car_server_core/coder/

no_change.rs

//! The gate behind `CoderState::Reported` — when a session may conclude that
//! **no code should change**, and when it may not.
//!
//! ## Nomination is not adjudication
//!
//! The model's `report_no_change` action records a *candidate* finding. It never
//! transitions the session. Everything that decides the outcome is checked here,
//! outside inference, against facts the runtime collected itself.
//!
//! That split is the whole defence. A verdict the model can reach by giving up
//! is an escape hatch from every hard contract, and that failure mode is worse
//! than the gap #1070 describes. So the interesting code in this module is all
//! refusal.
//!
//! ## Mutation history is monotonic
//!
//! [`MutationLedger`] latches on the first successful mutating tool call and
//! never clears. Editing and later reverting does **not** restore eligibility.
//!
//! This is deliberately unforgiving, because the alternative is a laundry: try
//! to fix it, fail, revert, declare the premise wrong, exit 0. The price is real
//! and worth paying — a session that explored by editing has to keep coding or
//! start over. A clean worktree is necessary but not sufficient; the ledger is
//! what makes it not sufficient.
//!
//! ## What the runtime can actually verify
//!
//! Only one of the three no-change shapes, and only narrowly:
//!
//! - [`NoChangeKind::PremiseWrong`] — verifiable *in the operational sense that
//!   a trusted contract already passed against the untouched baseline*. The
//!   verdict this licenses reads "the contract was already satisfied before any
//!   work", and NOT "the model's explanation is correct".
//! - [`NoChangeKind::DeliberateBehavior`] — not verifiable. Passing tests
//!   establish what the code does now, never that maintainers intended it or
//!   still want it.
//! - [`NoChangeKind::NonCodeDecision`] — not verifiable. Depends on policy,
//!   ownership, rollout and priorities that live outside the worktree.
//!
//! A red baseline proves none of them. Source inspection and model reasoning are
//! evidence for a human reviewer, never an independent gate.
//!
//! ## What the observation still cannot see, and why that is survivable
//!
//! [`worktree_fingerprint`] samples at shell-call *boundaries*, so a mutation
//! made and undone inside a single invocation — `sed -i …; cargo test;
//! git checkout -- .` — leaves `before == after` and records nothing. Writes to
//! `.gitignore`d paths are invisible too, deliberately: `--ignored` would make
//! every `cargo build` register as a mutation and put the terminal permanently
//! out of reach.
//!
//! Both falsify the weaker claim ("this session edited nothing"). Neither
//! reaches the terminal that matters, and the reason is worth being precise
//! about, because it is what the design actually rests on:
//!
//! **the autonomous path is gated on the baseline, and the baseline is computed
//! before the loop and held in memory.** No amount of editing, reverting,
//! committing or hiding during the session can retro-change it. A genuinely
//! hard task has a red baseline, so `baseline_gates_nothing` is false and the
//! nomination routes to a human at bucket 3 — a park, never an exit-0 success.
//! The give-up scenario this module exists to prevent therefore cannot be
//! reached by defeating the fingerprint; it can only be reached by defeating the
//! baseline, which the model never gets to touch.
//!
//! What the fingerprint and the ledger buy on top is the integrity of the
//! *report*: a session that says "I changed nothing" should not have changed
//! something. That is worth having, and it is a weaker claim than the terminal.

use super::contract::{baseline_gates_nothing, CheckResult};
use super::session::{ContractProvenance, NoChangeKind};

/// Upper bound on the nominated text. Generous for a real finding, small enough
/// that a transcript dump cannot be laundered into an issue body.
pub const MAX_FINDING_TEXT: usize = 4096;

/// Latches once a mutating tool has succeeded, and never clears.
///
/// See the module docs on why this does not reset when the worktree is restored.
#[derive(Debug, Default)]
pub struct MutationLedger {
    mutated: std::sync::atomic::AtomicBool,
}

impl MutationLedger {
    pub fn new() -> Self {
        Self::default()
    }

    /// Record that a mutating tool call succeeded. Idempotent, and one-way.
    pub fn record_mutation(&self) {
        self.mutated
            .store(true, std::sync::atomic::Ordering::SeqCst);
    }

    pub fn has_mutated(&self) -> bool {
        self.mutated.load(std::sync::atomic::Ordering::SeqCst)
    }
}

/// A cheap content-sensitive summary of the worktree's state.
///
/// Two git reads: `git status --porcelain -uall` (which paths are modified,
/// staged, or untracked) and `git diff HEAD` (the tracked content itself). Their
/// hash changes whenever a tool edits, creates or deletes anything git can see.
///
/// **Known gap**: editing the *contents* of a file that is already untracked
/// does not move the hash, because `status` lists the path and `diff HEAD` does
/// not cover it. That is a narrow hole and it is not the one that matters —
/// a nomination also requires a clean worktree, and an untracked file's mere
/// existence already shows up in `status`, so a session cannot arrive at a
/// "clean" fingerprint while carrying an edit git would report.
///
/// Returns `None` when git cannot answer, which callers must treat as
/// "cannot establish cleanliness" rather than as clean.
pub fn worktree_fingerprint(dir: &std::path::Path) -> Option<String> {
    use sha2::{Digest, Sha256};
    let mut hasher = Sha256::new();
    for part in observation(dir)? {
        hasher.update(&part);
        hasher.update([0u8]);
    }
    Some(format!("{:x}", hasher.finalize()))
}

/// The four git reads that together define "this session changed nothing".
///
/// Each one closes a hole the others do not, and all four were found by
/// reproducing the bypass against a real repository rather than by reasoning:
///
/// - `rev-parse HEAD` — **commit identity**. Without it,
///   `sed -i … && git add -A && git commit` leaves `status` and `diff HEAD`
///   both empty, so a real, persistent edit is invisible. The only thing
///   standing against that was a line in the system prompt asking the model not
///   to commit, and a prompt is not a control: under this feature's own threat
///   model the reader is a model looking for a non-loss exit.
/// - `status --porcelain -uall` — modified, staged and untracked paths.
/// - `diff HEAD` — tracked content, so an edit staged *and* unstaged still shows.
/// - `ls-files -v` — **index flags**. `git update-index --assume-unchanged f`
///   changes neither status nor diff, and every later edit to that path is then
///   invisible to both, permanently. The flag lives in `.git/`, outside
///   everything else here. Lowercase tags and `S` mark it.
fn observation(dir: &std::path::Path) -> Option<Vec<Vec<u8>>> {
    let read = |args: &[&str]| -> Option<Vec<u8>> {
        let out = std::process::Command::new("git")
            .current_dir(dir)
            .args(args)
            .output()
            .ok()?;
        out.status.success().then_some(out.stdout)
    };
    Some(vec![
        read(&["rev-parse", "HEAD"])?,
        read(&["status", "--porcelain", "-uall"])?,
        read(&["diff", "HEAD"])?,
        read(&["ls-files", "-v"])?,
    ])
}

/// Whether any path carries an index flag that hides later edits from
/// `status`/`diff` — `assume-unchanged` (lowercase tag) or `skip-worktree`
/// (`S`). `git ls-files -v` prefixes each path with its tag.
fn has_hiding_index_flags(ls_files_v: &[u8]) -> bool {
    String::from_utf8_lossy(ls_files_v).lines().any(|line| {
        line.chars()
            .next()
            .is_some_and(|c| c == 'S' || c.is_ascii_lowercase())
    })
}

/// Whether the worktree currently carries no change git can see.
///
/// `None` when git cannot answer. A caller must treat that as "cannot establish
/// cleanliness", never as clean — the whole point of the check is that the
/// burden of proof sits on the session claiming it changed nothing.
pub fn worktree_is_pristine(dir: &std::path::Path, expected_head: &str) -> Option<bool> {
    let parts = observation(dir)?;
    let head = String::from_utf8_lossy(&parts[0]).trim().to_string();
    Some(
        head == expected_head.trim()
            && parts[1].is_empty()
            && parts[2].is_empty()
            && !has_hiding_index_flags(&parts[3]),
    )
}

/// The commit a session started on, captured before any model turn so a later
/// `git commit` cannot pass itself off as a clean tree.
pub fn head_commit(dir: &std::path::Path) -> Option<String> {
    let out = std::process::Command::new("git")
        .current_dir(dir)
        .args(["rev-parse", "HEAD"])
        .output()
        .ok()?;
    out.status
        .success()
        .then(|| String::from_utf8_lossy(&out.stdout).trim().to_string())
}

/// Why a nomination was refused. Each variant is a distinct thing to tell the
/// model, because "try again differently" and "you cannot get here from where
/// you are" are different instructions.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum NominationRefusal {
    /// `summary` or `evidence` was empty, whitespace, or over the cap.
    UnusableText(&'static str),
    /// A mutating tool succeeded earlier in this session.
    WorktreeWasMutated,
    /// The worktree differs from the captured baseline right now.
    WorktreeNotClean,
    /// The baseline did not complete — starved by the session deadline, or a
    /// check killed by infrastructure. A baseline that did not finish is not
    /// evidence of anything.
    BaselineIncomplete,
}

impl NominationRefusal {
    /// The message handed back to the model as the tool result.
    pub fn message(&self) -> String {
        match self {
            Self::UnusableText(which) => format!(
                "report_no_change refused: `{which}` must be non-empty and under \
                 {MAX_FINDING_TEXT} characters. State the conclusion and what you \
                 examined to reach it."
            ),
            Self::WorktreeWasMutated => {
                "report_no_change refused: this session already made a successful edit. \
                 Reverting does not restore eligibility — a no-change finding is only \
                 available to a session that never changed anything. Continue toward a \
                 diff, or start a fresh session to investigate."
                    .to_string()
            }
            Self::WorktreeNotClean => {
                "report_no_change refused: the worktree differs from the baseline. \
                 Restore it before concluding that nothing should change."
                    .to_string()
            }
            Self::BaselineIncomplete => {
                "report_no_change refused: the baseline evaluation did not complete, so \
                 there is nothing to conclude from. This is not something you can fix — \
                 the session needs more time or a working check environment."
                    .to_string()
            }
        }
    }
}

/// What the runtime decided to do with a nomination.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum NominationVerdict {
    /// Terminal, with no human in the loop. The narrow path.
    Autonomous,
    /// Recorded, and parked for a human to accept or reject.
    NeedsHuman,
    /// Not recorded at all.
    Refused(NominationRefusal),
}

/// Whether a baseline run actually finished.
///
/// An empty result set is not a clean baseline — it is a baseline that never
/// ran. A check the *session clock* killed (`deadline_clamped` alongside
/// `timed_out`) is not a verdict on the work either, which is the same
/// distinction `code-task`'s event stream draws.
pub fn baseline_completed(results: &[CheckResult]) -> bool {
    !results.is_empty() && !results.iter().any(|r| r.timed_out && r.deadline_clamped)
}

/// The facts a nomination is judged against. All collected by the runtime; none
/// asserted by the model.
#[derive(Debug, Clone, Copy)]
pub struct NominationContext<'a> {
    pub kind: NoChangeKind,
    pub summary: &'a str,
    pub evidence: &'a str,
    pub baseline: &'a [CheckResult],
    pub provenance: ContractProvenance,
    /// The worktree matches the captured baseline right now.
    pub worktree_clean: bool,
    /// A mutating tool has succeeded at some point this session.
    pub mutated: bool,
}

/// Judge a nomination.
///
/// The ordering matters: the refusals that describe the *session's* history come
/// before the ones about the finding's content, so a model that has already
/// disqualified itself is told that rather than being invited to reword.
pub fn evaluate_nomination(ctx: NominationContext<'_>) -> NominationVerdict {
    use NominationRefusal::*;

    if ctx.mutated {
        return NominationVerdict::Refused(WorktreeWasMutated);
    }
    if !ctx.worktree_clean {
        return NominationVerdict::Refused(WorktreeNotClean);
    }
    if !baseline_completed(ctx.baseline) {
        return NominationVerdict::Refused(BaselineIncomplete);
    }
    if !usable(ctx.summary) {
        return NominationVerdict::Refused(UnusableText("summary"));
    }
    if !usable(ctx.evidence) {
        return NominationVerdict::Refused(UnusableText("evidence"));
    }

    // The one autonomous path. Every conjunct is load-bearing:
    //   - PremiseWrong is the only shape a green baseline can speak to at all.
    //   - baseline_gates_nothing means EVERY check passed untouched.
    //   - is_trusted means the model did not author the checks it is citing.
    // Drop any one and this becomes self-approval.
    let autonomous = matches!(ctx.kind, NoChangeKind::PremiseWrong)
        && baseline_gates_nothing(ctx.baseline)
        && ctx.provenance.is_trusted();

    if autonomous {
        NominationVerdict::Autonomous
    } else {
        NominationVerdict::NeedsHuman
    }
}

fn usable(text: &str) -> bool {
    let trimmed = text.trim();
    !trimmed.is_empty() && trimmed.len() <= MAX_FINDING_TEXT
}

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

    fn check(name: &str, passed: bool) -> CheckResult {
        CheckResult {
            name: name.to_string(),
            passed,
            exit_code: Some(if passed { 0 } else { 1 }),
            duration_ms: 1,
            output_tail: String::new(),
            timed_out: false,
            deadline_clamped: false,
        }
    }

    fn ctx<'a>(
        kind: NoChangeKind,
        baseline: &'a [CheckResult],
        provenance: ContractProvenance,
    ) -> NominationContext<'a> {
        NominationContext {
            kind,
            summary: "the code already handles this",
            evidence: "read handler.rs and ran the suite",
            baseline,
            provenance,
            worktree_clean: true,
            mutated: false,
        }
    }

    #[test]
    fn the_narrow_path_terminates_autonomously() {
        let green = vec![check("a", true), check("b", true)];
        assert_eq!(
            evaluate_nomination(ctx(
                NoChangeKind::PremiseWrong,
                &green,
                ContractProvenance::OperatorSupplied
            )),
            NominationVerdict::Autonomous
        );
    }

    #[test]
    fn a_model_authored_contract_never_self_approves() {
        // The whole escape hatch, in one assertion: same green baseline, same
        // kind, and the only difference is who wrote the checks.
        let green = vec![check("a", true)];
        assert_eq!(
            evaluate_nomination(ctx(
                NoChangeKind::PremiseWrong,
                &green,
                ContractProvenance::ModelDerived
            )),
            NominationVerdict::NeedsHuman
        );
    }

    #[test]
    fn a_runtime_generated_reproduction_is_trusted() {
        // This is what lets autonomous self-triage work at all: the contract
        // came from telemetry CAR collected before the session existed.
        let green = vec![check("repro", true)];
        assert_eq!(
            evaluate_nomination(ctx(
                NoChangeKind::PremiseWrong,
                &green,
                ContractProvenance::RuntimeGenerated
            )),
            NominationVerdict::Autonomous
        );
    }

    #[test]
    fn the_two_judgement_shapes_always_reach_a_human() {
        let green = vec![check("a", true)];
        for kind in [
            NoChangeKind::DeliberateBehavior,
            NoChangeKind::NonCodeDecision,
        ] {
            assert_eq!(
                evaluate_nomination(ctx(kind, &green, ContractProvenance::OperatorSupplied)),
                NominationVerdict::NeedsHuman,
                "{kind:?} is not runtime-verifiable"
            );
        }
    }

    #[test]
    fn a_red_baseline_proves_nothing_and_parks() {
        let mixed = vec![check("a", true), check("b", false)];
        assert_eq!(
            evaluate_nomination(ctx(
                NoChangeKind::PremiseWrong,
                &mixed,
                ContractProvenance::OperatorSupplied
            )),
            NominationVerdict::NeedsHuman
        );
    }

    #[test]
    fn editing_then_reverting_does_not_restore_eligibility() {
        let green = vec![check("a", true)];
        let mut c = ctx(
            NoChangeKind::PremiseWrong,
            &green,
            ContractProvenance::OperatorSupplied,
        );
        // Worktree is clean again — the revert worked. The ledger still says no.
        c.worktree_clean = true;
        c.mutated = true;
        assert_eq!(
            evaluate_nomination(c),
            NominationVerdict::Refused(NominationRefusal::WorktreeWasMutated)
        );
    }

    #[test]
    fn a_dirty_worktree_is_refused() {
        let green = vec![check("a", true)];
        let mut c = ctx(
            NoChangeKind::PremiseWrong,
            &green,
            ContractProvenance::OperatorSupplied,
        );
        c.worktree_clean = false;
        assert_eq!(
            evaluate_nomination(c),
            NominationVerdict::Refused(NominationRefusal::WorktreeNotClean)
        );
    }

    #[test]
    fn a_starved_baseline_is_not_evidence() {
        let mut killed = check("a", false);
        killed.timed_out = true;
        killed.deadline_clamped = true;
        let results = vec![killed];
        assert_eq!(
            evaluate_nomination(ctx(
                NoChangeKind::PremiseWrong,
                &results,
                ContractProvenance::OperatorSupplied
            )),
            NominationVerdict::Refused(NominationRefusal::BaselineIncomplete)
        );
    }

    #[test]
    fn an_empty_baseline_is_not_a_clean_one() {
        assert!(!baseline_completed(&[]));
        assert_eq!(
            evaluate_nomination(ctx(
                NoChangeKind::PremiseWrong,
                &[],
                ContractProvenance::OperatorSupplied
            )),
            NominationVerdict::Refused(NominationRefusal::BaselineIncomplete)
        );
    }

    #[test]
    fn empty_or_oversized_text_is_refused() {
        let green = vec![check("a", true)];
        let mut c = ctx(
            NoChangeKind::PremiseWrong,
            &green,
            ContractProvenance::OperatorSupplied,
        );
        c.summary = "   ";
        assert_eq!(
            evaluate_nomination(c),
            NominationVerdict::Refused(NominationRefusal::UnusableText("summary"))
        );

        let huge = "x".repeat(MAX_FINDING_TEXT + 1);
        let mut c2 = ctx(
            NoChangeKind::PremiseWrong,
            &green,
            ContractProvenance::OperatorSupplied,
        );
        c2.evidence = &huge;
        assert_eq!(
            evaluate_nomination(c2),
            NominationVerdict::Refused(NominationRefusal::UnusableText("evidence"))
        );
    }

    #[test]
    fn session_history_is_reported_before_content() {
        // A disqualified session must be told it is disqualified, not invited to
        // reword its way in.
        let green = vec![check("a", true)];
        let mut c = ctx(
            NoChangeKind::PremiseWrong,
            &green,
            ContractProvenance::OperatorSupplied,
        );
        c.mutated = true;
        c.summary = "";
        assert_eq!(
            evaluate_nomination(c),
            NominationVerdict::Refused(NominationRefusal::WorktreeWasMutated)
        );
    }

    /// Helper: a real git repo with one committed file.
    fn git_repo() -> tempfile::TempDir {
        let dir = tempfile::tempdir().unwrap();
        let git = |args: &[&str]| {
            std::process::Command::new("git")
                .current_dir(dir.path())
                .args(args)
                .output()
                .unwrap();
        };
        git(&["init", "-q"]);
        git(&["config", "user.email", "t@t"]);
        git(&["config", "user.name", "t"]);
        std::fs::write(dir.path().join("f.txt"), "original\n").unwrap();
        git(&["add", "-A"]);
        git(&["commit", "-qm", "init"]);
        dir
    }

    fn git(dir: &std::path::Path, args: &[&str]) {
        std::process::Command::new("git")
            .current_dir(dir)
            .args(args)
            .output()
            .unwrap();
    }

    /// Committing an edit leaves `status` and `diff HEAD` both empty, so an
    /// observation built only from those two reports a pristine tree over a
    /// real, persistent change. Binding HEAD is what closes it.
    ///
    /// The only other thing standing against this is a line in the system
    /// prompt asking the model not to commit — and under this feature's own
    /// threat model, a prompt is not a control.
    #[test]
    fn a_committed_edit_is_not_a_pristine_tree() {
        let dir = git_repo();
        let start = head_commit(dir.path()).expect("HEAD readable");
        let before = worktree_fingerprint(dir.path()).unwrap();
        assert_eq!(worktree_is_pristine(dir.path(), &start), Some(true));

        std::fs::write(dir.path().join("f.txt"), "mutated\n").unwrap();
        git(dir.path(), &["add", "-A"]);
        git(dir.path(), &["commit", "-qm", "sneak"]);

        assert_ne!(
            worktree_fingerprint(dir.path()).unwrap(),
            before,
            "a commit must move the fingerprint"
        );
        assert_eq!(
            worktree_is_pristine(dir.path(), &start),
            Some(false),
            "HEAD moved, so the tree is not the one this session started on"
        );
    }

    /// `git update-index --assume-unchanged` changes neither status nor diff,
    /// and every later edit to that path is then invisible to both —
    /// permanently, from metadata living in `.git/`. `ls-files -v` is the only
    /// one of the four reads that sees it.
    #[test]
    fn an_assume_unchanged_flag_is_not_a_pristine_tree() {
        let dir = git_repo();
        let start = head_commit(dir.path()).expect("HEAD readable");
        let before = worktree_fingerprint(dir.path()).unwrap();

        git(dir.path(), &["update-index", "--assume-unchanged", "f.txt"]);

        assert_ne!(
            worktree_fingerprint(dir.path()).unwrap(),
            before,
            "setting the flag must itself register as a mutation"
        );
        assert_eq!(
            worktree_is_pristine(dir.path(), &start),
            Some(false),
            "a tree that can hide later edits is not pristine"
        );

        // And the edit it was hiding stays invisible to status/diff — which is
        // exactly why the flag itself has to be what disqualifies the session.
        std::fs::write(dir.path().join("f.txt"), "tampered\n").unwrap();
        assert_eq!(worktree_is_pristine(dir.path(), &start), Some(false));
    }

    #[test]
    fn skip_worktree_is_caught_too() {
        let dir = git_repo();
        let start = head_commit(dir.path()).expect("HEAD readable");
        git(dir.path(), &["update-index", "--skip-worktree", "f.txt"]);
        assert_eq!(worktree_is_pristine(dir.path(), &start), Some(false));
    }

    #[test]
    fn an_ordinary_edit_still_shows() {
        let dir = git_repo();
        let start = head_commit(dir.path()).expect("HEAD readable");
        std::fs::write(dir.path().join("f.txt"), "edited\n").unwrap();
        assert_eq!(worktree_is_pristine(dir.path(), &start), Some(false));
    }

    #[test]
    fn a_head_that_does_not_match_is_never_pristine() {
        let dir = git_repo();
        assert_eq!(
            worktree_is_pristine(dir.path(), "0000000000000000000000000000000000000000"),
            Some(false)
        );
    }

    #[test]
    fn the_ledger_is_one_way() {
        let ledger = MutationLedger::new();
        assert!(!ledger.has_mutated());
        ledger.record_mutation();
        ledger.record_mutation();
        assert!(ledger.has_mutated());
    }

    #[test]
    fn trusted_provenance_has_exactly_one_untrusted_member() {
        assert!(ContractProvenance::OperatorSupplied.is_trusted());
        assert!(ContractProvenance::HumanConfirmed.is_trusted());
        assert!(ContractProvenance::RuntimeGenerated.is_trusted());
        assert!(
            !ContractProvenance::ModelDerived.is_trusted(),
            "a contract the session's own model wrote is never trusted — widening \
             this is how the gate stops working"
        );
    }
}