diff --git a/docs/ops/MUTATION_GATE_STRYKER_REGRESSION.md b/docs/ops/MUTATION_GATE_STRYKER_REGRESSION.md
new file mode 100644
index 0000000000..f3132b6cea
--- /dev/null
+++ b/docs/ops/MUTATION_GATE_STRYKER_REGRESSION.md
@@ -0,0 +1,84 @@
+---
+title: "Mutation gate — Stryker 9.6.1 OptionsValidator regression (npx stryker run fails at config validation)"
+---
+
+# Mutation gate is currently non-functional (Stryker 9.6.1 + ajv 8.18.0)
+
+> **Discovered:** 2026-06-17 (v3.8.29 cycle), re-running a scoped Stryker probe.
+> `npx stryker run` now fails at **config validation**, before any sandbox/mutant work.
+>
+> **This is a NEW toolchain regression, not a re-opening of the spike.** The Onda-1
+> Task 12 per-test (`killedBy`) attribution question was already settled **GO** in
+> v3.8.27, and the nightly mutation run executed as recently as the **v3.8.28** cycle
+> (it reached the run phase and timed out on **budget** — the standing Onda-2 blocker,
+> tracked separately and addressed by the god-file splits in Onda 3). Something in the
+> shared `node_modules` changed since that last nightly so that the validator now
+> rejects the (valid) config — stopping the nightly from even starting.
+
+## Symptom
+
+`npx stryker run` (and therefore `npm run test:mutation` and the
+`.github/workflows/nightly-mutation.yml` job) fails immediately with:
+
+```
+ERROR OptionsValidator Config option "concurrency" must match pattern "^(100|[1-9]?[0-9])%$"
+ERROR Stryker Please correct this configuration error and try again.
+  at OptionsValidator.schemaValidate (.../@stryker-mutator/core/dist/src/config/options-validator.js:161)
+```
+
+Reproduced with the **unmodified** `stryker.conf.json` (`npx stryker run -c stryker.conf.json --dryRunOnly`)
+and with a copy that **removes** `concurrency` entirely — the error persists either way.
+
+## Why it is a false positive (the project config is valid)
+
+- `stryker.conf.json` sets `"concurrency": 4`. The Stryker JSON schema
+  (`node_modules/@stryker-mutator/api/dist/schema/stryker-core.json`) defines
+  `concurrency` as `oneOf: [ {type:number, minimum:1}, {type:string, pattern:"^(100|[1-9]?[0-9])%$"} ]`
+  with **`examples: [4, "50%", "100%"]`** — so a number `4` is explicitly valid.
+- Validating the config (and a bare `{concurrency: 4}`) against that schema with a
+  **standalone ajv 8.18.0** — the exact version Stryker depends on (`~8.18.0`, no nested
+  copy, no `overrides`) — and even with Stryker's **exact** ajv options
+  (`useDefaults, allErrors, jsPropertySyntax, verbose, logger:false, strict:false`)
+  returns **valid**, no concurrency error.
+- A percentage string (`"30%"`, which _does_ match the pattern) is **also** rejected,
+  and the error appears even when `concurrency` is omitted. So the message is
+  **mis-attributed**: Stryker's compiled `validateFn` fails on the merged full-options
+  object and reports it against `concurrency`.
+
+## Likely root cause
+
+The interaction is inside Stryker 9.6.1's `OptionsValidator`, not the project config.
+`jsPropertySyntax: true` (line 18 of `options-validator.js`) is an **ajv 6** option that
+was **removed in ajv 7+**; under ajv 8.18.0 it is a no-op, and the error-path translation
+in `describeErrors` can mislabel the genuinely-failing field as `concurrency`. The true
+offending value is somewhere in the default-filled options object — not surfaced because
+`describeErrors` collapses the ajv `oneOf` errors to a single line.
+
+## Impact
+
+- The mutation gate is **nightly + advisory** (not PR-blocking), so this **reds no PR**.
+- But the nightly now **fails before it starts** — strictly worse than the prior state,
+  where it reached the run phase and only timed out on budget. It produces **zero scores**:
+  1. **P0 #1** — promoting `mutationScore` to a ratchet — has no fresh nightly values.
+  2. **Onda 2** (radiography + mutation-proved pruning) needs ≥1 complete run; this
+     config-validation regression must be cleared first, then the **budget** blocker
+     (god-files dominating ~⅔ of the 15k mutants) still remains, per the Onda-3 splits.
+
+## Recommended fix (focused follow-up — NOT in this PR)
+
+Out of scope for the oasdiff base-ref drift fix this PR carries, and it needs a
+dependency change + reinstall that must happen on a **non-shared** `node_modules`
+(the dev worktrees here symlink a shared `node_modules`; never `npm install` into it).
+
+1. Reproduce on a clean install: `npm ci` in an isolated checkout, then
+   `npx stryker run -c stryker.conf.json --dryRunOnly`.
+2. Surface the real failing field: temporarily log `validateFn.errors` (verbose) in
+   `options-validator.js`, or compile the schema with a plain ajv 8.18.0 and validate the
+   **full default options** object (not just the partial config) to see which field+value fails.
+3. Most probable remedies, in order of preference:
+   - Pin `@stryker-mutator/core` + `@stryker-mutator/tap-runner` to the last version whose
+     `OptionsValidator` validates cleanly against ajv 8.18.0 (bisect 9.6.1 ← down).
+   - Or add a tested `overrides` for the ajv version Stryker's validator actually expects.
+4. Verify by getting `npx stryker run --dryRunOnly` to exit 0 again (the conf comment claims
+   this passed on 2026-06-15/17 — confirm what changed in `node_modules` since), **then** run
+   the Task 12 spike to record the real `killedBy` verdict.
diff --git a/scripts/check/check-openapi-breaking.mjs b/scripts/check/check-openapi-breaking.mjs
index 445e4ffafc..168f5a07bd 100644
--- a/scripts/check/check-openapi-breaking.mjs
+++ b/scripts/check/check-openapi-breaking.mjs
@@ -29,13 +29,14 @@
 // neste repo: report → ratchet → block).
 //
 // Base ref:
-//   • CI passa BASE_REF=${{ github.base_ref }} (ex.: "release/v3.8.27").
-//   • Local: default origin/release/v3.8.27.
+//   • CI passa BASE_REF=${{ github.base_ref }} (ex.: "release/vX.Y.Z").
+//   • Local: default derivado da versão do package.json (releaseBranchForVersion),
+//     ex.: package 3.8.29 → "origin/release/v3.8.29" — nunca fica stale entre ciclos.
 // A spec base é extraída com `git show <BASE_REF>:docs/reference/openapi.yaml`.
 //
 // Uso:
 //   node scripts/check/check-openapi-breaking.mjs
-//   BASE_REF=origin/release/v3.8.27 node scripts/check/check-openapi-breaking.mjs
+//   BASE_REF=origin/release/vX.Y.Z node scripts/check/check-openapi-breaking.mjs
 //   node scripts/check/check-openapi-breaking.mjs --json    # imprime JSON bruto do oasdiff
 //   node scripts/check/check-openapi-breaking.mjs --quiet   # suprime logs de diagnóstico
 //   node scripts/check/check-openapi-breaking.mjs --ratchet # falha (exit 1) numa regressão
@@ -53,7 +54,35 @@ const RATCHET = process.argv.includes("--ratchet");
 
 const SPEC_REL = "docs/reference/openapi.yaml";
 const SPEC_PATH = path.join(ROOT, "docs", "reference", "openapi.yaml");
-const DEFAULT_BASE_REF = "origin/release/v3.8.27";
+
+/**
+ * Deriva o branch base de release a partir de uma versão semver
+ * (ex.: "3.8.29" → "origin/release/v3.8.29"). Mantém o default sincronizado com
+ * o ciclo de release SEM hard-code: o version-bump atualiza package.json a cada
+ * ciclo, então o default nunca fica stale (era "origin/release/v3.8.27" fixo).
+ * Ignora sufixos de prerelease/build (ex.: "3.8.29-dev.2" → v3.8.29).
+ *
+ * @param {string|null|undefined} version
+ * @returns {string|null} branch base (sem `origin/` ausente) ou null se não-semver
+ */
+export function releaseBranchForVersion(version) {
+  const m = String(version ?? "")
+    .trim()
+    .match(/^(\d+)\.(\d+)\.(\d+)/);
+  return m ? `origin/release/v${m[1]}.${m[2]}.${m[3]}` : null;
+}
+
+function readPackageVersion() {
+  try {
+    return JSON.parse(fs.readFileSync(path.join(ROOT, "package.json"), "utf8")).version;
+  } catch {
+    return null;
+  }
+}
+
+// CI sempre passa BASE_REF=${{ github.base_ref }} e vence; este default só vale
+// para runs locais. Derivado da versão para não re-driftar a cada release.
+const DEFAULT_BASE_REF = releaseBranchForVersion(readPackageVersion()) || "origin/release/v3.8.29";
 const BASELINE_PATH = path.join(ROOT, "config/quality/quality-baseline.json");
 
 // ---------------------------------------------------------------------------
diff --git a/tests/unit/check-openapi-breaking-ratchet.test.ts b/tests/unit/check-openapi-breaking-ratchet.test.ts
index cefd96cace..c9d1333be0 100644
--- a/tests/unit/check-openapi-breaking-ratchet.test.ts
+++ b/tests/unit/check-openapi-breaking-ratchet.test.ts
@@ -17,6 +17,7 @@ import { spawnSync } from "node:child_process";
 import {
   evaluateOpenapiRatchet,
   readBaselineOpenapiValue,
+  releaseBranchForVersion,
   // @ts-expect-error — .mjs helper has no type declarations; runtime shape is known.
 } from "../../scripts/check/check-openapi-breaking.mjs";
 
@@ -26,11 +27,34 @@ const evaluate = evaluateOpenapiRatchet as (args: {
   baseline: number | null;
 }) => RatchetVerdict;
 const readBaseline = readBaselineOpenapiValue as (p?: string) => number | null;
+const releaseBranch = releaseBranchForVersion as (v: string | null | undefined) => string | null;
 
 const SCRIPT_PATH = fileURLToPath(
   new URL("../../scripts/check/check-openapi-breaking.mjs", import.meta.url)
 );
 
+// ---------------------------------------------------------------------------
+// releaseBranchForVersion — the default base ref derives from the package
+// version so it never goes stale across release cycles (was a hard-coded
+// "origin/release/v3.8.27" that drifted into the v3.8.29 cycle).
+// ---------------------------------------------------------------------------
+
+test("releaseBranchForVersion: a clean semver derives the matching release branch", () => {
+  assert.equal(releaseBranch("3.8.29"), "origin/release/v3.8.29");
+});
+
+test("releaseBranchForVersion: a prerelease/build suffix is ignored", () => {
+  assert.equal(releaseBranch("3.8.29-dev.2"), "origin/release/v3.8.29");
+  assert.equal(releaseBranch("10.0.0+build.7"), "origin/release/v10.0.0");
+});
+
+test("releaseBranchForVersion: a non-semver value yields null (caller falls back)", () => {
+  assert.equal(releaseBranch(""), null);
+  assert.equal(releaseBranch(null), null);
+  assert.equal(releaseBranch(undefined), null);
+  assert.equal(releaseBranch("not-a-version"), null);
+});
+
 // ---------------------------------------------------------------------------
 // evaluateOpenapiRatchet — the three contract cases from the plan
 // ---------------------------------------------------------------------------