Rename RFCs to Agent Notes

2026-07-19 22:50:49 +08:00
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+# Agent Note: Mutation testing as the coverage counterweight
+
+Status: proposed
+
+## Problem
+
+The per-file 100% coverage gate ([the quality-gates decision](../../implemented/process/2026-06-11-quality-gates.md)) proves every line *executes* under test — not that any assertion would notice if the line were wrong. Under agent-written tests, coverage pressure can produce execution-without-assertion. Mutation testing measures what coverage cannot: whether the suite *kills* deliberately injected bugs.
+
+## Proposal
+
+Stryker (`@stryker-mutator/vitest-runner`) over `packages/*/src`:
+
+- **PR-scoped incremental runs** (changed files only) as a CI job — fast enough to gate merges once tuned.
+- **Nightly full runs** with a tracked mutation score; start by recording, then set the threshold at the observed baseline and ratchet upward (same policy as coverage: thresholds only ever tighten).
+- Surviving mutants are work items: an agent picks a survivor, writes the killing test, repeats — a well-shaped autonomous loop.
+- Equivalent mutants (provably behavior-preserving) get annotated exclusions with reasons, mirroring the `/* v8 ignore */` policy.
+
+## Plan
+
+1. Add Stryker config scoped to one package (llm — smallest, most algorithmic) and measure runtime.
+2. Expand to all packages; record baseline scores in the config.
+3. Wire the nightly job; add the incremental PR job once runtime is acceptable.
+
+## Acceptance criteria
+
+- A Stryker config runs over `packages/*/src` with the vitest runner; a nightly job records the mutation score, and a ratcheting threshold fails the run when the score drops below the recorded baseline.
+- PR-scoped incremental runs gate merges once runtime is acceptable — or are explicitly kept nightly-only, with that outcome recorded here.
+- Equivalent mutants carry annotated exclusions with reasons, mirroring the `/* v8 ignore */` policy.
+
+## Risks
+
+Runtime: mutation testing is expensive; per-file 100% coverage helps (every mutant is at least reached). If PR-scoped runs stay too slow, keep them nightly-only and rely on the score ratchet.
+
+<!-- agent-note-format: alternatives-not-recorded (pre-format Agent Note) -->