Mutation baseline — S-110 (2026-09-18)

Status: NO BASELINE OBTAINED. Stryker fails to start. Threshold (break 85 %) = not evaluated.

Run

pnpm exec stryker run      # repo root, stryker.config.mjs, Node 26.1.0, pnpm 12

Log: .copilot-tmp/stryker.log. Exit code 1 after ~5 s.

What worked

• Config loaded; incremental off (no .stryker-tmp/incremental.json).
• ProjectReader Found 14 of 389 file(s) to be mutated
• Instrumenter Instrumented 14 source file(s) with 922 mutant(s)
• Creating 4 test runner process(es)

So the mutate globs are correct and the instrumenter is fine: 922 mutants across 14 files is the population the baseline will measure once the runner loads.

Exact failure

WARN OptionsValidator Unknown stryker config option "vitest".
   * You might be missing a plugin that is supposed to use it. Stryker loaded plugins from: ["@stryker-mutator/*"]
...
ERROR Stryker Unexpected error occurred while running Stryker StrykerError: Error: Could not inject
[class ChildProcessTestRunnerWorker]. Cause: Cannot find TestRunner plugin "vitest".
In fact, no TestRunner plugins were loaded. Did you forget to install it?
    at ChildProcessProxyWorker.handleInit (.../@stryker-mutator+core@10.0.0_@types+node@26.6.1/.../child-process-proxy-worker.js:75:41)

Root cause (VERIFIED by reading the loader)

@stryker-mutator/vitest-runner@10.0.0 is installed (node_modules/@stryker-mutator/vitest-runner → symlink into .pnpm, version 10.0.0, dist/src/index.js present; catalog + lockfile agree). The plugin is not found because of how the default plugin expression @stryker-mutator/* is globbed:

@stryker-mutator/core/dist/src/di/plugin-loader.js:77-84 resolves the glob relative to core’s own location:

const pluginDirectory = path.resolve(fileURLToPath(new URL('../../../../../', import.meta.url)), org);
const plugins = (await fs.promises.readdir(pluginDirectory)) ...

Under pnpm’s isolated layout that directory is node_modules/.pnpm/@stryker-mutator+core@10.0.0_.../node_modules/@stryker-mutator/, which contains only api core instrumenter util (checked with Get-ChildItem). vitest-runner lives in a sibling .pnpm folder and is only linked from the root node_modules/@stryker-mutator/, which the glob never reads. Hence “no TestRunner plugins were loaded”, and the vitest option is then reported as unknown (nobody contributed its schema).

This is the documented Stryker + pnpm gotcha: the * glob does not work with pnpm’s non-hoisted store; plugins must be listed explicitly.

Fix to apply to stryker.config.mjs (NOT applied — read-only task)

Add an explicit plugins array so the loader uses bare import resolution (resolvePluginModules → import('@stryker-mutator/vitest-runner')), which goes through the root node_modules symlink and works under pnpm:

export default {
  packageManager: "pnpm",
  testRunner: "vitest",
  plugins: ["@stryker-mutator/vitest-runner"],   // ← add: pnpm isolates .pnpm/<core>/node_modules, the default "@stryker-mutator/*" glob never sees the runner
  vitest: { configFile: "vitest.config.ts" },
  ...
};

Alternative (worse): .npmrc public-hoist-pattern[]=@stryker-mutator/* + reinstall. Rejected — it changes the install layout for everyone to work around a one-line config issue.

Secondary items to check on the first real run (EXPECTED, not verified):

• concurrency: 4 × vitest worker pools on this box; if it thrashes, drop to 2.
• timeoutMS: 30_000 — apply.test.ts fast-check fault-injection and lock tests are the slowest; watch for Timeout mutants being misreported.
• reporters: ["progress"] downgrades to progress-append-only in a non-TTY (harmless).

Mutation population (for planning; no scores yet)

Instrumented files (14, 922 mutants), from mutate:

Area	Files
packages/canon/src	jcs.ts, pae.ts, statement.ts, digest.ts, index.ts (+ any non-test src)
packages/apply/src	contain.ts, write.ts
packages/checks/src/predicates	path.ts, content.ts, manifest.ts, deps.ts, repo.ts, guard.ts … (index.ts excluded)

Per-file score, survivor list and threshold verdict will be filled in once the runner starts. Recommended first triage targets, by expected survivor density (EXPECTED from reading the tests):

1. apply/src/write.ts — renameRetry EBUSY/EPERM/EACCES branches and retry counts have no test; chmod 0755 non-win32 branch untested.
2. apply/src/contain.ts:92 — \\?\ prefix strip on Windows is platform-conditional; the Linux CI job cannot kill those mutants.
3. checks/predicates/content.ts — SECRET_PATTERNS regex boundary mutants; predicates.test.ts has one positive sample per pattern but few negatives.
4. canon/src/jcs.ts — number formatting branches are pinned by RFC 8785 vectors, so high kill rate expected; string-escape edge cases less so.

Numbers

Metric	Value
Mutation score	n/a (runner failed to load)
Break threshold 85 %	not evaluated
Files instrumented	14
Mutants generated	922
Fix	add plugins: ["@stryker-mutator/vitest-runner"] to stryker.config.mjs