.first().copied().flatten().is_none() {
                let cache = &mut caches[0];
                if cache
                    .last_logits_dev
                    .as_ref()
                    .map(|d| d.len() < n_vocab)
                    .unwrap_or(true)
                {
                    cache.last_logits_dev = Some(e.uninit(n_vocab)?);
                }
                let dst = cache.last_logits_dev.as_mut().unwrap();
                e.dtod_copy_view(&logits.slice(0..n_vocab), dst)?;
            }
            vec![Vec::new()]
        } else if let Some(p) = pristine.as_ref() {
            vec![e.dtoh(p)?]
        } else {
            vec![e.dtoh(&logits)?]
        };
        // decode_layers_eager does NOT advance cache.pos (decode_step_h advances it after
        // the head); the batched path advances every cache at the tail — same here.
        caches[0].pos += 1;
        Ok((rows, next))
    }

    /// One batched greedy-decode step over B independent sequences.
    /// `tokens[b]` is sequence b's input token; `caches[b]` its private cache (position,
    /// quantized KV, GDN/conv state). Returns the B logits rows (host, [n_vocab] each).
    /// Each cache's pos/len advance exactly as `decode_step_h` would.
    pub fn decode_step_batch(
        &self,
        e: &Engine,
        tokens: &[u32],
        caches: &mut [&mut Cache],
    ) -> Result<Vec<Vec<f32>>, Box<dyn std::error::Error>> {
        let (rows, _) = self.decode_step_batch_sampled(e, tokens, caches, &[])?;
        Ok(rows)
    }

    /// `decode_step_batch` + DEVICE-SIDE SAMPLING for eligible rows (the batched-tick lever,
    /// 2026-08-01): the host sampler's temp-path is O(n_vocab) with a full-vocab exp per row
    /// (measured 1.36 ms/row at the 9B's 248320 vocab = 10.9 ms/tick at B=8 — the single
    /// largest component of the serving tick). Here each requested row samples ON DEVICE
    /// between the lm_head matmul and the logits D2H:
    ///   temp <= 0 (greedy): the 2-pass device argmax — bit-identical to host argmax
    ///     (argmax-gate contract, same kernels as the dc serving path).
    ///   temp > 0: gumbel_perturb(seed, ctr, temp) + the same argmax = ONE categorical draw
    ///     from softmax(logits/temp) — the sampled-spec Philox machinery. Deterministic per
    ///     (seed, ctr) and INDEPENDENT of batch composition (the isolation contract;
    ///     decode-batch-gate gate3). NOTE: the draw stream differs from the host sampler's
    ///     SplitMix64 (distribution-equal, seed-deterministic, NOT byte-equal to the old
    ///     host draws) — greedy rows are unchanged bit-exact.
    /// `samp[bi] = Some((temp, seed, ctr))` requests a device sample for row bi; the full
    /// logits rows are still returned (worker keeps last_logits semantics + fallback rows).
    pub fn decode_step_batch_sampled(
        &self,
        e: &Engine,
        tokens: &[u32],
        caches: &mut [&mut Cache],
        samp: &[Option<(f32, u64, u32)>],
    ) -> Result<(Vec<Vec<f32>>, Vec<Option<u32>>), Box<dyn std::error::Error>> {
        self.decode_step_batch_sampled_lean(e, tokens, caches, samp, false)
    }

    /// `decode_step_batch_sampled` + LEAN LOGITS (increment 2 component 3, 2026-08-01):
    /// with `lean`, device-sampled rows SKIP the [n_vocab] logits D2H (9.4%/32.5% of the
    /// pre-/post-inc2 tick profile) — their returned row is EMPTY. The audit-mapped
    /// consumers: (a) the next tick's host sample — never fires, `device_next` carries the
    /// token; (b) the graph-promotion argmax — reads only prefill logits (generated empty);
    /// (c) the KV-reuse pool park at retire — the REAL consumer, served by a per-cache
    /// device park: the row is dtod-copied into `cache.last_logits_dev` (device bandwidth)
    /// and D2H'd ONCE at retire by the worker. Rows without a device sample keep a per-row
    /// D2H. `lean=false` is bit-for-bit the previous method (gates + non-serving callers).
    pub fn decode_step_batch_sampled_lean(
        &self,
        e: &Engine,
        tokens: &[u32],
        caches: &mut [&mut Cache],
        samp: &[Option<(f32, u64, u32)>],
        lean: bool,
    ) -> Result<(Vec<Vec<f32>>, Vec<Option<u32>>), Box<dyn std::error::Error>> {
        self.decode_step_batch_sampled_lean_masked(e, tokens, caches, samp, &[], lean)
    }

    /// `decode_step_batch_sampled_lean` + GRAMMAR MASKS (constrained decoding, 2026-08-03):
    /// `masks[bi] = Some((packed_bitset, words))` bans every unset-bit vocab id on row bi
    /// (mask_logits_f32, -FLT_MAX) BETWEEN the lm_head matmul and the device sampler, so a
    /// constrained row rides the SAME device-sample/lean-logits tick as everyone else — no
    /// full-row D2H, no host O(n_vocab) sample. Contract: a masked row must also request a
    /// device sample. The row's PRISTINE logits are preserved for their consumers before the
    /// in-place ban: lean rows park the unmasked row into `cache.last_logits_dev` (the
    /// retire-time reuse-pool park stays unmasked — continuations resume grammar-free, the
    /// v1 host-path contract), non-lean rows D2H the unmasked row. `masks = &[]` is
    /// bit-for-bit the unmasked method.
    pub fn decode_step_batch_sampled_lean_masked(
        &self,
        e: &Engine,
        tokens: &[u32],
        caches: &mut [&mut Cache],
        samp: &[Option<(f32, u64, u32)>],
        masks: &[Option<(&CudaSlice<u32>, usize)>],
        lean: bool,
    ) -> Result<(Vec<Vec<f32>>, Vec<Option<u32>>), Box<dyn std::error::Error>> {
        self.decode_step_batch_sampled_lean_masked_schedule(
            e, tokens, caches, samp, masks, lean, None,
        )
    }

    /// Worker-scheduled twin of [`Self::decode_step_batch_sampled_lean_masked`]. The worker
    /// supplies the balanced dual-wave boundary it used when forming this tick. Direct engine
    /// callers keep the automatic midpoint above; the explicit seam makes scheduler chunking and
    /// engine execution one checked contract instead of two coincident width calculations.
    pub fn decode_step_batch_sampled_lean_masked_scheduled(
        &self,
        e: &Engine,
        tokens: &[u32],
        caches: &mut [&mut Cache],
        samp: &[Option<(f32, u64, u32)>],
        masks: &[Option<(&CudaSlice<u32>, usize)>],
        lean: bool,
        dual_wave_mid: usize,
    ) -> Result<(Vec<Vec<f32>>, Vec<Option<u32>>), Box<dyn std::error::Error>> {
        self.decode_step_batch_sampled_lean_masked_schedule(
            e,
            tokens,
            caches,
            samp,
            masks,
            lean,
            Some(dual_wave_mid),
        )
    }

    #[allow(clippy::too_many_arguments)]
    fn decode_step_batch_sampled_lean_masked_schedule(
        &self,
        e: &Engine,
        tokens: &[u32],
        caches: &mut [&mut Cache],
        samp: &[Option<(f32, u64, u32)>],
        masks: &[Option<(&CudaSlice<u32>, usize)>],
        lean: bool,
        scheduled_dual_mid: Option<usize>,
    ) -> Result<(Vec<Vec<f32>>, Vec<Option<u32>>), Box<dyn std::error::Error>> {
        // NOTE (inc3 3c, 2026-08-01, KILLED ARM): a deferred-token-readback variant (all
        // chunks of a tick writing device-sampled tokens into one shared buffer, ONE
        // dtoh_u32 after the last chunk instead of one per chunk) measured FLAT at serve
        // level on the 5090 (N=4 medians within +-0.7% at c=8/16/32 — 3 saved syncs
        // against a ~100 ms weight-bound tick is ~0.1%, below resolution). Killed per the
        // flags doctrine; receipts research/batched-tick-inc3-20260801 (serve-points.jsonl
        // base vs defer arms) are the record. The per-chunk [B]-u32 readback below IS the
        // tick's only steady-state D2H — one per chunk, none per seq.
        let b_n = tokens.len();
        assert!(
            b_n >= 1 && b_n == caches.len(),
            "tokens/caches length mismatch"
        );
        // ---- PP DOOR: THE BATCHED STAGE SPLIT (pp2-batch 2026-08-06) ----------------------
        // Until this increment this body had NO pp arm: it walked lo=0..n_layers on the
        // primary engine's stream, with no stage split, no boundary, and no `rt.enter()`. With
        // the door open and a sharded cross-device placement, every projection for the remote
        // stages' layers was read over PCIe, per step, silently — measured 7.4 vs 208.9 tok/s
        // at B=1 (28x), 47.4 vs 657.0 at B=8 (13.9x) on a PRO 6000 pair over Gen5 x16 P2P.
        // Nothing failed or warned, because peer reads return identical bytes and all three
        // `decode-batch-gate` gates PASS on that config — the failure mode was performance,
        // and a green exactness battery hid it. `pp2-hardening` made that regime FAIL CLOSED
        // (research/pp2-hardening-20260806); this lane makes it legitimately split, so the
        // refusal lifts for the batched path.
        //
        // `decode_step_batch_ppn` runs each stage's layer range through that stage's engine
        // and stream with a [B, n_embd] boundary transfer between them, i.e. every stage
        // touches only LOCAL weights and LOCAL cache state. The refusal below still guards
        // the residue: the door open with `MEMRA_PP_STREAMS=0` (the same-stream rollback,
        // which also disables the sharded loader, so nothing is remote — `pp_shard_off` and
        // `pp2_streams_off` both make `pp_sharded_cross_device()` false) or a placement whose
        // PpNRt fails to build. Keeping the call means a future path that reaches here in a
        // remote regime still refuses instead of regressing 28x.
        if let Some(fence) = crate::pp::pp_cuts(self.layers.len()) {
            if !crate::pp::pp2_streams_off() && crate::pp::batch_pp_on() {
                // Auto (flipped default) routes dual only in the re-gated regime and
                // degrades serially elsewhere; Forced keeps every ineligible placement on
                // the refusing dual body so the binding negative cells stay reachable.
                let route_dual = crate::pp::dual_pp_route(
                    crate::pp::dual_pp_mode(),
                    b_n,
                    fence.len() - 1,
                    crate::pp::pp2_overlap(),
                    crate::pp::pp_host_bounce_active(),
                );
                if route_dual {
                    let mid = scheduled_dual_mid
                        .or_else(|| crate::pp::dual_pp_wave_mid(b_n))
                        .expect("dual PP B>=2 must have a wave midpoint");
                    return self
                        .decode_step_batch_dual(e, tokens, caches, samp, masks, lean, &fence, mid);
                }
                return self.decode_step_batch_ppn(e, tokens, caches, samp, masks, lean, &fence);
            }
        }
        if scheduled_dual_mid.is_some() {
            return Err(
                "decode_step_batch: worker supplied a dual-wave schedule but the PP-2 dual path is unavailable"
                    .into(),
            );
        }
        crate::pp::refuse_unsplit_if_remote(
            "decode_step_batch",
            "drop MEMRA_PP_STREAMS=0 / MEMRA_BATCH_PP=0 so the batched path takes its OWN \
             stage split (decode_step_batch_ppn), or serve single-stream over the eager pp \
             arm (decode_step_h), which is also split",
        )?;
        // ---- H3: B=1 FAST-PATH (serve-path phase 2, 2026-08-05) ----------------------------
        // At b_n==1 every projection below calls `matmul_pre(.., b_n)` with m=1, which is
        // ALREADY the m=1 mmvq dispatch — so the m=1 *kernel family* was never the gap. What
        // this body does NOT have is the m=1 *fusion chain* that `decode_step_h` carries:
        //   - the cross-layer add+norm+quantize fusion (`add_rms_norm_q8_1`: 3 launches -> 1),
        //   - the fused SwiGLU epilogue (`silu_mul_scaled_q8_1`: folds ffn_down's quantize
        //     into its producer) and, with it, `matmul_pre_dual_noscale`'s gate+up pair
        //     fusion — i.e. phase-1 LEVER 1.
        // Routing b_n==1 through `decode_layers_eager` (the SHARED trunk `decode_step_h` and
        // the ppN stages already use, lifted verbatim — not a copy) makes every present and
        // future m=1 lever fire on the opt-in path automatically. The epilogue (grammar mask ->
        // device sample -> lean logits park) stays exactly as the batched path runs it; the trunk's
        // different FP composition is why this path cannot be a load-changing default.
        // BIT-IDENTITY: the trunk is the same function `decode_step_h` calls, and every
        // fusion it enables is kernel-check-pinned bit-identical to its unfused sequence
        // (add_rms_norm == add;rms_norm | _q8_1 == +quantize_q8_1 | dual_noscale == two
        // matmul_pre_noscale). Gate: decode-batch-gate B=1 vs decode_step_h + serve stream
        // identity. MEMRA_SERVE_B1FAST=1 is the fixed-solo opt-in/A-B seam; the default
        // stays on this function's generic body so batch-width changes cannot change the
        // FP program mid-request.
        if b_n == 1
            && Self::b1_fast_on()
            && self.b1_fast_arch_eligible()
            && !self.is_gemma4_e4b()
            && self.cfg.gemma4.is_none()
            && self.cfg.m3.is_none()
            && crate::pp::pp_cuts(self.layers.len()).is_none()
            && !e.verify_exact_on()
        {
            return self.decode_step_b1_fast(e, tokens[0], caches, samp, masks, lean);
        }
        // MEMRA_DECODE_BATCH_CAP (experimental door, serving-lane tier probe 2026-08-01):
        // default 8 keeps the v1 exactness policy — B=2..8 rides the verify-tier batched
        // mmvq arms, per-row bit-identical to isolated m=1 decode. Values >8 are a
        // MEASUREMENT DOOR ONLY: m=9..15 falls to the grid.y=m dp4a tail (m weight
        // re-reads + a different reduce shape) and m>=16 crosses into the GEMM tier
        // (block-scale f32 rounding) — BOTH break the "byte-identical to isolated"
        // serving contract. Never default this above 8 without the batched-tier
        // exactness policy landing.
        let cap = Self::decode_batch_cap();
        // EXACT-16 TIER (increment 3a): chunks of 9..=16 are admitted WITHOUT the env door
        // when every matmul has a bit-exact b16-class kernel (see decode_batch_exact16_ok).
        // The verify_exact scope below pins that dispatch for the whole step: it turns off
        // the m>=16 GEMM arms (qmatvec_gemm + MMQ + fp8/f16/fp4 — all block-scale/foreign
        // numeric configs) so every projection rides the batched-mmvq b16 tier, which is
        // per-(token,row) bit-identical to isolated m=1 decode (gate2 bit-strength PASS at
        // B=12/16, s32+s160, 5090 receipts research/batched-tick-inc3-20260801). Without
        // the exact tier, B>cap stays refused; the env door (MEMRA_DECODE_BATCH_CAP) keeps
        // its old meaning as the non-exact measurement probe.
        let exact16 = b_n > 8 && b_n <= 16 && self.decode_batch_exact16_ok();
        assert!(
            b_n <= cap || exact16,
            "decode_step_batch: B={b_n} > cap {cap} with no exact tier — refused. Either \
             B>16 (there is NO exact kernel class above 16: m>16 crosses GEMM/dp4a numeric \
             configs; the serve scheduler chunks wider concurrency into <=16 groups instead), \
             or some matmul in this checkpoint has no bit-exact b16 kernel — run with \
             MEMRA_EXACT16_WHY=1 to see which tensor and qtype refuses"
        );
        struct ExactScope<'a>(&'a Engine, bool);
        impl Drop for ExactScope<'_> {
            fn drop(&mut self) {
                if self.1 {
                    self.0.set_verify_exact(false);
                }
            }
        }
        let _exact_scope = ExactScope(e, exact16);
        if exact16 {
            e.set_verify_exact(true);
        }
        // gemma4: NO batched arm at any B (per-layer SWA/global geometry, hd-512 MQA globals,
        // weightless V-norm, softcapped head — none of it in the generic body below). This was
        // an assert until 2026-08-07: one serve request panicked the worker, the respawn
        // re-panicked on the queued request, and the process FATALed
        // (research/gemma4-serve-20260807/raw/repro-panic-server-*.log). The worker now routes
        // gemma4 sessions to the per-session eager loop and never calls here; this Err is the
        // defense-in-depth backstop — a future path that reaches it refuses PER-REQUEST
        // instead of killing the process. The eager arm (gemma4_decode_step_h) is the
        // supported decode.
        if self.is_gemma4_e4b() || self.cfg.gemma4.is_some() {
            return Err(
                "decode_step_batch has no gemma4 arm (per-layer swa/global geometry, \
                        softcapped head) — serve gemma4 on the eager per-session path"
                    .into(),
            );
        }
        // step35 (lane/step35-batched-decode, 2026-08-08): its OWN batched walk. The generic
        // body below is the uniform Full arm — global n_head, 128-dim rope on every layer, no
        // SWA window, no head-wise gate — which on step35 produced HTTP-200 GARBAGE at c>1
        // (research/step-sku-20260807/raw/b2ab-pre-*.log), so step35 NEVER enters it at any B.
        // `step35_decode_batch_layers` carries the real geometry: per-layer n_head (64/96),
        // partial rope (64 full / 128 SWA, dual base, rope_freqs on FULL only), per-SESSION
        // SWA view offsets from each session's own kvl.len, the separate head-wise gate at
        // m=B, and the sigmoid-router MoE via the same moe_ffn_il_zq8 the eager path uses.
        // MEMRA_STEP35_BATCH=0 = the fail-closed rollback seam. The server caps chunks at
        // B=1; on PP-N the B=1 correctness default also refuses the eager numeric class, while
        // an unsplit deployment can still use its existing eager B=1 route.
        if self.cfg.step35.is_some() {
            if !Self::step35_batch_on() {
                return Err(
                    "step35 batched decode is disabled (MEMRA_STEP35_BATCH=0) — \
                            only a non-PP eager B=1 route remains available"
                        .into(),
                );
            }
            let n_embd = self.cfg.n_embd as usize;
            let eps = self.cfg.rms_eps;
            let mut ph_last = std::time::Instant::now();
            let pos_v: Vec<i32> = caches.iter().map(|c| c.pos as i32).collect();
            let pos_d = e.htod_i32(&pos_v)?;
            let x = e.htod(&self.embd.gather(n_embd, tokens))?;
            ph_mark(e, 0, &mut ph_last)?;
            let x = self.step35_decode_batch_layers(
                e,
                x,
                caches,
                &pos_d,
                0,
                self.layers.len(),
                &mut ph_last,
            )?;
            let mut hn = e.uninit(b_n * n_embd)?;
            e.rms_norm(&x, self.output_norm.float_data(), &mut hn, n_embd, b_n, eps)?;
            let logits = e.matmul(&self.output, &hn, b_n)?;
            ph_mark(e, 10, &mut ph_last)?;
            return self.decode_batch_epilogue(
                e,
                caches,
                samp,
                masks,
                lean,
                logits,
                b_n,
                &mut ph_last,
            );
        }
        let n_embd = self.cfg.n_embd as usize;
        let eps = self.cfg.rms_eps;

        // MEMRA_BATCH_PHASE=1: sync-bounded phase accumulation (diagnostics — see header note).
        // Initialized BEFORE the tick-input assembly below so slot 0 covers the HOST side of
        // setup (pos_v/ptr-table builds, embed gather) as well as the H2D sync — the audit-fix
        // lane's Q6 instrumentation gap (research/audit-fixes2-20260805): the old placement
        // started the clock after the assembly, so slot 0 under-reported setup.
        let mut ph_last = std::time::Instant::now();

        // Per-row rope positions (each sequence at its own depth).
        let pos_v: Vec<i32> = caches.iter().map(|c| c.pos as i32).collect();
        let pos_d = e.htod_i32(&pos_v)?;

        // Per-step, whole-trunk layer context: state pointer table + arm picks. Under a pp
        // split this call is made once PER STAGE with that stage's engine and range instead
        // (see `batch_layer_ctx`'s doc for why the table cannot be shared across devices).
        let n_layers = self.layers.len();
        let ctx = self.batch_layer_ctx(e, caches, 0, n_layers)?;

        // Embed all B tokens -> x [B, n_embd] (host gather, one H2D).
        let x = e.htod(&self.embd.gather(n_embd, tokens))?;
        ph_mark(e, 0, &mut ph_last)?;

        let x = self.decode_batch_layers(e, x, caches, &ctx, &pos_d, &mut ph_last)?;

        // ---- output norm + lm_head at m=B, one D2H ----
        let mut hn = e.uninit(b_n * n_embd)?;
        e.rms_norm(&x, self.output_norm.float_data(), &mut hn, n_embd, b_n, eps)?;
        let logits = e.matmul(&self.output, &hn, b_n)?;
        ph_mark(e, 10, &mut ph_last)?;

        self.decode_batch_epilogue(e, caches, samp, masks, lean, logits, b_n, &mut ph_last)
    }

    /// DUAL-ACTIVE PP-2 DECODE (increment 0): split one batch into wave A/B and drive
    /// stage 0(B) from a scoped host walker while this thread drives stage 1(A). Step's
    /// per-layer router readback synchronizes the host, so two CUDA streams issued by one
    /// host thread would remain serial; this mirrors the proven prime PP-2 host schedule.
    ///
    /// This arm is the naked PP-2 default since the 2026-08-11 owner flip (`MEMRA_DUAL_PP`
    /// unset = Auto; `0` is the serial rollback seam). It is fail-closed unless the
    /// double-slot door is open, prewarms both slots, and uses `tx_pipelined` exclusively.
    #[allow(clippy::too_many_arguments)]
    fn decode_step_batch_dual(
        &self,
        e: &Engine,
        tokens: &[u32],
        caches: &mut [&mut Cache],
        samp: &[Option<(f32, u64, u32)>],
        masks: &[Option<(&CudaSlice<u32>, usize)>],
        lean: bool,
        fence: &[usize],
        mid: usize,
    ) -> Result<(Vec<Vec<f32>>, Vec<Option<u32>>), Box<dyn std::error::Error>> {
        let b_n = tokens.len();
        assert!(
            b_n >= 1 && b_n == caches.len(),
            "tokens/caches length mismatch"
        );
        let Some(expected_mid) = crate::pp::dual_pp_wave_mid(b_n) else {
            return self.decode_step_batch_ppn(e, tokens, caches, samp, masks, lean, fence);
        };
        if mid != expected_mid {
            return Err(format!(
                "decode_step_batch_dual: worker midpoint {mid} is not the balanced midpoint {expected_mid} for B={b_n}"
            ).into());
        }
        if self.is_gemma4_e4b() || self.cfg.gemma4.is_some() {
            return Err(
                "decode_step_batch_dual has no gemma4 arm — serve gemma4 on the eager \
                        per-session path"
                    .into(),
            );
        }
        assert!(
            samp.is_empty() || samp.len() == b_n,
            "decode_step_batch_dual: samp must be empty or have one entry per row"
        );
        assert!(
            masks.is_empty() || masks.len() == b_n,
            "decode_step_batch_dual: masks must be empty or have one entry per row"
        );

        let cap = Self::decode_batch_cap();
        let max_wave = mid.max(b_n - mid);
        let exact16 = max_wave > 8 && max_wave <= 16 && self.decode_batch_exact16_ok();
        if max_wave > cap && !exact16 {
            return Err(format!(
                "decode_step_batch_dual: B={b_n} waves {mid}+{} exceed per-wave cap {cap} with no exact tier — refused",
                b_n - mid,
            ).into());
        }
        let n_st = fence.len() - 1;
        crate::pp::dual_pp_eligibility(
            n_st,
            crate::pp::pp2_overlap(),
            crate::pp::pp_host_bounce_active(),
        )
        .map_err(|msg| -> Box<dyn std::error::Error> { msg.into() })?;
        let rt = crate::pp::PpNRt::get(e)?;
        assert_eq!(
            rt.n_stages(),
            n_st,
            "PpNRt stage count {} != fence stages {n_st}",
            rt.n_stages()
        );
        let caller_stream = e.stream();
        rt.fence_stages_behind(&caller_stream)?;

        let n_embd = self.cfg.n_embd as usize;
        let wave_cap = mid.max(b_n - mid) * n_embd;
        rt.prepare_overlap_slots(0, wave_cap)?;

        // EXACT-16 is a property of either scheduled wave, not the combined live width. Keep
        // the scope live across both host walkers and set it on both stage-owned Engines.
        struct ExactScopeN<'a>(Vec<&'a Engine>);
        impl Drop for ExactScopeN<'_> {
            fn drop(&mut self) {
                for eng in &self.0 {
                    eng.set_verify_exact(false);
                }
            }
        }
        let _exact_scope = if exact16 {
            let engines: Vec<&Engine> = (0..n_st).map(|s| rt.engine(s, e)).collect();
            for eng in &engines {
                eng.set_verify_exact(true);
            }
            Some(ExactScopeN(engines))
        } else {
            None
        };

        let step35_batched = self.cfg.step35.is_some();
        if step35_batched && !Self::step35_batch_on() {
            return Err(
                "step35 batched decode is disabled (MEMRA_STEP35_BATCH=0) — \
                        dual-active PP-2 decode has no correct fallback trunk"
                    .into(),
            );
        }

        let (tokens_a, tokens_b) = tokens.split_at(mid);
        let (caches_a, caches_b) = caches.split_at_mut(mid);
        let (samp_a, samp_b) = if samp.is_empty() {
            (&[][..], &[][..])
        } else {
            samp.split_at(mid)
        };
        let (masks_a, masks_b) = if masks.is_empty() {
            (&[][..], &[][..])
        } else {
            masks.split_at(mid)
        };

        let (slot_a, ph_a, span_a0) = self.decode_step_batch_dual_stage0(
            e,
            rt,
            tokens_a,
            caches_a,
            fence,
            step35_batched,
            false,
        )?;

        static LOGGED: std::sync::Once = std::sync::Once::new();
        LOGGED.call_once(|| {
            eprintln!("[dual-pp] dual-active PP-2 decode engaged (naked default since 2026-08-11; two waves)");
        });

        let (out_a, out_b, span_b0, span_b1) = std::thread::scope(
            |scope| -> Result<_, Box<dyn std::error::Error>> {
                let stage0_b = scope.spawn(move || {
                    let staged = self
                        .decode_step_batch_dual_stage0(
                            e,
                            rt,
                            tokens_b,
                            caches_b,
                            fence,
                            step35_batched,
                            true,
                        )
                        .map_err(|err| err.to_string())?;
                    Ok::<_, String>((staged, caches_b))
                });

                let out_a = self.decode_step_batch_dual_stage1(
                    e,
                    rt,
                    slot_a,
                    caches_a,
                    samp_a,
                    masks_a,
                    lean,
                    fence,
                    step35_batched,
                    ph_a,
                    true,
                )?;
                let ((slot_b, ph_b, span_b0), caches_b) = stage0_b
                    .join()
                    .map_err(|_| "dual PP stage-0 wave-B host walker panicked")?
                    .map_err(|err| -> Box<dyn std::error::Error> { err.into() })?;
                if !crate::pp::record_dual_pp_slot_pair(slot_a, slot_b) {
                    return Err(format!(
                        "decode_step_batch_dual: refused: wave A and B both selected boundary slot {slot_a}"
                    ).into());
                }
                let (out_b, span_b1) = self.decode_step_batch_dual_stage1(
                    e,
                    rt,
                    slot_b,
                    caches_b,
                    samp_b,
                    masks_b,
                    lean,
                    fence,
                    step35_batched,
                    ph_b,
                    false,
                )?;
                Ok((out_a, out_b, span_b0, span_b1))
            },
        )?;

        // Wave B is the final producer. One event publishes all last-stage work back to the
        // caller after both epilogues, preserving the ordinary PP-N exit law.
        rt.publish_to(1, &caller_stream)?;
        let (out_a, span_a1) = out_a;
        for (stage, span) in [span_a0, span_a1, span_b0, span_b1].into_iter().enumerate() {
            if let Some((start, end)) = span {
                crate::pp::record_dual_pp_stage_result(stage, start.elapsed_ms(&end));
            }
        }
        let (mut rows, mut next) = out_a;
        rows.extend(out_b.0);
        next.extend(out_b.1);
        Ok((rows, next))
    }

    #[allow(clippy::too_many_arguments)]
    fn decode_step_batch_dual_stage0(
        &self,
        e: &Engine,
        rt: &crate::pp::PpNRt,
        tokens: &[u32],
        caches: &mut [&mut Cache],
        fence: &[usize],
        step35_batched: bool,
        track_overlap: bool,
    ) -> Result<(usize, std::time::Instant, DualPpCudaSpan), Box<dyn std::error::Error>> {
        let b_n = tokens.len();
        let n_embd = self.cfg.n_embd as usize;
        let mut ph_last = std::time::Instant::now();
        rt.bind_stage(0)?;
        let _st0 = rt.enter(0);
        let e0 = rt.engine(0, e);
        let pos_v: Vec<i32> = caches.iter().map(|c| c.pos as i32).collect();
        let pos_d = e0.htod_i32(&pos_v)?;
        let x = e0.htod(&self.embd.gather(n_embd, tokens))?;
        ph_mark(e0, 0, &mut ph_last)?;
        let timing_start = dual_pp_timing_event(e0, "stage0 start event");
        let x = {
            let _overlap = track_overlap.then(crate::pp::enter_dual_pp_stage);
            if step35_batched {
                self.step35_decode_batch_layers(
                    e0,
                    x,
                    caches,
                    &pos_d,
                    fence[0],
                    fence[1],
                    &mut ph_last,
                )?
            } else {
                let ctx = self.batch_layer_ctx(e0, caches, fence[0], fence[1])?;
                self.decode_batch_layers(e0, x, caches, &ctx, &pos_d, &mut ph_last)?
            }
        };
        let timing = timing_start
            .and_then(|start| dual_pp_timing_event(e0, "stage0 end event").map(|end| (start, end)));
        let slot = rt.tx_pipelined(0, &x, b_n * n_embd)?;
        Ok((slot, ph_last, timing))
    }

    #[allow(clippy::too_many_arguments)]
    fn decode_step_batch_dual_stage1(
        &self,
        e: &Engine,
        rt: &crate::pp::PpNRt,
        slot: usize,
        caches: &mut [&mut Cache],
        samp: &[Option<(f32, u64, u32)>],
        masks: &[Option<(&CudaSlice<u32>, usize)>],
        lean: bool,
        fence: &[usize],
        step35_batched: bool,
        mut ph_last: std::time::Instant,
        track_overlap: bool,
    ) -> Result<((Vec<Vec<f32>>, Vec<Option<u32>>), DualPpCudaSpan), Box<dyn std::error::Error>>
    {
        let b_n = caches.len();
        let n_embd = self.cfg.n_embd as usize;
        let eps = self.cfg.rms_eps;
        rt.bind_stage(1)?;
        let _st1 = rt.enter(1);
        let e1 = rt.engine(1, e);
        let pos_v: Vec<i32> = caches.iter().map(|c| c.pos as i32).collect();
        let pos_d = e1.htod_i32(&pos_v)?;
        let x = rt.rx(0, slot, b_n * n_embd)?;
        let timing_start = dual_pp_timing_event(e1, "stage1 start event");
        let x = {
            let _overlap = track_overlap.then(crate::pp::enter_dual_pp_stage);
            if step35_batched {
                self.step35_decode_batch_layers(
                    e1,
                    x,
                    caches,
                    &pos_d,
                    fence[1],
                    fence[2],
                    &mut ph_last,
                )?
            } else {
                let ctx = self.batch_layer_ctx(e1, caches, fence[1], fence[2])?;
                self.decode_batch_layers(e1, x, caches, &ctx, &pos_d, &mut ph_last)?
            }
        };
        let timing = timing_start
            .and_then(|start| dual_pp_timing_event(e1, "stage1 end event").map(|end| (start, end)));
        let mut hn = e1.uninit(b_n * n_embd)?;
        e1.rms_norm(&x, self.output_norm.float_data(), &mut hn, n_embd, b_n, eps)?;
        let logits = e1.matmul(&self.output, &hn, b_n)?;
        ph_mark(e1, 10, &mut ph_last)?;
        Ok((
            self.decode_batch_epilogue(e1, caches, samp, masks, lean, logits, b_n, &mut ph_last)?,
            timing,
        ))
    }

    /// THE BATCHED PP-N STEP (pp2-batch increment 2, 2026-08-06): the batched tick split
    /// across `fence.len()-1` stages, each stage running ONLY its own layer range through
    /// ITS OWN engine and stream, with a `[B, n_embd]` boundary activation between them.
    /// The batched twin of `decode_step_h_ppn`, and the #1 item on the PP-2 serving bill —
    /// without it a >VRAM SKU (Step-3.7-Flash: 105 GB, fits only across two cards) serves
    /// SINGLE-STREAM only, because the batched path was the one loop with no stage split.
    ///
    /// STRUCTURE (mirrors the eager arm exactly, so the two stay comparable):
    ///   stage 0        `rt.enter(0)` -> per-stage pos_d + embed -> range -> `rt.tx`
    ///   middle stages  `rt.rx` -> per-stage pos_d -> range -> `rt.tx`
    ///   last stage     `rt.rx` -> per-stage pos_d -> range -> output_norm + lm_head ->
    ///                  the batched serving epilogue (masks, device sample, lean park)
    ///
    /// FOUR THINGS ARE PER-STAGE, and each is per-stage for a measured reason:
    ///
    /// 1. THE ENGINE (`rt.engine(s, e)`). Not just for the remote device: `Engine` owns
    ///    lazily-grown stable-pointer scratch pools (`fa_part_pool`, `fa_vf16_scratch`,
    ///    `argmax_partials`) that are single-stream-safe BY DESIGN. Two stage streams
    ///    through one Engine is the shared-scratch race the pp2 lane hit (2026-08-02
    ///    nondeterministic all-logits divergence, 35% flake). `PpNRt::build` already gives
    ///    every stage s>0 its own Engine even on the primary device, so honouring
    ///    `rt.engine(s, e)` here is what scopes the pools per stage — the batched path
    ///    allocates MORE of that scratch than the eager one (fa at m=B), so this is the
    ///    load-bearing half of the trap's mitigation, not an inherited nicety.
    ///
    /// 2. THE POINTER TABLE (`batch_layer_ctx(es, caches, lo, hi)`). See [`BatchLayerCtx`]:
    ///    it holds DEVICE ADDRESSES of that range's cache state, uploaded through that
    ///    stage's engine. One step-wide table on the primary would put every stage's kernel
    ///    arguments in stage-0's HBM — a peer read per pointer fetch, the exact cliff this
    ///    whole lane exists to remove.
    ///
    /// 3. `pos_d` (the M2 pipelining law, learned on the eager arm): each stage uploads its
    ///    own copy of the step's per-row positions on ITS stream, so the buffer is
    ///    allocated, consumed and freed on one stream. A shared stage-0 `pos_d` freed at fn
    ///    return breaks under deferred readback — the free enqueues on stream 0 while later
    ///    stages still dereference it.
    ///
    /// 4. THE HEAD + EPILOGUE run on the LAST stage: `output_norm`/`output` were uploaded
    ///    through the last stage's engine by the sharded loader (`hybrid.rs`: `e_head =
    ///    layer_engine(e, n_trunk, n_trunk-1)`), and `cache.last_logits_dev` must be
    ///    allocated where the logits are.
    ///
    /// EXACTNESS: PP-N adds ZERO deviation. Each stage runs the SAME kernels on the SAME
    /// bytes in the same order — the split only moves where the residual is materialized,
    /// and the boundary is a straight f32 copy (dtod same-device / `cudaMemcpyPeerAsync`
    /// cross-device, no conversion). So batched PP-N must be BIT-IDENTICAL to single-device
    /// batched at the same B, in both placement orders. Gate: `decode-batch-gate --mode
    /// pp` (logit-dump, both orders) — the batched analogue of the eager arm's 48 steps x
    /// 248,320 f32 logits with zero differing bits.
    ///
    /// The B=1 fast path is NOT taken here (its condition already excludes an open door):
    /// it routes through `decode_layers_eager` whole-trunk on one engine, which is exactly
    /// the unsplit walk. B=1 under the door rides this function's B=1 case instead — the
    /// same trade the eager arm's own ppn step makes, and the reason the pp2 lane measured
    /// B=1 door-open at 0.854x (the lost fusion chain), not a cliff.
    #[allow(clippy::too_many_arguments)]
    fn decode_step_batch_ppn(
        &self,
        e: &Engine,
        tokens: &[u32],
        caches: &mut [&mut Cache],
        samp: &[Option<(f32, u64, u32)>],
        masks: &[Option<(&CudaSlice<u32>, usize)>],
        lean: bool,
        fence: &[usize],
    ) -> Result<(Vec<Vec<f32>>, Vec<Option<u32>>), Box<dyn std::error::Error>> {
        let b_n = tokens.len();
        assert!(
            b_n >= 1 && b_n == caches.len(),
            "tokens/caches length mismatch"
        );
        // gemma4: same no-arm refusal as the unsplit body (see decode_step_batch), Err not
        // assert — a request must never kill the worker process.
        if self.is_gemma4_e4b() || self.cfg.gemma4.is_some() {
            return Err(
                "decode_step_batch_ppn has no gemma4 arm — serve gemma4 on the eager \
                        per-session path"
                    .into(),
            );
        }
        // Same width policy as the unsplit body — the stage split changes WHERE kernels run,
        // never WHICH tier admits the width. Duplicated deliberately rather than hoisted:
        // the exact-16 scope must wrap the whole multi-stage walk (`set_verify_exact` is
        // per-Engine state read at dispatch on every stage), so it has to be established
        // here, and a shared helper returning a guard would have to own `e` plus the flag.
        let cap = Self::decode_batch_cap();
        let exact16 = b_n > 8 && b_n <= 16 && self.decode_batch_exact16_ok();
        assert!(
            b_n <= cap || exact16,
            "decode_step_batch_ppn: B={b_n} > cap {cap} with no exact tier — refused"
        );
        let rt = crate::pp::PpNRt::get(e)?;
        let n_st = fence.len() - 1;
        assert_eq!(
            rt.n_stages(),
            n_st,
            "PpNRt stage count {} != fence stages {n_st}",
            rt.n_stages()
        );
        // #87 REVERSE PUBLICATION (lane/pp2spec-crash): order every stage stream behind
        // the caller before this body's first stage allocation can reuse a pool block
        // whose queued primary-stream consumer has not read it yet. Anatomy:
        // `PpNRt::fence_stages_behind`. (This body dtoh+syncs its own logits, but its
        // PP-mode callers interleave with the spec verify's device-resident outputs in
        // the same worker, so the entry fence is the uniform law, not an optimization.)
        rt.fence_stages_behind(&e.stream())?;
        let n_embd = self.cfg.n_embd as usize;
        let eps = self.cfg.rms_eps;
        let payload = b_n * n_embd;

        // EXACT-16 SCOPE, PER STAGE ENGINE: `verify_exact` is per-Engine state (an AtomicBool
        // on the Engine the dispatch reads), and each stage runs through a DIFFERENT Engine —
        // so setting it on the primary alone would leave stages 1..N-1 dispatching the m>=16
        // GEMM/MMQ arms while stage 0 used the exact b16 tier. That is a silent per-stage
        // numeric split (the failure this tier exists to prevent), so the flag is set on
        // every stage engine and cleared on all of them at scope exit.
        struct ExactScopeN<'a>(Vec<&'a Engine>);
        impl Drop for ExactScopeN<'_> {
            fn drop(&mut self) {
                for eng in &self.0 {
                    eng.set_verify_exact(false);
                }
            }
        }
        let _exact_scope = if exact16 {
            let engines: Vec<&Engine> = (0..n_st).map(|s| rt.engine(s, e)).collect();
            for eng in &engines {
                eng.set_verify_exact(true);
            }
            Some(ExactScopeN(engines))
        } else {
            None
        };

        let mut ph_last = std::time::Instant::now();

        // B=1 PER-STAGE FAST PATH (measured 2026-08-06, PRO 6000 pair). The unsplit body's
        // b1_fast guard includes `pp_cuts().is_none()`, so opening the pp door dropped every
        // solo session off the m=1 FUSION chain (cross-layer add+norm+q8_1, fused SwiGLU,
        // lever 1's gate+up dual) and onto the batched m=1 walk. Cost, arm A vs arm C at B=1:
        // 208.5 vs 177.3 tok/s = -15.0% — and NOT a split cost, since arm B (stages=2 on ONE
        // card) pays the same 177, and the prior lane's `MEMRA_PP_SHARD=0` batched-body B=1
        // was 178.5. It was the fusion chain going missing, on the config the Step SKU serves
        // solo requests from.
        //
        // `decode_layers_eager(lo, hi)` is ALREADY range-scoped and is exactly what the eager
        // ppn arm (`decode_step_h_ppn`) calls per stage, so B=1 rides the same per-stage
        // structure: same engines, same streams, same [1, n_embd] boundary slots, same
        // stage-owned caches. Only the trunk kernels differ, and they differ identically to
        // how they differ off-door. Exactness is therefore the SAME accepted decode-config FP
        // class the unsplit b1_fast lever already carries (strict gate1 PASSes with it on,
        // FAILs with it off at maxdiff 1.591e-1) — which is why the pp gate pins
        // `set_b1_fast(false)`: with it on, the B=1 reference and the split arm would
        // legitimately sit on opposite sides of that gap and the bit-identity arm would
        // report a fake stage-split failure.
        //
        // Step3.5/Step3.7 are an exception (lane/cx-b1fix, 2026-08-10): their B>1 route is
        // `step35_decode_batch_layers`, and the live scheduler may move a session from B=1
        // to B>1. The eager/fused class and that batched class produce different greedy bytes,
        // so selecting the eager arm at B=1 made output depend on load history. Keep one
        // numeric class for this model family: Step35 always takes its stage-scoped batched
        // trunk at every width. The live transition gate in step35-b2-geometry-gate pins it.
        // Qwen35-MoE is the second exception (lane/cx-q35bug, 2026-08-12): on the Q35
        // sellgate workload the eager-B1 -> batched-B2 transition changed emitted token ids and
        // selected EOS at tokens 15/17/25. Keep that family on this generic batched trunk at B=1
        // too; dense Qwen35 retains the measured eager fast path.
        let b1_stage_fast = b_n == 1
            && Self::b1_fast_on()
            && self.b1_fast_arch_eligible()
            && !self.is_gemma4_e4b()
            && self.cfg.gemma4.is_none()
            && self.cfg.m3.is_none()
            && self.cfg.step35.is_none()
            && !e.verify_exact_on();
        // step35 (lane/step35-batched-decode, 2026-08-08): B>1 rides its OWN stage-scoped
        // batched walk (`step35_decode_batch_layers`) — the generic `decode_batch_layers`
        // remains OFF-LIMITS for this arch at every B (its uniform geometry produced the
        // b2ab HTTP-200 garbage: research/step-sku-20260807/raw/b2ab-pre-*.log). Since
        // lane/cx-b1fix, B=1 also takes this walk: a Step35 PP-N session must not change
        // numeric class when live decode width changes. The refusal below guards the
        // rollback residue; under PP-N, disabling the only correct trunk makes Step35
        // requests fail closed instead of falling back to the eager class.
        let step35_batched = self.cfg.step35.is_some();
        if step35_batched && !Self::step35_batch_on() {
            return Err(
                "step35 batched decode is disabled (MEMRA_STEP35_BATCH=0) — \
                        PP-N Step35 decode is unavailable because eager B=1 is a different \
                        numeric class"
                    .into(),
            );
        }
        // Hoisted: `caches[0].pos` as a value argument alongside `caches[0]` as `&mut` in one
        // call is a borrow conflict; `pos` is Copy and the epilogue is what advances it.
        let pos0 = if b1_stage_fast { caches[0].pos } else { 0 };

        // ---- STAGE 0: embed (the table lives with stage 0) + layers [0, fence[1]) + TX ----
        let mut slot = {
            let _st0 = rt.enter(0);
            let e0 = rt.engine(0, e);
            let pos_v: Vec<i32> = caches.iter().map(|c| c.pos as i32).collect();
            let pos_d = e0.htod_i32(&pos_v)?;
            let x = e0.htod(&self.embd.gather(n_embd, tokens))?;
            ph_mark(e0, 0, &mut ph_last)?;
            let x = if b1_stage_fast {
                self.decode_layers_eager(e0, x, fence[0], fence[1], &pos_d, pos0, caches[0])?
            } else if step35_batched {
                self.step35_decode_batch_layers(
                    e0,
                    x,
                    caches,
                    &pos_d,
                    fence[0],
                    fence[1],
                    &mut ph_last,
                )?
            } else {
                let ctx = self.batch_layer_ctx(e0, caches, fence[0], fence[1])?;
                self.decode_batch_layers(e0, x, caches, &ctx, &pos_d, &mut ph_last)?
            };
            rt.tx(0, &x, payload)?
            // x + pos_d + ctx.ptr_table drop here: freed stream-ordered on stage-0's stream.
        };

        // ---- MIDDLE STAGES: RX boundary s-1 -> range -> TX boundary s ----
        for s in 1..n_st - 1 {
            let _st = rt.enter(s);
            let es = rt.engine(s, e);
            let pos_v: Vec<i32> = caches.iter().map(|c| c.pos as i32).collect();
            let pos_d = es.htod_i32(&pos_v)?;
            let x = rt.rx(s - 1, slot, payload)?;
            let x = if b1_stage_fast {
                self.decode_layers_eager(es, x, fence[s], fence[s + 1], &pos_d, pos0, caches[0])?
            } else if step35_batched {
                self.step35_decode_batch_layers(
                    es,
                    x,
                    caches,
                    &pos_d,
                    fence[s],
                    fence[s + 1],
                    &mut ph_last,
                )?
            } else {
                let ctx = self.batch_layer_ctx(es, caches, fence[s], fence[s + 1])?;
                self.decode_batch_layers(es, x, caches, &ctx, &pos_d, &mut ph_last)?
            };
            slot = rt.tx(s, &x, payload)?;
        }

        // ---- LAST STAGE: RX + final range + head + the batched serving epilogue ----
        let _stl = rt.enter(n_st - 1);
        let el = rt.engine(n_st - 1, e);
        let pos_v: Vec<i32> = caches.iter().map(|c| c.pos as i32).collect();
        let pos_d = el.htod_i32(&pos_v)?;
        let x = rt.rx(n_st - 2, slot, payload)?;
        let x = if b1_stage_fast {
            self.decode_layers_eager(el, x, fence[n_st - 1], fence[n_st], &pos_d, pos0, caches[0])?
        } else if step35_batched {
            self.step35_decode_batch_layers(
                el,
                x,
                caches,
                &pos_d,
                fence[n_st - 1],
                fence[n_st],
                &mut ph_last,
            )?
        } else {
            let ctx = self.batch_layer_ctx(el, caches, fence[n_st - 1], fence[n_st])?;
            self.decode_batch_layers(el, x, caches, &ctx, &pos_d, &mut ph_last)?
        };

        let mut hn = el.uninit(payload)?;
        el.rms_norm(&x, self.output_norm.float_data(), &mut hn, n_embd, b_n, eps)?;
        let logits = el.matmul(&self.output, &hn, b_n)?;
        ph_mark(el, 10, &mut ph_last)?;

        self.decode_batch_epilogue(el, caches, samp, masks, lean, logits, b_n, &mut ph_last)
    }

    /// Build the per-step layer context for layers `[lo, hi)`: the device state-pointer
    /// table plus the step's arm picks. See [`BatchLayerCtx`] for why this is RANGE-scoped
    /// (the table holds device addresses and must be uploaded through the engine whose
    /// device runs those layers).
    ///
    /// Table layout is unchanged from the whole-trunk version — `lin_base`/`attn_base` are
    /// still indexed by ABSOLUTE layer id, so `decode_batch_layers`' body indexes them
    /// exactly as the old inline loop did. Only layers in `[lo, hi)` contribute entries; the
    /// rest stay `None`, which is a loud `expect` if a range ever reads outside its own.
    pub(crate) fn batch_layer_ctx(
        &self,
        e: &Engine,
        caches: &[&mut Cache],
        lo: usize,
        hi: usize,
    ) -> Result<BatchLayerCtx, Box<dyn std::error::Error>> {
        let cfg = &self.cfg;
        let head_dim = cfg.head_dim_k as usize;
        // Per-step STATE POINTER TABLE (one H2D): for every linear layer, [conv x B]
        // [ssm_in x B][ssm_out x B] device addresses. The batched state kernels read their
        // sequence's pointer from these arrays — states stay per-cache (no pooling refactor),
        // yet conv/prep/scan collapse from 3xB launches per layer to 3. Rebuilt every step
        // because the ssm ping-pong swaps pointers host-side after each scan.
        // INCREMENT 2 (2026-08-01): the SAME table now also carries, for every FULL-attn
        // layer, [k0,v0,k1,v1,...] cache base addresses — the z-batched seqs append and
        // seqs fa_decode kernels read their sequence's cache through it (the MoE
        // expert-table pattern), collapsing 2xB launches per attn layer to 2.
        let mut lin_base: Vec<Option<usize>> = vec![None; self.layers.len()];
        let mut attn_base: Vec<Option<usize>> = vec![None; self.layers.len()];
        let mut ptrs: Vec<u64> = Vec::new();
        {
            use cudarc::driver::DevicePtr;
            let s = &e.gpu.stream();
            for il in lo..hi {
                match &self.layers[il].mixer {
                    Mixer::Linear(_) => {
                        lin_base[il] = Some(ptrs.len());
                        for c in caches.iter() {
                            let rl = c.recur[il].as_ref().unwrap();
                            let (p, _g) = rl.conv_state.device_ptr(s);
                            ptrs.push(p as u64);
                        }
                        for c in caches.iter() {
                            let rl = c.recur[il].as_ref().unwrap();
                            let (p, _g) = rl.ssm_state.device_ptr(s);
                            ptrs.push(p as u64);
                        }
                        for c in caches.iter() {
                            let rl = c.recur[il].as_ref().unwrap();
                            let (p, _g) = rl.ssm_state_alt.device_ptr(s);
                            ptrs.push(p as u64);
                        }
                    }
                    Mixer::Full(_) => {
                        attn_base[il] = Some(ptrs.len());
                        for c in caches.iter() {
                            let kvl = c.kv[il].as_ref().unwrap();
                            let (pk, _g) = kvl.k.device_ptr(s);
                            let (pv, _g2) = kvl.v.device_ptr(s);
                            ptrs.push(pk as u64);
                            ptrs.push(pv as u64);
                        }
                    }
                    Mixer::Mla(_) => crate::hybrid::mla_forward_unimplemented(),
                }
            }
        }
        let ptr_table = if ptrs.is_empty() {
            None
        } else {
            Some(e.htod_u64(&ptrs)?)
        };

        // INCREMENT 2 arm picks (per STEP — t_kv is layer-invariant within a tick):
        // - seqs APPEND: format-only condition (per-row program is t_kv-independent);
        //   default flash module only (fp8-KV rides the per-seq g-module path).
        // - seqs FA: every row must take the v4 eager arm at ITS OWN t_kv AND all rows
        //   must share ONE fa_split_keys rung (the rows-twins' straddle law) — a rung
        //   crossing inside the batch keeps the per-seq loop for that step, so each
        //   sequence always executes the exact program its isolated run would.
        // MEMRA_BATCH_APPEND=0 / MEMRA_BATCH_FA=0 are the rollback/A-B seams.
        //
        // The picks are t_kv-driven, and t_kv is layer-INVARIANT within a step, so every
        // stage of a pp split independently computes the SAME arms from the same `caches`
        // — a stage cannot silently take a different program than its unsplit self.
        let t_kvs: Vec<usize> = caches.iter().map(|c| c.pos + 1).collect();
        let t_kv_max = *t_kvs.iter().max().unwrap();
        let seqs_append = {
            static ON: std::sync::OnceLock<bool> = std::sync::OnceLock::new();
            *ON.get_or_init(|| std::env::var("MEMRA_BATCH_APPEND").as_deref() != Ok("0"))
        } && !Engine::kv_fp8_on();
        let sp0 = crate::fa_split_keys(t_kvs[0], cfg.n_head_kv as usize);
        let seqs_fa = {
            static ON: std::sync::OnceLock<bool> = std::sync::OnceLock::new();
            *ON.get_or_init(|| std::env::var("MEMRA_BATCH_FA").as_deref() != Ok("0"))
        } && t_kvs.iter().all(|&t| crate::fa_seqs_eligible(t, head_dim))
            && t_kvs
                .iter()
                .all(|&t| crate::fa_split_keys(t, cfg.n_head_kv as usize) == sp0);

        Ok(BatchLayerCtx {
            lin_base,
            attn_base,
            ptr_table,
            t_kvs,
            t_kv_max,
            sp0,
            seqs_append,
            seqs_fa,
            lo,
            hi,
        })
    }

    /// THE PP SEAM (pp2-batch increment 1, 2026-08-06): run the batched trunk over layers
    /// `[ctx.lo, ctx.hi)`, entering with a materialized `[B, n_embd]` residual and exiting
    /// with the range's final residual materialized. The batched twin of
    /// `decode_layers_eager` — the eager arm has had this seam since M1-PP2 and every ppN
    /// stage calls it; the batched body had no equivalent, which is why every later PP-2
    /// increment (and spec-over-PP2, whose verify is a batched T=K+1 forward) waited on this
    /// extraction (`research/pp2-hardening-20260806/PROGRESS.md` bill item 1).
    ///
    /// SINGLE-DEVICE SEMANTICS ARE UNCHANGED BY CONSTRUCTION: the body is the old
    /// `for (il, layer) in self.layers.iter().enumerate()` loop moved verbatim, with `for il
    /// in ctx.lo..ctx.hi` as the header and the per-step invariants (`ptr_table`, arm picks,
    /// `t_kv`) read from `ctx` instead of enclosing locals. At `lo=0, hi=n_layers` — every
    /// call today — the launch sequence is identical, so the exactness contract in this
    /// module's header carries over untouched rather than needing a re-proof.
    ///
    /// UNLIKE the eager seam, this one is NOT yet stage-callable: `caches` is `&mut [&mut
    /// Cache]` mutated in place (KV `len` bumps, ssm ping-pong swaps), and `pos_d`/`x` come
    /// from the caller's device. Wiring a stage split means per-stage `pos_d` + a boundary
    /// `[B, n_embd]` transfer around this call, which is the NEXT increment. The seam exists
    /// so that increment is a call-site change, not a 250-line surgery.
    #[allow(clippy::too_many_arguments)]
    pub(crate) fn decode_batch_layers(
        &self,
        e: &Engine,
        mut x: CudaSlice<f32>,
        caches: &mut [&mut Cache],
        ctx: &BatchLayerCtx,
        pos_d: &CudaSlice<i32>,
        ph_last: &mut std::time::Instant,
    ) -> Result<CudaSlice<f32>, Box<dyn std::error::Error>> {
        let b_n = caches.len();
        let cfg = &self.cfg;
        let n_embd = cfg.n_embd as usize;
        let eps = cfg.rms_eps;
        let (lin_base, attn_base) = (&ctx.lin_base, &ctx.attn_base);
        let ptr_table = &ctx.ptr_table;
        let (seqs_append, seqs_fa, sp0, t_kv_max) =
            (ctx.seqs_append, ctx.seqs_fa, ctx.sp0, ctx.t_kv_max);
        debug_assert_eq!(
            ctx.t_kvs.len(),
            b_n,
            "ctx built for a different batch width"
        );

        for il in ctx.lo..ctx.hi {
            let layer = &self.layers[il];
            // ---- attn_norm + q8_1 quantize, batched (B rows) ----
            let anorm = layer.attn_norm.float_data();
            let mut xn = e.uninit(b_n * n_embd)?;
            e.rms_norm(&x, anorm, &mut xn, n_embd, b_n, eps)?;
            let (hq, hd) = e.quantize_q8_1(&xn, b_n, n_embd)?;

            // ---- mixer ----
            let mixed: CudaSlice<f32> = match &layer.mixer {
                Mixer::Mla(_) => crate::hybrid::mla_forward_unimplemented(),
                Mixer::Full(fa) => {
                    let geometry = cfg.full_attention_geometry_at(il as u32);
                    let n_head = geometry.n_head as usize;
                    let n_head_kv = geometry.n_head_kv as usize;
                    let head_dim = geometry.head_dim_k as usize;
                    let rope_dims = geometry.n_rot as usize;
                    let rope_base = geometry.rope_base;
                    let scale = geometry.attention_scale();
                    // Batched projections: one weight read serves all B rows.
                    let qf = e.matmul_pre(&fa.wq, &hq, &hd, &xn, b_n)?;
                    let mut k = e.matmul_pre(&fa.wk, &hq, &hd, &xn, b_n)?;
                    let v = e.matmul_pre(&fa.wv, &hq, &hd, &xn, b_n)?;

                    let gated =
                        geometry.attention_gate == memra_gguf::config::AttentionGateKind::FusedQ;
                    let (mut q, gate) = if gated {
                        let mut qs = e.uninit(b_n * n_head * head_dim)?;
                        let mut gs = e.uninit(b_n * n_head * head_dim)?;
                        e.q_gate_split(&qf, &mut qs, &mut gs, head_dim, n_head, b_n)?;
                        (qs, Some(gs))
                    } else {
                        (qf, None)
                    };

                    // QK-norm over B*n_head rows, rope with per-row positions.
                    let mut qn = e.uninit(b_n * n_head * head_dim)?;
                    e.rms_norm(
                        &q,
                        fa.q_norm.float_data(),
                        &mut qn,
                        head_dim,
                        b_n * n_head,
                        eps,
                    )?;
                    q = qn;
                    let mut kn = e.uninit(b_n * n_head_kv * head_dim)?;
                    e.rms_norm(
                        &k,
                        fa.k_norm.float_data(),
                        &mut kn,
                        head_dim,
                        b_n * n_head_kv,
                        eps,
                    )?;
                    k = kn;
                    e.rope_neox(
                        &mut q, &pos_d, head_dim, rope_dims, n_head, b_n, rope_base, 1.0,
                    )?;
                    e.rope_neox(
                        &mut k, &pos_d, head_dim, rope_dims, n_head_kv, b_n, rope_base, 1.0,
                    )?;
                    ph_mark(e, 1, ph_last)?;

                    // INCREMENT 2 (2026-08-01): the per-seq (append, attend) launch train
                    // becomes two phases. Phase A appends all B rows (one z-batched launch,
                    // or the per-seq loop on the seam/fp8 path); phase B attends all B
                    // sequences (one blockIdx.z launch + one combine on the batched arm —
                    // which also reads q / writes attn at row offsets, killing the per-seq
                    // q/a dtod copies — or the per-seq loop when any row is outside the v4
                    // arm / a split rung crosses inside the batch). Caches are disjoint per
                    // sequence, so the phase split leaves every row's math untouched.
                    let q_dim = n_head * head_dim;
                    let kv_dim = n_head_kv * head_dim;
                    let mut attn = e.uninit(b_n * q_dim)?;
                    // ---- phase A: KV append (all B rows) ----
                    if seqs_append {
                        let (kdk, kdv, ktb, vtb) = {
                            let kvl = caches[0].kv[il].as_ref().unwrap();
                            (kvl.kv_dim_k, kvl.kv_dim_v, kvl.k_tok_bytes, kvl.v_tok_bytes)
                        };
                        let base = attn_base[il].expect("full layer missing from pointer table");
                        let table = ptr_table.as_ref().expect("pointer table missing");
                        let kv_view = table.slice(base..base + 2 * b_n);
                        e.append_kv_quantized_seqs(
                            &k, &v, &kv_view, &pos_d, b_n, kdk, kdv, ktb, vtb,
                        )?;
                        for cache in caches.iter_mut() {
                            let kvl = cache.kv[il].as_mut().unwrap();
                            debug_assert_eq!(kvl.len, cache.pos, "kv len / pos out of lockstep");
                            kvl.len += 1;
                        }
                    } else {
                        for (bi, cache) in caches.iter_mut().enumerate() {
                            let kvl = cache.kv[il].as_mut().unwrap();
                            let k_row = k.slice(bi * kv_dim..(bi + 1) * kv_dim);
                            let v_row = v.slice(bi * kv_dim..(bi + 1) * kv_dim);
                            e.append_kv_quantized_view(
                                &k_row,
                                &v_row,
                                &mut kvl.k,
                                &mut kvl.v,
                                kvl.len,
                                kvl.kv_dim_k,
                                kvl.kv_dim_v,
                                kvl.k_tok_bytes,
                                kvl.v_tok_bytes,
                                Engine::kv_fp8_on(),
                            )?;
                            kvl.len += 1;
                        }
                    }
                    ph_mark(e, 2, ph_last)?;
                    // ---- phase B: attention (all B sequences) ----
                    if seqs_fa {
                        let (ktb, vtb) = {
                            let kvl = caches[0].kv[il].as_ref().unwrap();
                            (kvl.k_tok_bytes, kvl.v_tok_bytes)
                        };
                        let base = attn_base[il].expect("full layer missing from pointer table");
                        let table = ptr_table.as_ref().expect("pointer table missing");
                        let kv_view = table.slice(base..base + 2 * b_n);
                        e.fa_decode_batch_seqs_v4(
                            &q, &kv_view, &pos_d, &mut attn, head_dim, n_head, n_head_kv, b_n,
                            t_kv_max, scale, sp0, ktb, vtb,
                        )?;
                        ph_mark(e, 4, ph_last)?;
                    } else {
                        for (bi, cache) in caches.iter_mut().enumerate() {
                            let kvl = cache.kv[il].as_mut().unwrap();
                            let t_kv = kvl.len;
                            let k_view = e.view_u8(&kvl.k, t_kv * kvl.k_tok_bytes);
                            let v_view = e.view_u8(&kvl.v, t_kv * kvl.v_tok_bytes);
                            // The fallback keeps one FA launch per distinct KV view, but Q and
                            // attention already live in packed row-major buffers. Pass those row
                            // views directly; only the arithmetic-free materialization copies go.
                            let q_row = q.slice(bi * q_dim..(bi + 1) * q_dim);
                            let mut a_row = attn.slice_mut(bi * q_dim..(bi + 1) * q_dim);
                            e.fa_decode_kvmod_view(
                                &q_row,
                                &k_view,
                                &v_view,
                                &mut a_row,
                                head_dim,
                                n_head,
                                n_head_kv,
                                t_kv,
                                scale,
                                kvl.k_tok_bytes,
                                kvl.v_tok_bytes,
                                Engine::kv_fp8_on(),
                            )?;
                            ph_mark(e, 4, ph_last)?;
                        }
                    }

                    // Output gate (element-wise — batches whole) + o-proj at m=B.
                    let attn_g = match &gate {
                        Some(g) => {
                            let n = b_n * q_dim;
                            let mut gsig = e.uninit(n)?;
                            e.sigmoid(g, &mut gsig, n)?;
                            let mut ag = e.uninit(n)?;
                            e.mul(&attn, &gsig, &mut ag, n)?;
                            ag
                        }
                        None => attn,
                    };
                    let o = e.matmul(&fa.wo, &attn_g, b_n)?;
                    ph_mark(e, 5, ph_last)?;
                    o
                }
                Mixer::Linear(la) => {
                    // v2 (the B-scaling fix): the GDN mixer's PROJECTIONS carry the layer's
                    // weight mass — batch them at m=B so wqkv/gate/beta/alpha/ssm_out stream
                    // ONCE per step instead of once per sequence. Only the recurrent state ops
                    // (fused conv ring, gdn prep, gdn scan) stay per-seq — they are state-bound
                    // micro-kernels, not weight readers. Composition unchanged vs v1 (matmul_pre
                    // == fused2 per (tensor,row); _bN mmvq per-row == m=1): same numeric config.
                    let ssm = cfg.ssm.as_ref().expect("linear mixer requires ssm cfg");
                    let d_state = ssm.state_size as usize;
                    let num_k = ssm.group_count as usize;
                    let num_v = ssm.time_step_rank as usize;
                    let d_conv = ssm.conv_kernel as usize;
                    let key_dim = d_state * num_k;
                    let value_dim = d_state * num_v;
                    let conv_dim = key_dim * 2 + value_dim;
                    let gdn_scale = 1.0 / (d_state as f32).sqrt();

                    // ---- batched projections (the weight win) ----
                    let qkv_mixed = e.matmul_pre(&la.wqkv, &hq, &hd, &xn, b_n)?;
                    let z = e.matmul_pre(&la.wqkv_gate, &hq, &hd, &xn, b_n)?;
                    let beta_raw = e.matmul_pre(&la.ssm_beta, &hq, &hd, &xn, b_n)?;
                    let alpha = e.matmul_pre(&la.ssm_alpha, &hq, &hd, &xn, b_n)?;
                    ph_mark(e, 6, ph_last)?;

                    // ---- batched recurrent state ops (3 launches for all B sequences) ----
                    let base = lin_base[il].expect("linear layer missing from pointer table");
                    let table = ptr_table.as_ref().expect("pointer table missing");
                    let conv_view = table.slice(base..base + b_n);
                    let in_view = table.slice(base + b_n..base + 2 * b_n);
                    let out_view = table.slice(base + 2 * b_n..base + 3 * b_n);
                    let mut conv_outs = e.uninit(b_n * conv_dim)?;
                    e.ssm_conv1d_fused_decode_b(
                        &qkv_mixed,
                        &conv_view,
                        la.ssm_conv1d.float_data(),
                        &mut conv_outs,
                        conv_dim,
                        d_conv,
                        b_n,
                    )?;
                    let mut q_l2 = e.uninit(b_n * value_dim)?;
                    let mut k_l2 = e.uninit(b_n * value_dim)?;
                    let mut v_gd = e.uninit(b_n * value_dim)?;
                    let mut beta_b = e.uninit(b_n * num_v)?;
                    let mut g_log = e.uninit(b_n * num_v)?;
                    e.gdn_prep_decode_b(
                        &conv_outs,
                        &beta_raw,
                        &alpha,
                        la.ssm_dt.float_data(),
                        la.ssm_a.float_data(),
                        &mut q_l2,
                        &mut k_l2,
                        &mut v_gd,
                        &mut beta_b,
                        &mut g_log,
                        d_state,
                        num_v,
                        num_k,
                        key_dim,
                        eps,
                        conv_dim,
                        b_n,
                    )?;
                    let mut o_all = e.uninit(b_n * value_dim)?;
                    e.gdn_scan_s128_batched(
                        &q_l2, &k_l2, &v_gd, &g_log, &beta_b, &in_view, &out_view, &mut o_all,
                        num_v, b_n, gdn_scale,
                    )?;
                    // ping-pong: scan wrote each seq's alt buffer; swap host handles (the
                    // NEXT step's table rebuild picks up the new canonical pointers).
                    for cache in caches.iter_mut() {
                        let rl = cache.recur[il].as_mut().unwrap();
                        std::mem::swap(&mut rl.ssm_state, &mut rl.ssm_state_alt);
                    }
                    ph_mark(e, 7, ph_last)?;

                    // ---- batched gated norm + out-projection ----
                    let o = if e.uses_q8_1_fast(&la.ssm_out) {
                        let (gq, gd) = e.gated_rmsnorm_q8_1(
                            &o_all,
                            la.ssm_norm.float_data(),
                            &z,
                            d_state,
                            b_n * num_v,
                            eps,
                        )?;
                        let g0 = e.zeros(0)?;
                        e.matmul_pre(&la.ssm_out, &gq, &gd, &g0, b_n)?
                    } else {
                        let mut gn = e.uninit(b_n * value_dim)?;
                        e.gated_rmsnorm(
                            &o_all,
                            la.ssm_norm.float_data(),
                            &z,
                            &mut gn,
                            d_state,
                            b_n * num_v,
                            eps,
                        )?;
                        e.matmul(&la.ssm_out, &gn, b_n)?
                    };
                    ph_mark(e, 8, ph_last)?;
                    o
                }
            };

            // ---- residual add + post_attn_norm + FFN, batched ----
            let pnorm = layer.post_attn_norm.float_data();
            let mut x1 = e.uninit(b_n * n_embd)?;
            let mut z = e.uninit(b_n * n_embd)?;
            e.add_rms_norm(&x, &mixed, pnorm, &mut x1, &mut z, n_embd, b_n, eps)?;
            let ffn_out = match &layer.ffn {
                crate::hybrid::Ffn::Dense {
                    ffn_gate,
                    ffn_up,
                    ffn_down,
                } => {
                    // v1 covers the SiLU family; M3's swigluoai clamp rides a scaled epilogue
                    // (m=1 fused tier) — batched M3 lands with the batched-fusion pass.
                    assert!(
                        self.cfg.m3.is_none(),
                        "decode_step_batch v1: M3 swigluoai FFN not yet batched"
                    );
                    let n_ff = ffn_gate.out_features();
                    let (zq, zd) = e.quantize_q8_1(&z, b_n, n_embd)?;
                    // REFUTED ARM (lane/q27-deepdive, 2026-08-05): fusing this gate+up pair
                    // into `matmul_q8_fused2_t` (the fused2_b8 tier) measured FLAT-TO-NEGATIVE
                    // at the serving tick — bench c=8 213.1/213.8, 213.9/214.4, 214.4/213.5
                    // (sign flips) and serve c=8 paired mean −0.20% over 3 passes. Mechanism:
                    // unlike m=1 (where the pair is 128 of 1015 launches in a 7.67%-gap tick),
                    // the c=8 tick is 73.2% one weight-bound kernel class with launch cost
                    // already hidden — halving 128 launches of ~28k buys nothing. The m=1 arm
                    // in `matmul_pre_dual_noscale` (+0.94%) stays; this call site keeps the two
                    // launches. Kernel + fused2_b8 wrapper retained: kernel-check gates it at
                    // m=5/8 and matmul_q8_fused2_t serves the verify tier. Receipts:
                    // research/q27-deepdive-20260805/ (lever3-bench-*, serve-points.jsonl).
                    let g = e.matmul_pre(ffn_gate, &zq, &zd, &z, b_n)?;
                    let u = e.matmul_pre(ffn_up, &zq, &zd, &z, b_n)?;
                    let mut act = e.uninit(b_n * n_ff)?;
                    e.silu_mul(&g, &u, &mut act, b_n * n_ff)?;
                    let (aq, ad) = e.quantize_q8_1(&act, b_n, n_ff)?;
                    e.matmul_pre(ffn_down, &aq, &ad, &act, b_n)?
                }
                crate::hybrid::Ffn::Moe(m) => {
                    self.moe_ffn_il_zq8(e, m, &z, None, b_n, il as u16)?
                }
            };
            // next-layer input x = x1 + ffn_out (batched element-wise add)
            let mut x2 = e.uninit(b_n * n_embd)?;
            e.add(&x1, &ffn_out, &mut x2, b_n * n_embd)?;
            x = x2;
            ph_mark(e, 9, ph_last)?;
        }
        Ok(x)
    }

    /// Rollback seam for the step35 batched decode arm (lane/step35-batched-decode,
    /// 2026-08-08). Default ON; `MEMRA_STEP35_BATCH=0` caps serving at B=1 and makes the
    /// batched bodies return Err. Since lane/cx-b1fix, PP-N also refuses the eager B=1
    /// numeric class, so the seam disables PP-N Step35 decode rather than serving unstable
    /// bytes. Also the b2geo35 gate's CANARY seam — the live assertions must fail under it.
    pub fn step35_batch_on() -> bool {
        static ON: std::sync::OnceLock<bool> = std::sync::OnceLock::new();
        *ON.get_or_init(|| std::env::var("MEMRA_STEP35_BATCH").as_deref() != Ok("0"))
    }

    /// THE step35 BATCHED LAYER WALK (lane/step35-batched-decode, 2026-08-08): B sequences
    /// share one pass over layers `[lo, hi)` with the REAL step35 geometry — the arm that
    /// kills the B=1 pin (34 tok/s aggregate FLAT across c=1..8, round-robin serialized;
    /// research/step-sku-20260807 §4) without re-opening the b2ab garbage hole (the generic
    /// `decode_batch_layers` ran uniform n_head/full-width rope/no window/no gate over
    /// step35 weights and returned HTTP-200 garbage at c>1).
    ///
    /// SHAPE — batched where the weights are, per-session where the state is:
    ///   * attn_norm + quantize + wq/wk/wv/attn_gate projections + q/k norms + rope + head
    ///     gate + wo + residual/post-norm + FFN all run at m=B: ONE weight stream serves B
    ///     rows (decode is weight-BW-bound; this is the entire win).
    ///   * KV append + fa_decode stay a per-session loop — the SWA window makes each
    ///     session's KV view a function of ITS OWN `kvl.len` (`off = len-win` when past the
    ///     window), and the z-batched seqs kernels take one shared t_kv/rung, not per-row
    ///     offsets. This is the same shape as `decode_batch_layers`' per-seq fallback arm,
    ///     and it costs launches, not weight bandwidth (KV is per-session state either way).
    ///
    /// PER-LAYER GEOMETRY (the five mechanisms that make the generic body wrong here, all
    /// from `step35_geom`/cfg): n_head 64 full / 96 SWA (wq/wo/attn_gate widths per layer),
    /// partial rope (n_rot 64 full / 128 SWA), dual base (5e6/1e4) + `rope_freqs` factors
    /// on FULL layers only, SWA window 512 with per-SESSION view offsets, and the separate
    /// head-wise `attn_gate` (one pre-sigmoid scalar per (token, head), input = the
    /// post-attn_norm hidden, applied before wo).
    ///
    /// EXACTNESS (the isolation contract, decode-batch-gate gate2's bar): every kernel here
    /// is row-independent at m=B or per-session:
    ///   * `rms_norm`/`add_rms_norm`/`quantize_q8_1`/`attn_head_gate`/activations: per-row
    ///     programs, grid over rows — row bi's bytes are the 1-row call's bytes.
    ///   * projections via `matmul_pre` at m=2..8: Q8_0/Q6_K-class rides the b2/b4/b8
    ///     batched-mmvq tier (bit-identical per (token,row) to m=1 mmvq); IQ4_XS — this
    ///     SKU's trunk class — has no mmvq/batched kernel, so BOTH m=1 decode and the m=B
    ///     walk ride `qmatvec_iq4_XS_dp4a` (grid (out_f, m): each column IS the m=1 dp4a
    ///     program). Same class at every width = the decode-parity law by construction.
    ///   * `rope_neox2` takes per-row positions (tok = row / n_heads) — row bi rotates at
    ///     ITS pos with the layer's (n_rot, base, ff), same bits as its solo call.
    ///   * per-session append/fa_decode_kvmod: literally the eager arm's calls on that
    ///     session's own cache and views.
    ///   * MoE (`moe_ffn_il_zq8` at t=B): the router is per-column decode-exact at
    ///     t < PRIME_MIN_T (m=1 program per column), sigmoid routing + expert dispatch are
    ///     per-token — a session's experts are a function of its own row only.
    /// The known eager-vs-batched FP gap is why PP-N Step35 deliberately serves THIS walk at
    /// B=1 too: the scheduler can change width during a session, so one numeric class must
    /// cover every live width. `b2geo35` pins static widths and an explicit B=1 -> B>1
    /// transition under live defaults.
    ///
    /// STAGE-SCOPED FROM BIRTH: `[lo, hi)` + caller-supplied engine/pos_d, so
    /// `decode_step_batch_ppn` calls it per stage (per-stage engine, per-stage pos_d, the
    /// #87 entry fence and boundary slots unchanged) — the pp2-batch seam lesson.
    #[allow(clippy::too_many_arguments)]
    pub(crate) fn step35_decode_batch_layers(
        &self,
        e: &Engine,
        x: CudaSlice<f32>,
        caches: &mut [&mut Cache],
        pos_d: &CudaSlice<i32>,
        lo: usize,
        hi: usize,
        ph_last: &mut std::time::Instant,
    ) -> Result<CudaSlice<f32>, Box<dyn std::error::Error>> {
        self.step35_decode_rows_layers(e, x, caches, pos_d, None, lo, hi, ph_last)
    }

    /// Diagnostic generalization of the serving walk: `row_to_cache[r]` names the session
    /// whose KV row is consumed by hidden row `r`. Serving passes `None`, preserving the
    /// identity mapping and its launch sequence. The MoESD harness passes B groups of gamma
    /// consecutive rows so each session's verify columns append causally while projections and
    /// MoE dispatch see the full B*gamma target width.
    #[allow(clippy::too_many_arguments)]
    fn step35_decode_rows_layers(
        &self,
        e: &Engine,
        mut x: CudaSlice<f32>,
        caches: &mut [&mut Cache],
        pos_d: &CudaSlice<i32>,
        row_to_cache: Option<&[usize]>,
        lo: usize,
        hi: usize,
        ph_last: &mut std::time::Instant,
    ) -> Result<CudaSlice<f32>, Box<dyn std::error::Error>> {
        let b_n = row_to_cache.map_or(caches.len(), |rows| rows.len());
        let cfg = &self.cfg;
        let n_embd = cfg.n_embd as usize;
        let eps = cfg.rms_eps;
        cfg.step35
            .as_ref()
            .ok_or("step35_decode_batch_layers requires step35 cfg")?;
        if b_n == 0 || x.len() != b_n * n_embd || pos_d.len() != b_n {
            return Err(format!(
                "step35 row mapping shape mismatch: rows={b_n} x={} pos={} n_embd={n_embd}",
                x.len(),
                pos_d.len(),
            )
            .into());
        }
        if row_to_cache.is_some_and(|rows| rows.iter().any(|&ci| ci >= caches.len())) {
            return Err("step35 row mapping names a missing cache".into());
        }
        let cache_index = |row: usize| row_to_cache.map_or(row, |rows| rows[row]);
        // b2geo35 gate evidence: one line, first B>1 walk only (grep-stable prefix).
        if b_n > 1 {
            static ONCE: std::sync::Once = std::sync::Once::new();
            ONCE.call_once(|| {
                eprintln!(
                    "[step35-batch] first B>1 batched step35 walk: B={b_n} layers=[{lo},{hi})"
                );
            });
        }

        for il in lo..hi {
            let layer = &self.layers[il];
            let Mixer::Full(fa) = &layer.mixer else {
                return Err(format!("step35 layer {il} is not full-attn — corrupt config").into());
            };
            let geometry = self.step35_geom(il);
            let hd = geometry.head_dim_k as usize;
            let nkv = geometry.n_head_kv as usize;
            let nh = geometry.n_head as usize;
            let rbase = geometry.rope_base;
            let scale = geometry.attention_scale();
            let swa = geometry.window.is_some();
            let win = geometry.window.unwrap_or(0) as usize;
            let n_rot = geometry.n_rot as usize;
            let q_dim = nh * hd;
            let kv_dim = nkv * hd;

            // ---- attn_norm + q8_1 quantize, batched (B rows) ----
            let anorm = layer.attn_norm.float_data();
            let mut xn = e.uninit(b_n * n_embd)?;
            e.rms_norm(&x, anorm, &mut xn, n_embd, b_n, eps)?;
            let (hq, hdq) = e.quantize_q8_1(&xn, b_n, n_embd)?;

            // ---- batched projections: q/k/v + the separate head-wise gate (one weight
            // stream for B rows; xn is the live f32 fallback for non-q8_1-fast classes) ----
            let q0 = e.matmul_pre(&fa.wq, &hq, &hdq, &xn, b_n)?;
            let k0 = e.matmul_pre(&fa.wk, &hq, &hdq, &xn, b_n)?;
            let v0 = e.matmul_pre(&fa.wv, &hq, &hdq, &xn, b_n)?;
            let gw = fa
                .attn_gate
                .as_ref()
                .ok_or("step35 layer is missing attn_gate.weight (head-wise attention gate)")?;
            // gate input = the post-attn_norm hidden (upstream `cur`) — same xn/q8 pair.
            let gt = e.matmul_pre(gw, &hq, &hdq, &xn, b_n)?;

            // ---- q/k RMSNorm over head_dim rows + the per-layer PARTIAL rope ----
            let mut q = e.uninit(b_n * q_dim)?;
            e.rms_norm(&q0, fa.q_norm.float_data(), &mut q, hd, b_n * nh, eps)?;
            let mut k = e.uninit(b_n * kv_dim)?;
            e.rms_norm(&k0, fa.k_norm.float_data(), &mut k, hd, b_n * nkv, eps)?;
            let ff = if geometry.rope_factors {
                self.step35_aux.as_ref().and_then(|a| a.rope_freqs(e))
            } else {
                None
            };
            e.rope_neox2(
                &mut q, &mut k, pos_d, hd, n_rot, nh, nkv, b_n, rbase, 1.0, ff,
            )?;
            ph_mark(e, 1, ph_last)?;

            // ---- per-session: KV append + windowed/global fa_decode (each session's OWN
            // len drives its view offset — the iso-gap law, no cross-session term) ----
            let mut attn = e.uninit(b_n * q_dim)?;
            if b_n == 1 {
                // B=1 SPECIALIZED ENTRY (lane/cx-eagerpar): the general row loop below
                // materializes q_row and a_row because a B>1 FA call consumes/produces one
                // contiguous row at a time. At B=1, q and attn already ARE those whole rows.
                // Pass them directly to the same fa_decode_kvmod call: this removes two
                // arithmetic-free D2D copies (90 launches/token on Step3.7's 45 layers)
                // without changing any arithmetic kernel, shape, argument value, or order.
                // Keep the B>1 body verbatim below; b1fix's one-class/transition gates are
                // the promotion bar, not an FP-similarity tolerance.
                let kvl = caches[cache_index(0)].kv[il].as_mut().unwrap();
                let k_row = k.slice(0..kv_dim);
                let v_row = v0.slice(0..kv_dim);
                let next_len = kvl.len + 1;
                let (off, t_kv) = if swa && next_len > win {
                    (next_len - win, win)
                } else {
                    (0, next_len)
                };
                let write_row = e.prepare_kv_append(kvl, off & !31usize, 1)?;
                e.append_kv_quantized_view(
                    &k_row,
                    &v_row,
                    &mut kvl.k,
                    &mut kvl.v,
                    write_row,
                    kvl.kv_dim_k,
                    kvl.kv_dim_v,
                    kvl.k_tok_bytes,
                    kvl.v_tok_bytes,
                    Engine::kv_fp8_on(),
                )?;
                kvl.len = next_len;
                ph_mark(e, 2, ph_last)?;
                let physical = kvl.physical_rows(off, off + t_kv)?;
                let k_view = e.view_u8_range(
                    &kvl.k,
                    physical.start * kvl.k_tok_bytes,
                    physical.end * kvl.k_tok_bytes,
                );
                let v_view = e.view_u8_range(
                    &kvl.v,
                    physical.start * kvl.v_tok_bytes,
                    physical.end * kvl.v_tok_bytes,
                );
                e.fa_decode_kvmod(
                    &q,
                    &k_view,
                    &v_view,
                    &mut attn,
                    hd,
                    nh,
                    nkv,
                    t_kv,
                    scale,
                    kvl.k_tok_bytes,
                    kvl.v_tok_bytes,
                    Engine::kv_fp8_on(),
                )?;
                ph_mark(e, 4, ph_last)?;
            } else {
                for bi in 0..b_n {
                    let cache = &mut caches[cache_index(bi)];
                    let kvl = cache.kv[il].as_mut().unwrap();
                    let k_row = k.slice(bi * kv_dim..(bi + 1) * kv_dim);
                    let v_row = v0.slice(bi * kv_dim..(bi + 1) * kv_dim);
                    let next_len = kvl.len + 1;
                    let (off, t_kv) = if swa && next_len > win {
                        (next_len - win, win)
                    } else {
                        (0, next_len)
                    };
                    let write_row = e.prepare_kv_append(kvl, off & !31usize, 1)?;
                    e.append_kv_quantized_view(
                        &k_row,
                        &v_row,
                        &mut kvl.k,
                        &mut kvl.v,
                        write_row,
                        kvl.kv_dim_k,
                        kvl.kv_dim_v,
                        kvl.k_tok_bytes,
                        kvl.v_tok_bytes,
                        Engine::kv_fp8_on(),
                    )?;
                    kvl.len = next_len;
                    ph_mark(e, 2, ph_last)?;
                    // the eager arm's SWA view arithmetic, verbatim (step35_decode_attn):
                    // token-aligned offset, keys carry absolute rope, mask is positional.
                    let physical = kvl.physical_rows(off, off + t_kv)?;
                    let k_view = e.view_u8_range(
                        &kvl.k,
                        physical.start * kvl.k_tok_bytes,
                        physical.end * kvl.k_tok_bytes,
                    );
                    let v_view = e.view_u8_range(
                        &kvl.v,
                        physical.start * kvl.v_tok_bytes,
                        physical.end * kvl.v_tok_bytes,
                    );
                    // The per-session cache view remains authoritative (including SWA's
                    // physical-row rebase), while Q/O use their existing packed row views.
                    // This preserves the exact FA program and removes only the two D2D copies.
                    let q_row = q.slice(bi * q_dim..(bi + 1) * q_dim);
                    let mut a_row = attn.slice_mut(bi * q_dim..(bi + 1) * q_dim);
                    e.fa_decode_kvmod_view(
                        &q_row,
                        &k_view,
                        &v_view,
                        &mut a_row,
                        hd,
                        nh,
                        nkv,
                        t_kv,
                        scale,
                        kvl.k_tok_bytes,
                        kvl.v_tok_bytes,
                        Engine::kv_fp8_on(),
                    )?;
                    ph_mark(e, 4, ph_last)?;
                }
            }

            // ---- head-wise gate (one sigmoid per (token, head), pre-wo) + o-proj at m=B ----
            let mut ag = e.uninit(b_n * q_dim)?;
            e.attn_head_gate(&attn, &gt, &mut ag, None, hd, nh, b_n)?;
            let mixed = e.matmul(&fa.wo, &ag, b_n)?;
            ph_mark(e, 5, ph_last)?;

            // ---- residual add + post_attn_norm + FFN, batched ----
            let pnorm = layer.post_attn_norm.float_data();
            let mut x1 = e.uninit(b_n * n_embd)?;
            let mut z = e.uninit(b_n * n_embd)?;
            e.add_rms_norm(&x, &mixed, pnorm, &mut x1, &mut z, n_embd, b_n, eps)?;
            let ffn_out = match &layer.ffn {
                crate::hybrid::Ffn::Dense {
                    ffn_gate,
                    ffn_up,
                    ffn_down,
                } => {
                    // A dense step35 FFN's clamp is the SHEXP array (upstream's one
                    // build_ffn serves dense + shared expert, llama-graph.cpp:1751);
                    // ffn_act_lim dispatches clamped/plain per layer. Layers 0-2 (the
                    // leading dense) have no live limit on this artifact, but the route
                    // is correct by construction, not by artifact.
                    let n_ff = ffn_gate.out_features();
                    let (zq, zd) = e.quantize_q8_1(&z, b_n, n_embd)?;
                    let g = e.matmul_pre(ffn_gate, &zq, &zd, &z, b_n)?;
                    let u = e.matmul_pre(ffn_up, &zq, &zd, &z, b_n)?;
                    let mut act = e.uninit(b_n * n_ff)?;
                    Self::ffn_act_lim(
                        e,
                        cfg,
                        &g,
                        &u,
                        1.0,
                        1.0,
                        cfg.clamp_shexp_at(il as u32),
                        &mut act,
                        b_n * n_ff,
                    )?;
                    let (aq, ad) = e.quantize_q8_1(&act, b_n, n_ff)?;
                    e.matmul_pre(ffn_down, &aq, &ad, &act, b_n)?
                }
                // t=B < PRIME_MIN_T: per-column decode-exact router + host sigmoid routing
                // + per-token expert dispatch — the same per-token program as eager t=1,
                // including the per-layer SwiGLU clamp (43/44) via the sequential path's
                // ffn_act_lim. The sigmoid-router deny on dev/pairs holds by predicate.
                crate::hybrid::Ffn::Moe(m) => {
                    self.moe_ffn_il_zq8(e, m, &z, None, b_n, il as u16)?
                }
            };
            let mut x2 = e.uninit(b_n * n_embd)?;
            e.add(&x1, &ffn_out, &mut x2, b_n * n_embd)?;
            x = x2;
            ph_mark(e, 9, ph_last)?;
        }
        Ok(x)
    }

    /// Standalone MoESD target forward. This entrypoint is not used by serving: it widens the
    /// existing Step-3.7 batched layer walk to B*gamma rows while preserving one causal KV chain
    /// per session. It returns device logits and performs no sampling or logits D2H, matching the
    /// target-model term T_T measured by the paper.
    pub fn moesd_target_forward(
        &self,
        e: &Engine,
        tokens: &[u32],
        batch: usize,
        gamma: usize,
        caches: &mut [&mut Cache],
    ) -> Result<CudaSlice<f32>, Box<dyn std::error::Error>> {
        if self.cfg.step35.is_none() {
            return Err("MoESD target forward currently requires Step-3.7/Step35 geometry".into());
        }
        if batch == 0 || gamma == 0 || caches.len() != batch || tokens.len() != batch * gamma {
            return Err(format!(
                "MoESD shape mismatch: B={batch} gamma={gamma} caches={} tokens={}",
                caches.len(),
                tokens.len(),
            )
            .into());
        }
        let rows = batch * gamma;
        if rows > 256 {
            return Err(format!("MoESD target width {rows} exceeds the frozen 32*8 matrix").into());
        }
        let n_embd = self.cfg.n_embd as usize;
        let eps = self.cfg.rms_eps;
        let payload = rows * n_embd;
        let row_to_cache: Vec<usize> = (0..batch)
            .flat_map(|session| (0..gamma).map(move |_| session))
            .collect();
        let positions: Vec<i32> = row_to_cache
            .iter()
            .enumerate()
            .map(|(row, &session)| (caches[session].pos + row % gamma) as i32)
            .collect();
        let mut ph_last = std::time::Instant::now();

        let logits = if let Some(fence) = crate::pp::pp_cuts(self.layers.len()) {
            if fence.len() != 3 || crate::pp::pp2_streams_off() {
                return Err(
                    "MoESD PP target forward requires the live two-stage stream split".into(),
                );
            }
            let rt = crate::pp::PpNRt::get(e)?;
            if rt.n_stages() != 2 {
                return Err(format!("MoESD expected two PP stages, got {}", rt.n_stages()).into());
            }
            let caller_stream = e.stream();
            rt.fence_stages_behind(&caller_stream)?;
            let slot = {
                let _st0 = rt.enter(0);
                let e0 = rt.engine(0, e);
                let pos_d = e0.htod_i32(&positions)?;
                let x = e0.htod(&self.embd.gather(n_embd, tokens))?;
                ph_mark(e0, 0, &mut ph_last)?;
                let x = self.step35_decode_rows_layers(
                    e0,
                    x,
                    caches,
                    &pos_d,
                    Some(&row_to_cache),
                    fence[0],
                    fence[1],
                    &mut ph_last,
                )?;
                rt.tx(0, &x, payload)?
            };
            let logits = {
                let _st1 = rt.enter(1);
                let e1 = rt.engine(1, e);
                let pos_d = e1.htod_i32(&positions)?;
                let x = rt.rx(0, slot, payload)?;
                let x = self.step35_decode_rows_layers(
                    e1,
                    x,
                    caches,
                    &pos_d,
                    Some(&row_to_cache),
                    fence[1],
                    fence[2],
                    &mut ph_last,
                )?;
                let mut hn = e1.uninit(payload)?;
                e1.rms_norm(
                    &x,
                    self.output_norm.float_data(),
                    &mut hn,
                    n_embd,
                    rows,
                    eps,
                )?;
                let logits = e1.matmul(&self.output, &hn, rows)?;
                rt.publish_to(1, &caller_stream)?;
                logits
            };
            logits
        } else {
            let pos_d = e.htod_i32(&positions)?;
            let x = e.htod(&self.embd.gather(n_embd, tokens))?;
            ph_mark(e, 0, &mut ph_last)?;
            let x = self.step35_decode_rows_layers(
                e,
                x,
                caches,
                &pos_d,
                Some(&row_to_cache),
                0,
                self.layers.len(),
                &mut ph_last,
            )?;
            let mut hn = e.uninit(payload)?;
            e.rms_norm(
                &x,
                self.output_norm.float_data(),
                &mut hn,
                n_embd,
                rows,
                eps,
            )?;
            e.matmul(&self.output, &hn, rows)?
        };
        for cache in caches.iter_mut() {
            cache.pos += gamma;
        }
        Ok(logits)
    }

    /// The batched tick's TAIL, after the trunk: grammar masks -> device sampling -> lean
    /// logits park -> `pos` bump. Split out with the pp seam (`decode_batch_layers`) because
    /// under a stage split this runs on the LAST stage's engine and device — the lm_head, the
    /// masks, the sampler, and `cache.last_logits_dev` all live where the final residual
    /// lands, and the caller must be able to place them there without duplicating 90 lines of
    /// serving contract. `logits` is `[b_n, n_vocab]` already computed by the caller (the
    /// output_norm + lm_head pair stays at the call site so a stage split can fence around
    /// it); everything after it is here, verbatim.
    #[allow(clippy::too_many_arguments)]
    fn decode_batch_epilogue(
        &self,
        e: &Engine,
        caches: &mut [&mut Cache],
        samp: &[Option<(f32, u64, u32)>],
        masks: &[Option<(&CudaSlice<u32>, usize)>],
        lean: bool,
        logits: CudaSlice<f32>,
        b_n: usize,
        ph_last: &mut std::time::Instant,
    ) -> Result<(Vec<Vec<f32>>, Vec<Option<u32>>), Box<dyn std::error::Error>> {
        // GRAMMAR MASKS (constrained decoding): preserve each masked row's PRISTINE logits
        // for its consumer (lean park into cache.last_logits_dev — the reuse-pool park stays
        // unmasked, the v1 contract — or the non-lean D2H), then ban in place BEFORE the
        // device sampler reads the row. All stream-ordered; masks=&[] takes no new branch.
        let n_vocab = self.output.out_features();
        let mut logits = logits;
        let mut pristine: Vec<Option<CudaSlice<f32>>> = Vec::new();
        if masks.iter().take(b_n).any(|m| m.is_some()) {
            pristine.resize_with(b_n, || None);
            for (bi, m) in masks.iter().take(b_n).enumerate() {
                let Some((mask, words)) = m else { continue };
                assert!(
                    samp.get(bi).copied().flatten().is_some(),
                    "grammar-masked row {bi} must request a device sample"
                );
                if lean {
                    let cache = &mut caches[bi];
                    if cache
                        .last_logits_dev
                        .as_ref()
                        .map(|d| d.len() < n_vocab)
                        .unwrap_or(true)
                    {
                        cache.last_logits_dev = Some(e.uninit(n_vocab)?);
                    }
                    let dst = cache.last_logits_dev.as_mut().unwrap();
                    e.dtod_copy_view(&logits.slice(bi * n_vocab..(bi + 1) * n_vocab), dst)?;
                } else {
                    let mut p = e.uninit(n_vocab)?;
                    e.dtod_copy_view(&logits.slice(bi * n_vocab..(bi + 1) * n_vocab), &mut p)?;
                    pristine[bi] = Some(p);
                }
                e.mask_logits_col(&mut logits, mask, bi, n_vocab, *words)?;
            }
        }

        // Device-side sampling for requested rows (see the method doc). Enqueued before the
        // big logits D2H so the tiny [B] token readback rides the same sync.
        let mut next: Vec<Option<u32>> = vec![None; b_n];
        if samp.iter().take(b_n).any(|s| s.is_some()) {
            let mut toks = e.alloc_u32_zeroed(b_n)?;
            let mut perturb: Option<CudaSlice<f32>> = None;
            for (bi, s) in samp.iter().take(b_n).enumerate() {
                let Some((temp, seed, ctr)) = s else { continue };
                if *temp <= 0.0 {
                    e.argmax_token_device_col(&logits, bi, n_vocab, &mut toks, bi)?;
                } else {
                    if perturb.is_none() {
                        perturb = Some(e.zeros(n_vocab)?);
                    }
                    let pb = perturb.as_mut().unwrap();
                    e.gumbel_perturb_col(&logits, bi, pb, n_vocab, *seed, *ctr, *temp)?;
                    e.argmax_token_device_col(pb, 0, n_vocab, &mut toks, bi)?;
                }
            }
            let host_toks = e.dtoh_u32(&toks)?;
            for (bi, s) in samp.iter().take(b_n).enumerate() {
                if s.is_some() {
                    next[bi] = Some(host_toks[bi]);
                }
            }
        }

        let lean_any = lean && samp.iter().take(b_n).any(|s| s.is_some());
        let rows: Vec<Vec<f32>> = if lean_any {
            // LEAN: park device-sampled rows on-device (per-cache buffer, dtod); D2H only
            // the rows that still need host logits. No sampled rows + no fallback rows =
            // the big D2H disappears (the [B] token readback above already synced).
            for (bi, s) in samp.iter().take(b_n).enumerate() {
                if s.is_none() {
                    continue;
                }
                // grammar-masked rows already parked their PRISTINE copy above — the
                // in-place ban has since poisoned this row for the reuse-pool consumer.
                if masks.get(bi).copied().flatten().is_some() {
                    continue;
                }
                let cache = &mut caches[bi];
                if cache
                    .last_logits_dev
                    .as_ref()
                    .map(|d| d.len() < n_vocab)
                    .unwrap_or(true)
                {
                    cache.last_logits_dev = Some(e.uninit(n_vocab)?);
                }
                let dst = cache.last_logits_dev.as_mut().unwrap();
                e.dtod_copy_view(&logits.slice(bi * n_vocab..(bi + 1) * n_vocab), dst)?;
            }
            (0..b_n)
                .map(|bi| {
                    if samp.get(bi).copied().flatten().is_some() {
                        Ok(Vec::new())
                    } else {
                        e.dtoh_view(&logits.slice(bi * n_vocab..(bi + 1) * n_vocab))
                    }
                })
                .collect::<Result<_, _>>()?
        } else {
            let host = e.dtoh(&logits)?;
            (0..b_n)
                .map(|bi| {
                    // grammar-masked non-lean rows return the PRISTINE copy (the in-place ban
                    // must never leak into last_logits — reuse-pool/park semantics unchanged).
                    if let Some(p) = pristine.get(bi).and_then(|p| p.as_ref()) {
                        return e.dtoh(p);
                    }
                    Ok(host[bi * n_vocab..(bi + 1) * n_vocab].to_vec())
                })
                .collect::<Result<_, _>>()?
        };
        for c in caches.iter_mut() {
            c.pos += 1;
        }
        ph_mark(e, 11, ph_last)?;
        Ok((rows, next))
    }
}

fn b1_fast_arch_eligible(arch: &Arch) -> bool {
    // The whole qwen35 family is excluded, not just MoE: spec verify for these archs runs
    // the generic batched numeric class (spec.rs qwen35_serving_class), so live B=1 serving
    // must stay in that same class. B1FAST's eager program would reopen the near-tie-flip
    // divergence the 2026-08-14 exactness fix closed (1 ULP at layer 2 -> 2.3e-1 head
    // maxdiff, amplified by the GDN recurrence).
    !matches!(arch, Arch::Qwen35 | Arch::Qwen35Moe)
}

fn b1_fast_env_on(value: Option<&str>) -> bool {
    value == Some("1")
}

#[cfg(test)]
mod tests {
    use super::{b1_fast_arch_eligible, b1_fast_env_on};
    use memra_gguf::config::Arch;

    #[test]
    fn qwen35_family_stays_in_one_decode_numeric_class_across_widths() {
        assert!(!b1_fast_arch_eligible(&Arch::Qwen35Moe));
        assert!(!b1_fast_arch_eligible(&Arch::Qwen35));
        assert!(b1_fast_arch_eligible(&Arch::Qwen3Moe));
    }

    #[test]
    fn b1_eager_program_requires_explicit_opt_in() {
        assert!(!b1_fast_env_on(None));
        assert!(!b1_fast_env_on(Some("0")));
        assert!(!b1_fast_env_on(Some("true")));
        assert!(b1_fast_env_on(Some("1")));
    }
}