rpi-plugin-sdk 0.1.4

//! Stable `#[repr(C)]` ABI contract for rpi **Rust-native (cdylib) plugins**.
//!
//! rpi loads extensions as compiled Rust cdylibs (`.dll`/`.so`/`.dylib`) via
//! `libloading` — **not** TS/jiti. Because we control both sides, the plugin is
//! Rust, but the *boundary* is still a hand-defined C ABI: the two sides may be
//! compiled with different Rust versions / crate versions, so no Rust type with
//! a non-`C` repr or a `Drop` impl may cross. This crate defines exactly those
//! crossing types and the registration contract.
//!
//! ## Soundness rules (load-bearing — verified by an adversarial review)
//!
//! 1. **Every crossing type is `#[repr(C)]`.** Enums used in unions carry
//!    `#[repr(u32)]` so the discriminant width is pinned.
//! 2. **No `Drop` type crosses.** `Vec`/`String`/`serde_json::Value`/`Option` of
//!    those / `Result` never appear in the ABI. Owned data crosses as
//!    [`StbString`] (ptr+len) with an **explicit `free_string`** the producer
//!    exports. [`StbString`] is `Copy` (raw pointers are `Copy`); copying
//!    duplicates the *pointer*, not the allocation, so each allocation is freed
//!    **exactly once** by the side that received it (see [`StbString`] docs).
//! 3. **Owned → JSON round-trip.** Structured host data ([`StableJsonValue`],
//!    tool params, `AgentToolResult`, events) crosses as a JSON string in a
//!    [`StbString`]. `serde_json` with `preserve_order` + `arbitrary_precision`
//!    must be enabled **consistently on host AND plugin** or integers >
//!    `u64`/`i64` lose precision and object keys may reorder — documented as a
//!    v1 limit. Tool args from the model rarely carry overflow ints, but it is
//!    never silent.
//! 4. **Unions are all-`Copy` payloads.** [`EventPayload`] / [`StepResultPayload`]
//!    variants are `#[repr(C)]` structs of primitives or [`StbString`] only, so
//!    the union is `Copy`-able and a wrong-variant read is `unsafe` (caller
//!    discriminates by `tag`).
//! 5. **Unwinding never crosses the ABI.** Every host→plugin and plugin→host
//!    call is `extern "C"`; both sides wrap dispatch in `catch_unwind`
//!    (abort-on-unwind / log-and-drop). A poisoned mutex or panicking emitter
//!    cannot unwind into the other side.
//!
//! ## Lifetime: the 4-function handle
//!
//! A registered tool drives an execution through **four** plugin-exported
//! functions (see [`ToolExecuteFn`] / [`ToolPollFn`] / [`ToolCancelFn`] /
//! [`ToolDestroyFn`]) — `execute`→[`StepHandle`] (plugin-allocates), `poll`
//! (non-blocking, **borrows** the handle, returns [`StepResult`]), `cancel`
//! (sets an internal `AtomicBool` flag; **idempotent; does NOT free;
//! thread-safe**), `destroy` (frees; **idempotent; called exactly once by the
//! blocking driver**). `cancel` ≠ `destroy`: conflating them is a UAF /
//! double-free. The adapter's blocking driver calls `poll` in a loop until
//! `Done`/`Err`, forwards `Pending` partials, and calls `destroy` once on exit.
//!
//! `poll` is **non-blocking** and MUST observe the cancel flag and return
//! `Done`/`Err` within a bounded number of polls; otherwise a cancelled call
//! leaks a `spawn_blocking` thread forever (those tasks run to completion
//! regardless of outer-future drop).
//!
//! The crate is `std` (not `no_std`): the ABI *types* are `#[repr(C)]` POD with
//! no `Drop` — that is what makes the boundary sound — but plugins and the host
//! are ordinary binaries with `std`, so the constructor/reader helpers and tests
//! use `String`/`Vec`/`serde_json` directly. The `json` feature keeps
//! `serde_json` optional for a plugin that wants to skip it.

use core::ffi::c_char;
use core::ffi::c_void;
use core::ptr;

// ---------------------------------------------------------------------------
// StbString — owned UTF-8 crossing as ptr+len with an explicit free
// ---------------------------------------------------------------------------

/// An owned UTF-8 string crossing the ABI as a `(ptr, len)` pair.
///
/// `Copy` (raw pointers are `Copy`): copying a `StbString` duplicates the
/// **pointer**, not the allocation. The **receiver** of a `StbString` owns the
/// allocation and MUST free it **exactly once** by calling the producer's
/// [`FreeStringFn`] (or [`StbString::free_with`] / [`StbString::free_host`]).
/// Never `free` a `StbString` you did not receive as an owner (e.g. one built
/// from a borrow via [`StbString::from_ref`], which is non-owning — its `free`
/// is a no-op only if the producer guarantees the buffer outlives the call; in
/// practice inputs cross as [`StbStringRef`] instead).
///
/// **Construction ownership contract:**
/// - [`StbString::from_owned`] — takes a `Box<[u8]>` the caller allocated; the
///   `StbString` now owns it; `free` deallocates.
/// - [`StbString::from_boxed_str`] / [`StbString::from_string`] — convenience
///   over `from_owned` (host side, needs `alloc`).
/// - [`StbString::empty`] — null/0; `free` is a no-op.
///
/// **Null `ptr` ⇒ empty** (`len` MUST be 0). A null pointer is never
/// dereferenced.
#[repr(C)]
#[derive(Clone, Copy)]
pub struct StbString {
    /// UTF-8 bytes. Null when `len == 0` (empty string).
    pub ptr: *mut c_char,
    /// Byte length (NOT a NUL terminator — the buffer is NOT NUL-terminated).
    pub len: usize,
}

// SAFETY: `StbString` is a plain `(ptr, len)` of raw pointers — no ownership
// transferred across threads by the type itself, and lifetime/aliasing is the
// caller's contract (documented above). It is `Send`+`Sync` so the host and the
// blocking driver can pass it across threads; the *allocation* ownership rules
// above still apply regardless of thread.
unsafe impl Send for StbString {}
unsafe impl Sync for StbString {}

impl StbString {
    /// An empty string: null pointer, zero length. `free` is a no-op.
    pub const fn empty() -> Self {
        Self { ptr: ptr::null_mut(), len: 0 }
    }

    /// Whether this is the empty/null string.
    pub const fn is_empty(&self) -> bool {
        self.len == 0
    }
}

// Allocating constructors + safe reader need `std` (Box/Vec/String). The ABI
// type itself (`StbString { ptr, len }`) is POD with no `Drop` — that is what
// makes the boundary sound. These helpers are available whenever `std` is (the
// crate is std-using). When the `json` feature is off a plugin still gets these
// because the crate links std; the gate here keeps `serde_json` truly optional.
#[cfg(any(feature = "json", test))]
impl StbString {
    /// Wrap an allocation the caller already boxed. The `StbString` takes
    /// ownership: a later `free_with(free_fn)` will deallocate it via the
    /// producer's `free_string`.
    ///
    /// The buffer MUST be UTF-8.
    pub fn from_owned(buf: Box<[u8]>) -> Self {
        let len = buf.len();
        // Stabilize the pointer via `Box::into_raw`; the free fn reconstructs a
        // slice from ptr+len and drops it. Store the element pointer as
        // `*mut c_char` (u8 ↔ c_char on every platform rpi targets). Ownership
        // moves into the `StbString` (we do NOT drop here).
        let ptr = Box::into_raw(buf) as *mut [u8] as *mut u8 as *mut c_char;
        let _ = len;
        Self { ptr, len }
    }

    /// Convenience: from a `String`, transferring ownership. After this the
    /// passed `String` is consumed and must not be reused.
    pub fn from_string(s: String) -> Self {
        Self::from_vec(s.into_bytes())
    }

    /// Convenience: from a `Vec<u8>` (UTF-8), transferring ownership.
    pub fn from_vec(v: Vec<u8>) -> Self {
        Self::from_owned(v.into_boxed_slice())
    }

    /// Convenience: from a boxed `str`.
    pub fn from_boxed_str(s: Box<str>) -> Self {
        let string: String = s.into();
        Self::from_vec(string.into_bytes())
    }

    /// Copy this `StbString`'s bytes into an owned `String` (**without** freeing
    /// the original — the caller still owns the original allocation). Use this
    /// to *read* a received `StbString` into safe Rust; then `free` the original.
    pub fn to_string_lossy(&self) -> String {
        if self.len == 0 || self.ptr.is_null() {
            return String::new();
        }
        // SAFETY: the producer guarantees `ptr` is valid for `len` bytes and the
        // bytes are UTF-8. We only read (no free) here.
        let slice = unsafe { core::slice::from_raw_parts(self.ptr as *const u8, self.len) };
        String::from_utf8_lossy(slice).into_owned()
    }
}

/// A **borrowed**, non-owning view of a string passed as an *input* to an FFI
/// call. The callee MUST NOT free it and MUST NOT retain it past the call.
///
/// Built from a `&str` on the calling side; the buffer is valid for the
/// duration of the call (the caller's borrow).
#[repr(C)]
#[derive(Clone, Copy)]
pub struct StbStringRef {
    /// UTF-8 bytes (valid for the call; NUL-terminated not required).
    pub ptr: *const c_char,
    /// Byte length.
    pub len: usize,
}

unsafe impl Send for StbStringRef {}
unsafe impl Sync for StbStringRef {}

impl StbStringRef {
    /// Empty input.
    pub const fn empty() -> Self {
        Self { ptr: ptr::null(), len: 0 }
    }

    /// Borrow a `&str` for the duration of a call. The caller must outlive the
    /// call (normal borrow rules).
    pub fn from_str(s: &str) -> Self {
        Self { ptr: s.as_ptr() as *const c_char, len: s.len() }
    }

    /// Read into a safe `&str` for the callee's lifetime `'a`.
    ///
    /// # Safety
    /// The caller guarantees `ptr` is valid for `len` bytes and they are UTF-8,
    /// and the borrow survives `'a`.
    pub unsafe fn as_str<'a>(&self) -> &'a str {
        if self.len == 0 || self.ptr.is_null() {
            return "";
        }
        let slice = unsafe { core::slice::from_raw_parts(self.ptr as *const u8, self.len) };
        unsafe { core::str::from_utf8_unchecked(slice) }
    }
}

/// Function pointer a plugin exports to free a [`StbString`] it produced.
/// Idempotent: freeing an already-freed or empty `StbString` is a no-op.
///
/// The host calls this for every [`StbString`] it receives from the plugin; the
/// plugin calls the **host's** `free_string` (from [`PluginApiVt`]) for every
/// [`StbString`] it receives from the host.
pub type FreeStringFn = extern "C" fn(s: StbString);

impl StbString {
    /// Free this `StbString` via the given `free_string` fn, if non-null and
    /// non-empty. Consumes ownership (the value is `Copy`, but semantically the
    /// caller relinquishes the allocation).
    ///
    /// After this call the bytes are invalid; do not use the `StbString` again.
    pub fn free_with(self, free_fn: Option<FreeStringFn>) {
        if let Some(free_fn) = free_fn {
            if !self.is_empty() {
                free_fn(self);
            }
        }
    }
}

// ---------------------------------------------------------------------------
// StableJsonValue — JSON round-trip helpers (json feature)
// ---------------------------------------------------------------------------

/// Helpers to cross structured data as JSON-in-`StbString`. See the module docs
/// for the precision/order limit.
#[cfg(any(feature = "json", doc))]
pub mod json {
    use super::StbString;
    use serde_json::Value;

    /// Serialize a `serde_json::Value` into an owning [`StbString`] the receiver
    /// must `free` via the producer's `free_string`.
    pub fn to_stable(value: &Value, free_fn: Option<super::FreeStringFn>) -> StbString {
        let s = serde_json::to_string(value).unwrap_or_else(|_| "null".to_string());
        let stb = StbString::from_string(s);
        // `free_fn` is advisory metadata the receiver needs; the StbString
        // itself carries only ptr+len. Stash nothing — the receiver must know
        // which free fn to use (host's vs plugin's) by direction.
        let _ = free_fn;
        stb
    }

    /// Parse a received [`StbString`] back into a `Value`. Does **not** free the
    /// input — the caller still owns it.
    pub fn from_stable(s: &StbString) -> Value {
        let text = s.to_string_lossy();
        if text.is_empty() {
            return Value::Null;
        }
        serde_json::from_str(&text).unwrap_or(Value::Null)
    }
}

// ---------------------------------------------------------------------------
// StableToolSchema — the provider-facing tool definition
// ---------------------------------------------------------------------------

/// A tool's provider-facing schema crossing the ABI. `name` / `description` are
/// raw strings; `parameters` is a JSON Schema serialized to a JSON string (the
/// host parses it into its native `schemars::Schema`).
///
/// All three are owning [`StbString`]s the **plugin** produced; the **host**
/// frees them via the plugin's `free_string` (passed to [`ToolExecuteFn`] /
/// `register_tool`).
#[repr(C)]
#[derive(Clone, Copy)]
pub struct StableToolSchema {
    pub name: StbString,
    pub description: StbString,
    /// JSON-encoded JSON Schema for the tool's `parameters`.
    pub parameters: StbString,
}

unsafe impl Send for StableToolSchema {}
unsafe impl Sync for StableToolSchema {}

// ---------------------------------------------------------------------------
// StepResult — the poll() return: Pending | Done | Err (explicit tag + union)
// ---------------------------------------------------------------------------

/// Opaque, plugin-allocated handle for one tool execution drive. Produced by
/// [`ToolExecuteFn`], polled by [`ToolPollFn`], cancelled by [`ToolCancelFn`],
/// freed by [`ToolDestroyFn`] (exactly once, idempotent).
///
/// The handle's interior layout is entirely plugin-private; the host treats it
/// as an opaque pointer.
pub type StepHandle = *mut c_void;

/// Discriminant for [`StepResult`]. `#[repr(u32)]` pins the width so the union
/// payload is sound across compilers.
#[repr(u32)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum StepResultTag {
    /// `poll` has no terminal result yet; the partial payload may carry progress.
    Pending = 0,
    /// Terminal success; `done.result` is the JSON `AgentToolResult`.
    Done = 1,
    /// Terminal failure; `err.message` is a UTF-8 error string.
    Err = 2,
}

/// A partial/progress result emitted during `Pending`. `progress` is a JSON
/// `AgentToolResult` (the same shape `on_update` carries) — the host forwards it
/// to the adapter's `on_update` callback. May be empty.
#[repr(C)]
#[derive(Clone, Copy)]
pub struct StbPending {
    pub progress: StbString,
}

/// Terminal success payload. `result` is a JSON `AgentToolResult`.
#[repr(C)]
#[derive(Clone, Copy)]
pub struct StbDone {
    pub result: StbString,
}

/// Terminal failure payload. `message` is a UTF-8 error string.
#[repr(C)]
#[derive(Clone, Copy)]
pub struct StbErr {
    pub message: StbString,
}

/// The `poll()` return value. Read the `payload` variant matching `tag`.
///
/// All payload variants are `#[repr(C)]` structs of [`StbString`] (Copy), so the
/// union is `Copy`. A wrong-variant read is `unsafe`; always match on `tag`.
#[repr(C)]
#[derive(Clone, Copy)]
pub union StepResultPayload {
    pub pending: StbPending,
    pub done: StbDone,
    pub err: StbErr,
}

/// Return value of [`ToolPollFn`]. The blocking driver matches on `tag`, reads
/// the matching payload, and breaks the loop on `Done`/`Err`.
#[repr(C)]
#[derive(Clone, Copy)]
pub struct StepResult {
    pub tag: StepResultTag,
    pub payload: StepResultPayload,
}

impl StepResult {
    /// Build a `Pending` with a progress JSON string (may be empty).
    pub fn pending(progress: StbString) -> Self {
        Self { tag: StepResultTag::Pending, payload: StepResultPayload { pending: StbPending { progress } } }
    }

    /// Build a `Done` with the terminal JSON `AgentToolResult`.
    pub fn done(result: StbString) -> Self {
        Self { tag: StepResultTag::Done, payload: StepResultPayload { done: StbDone { result } } }
    }

    /// Build an `Err` with an error message.
    pub fn err(message: StbString) -> Self {
        Self { tag: StepResultTag::Err, payload: StepResultPayload { err: StbErr { message } } }
    }

    /// Access the `pending` payload. Caller MUST guarantee `tag == Pending`.
    ///
    /// # Safety
    /// Undefined behavior if `tag != StepResultTag::Pending`.
    pub unsafe fn pending_payload(&self) -> &StbPending {
        unsafe { &self.payload.pending }
    }

    /// Access the `done` payload. Caller MUST guarantee `tag == Done`.
    ///
    /// # Safety
    /// Undefined behavior if `tag != StepResultTag::Done`.
    pub unsafe fn done_payload(&self) -> &StbDone {
        unsafe { &self.payload.done }
    }

    /// Access the `err` payload. Caller MUST guarantee `tag == Err`.
    ///
    /// # Safety
    /// Undefined behavior if `tag != StepResultTag::Err`.
    pub unsafe fn err_payload(&self) -> &StbErr {
        unsafe { &self.payload.err }
    }
}

// ---------------------------------------------------------------------------
// The 4-function tool lifecycle fn-pointer types
// ---------------------------------------------------------------------------

/// Partial-result callback the **blocking driver** passes to `poll`, wrapped in
/// `catch_unwind` on the host side. The plugin invokes it **synchronously
/// inside `poll()`** when it has a `Pending` partial — never retained, never
/// invoked after `Done`/`Err`.
///
/// `partial` is a JSON `AgentToolResult`; ownership passes to the callback (the
/// host frees it via the host's `free_string`).
pub type ToolPartialCb = extern "C" fn(partial: StbString, user_data: *mut c_void);

/// `execute(tool_call_id, params) -> StepHandle`. Plugin-allocates a drive
/// handle and begins the work (non-blocking — the real progress comes via
/// `poll`). `tool_call_id` is a borrowed [`StbStringRef`] (valid for the call);
/// `params` is an owning JSON string of the tool-call arguments (the plugin
/// frees it via the host's `free_string`). Returns null on allocation failure.
pub type ToolExecuteFn = extern "C" fn(
    tool_call_id: StbStringRef,
    params: StbString,
    free_params: Option<FreeStringFn>,
) -> StepHandle;

/// `poll(handle, partial_cb, user_data) -> StepResult`. **Non-blocking.** Must
/// observe the cancel flag (set by [`ToolCancelFn`]) and return `Done`/`Err`
/// within a bounded number of polls. Borrows `handle` (does not free it).
pub type ToolPollFn =
    extern "C" fn(handle: StepHandle, partial_cb: Option<ToolPartialCb>, user_data: *mut c_void) -> StepResult;

/// `cancel(handle)`. Sets an internal `AtomicBool` (SeqCst) cancel flag.
/// **Idempotent, thread-safe, does NOT free.** The poll loop observes it.
pub type ToolCancelFn = extern "C" fn(handle: StepHandle);

/// `destroy(handle)`. Frees the handle. **Idempotent; called exactly once by the
/// blocking driver on exit** (after the loop sees `Done`/`Err`, or after cancel
/// propagated). Null handle is a no-op.
pub type ToolDestroyFn = extern "C" fn(handle: StepHandle);

// ---------------------------------------------------------------------------
// StablePluginEvent — 33 on() categories (explicit tag + union)
// ---------------------------------------------------------------------------

/// Discriminant for [`StablePluginEvent`], one variant per pi `on()` category
/// (33 total). `#[repr(u32)]` pins the discriminant width.
///
/// The 33 categories (verified against `extensions/types.ts:1203-1244`):
/// project_trust, resources_discover, session_start, session_info_changed,
/// session_before_switch, session_before_fork, session_before_compact,
/// session_compact, session_shutdown, session_before_tree, session_tree,
/// context, before_provider_request, before_provider_headers,
/// after_provider_response, before_agent_start, agent_start, agent_end,
/// agent_settled, turn_start, turn_end, message_start, message_update,
/// message_end, tool_execution_start, tool_execution_update,
/// tool_execution_end, model_select, thinking_level_select, tool_call,
/// tool_result, user_bash, input.
#[repr(u32)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum EventTag {
    ProjectTrust = 0,
    ResourcesDiscover = 1,
    SessionStart = 2,
    SessionInfoChanged = 3,
    SessionBeforeSwitch = 4,
    SessionBeforeFork = 5,
    SessionBeforeCompact = 6,
    SessionCompact = 7,
    SessionShutdown = 8,
    SessionBeforeTree = 9,
    SessionTree = 10,
    Context = 11,
    BeforeProviderRequest = 12,
    BeforeProviderHeaders = 13,
    AfterProviderResponse = 14,
    BeforeAgentStart = 15,
    AgentStart = 16,
    AgentEnd = 17,
    AgentSettled = 18,
    TurnStart = 19,
    TurnEnd = 20,
    MessageStart = 21,
    MessageUpdate = 22,
    MessageEnd = 23,
    ToolExecutionStart = 24,
    ToolExecutionUpdate = 25,
    ToolExecutionEnd = 26,
    ModelSelect = 27,
    ThinkingLevelSelect = 28,
    ToolCall = 29,
    ToolResult = 30,
    UserBash = 31,
    Input = 32,
}

/// Number of `on()` event categories — `33`. A test asserts
/// `EVENT_TAG_COUNT == 33` so a future edit that adds/removes a tag is caught.
pub const EVENT_TAG_COUNT: usize = 33;

/// No-payload marker for events that carry none (e.g. `session_shutdown`).
/// Carries a dummy byte so the empty-struct isn't flagged FFI-unsafe by
/// `improper_ctypes` (zero-sized C structs are rejected regardless of `repr(C)`).
#[repr(C)]
#[derive(Clone, Copy)]
pub struct EventEmpty {
    _opaque: u8,
}

impl EventEmpty {
    /// The one no-payload instance.
    pub const INSTANCE: EventEmpty = EventEmpty { _opaque: 0 };
}

impl Default for EventEmpty {
    fn default() -> Self {
        Self::INSTANCE
    }
}

/// A serialized message payload (`message_start`/`update`/`end`, tool-result
/// messages). `message` is a JSON `AgentMessage`.
#[repr(C)]
#[derive(Clone, Copy)]
pub struct EventMessage {
    pub message: StbString,
}

/// A tool-call payload (`tool_call`, `tool_execution_start`/`update`).
/// `tool_call_id` + `tool_name` are raw strings; `params` is the JSON args.
#[repr(C)]
#[derive(Clone, Copy)]
pub struct EventToolCall {
    pub tool_call_id: StbString,
    pub tool_name: StbString,
    pub params: StbString,
}

/// A tool-result payload (`tool_result`, `tool_execution_end`).
#[repr(C)]
#[derive(Clone, Copy)]
pub struct EventToolResult {
    pub tool_call_id: StbString,
    pub tool_name: StbString,
    pub result: StbString,
    pub is_error: u8,
}

/// An error/failure payload.
#[repr(C)]
#[derive(Clone, Copy)]
pub struct EventError {
    pub message: StbString,
}

/// A generic JSON-data payload for the long-tail events whose structured shape
/// the host serializes wholesale (`context`, `before_provider_request`, model
/// select, resources_discover response, etc.). The plugin reads the fields it
/// needs.
#[repr(C)]
#[derive(Clone, Copy)]
pub struct EventData {
    pub data: StbString,
}

/// Payload union for [`StablePluginEvent`]. All variants are `#[repr(C)]` structs
/// of [`StbString`] / primitives (Copy), so the union is Copy. Discriminate by
/// [`StablePluginEvent::tag`] before reading.
#[repr(C)]
#[derive(Clone, Copy)]
pub union EventPayload {
    pub empty: EventEmpty,
    pub message: EventMessage,
    pub tool_call: EventToolCall,
    pub tool_result: EventToolResult,
    pub error: EventError,
    pub data: EventData,
}

/// One event dispatched to a plugin handler. The host translates its native
/// `AgentEvent` / `HarnessEvent` into this and calls every registered handler
/// for the `tag` (dispatch wrapped in `catch_unwind`). Ownership of the
/// [`StbString`]s passes to the handler; the handler frees them via the host's
/// `free_string`.
#[repr(C)]
#[derive(Clone, Copy)]
pub struct StablePluginEvent {
    pub tag: EventTag,
    pub payload: EventPayload,
}

impl StablePluginEvent {
    /// Build a no-payload event.
    pub fn empty(tag: EventTag) -> Self {
        Self { tag, payload: EventPayload { empty: EventEmpty::INSTANCE } }
    }

    /// Build a message event.
    pub fn message(tag: EventTag, message: StbString) -> Self {
        debug_assert!(matches!(
            tag,
            EventTag::MessageStart | EventTag::MessageUpdate | EventTag::MessageEnd
        ));
        Self { tag, payload: EventPayload { message: EventMessage { message } } }
    }

    /// Build a tool-call event.
    pub fn tool_call(tag: EventTag, tool_call_id: StbString, tool_name: StbString, params: StbString) -> Self {
        debug_assert!(matches!(
            tag,
            EventTag::ToolCall | EventTag::ToolExecutionStart | EventTag::ToolExecutionUpdate
        ));
        Self { tag, payload: EventPayload { tool_call: EventToolCall { tool_call_id, tool_name, params } } }
    }

    /// Build a tool-result event.
    pub fn tool_result(
        tag: EventTag,
        tool_call_id: StbString,
        tool_name: StbString,
        result: StbString,
        is_error: bool,
    ) -> Self {
        debug_assert!(matches!(tag, EventTag::ToolResult | EventTag::ToolExecutionEnd));
        Self {
            tag,
            payload: EventPayload {
                tool_result: EventToolResult { tool_call_id, tool_name, result, is_error: is_error as u8 },
            },
        }
    }

    /// Build an error event.
    pub fn error(tag: EventTag, message: StbString) -> Self {
        Self { tag, payload: EventPayload { error: EventError { message } } }
    }

    /// Build a generic data event (JSON in `data`).
    pub fn data(tag: EventTag, data: StbString) -> Self {
        Self { tag, payload: EventPayload { data: EventData { data } } }
    }
}

/// Handler fn pointer registered via `register_event_handler(tag, handler)`.
/// `user_data` is the plugin's opaque context. Return `0` on success; nonzero
/// signals a handled error (the host logs it; dispatch continues to other
/// handlers — one handler's error does not abort the fan-out).
pub type EventHandlerFn = extern "C" fn(event: StablePluginEvent, user_data: *mut c_void) -> i32;

/// `resources_discover` handler signature (B5b). Unlike [`EventHandlerFn`]
/// (fire-and-forget, `i32` only), this carries an owning `out` so the plugin
/// can hand `{skillPaths, promptPaths, themePaths}` back to the host. `cwd` and
/// `reason` are borrowed inputs ([`StbStringRef`]); `out` is plugin-produced
/// and reclaimed via the `plugin_free_string` the host stored alongside the
/// handler at registration. `user_data` is the plugin's opaque context. Returns
/// `0` on success (host reads `out`); nonzero on a handled error (host logs +
/// skips this handler, fan-out continues — mirrors pi `runner.ts:1179-1188`).
pub type ResourcesDiscoverFn = extern "C" fn(
    cwd: StbStringRef,
    reason: StbStringRef,
    out: *mut StbString,
    user_data: *mut c_void,
) -> i32;

// ---------------------------------------------------------------------------
// Runtime actions — uniform JSON-RPC dispatch by RuntimeActionId
// ---------------------------------------------------------------------------

/// Identifier for a host runtime action the plugin may invoke via
/// `PluginApiVt::runtime_action`. One slot dispatches all actions — forward-
/// compatible (new actions add ids, not vtable slots). Args/results cross as
/// JSON strings.
#[repr(u32)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum RuntimeActionId {
    SendMessage = 0,
    SendUserMessage = 1,
    AppendEntry = 2,
    SetSessionName = 3,
    GetActiveTools = 4,
    SetActiveTools = 5,
    SetModel = 6,
    GetThinkingLevel = 7,
    SetThinkingLevel = 8,
    Compact = 9,
    GetSystemPrompt = 10,
    NewSession = 11,
    Fork = 12,
    NavigateTree = 13,
    SwitchSession = 14,
    Reload = 15,
}

/// Runtime-action signature: `runtime_action(action_id, args_json, out,
/// user_data) -> i32`. `args_json` is a borrowed input ([`StbStringRef`]); `out`
/// is an owning output ([`StbString`]) the host produces and the plugin frees
/// via the host's `free_string`. Returns `0` on success, nonzero on error.
pub type RuntimeActionFn = extern "C" fn(
    action: RuntimeActionId,
    args_json: StbStringRef,
    out: *mut StbString,
    user_data: *mut c_void,
) -> i32;

// ---------------------------------------------------------------------------
// PluginApiVt — host-provided vtable of fn pointers the plugin calls
// ---------------------------------------------------------------------------

/// A generic command-handler fn (for `register_command`). `args_json` is
/// borrowed input; `out` is owning output the plugin frees via host `free_string`.
pub type CommandHandlerFn =
    extern "C" fn(args_json: StbStringRef, out: *mut StbString, user_data: *mut c_void) -> i32;

/// A render/transform fn (for the renderer registrars). `input_json` is borrowed;
/// `out` is owning output the plugin frees via host `free_string`.
pub type RenderFn = extern "C" fn(input_json: StbStringRef, out: *mut StbString, user_data: *mut c_void) -> i32;

/// A provider-injection factory fn (for `register_provider`). `req_json` is a
/// borrowed request envelope; `out` is an owning response the plugin frees.
/// The host wraps this into a `Provider` impl (B4/B5).
pub type ProviderRequestFn =
    extern "C" fn(req_json: StbStringRef, out: *mut StbString, user_data: *mut c_void) -> i32;

/// The host-provided vtable, passed to [`rpi_plugin_register`] as a `*const`.
///
/// The plugin reads it during `register` and may copy fn pointers it needs (the
/// struct is POD/Copy). **Every slot is nullable**: a null fn pointer means the
/// host does not support that capability yet — the plugin MUST null-check
/// before calling and degrade gracefully. This keeps the vtable forward-
/// compatible across rpi versions without re-ABI bumps within one
/// `RPI_PLUGIN_ABI_VERSION`.
///
/// `user_data` is the host's opaque context, passed back to every host-provided
/// fn (so the host can recover its session/harness state). The plugin stores it
/// and passes it through unchanged.
#[repr(C)]
pub struct PluginApiVt {
    /// Host's `free_string` — the plugin calls this for every [`StbString`] it
    /// *receives* from the host (outputs of actions, event payloads, inputs to
    /// execute). Never null.
    pub free_string: FreeStringFn,

    // --- 8 registrars (plugin → host "register X into the host") ---

    /// Register a tool. `schema` + the four lifecycle fns + the plugin's own
    /// `free_string` (for the [`StbString`]s in `schema`). Returns `0` on
    /// success. Nullable: host not yet wired for tool registration.
    pub register_tool: Option<
        extern "C" fn(
            schema: *const StableToolSchema,
            execute_fn: ToolExecuteFn,
            poll_fn: ToolPollFn,
            cancel_fn: ToolCancelFn,
            destroy_fn: ToolDestroyFn,
            plugin_free_string: FreeStringFn,
        ) -> i32,
    >,

    /// Register a slash command. Nullable.
    pub register_command: Option<extern "C" fn(name: StbStringRef, description: StbStringRef, handler: CommandHandlerFn) -> i32>,

    /// Register a keyboard shortcut. Nullable.
    pub register_shortcut: Option<extern "C" fn(key: StbStringRef, description: StbStringRef) -> i32>,

    /// Register a CLI flag. Nullable.
    pub register_flag: Option<extern "C" fn(name: StbStringRef, description: StbStringRef) -> i32>,

    /// Register a custom provider. The host stores `provider_id`/`base_url`/
    /// `api_style` + the plugin's `request_fn` + the plugin's own
    /// `plugin_free_string` (the `out` [`StbString`] `request_fn` *produces* is
    /// plugin-owned and the host must reclaim it — same ownership rule as
    /// `register_resources_discover`) + the plugin's `user_data` (which
    /// `request_fn` receives back unmodified on every call). Nullable (B5c).
    pub register_provider: Option<
        extern "C" fn(
            provider_id: StbStringRef,
            base_url: StbStringRef,
            api_style: StbStringRef,
            request_fn: ProviderRequestFn,
            plugin_free_string: FreeStringFn,
            user_data: *mut c_void,
        ) -> i32,
    >,

    /// Register a message renderer. `plugin_free_string` reclaims the `out`
    /// [`StbString`] `render_fn` produces; `user_data` is passed back to it on
    /// every render call. Nullable (B5c; TUI consumption deferred).
    pub register_message_renderer: Option<
        extern "C" fn(
            name: StbStringRef,
            render_fn: RenderFn,
            plugin_free_string: FreeStringFn,
            user_data: *mut c_void,
        ) -> i32,
    >,

    /// Register a markdown transformer. Same ownership shape as
    /// `register_message_renderer`. Nullable (B5c; TUI wiring in B5e).
    pub register_markdown_transformer: Option<
        extern "C" fn(
            name: StbStringRef,
            render_fn: RenderFn,
            plugin_free_string: FreeStringFn,
            user_data: *mut c_void,
        ) -> i32,
    >,

    /// Register an entry renderer. Same ownership shape as
    /// `register_message_renderer`. Nullable (B5c; TUI consumption deferred).
    pub register_entry_renderer: Option<
        extern "C" fn(
            name: StbStringRef,
            render_fn: RenderFn,
            plugin_free_string: FreeStringFn,
            user_data: *mut c_void,
        ) -> i32,
    >,

    // --- on() event handler registration (the 33-category subscription) ---

    /// Subscribe a handler to one event `tag`. Nullable: host not yet wiring
    /// events. The host dispatches [`StablePluginEvent`]s of that tag to the
    /// handler (`catch_unwind`-wrapped).
    pub register_event_handler: Option<extern "C" fn(tag: EventTag, handler: EventHandlerFn, user_data: *mut c_void) -> i32>,

    /// Register a `resources_discover` handler (B5b). The host stores `handler`
    /// + the plugin's own `plugin_free_string` (the `out` [`StbString`] the
    /// handler produces is plugin-owned and the host must reclaim it) +
    /// `user_data`. On discovery (`startup`/`reload`) the host fans the event to
    /// every registered handler in order, concatenating their returned
    /// `{skillPaths, promptPaths, themePaths}` (errors per-handler do NOT abort
    /// the fan-out). Nullable: a host without the resources-discover path leaves
    /// this null and the plugin must degrade (no dynamic resource contribution).
    pub register_resources_discover: Option<
        extern "C" fn(handler: ResourcesDiscoverFn, plugin_free_string: FreeStringFn, user_data: *mut c_void) -> i32,
    >,

    // --- runtime actions (~14, uniform dispatch) ---

    /// Invoke a host runtime action. See [`RuntimeActionId`] / [`RuntimeActionFn`].
    /// Nullable: host not yet exposing actions.
    pub runtime_action: RuntimeActionFn,

    // --- event dispatch (plugin → host "emit an event upstream") ---

    /// Emit an event upstream (e.g. a tool announcing a custom UI event). The
    /// host forwards to interested subscribers. Ownership of the event's
    /// [`StbString`]s passes to the host (freed via `free_string`). Nullable.
    pub dispatch_event: Option<extern "C" fn(event: StablePluginEvent, user_data: *mut c_void) -> i32>,

    /// The host's opaque context, passed through to every host-provided fn.
    /// The plugin stores this and hands it back unmodified on each call.
    pub user_data: *mut c_void,
}

// SAFETY: the vtable is a POD struct of fn pointers + one raw `user_data`
// pointer. It is `Send`+`Sync` so the host can hand it to the plugin's register
// thread and the plugin can call its fns from the blocking driver thread; the
// host guarantees the `user_data` is valid across those calls.
unsafe impl Send for PluginApiVt {}
unsafe impl Sync for PluginApiVt {}

// ---------------------------------------------------------------------------
// Register contract
// ---------------------------------------------------------------------------

/// The ABI version this SDK publishes. The host refuses to load a plugin whose
/// declared `RPI_PLUGIN_ABI_VERSION` differs from its own (skip + diagnostic,
/// never load — no half-compatible call surface). Bump only on a breaking ABI
/// change (reorder/retype a vtable slot, change a crossing struct layout);
/// adding a nullable vtable slot **or widening an existing nullable slot**'s
/// parameter list within a version is *not* a bump — the plugin and host are
/// both recompiled from this same SDK, and a nullable slot a plugin never calls
/// is unaffected by a wider callee signature. (B5c widens the four
/// renderer/provider registrar slots within ABI v1 on this basis.)
pub const RPI_PLUGIN_ABI_VERSION: u32 = 1;

/// The symbol the host looks up in each cdylib via `libloading::Library::get`.
/// Must be an `extern "C" fn(*const PluginApiVt, u32) -> i32`.
pub const REGISTER_SYMBOL: &[u8] = b"rpi_plugin_register\0";

/// Plugin entrypoint signature. The host loads the cdylib, looks up
/// `rpi_plugin_register`, and calls it with the host `PluginApiVt` and the
/// host's current `RPI_PLUGIN_ABI_VERSION`.
///
/// Return `0` on successful registration; nonzero is a plugin-defined error
/// code (the host logs it and skips the plugin). The host checks
/// `abi_version` **before** calling — if the plugin was compiled against a
/// different `RPI_PLUGIN_ABI_VERSION` it must itself refuse (return nonzero) if
/// it sees an unrecognized version; idiomatically the plugin stores the passed
/// `api` only when `abi_version == RPI_PLUGIN_ABI_VERSION`.
pub type RpiPluginRegister = extern "C" fn(api: *const PluginApiVt, abi_version: u32) -> i32;

/// Convenience for host + plugin: declare the register entrypoint.
///
/// A plugin crate writes:
/// ```ignore
/// #[no_mangle]
/// pub extern "C" fn rpi_plugin_register(api: *const PluginApiVt, abi_version: u32) -> i32 {
///     rpi_plugin_sdk::register_entrypoint(api, abi_version, |api| {
///         // ... register tools / handlers using `api` ...
///         0
///     })
/// }
/// ```
/// The helper performs the version check (return nonzero on mismatch) and
/// null-checks `api` before invoking the plugin body.
pub fn register_entrypoint(
    api: *const PluginApiVt,
    abi_version: u32,
    body: impl FnOnce(&PluginApiVt) -> i32,
) -> i32 {
    if abi_version != RPI_PLUGIN_ABI_VERSION {
        // Mismatch: refuse to register. The host logs "ABI version mismatch"
        // and skips loading this plugin.
        return 1;
    }
    if api.is_null() {
        return 2;
    }
    // SAFETY: the host guarantees `api` is valid for the register call and the
    // plugin does not retain the borrow past `body` (it copies the fn pointers
    // it needs).
    let api = unsafe { &*api };
    body(api)
}

// ===========================================================================
// Tests (need std + serde_json)
// ===========================================================================

#[cfg(test)]
mod tests {
    use super::*;

    // A test allocator + free fn so we can verify the own/free contract
    // without a real plugin's free_string.
    static FREED: std::sync::atomic::AtomicUsize = std::sync::atomic::AtomicUsize::new(0);

    extern "C" fn test_free(s: StbString) {
        if s.is_empty() || s.ptr.is_null() {
            return;
        }
        // Reconstruct the boxed slice and drop it.
        unsafe {
            let slice = core::slice::from_raw_parts(s.ptr as *const u8, s.len);
            let _ = Box::from_raw(slice as *const [u8] as *mut [u8]);
        }
        FREED.fetch_add(1, std::sync::atomic::Ordering::SeqCst);
    }

    fn reset_freed() -> usize {
        FREED.swap(0, std::sync::atomic::Ordering::SeqCst)
    }

    // A no-op `runtime_action` impl for the vtable-construction tests (closures
    // can't coerce to `extern "C" fn`, so we use a real fn).
    extern "C" fn noop_runtime_action(
        _action: RuntimeActionId,
        _args: StbStringRef,
        _out: *mut StbString,
        _user_data: *mut c_void,
    ) -> i32 {
        0
    }

    #[test]
    fn stbstring_round_trip_and_free_once() {
        let prev = reset_freed();
        let _ = prev;
        let s = StbString::from_string("hello, pi".to_string());
        assert_eq!(s.len, 9);
        assert_eq!(s.to_string_lossy(), "hello, pi");
        s.free_with(Some(test_free));
        assert_eq!(FREED.load(std::sync::atomic::Ordering::SeqCst), 1);
    }

    #[test]
    fn empty_stbstring_free_is_noop() {
        let _ = reset_freed();
        StbString::empty().free_with(Some(test_free));
        assert_eq!(FREED.load(std::sync::atomic::Ordering::SeqCst), 0);
    }

    #[test]
    fn json_round_trip_preserves_structure() {
        let val = serde_json::json!({ "name": "echo", "args": [1, 2, 3], "ok": true });
        let stb = json::to_stable(&val, None);
        let back = json::from_stable(&stb);
        assert_eq!(val, back);
        stb.free_with(Some(test_free));
        let _ = reset_freed();
    }

    #[test]
    fn step_result_done_round_trip() {
        let result_json = StbString::from_string(r#"{"content":[{"text":"hi"}]}"#.to_string());
        let sr = StepResult::done(result_json);
        assert_eq!(sr.tag, StepResultTag::Done);
        // SAFETY: tag == Done.
        let done = unsafe { sr.done_payload() };
        assert_eq!(done.result.to_string_lossy(), r#"{"content":[{"text":"hi"}]}"#);
        done.result.free_with(Some(test_free));
        let _ = reset_freed();
    }

    #[test]
    fn step_result_pending_and_err() {
        let prog = StbString::from_string("...".to_string());
        let srp = StepResult::pending(prog);
        assert_eq!(srp.tag, StepResultTag::Pending);
        // SAFETY: tag == Pending.
        unsafe {
            assert_eq!(srp.pending_payload().progress.to_string_lossy(), "...");
        }
        unsafe { srp.pending_payload().progress.free_with(Some(test_free)) };

        let msg = StbString::from_string("boom".to_string());
        let sre = StepResult::err(msg);
        assert_eq!(sre.tag, StepResultTag::Err);
        // SAFETY: tag == Err.
        unsafe {
            assert_eq!(sre.err_payload().message.to_string_lossy(), "boom");
            sre.err_payload().message.free_with(Some(test_free));
        }
        let _ = reset_freed();
    }

    #[test]
    fn event_tag_count_is_33() {
        // Enumerate every tag; a compile-time + runtime guarantee that the
        // 33-category surface is intact.
        let tags = [
            EventTag::ProjectTrust,
            EventTag::ResourcesDiscover,
            EventTag::SessionStart,
            EventTag::SessionInfoChanged,
            EventTag::SessionBeforeSwitch,
            EventTag::SessionBeforeFork,
            EventTag::SessionBeforeCompact,
            EventTag::SessionCompact,
            EventTag::SessionShutdown,
            EventTag::SessionBeforeTree,
            EventTag::SessionTree,
            EventTag::Context,
            EventTag::BeforeProviderRequest,
            EventTag::BeforeProviderHeaders,
            EventTag::AfterProviderResponse,
            EventTag::BeforeAgentStart,
            EventTag::AgentStart,
            EventTag::AgentEnd,
            EventTag::AgentSettled,
            EventTag::TurnStart,
            EventTag::TurnEnd,
            EventTag::MessageStart,
            EventTag::MessageUpdate,
            EventTag::MessageEnd,
            EventTag::ToolExecutionStart,
            EventTag::ToolExecutionUpdate,
            EventTag::ToolExecutionEnd,
            EventTag::ModelSelect,
            EventTag::ThinkingLevelSelect,
            EventTag::ToolCall,
            EventTag::ToolResult,
            EventTag::UserBash,
            EventTag::Input,
        ];
        assert_eq!(tags.len(), EVENT_TAG_COUNT);
        assert_eq!(EVENT_TAG_COUNT, 33);
        // Distinct discriminants 0..32.
        let mut discs: Vec<u32> = tags.iter().map(|t| *t as u32).collect();
        discs.sort();
        assert_eq!(discs, (0..33).collect::<Vec<u32>>());
    }

    #[test]
    fn event_payloads_construct_and_free() {
        let m = StbString::from_string("msg".to_string());
        let ev = StablePluginEvent::message(EventTag::MessageEnd, m);
        assert_eq!(ev.tag, EventTag::MessageEnd);
        // SAFETY: tag == MessageEnd (message variant).
        unsafe {
            assert_eq!(ev.payload.message.message.to_string_lossy(), "msg");
            ev.payload.message.message.free_with(Some(test_free));
        }

        let tc = StablePluginEvent::tool_call(
            EventTag::ToolCall,
            StbString::from_string("call_1".to_string()),
            StbString::from_string("echo".to_string()),
            StbString::from_string("{}".to_string()),
        );
        // SAFETY: tag == ToolCall (tool_call variant).
        unsafe {
            assert_eq!(tc.payload.tool_call.tool_name.to_string_lossy(), "echo");
            tc.payload.tool_call.tool_call_id.free_with(Some(test_free));
            tc.payload.tool_call.tool_name.free_with(Some(test_free));
            tc.payload.tool_call.params.free_with(Some(test_free));
        }
        let _ = reset_freed();
    }

    #[test]
    fn plugin_api_vt_is_pod_and_sized() {
        // The vtable must be a plain old data struct: every fn pointer is
        // non-Drop, the struct has no Drop impl. We exercise that it can be
        // zeroed and read without UB.
        let vt = PluginApiVt {
            free_string: test_free,
            register_tool: None,
            register_command: None,
            register_shortcut: None,
            register_flag: None,
            register_provider: None,
            register_message_renderer: None,
            register_markdown_transformer: None,
            register_entry_renderer: None,
            register_event_handler: None,
            register_resources_discover: None,
            runtime_action: noop_runtime_action,
            dispatch_event: None,
            user_data: core::ptr::null_mut(),
        };
        // All optional slots are null → plugin must degrade.
        assert!(vt.register_tool.is_none());
        assert!(vt.register_event_handler.is_none());
        assert!(vt.register_resources_discover.is_none());
        // Copy (POD) — no UB from a plain copy.
        let _copy = vt;
        // `assert!(core::mem::needs_drop::<PluginApiVt>() == false)` — verified
        // by the absence of a Drop impl + all-Copy fields.
        assert!(!core::mem::needs_drop::<PluginApiVt>());
        assert!(!core::mem::needs_drop::<StbString>());
        assert!(!core::mem::needs_drop::<StepResult>());
        assert!(!core::mem::needs_drop::<StablePluginEvent>());
        assert!(!core::mem::needs_drop::<StableToolSchema>());
    }

    #[test]
    fn register_entrypoint_version_mismatch_refuses() {
        let vt = PluginApiVt {
            free_string: test_free,
            register_tool: None,
            register_command: None,
            register_shortcut: None,
            register_flag: None,
            register_provider: None,
            register_message_renderer: None,
            register_markdown_transformer: None,
            register_entry_renderer: None,
            register_event_handler: None,
            register_resources_discover: None,
            runtime_action: noop_runtime_action,
            dispatch_event: None,
            user_data: core::ptr::null_mut(),
        };
        // Wrong version → refuse (nonzero), body never runs.
        let rc = register_entrypoint(&vt, RPI_PLUGIN_ABI_VERSION.wrapping_add(1), |_| {
            panic!("body must not run on version mismatch");
        });
        assert_ne!(rc, 0);

        // Right version → body runs, rc propagated.
        let rc = register_entrypoint(&vt, RPI_PLUGIN_ABI_VERSION, |_| 0);
        assert_eq!(rc, 0);
        let rc = register_entrypoint(&vt, RPI_PLUGIN_ABI_VERSION, |_| 42);
        assert_eq!(rc, 42);

        // Null api → refuse.
        let rc = register_entrypoint(core::ptr::null(), RPI_PLUGIN_ABI_VERSION, |_| 0);
        assert_ne!(rc, 0);
        let _ = reset_freed();
    }
}