Skill: Reviewer-Order Gate

The implementer goes first. A reviewer starts only when the implementer says it is done. (Etan, 2026-09-27, ~/.claude/CLAUDE.md § Merging.)

What It Enforces

A Claude Code PreToolUse hook on mcp__cmux(layer)?__spawn_agent. It acts only when tool_input.role is exactly "reviewer" (no case or whitespace folding); every other spawn passes untouched.

The boot brief is the inline prompt, or the file at boot_prompt_path, plus any file the brief points at with Read and follow <path> (one level deep). The spawn is allowed when the brief cites either:

Every PR the brief cites must be finished (golemsLead ruling, #296 R1). One cited PR with running or queued checks, no checks reported yet, or a fresh cloud head denies the spawn, even when another cited PR or a DONE report would allow it. This includes merged and closed PRs: pending checks deny whatever the PR's state. A merged or closed PR with finished checks, or a PR not found, neither allows nor blocks. An observed pending PR still denies when another cited PR's gh lookup fails, times out, or returns a malformed check rollup; the gate fails open only when no cited PR was seen blocking.

Completed means finished, not green: a failed check still counts as completed. A bare #N resolves against the spawn's cwd (then the hook's cwd).

Reports are read from their last 256 KiB; a tail that starts mid-line drops that partial line, so a marker is only trusted on a whole line. Briefs are read whole, up to 1 MiB.

Otherwise the spawn is denied with permissionDecision: "deny" and a one-line reason: REVIEWER-ORDER-GATE: implementer not done — <what was missing>.

Fail-Open

The gate never blocks on its own failure. Each of these allows the spawn and adds additionalContext starting REVIEWER-ORDER-GATE advisory::

• missing gh, a gh timeout, a gh error other than "PR not found", or unparseable output;
• an unreadable brief file, or a brief over 1 MiB;
• a brief citing more than six PRs, which cannot all be checked in time;
• malformed hook input.

All gh calls share one budget (REVIEWER_ORDER_GATE_GH_BUDGET_MS, default and ceiling 3.5 s). A timed-out gh is sent SIGKILL, because SIGTERM can be ignored. That keeps the hook inside its 5 s manifest timeout. Measured live on 2026-09-27: about 0.5–0.7 s per PR check.

Output Schema

• allow, or not a reviewer spawn: {}
• deny: {"hookSpecificOutput":{"hookEventName":"PreToolUse","permissionDecision":"deny","permissionDecisionReason":"REVIEWER-ORDER-GATE: implementer not done — ..."}}
• fail-open: {"hookSpecificOutput":{"hookEventName":"PreToolUse","additionalContext":"REVIEWER-ORDER-GATE advisory: ..."}}

Install

Registered in scripts/hooks/manifest.json for the mbp and m1 hosts, next to model-pin-gate, with timeout: 5. The hook installer wires it; nothing else to do.

Run It

bun test skills/golem-powers/reviewer-order-gate/evals/

The suite replays evals/fixtures/*.json through the real hook with gh stubbed on PATH. It makes no network calls. With the hook file absent, as on master before this gate, the harness reports the ungated result {} for every fixture. That is the behavioural RED: every DENY and ADVISORY fixture fails.

Known Gaps

• send_to is not gated. Queueing a new PR to an already-running reviewer with send_to bypasses this gate; the lead's discipline covers that case. The structural fix is a cmuxlayer spawn_agent({..., after_done: <agent_id>}) primitive, worth building only if this gate fires often.
• Evidence is not bound to the review target. Every cited PR must be finished, so a running target PR always denies. But a brief that cites only an unrelated finished PR, and never names the real target, still passes.
• Head age is the commit date. For cloud branches, "stable ≥10 min" uses the head commit's committedDate as a stand-in for push time.