@@ -6,9 +6,9 @@
   code-quality/dead-code risk is relevant, CI guards are green, and no conflict markers or unmerged
   paths remain.
 - A local unit-only run is not CI-green. Use `pnpm test:unit` for the repo unit bundle, or
-  `vitest run --project unit-core --project subprocess-stub` when invoking Vitest directly. The
-  **Integration Tests** and **Coverage** jobs run the `provider-integration` project — verify those
-  green on the actual PR head.
+  `vitest run --project unit-core --project subprocess-stub --project fuzz-worker` when invoking
+  Vitest directly. The **Integration Tests** and **Coverage** jobs run the `provider-integration`
+  project — verify those green on the actual PR head.
 - Device-facing behavior is not merge-ready without real simulator/emulator/device evidence for the
   changed path. Fixture-backed tests prove contracts; they do not replace a live run that creates or
   observes the artifact/state the feature claims to handle. If live verification is blocked, state
 
@@ -171,10 +171,11 @@
     "test-app:maestro:ios": "pnpm test-app:maestro --platform ios",
     "test-app:maestro:android": "pnpm test-app:maestro --platform android",
     "test": "pnpm test:unit",
-    "test:unit": "vitest run --project unit-core --project subprocess-stub",
+    "test:unit": "vitest run --project unit-core --project subprocess-stub --project fuzz-worker",
     "test:maestro-compat": "vitest run --project unit-core packages/maestro src/daemon/adapters/maestro src/compat/__tests__/replay-input.test.ts",
-    "test:coverage": "vitest run --coverage",
-    "test:coverage:ci": "vitest run --coverage",
+    "test:coverage": "vitest run --coverage --project=!fuzz-worker && pnpm test:fuzz-worker",
+    "test:coverage:ci": "vitest run --coverage --project=!fuzz-worker && pnpm test:fuzz-worker",
+    "test:fuzz-worker": "AGENT_DEVICE_COVERAGE_SHARD= AGENT_DEVICE_COVERAGE_MERGE= vitest run --project fuzz-worker",
     "test:integration:provider": "vitest run --project provider-integration",
     "test:integration:progress": "node --experimental-strip-types scripts/integration-progress.ts",
     "test:integration:progress:check": "node --experimental-strip-types scripts/integration-progress.ts --check",
 
@@ -65,6 +65,31 @@ test('a bare Vitest run spans every configured project', () => {
   ]);
 });
 
+test('a negated --project subtracts from the configured set, so the skipped one is not credited', () => {
+  assert.deepEqual(
+    scriptUnits('cov', scriptModel({ cov: 'vitest run --coverage --project=!subprocess-stub' })),
+    ['vitest:unit-core'],
+  );
+});
+
+// The real `test:coverage:ci` shape: the second leg is a nested script whose body carries an env
+// prefix (it blanks the coverage-shard switches). Both indirections have to survive, or the lane
+// stops owning the project it hands to that leg.
+test('the two halves of test:coverage:ci together still own every project', () => {
+  assert.deepEqual(
+    scriptUnits(
+      'test:coverage:ci',
+      scriptModel({
+        'test:coverage:ci':
+          'vitest run --coverage --project=!subprocess-stub && pnpm test:subprocess-stub',
+        'test:subprocess-stub':
+          'AGENT_DEVICE_COVERAGE_SHARD= AGENT_DEVICE_COVERAGE_MERGE= vitest run --project subprocess-stub',
+      }),
+    ),
+    ['vitest:unit-core', 'vitest:subprocess-stub'],
+  );
+});
+
 test('aggregates expand transitively, so a lane running the aggregate owns its parts', () => {
   const units = scriptUnits(
     'check:all',
 
@@ -70,9 +70,24 @@ function vitestArgs(parts: readonly string[]): {
 
 const RUNNER_TOKENS = /^(?:pnpm|exec|vitest|run)$/;
 
+/**
+ * Vitest's own `--project` semantics: bare names select, `!name` excludes, and a run with only
+ * exclusions starts from every configured project. The model has to read the negated form or it
+ * would credit a lane with a project it skips — `test:coverage:ci` runs `--project=!fuzz-worker`
+ * and hands that project to a second, uninstrumented invocation.
+ */
+function selectedProjects(named: readonly string[], projects: readonly string[]): string[] {
+  const excluded = new Set(
+    named.filter((name) => name.startsWith('!')).map((name) => name.slice(1)),
+  );
+  const included = named.filter((name) => !name.startsWith('!'));
+  const base = included.length > 0 ? included : projects;
+  return base.filter((name) => !excluded.has(name));
+}
+
 function vitestUnits(parts: readonly string[], projects: readonly string[]): Unit[] {
   const { named, files } = vitestArgs(parts);
-  const selected = named.length > 0 ? named : projects;
+  const selected = named.length > 0 ? selectedProjects(named, projects) : projects;
   const suffix = files.length > 0 ? `@${files.join(',')}` : '';
   return selected.map((project) => `vitest:${project}${suffix}`);
 }
 
@@ -8,14 +8,52 @@ import slowTestGateReporter from './scripts/vitest-slow-test-reporter.ts';
 // them one at a time to bound that contention; per-file `process.env` isolation is
 // already delivered by `pool: forks` + `isolate: true` on every project.
 // Membership and the project's deletion test live in issue #1823.
-export const SUBPROCESS_STUB_TESTS: readonly string[] = [
+const SUBPROCESS_STUB_TESTS: readonly string[] = [
   // Stubs npx plus the package managers and spawns a real Metro dev server per case.
   'src/__tests__/client-metro.test.ts',
   // The SUT is the subprocess watchdog: a node subprocess per case, one hangs on purpose (#1414).
   'scripts/fuzz/harness.test.ts',
-  // Replays the fuzz corpus through that same worker watchdog, waiting its per-case budget.
+];
+
+// The fuzz corpus replay, which must not run under V8 coverage instrumentation.
+//
+// #1824 found two causes behind `Worker exited unexpectedly`. Shape (A) — a test signalling
+// a fabricated pid that landed on a sibling fork — was fixed at the source in #1854. Shape
+// (B) was left open: one fork dies mid-file, alone, with no test attributed. Scanning every
+// failed Coverage job across the 120 CI runs after #1854 merged found the signature five
+// times, and the vanished file was this one all five (plus #1866's, so six for six) — 23% of
+// Coverage failures in that window. The uninstrumented unit lane has never lost it.
+//
+// So the coverage run skips this project and a second, uninstrumented Vitest invocation owns
+// it — see `test:coverage:ci`. That costs no coverage at all, which is measured rather than
+// assumed: the cases execute inside worker threads, a separate isolate the fork's inspector
+// session never instruments, so this file reports the same lines with or without it.
+//
+// The second leg goes through `test:fuzz-worker`, which blanks AGENT_DEVICE_COVERAGE_SHARD and
+// AGENT_DEVICE_COVERAGE_MERGE. Those are the switches #1996 introduces for a sharded coverage
+// lane, and it sets them as *job*-level env over a single `gate: unit-ci` step — so both legs
+// would inherit them. Blanking them is inert until that lands and required the moment it does:
+// verified on a local merge of the two branches, the shared leg otherwise takes the whole shard
+// down, because Vitest refuses `--shard=1/2` over this one-file project ("must be a smaller than
+// count of test files") and the blob reporter overwrites the instrumented shard's report on the
+// way out.
+//
+// Membership is by demonstrated failure, not by a property of the code. In particular it is
+// NOT "constructs a `node:worker_threads` Worker": `session-replay-runtime-maestro.test.ts`
+// does exactly that and stays in `unit-core`, instrumented and green. The proximate cause was
+// never reproduced — what these entries share is an observed record of vanishing from the
+// Coverage lane, and that record is the only thing that admits a file here. A new entry needs
+// its own run URLs; a theory about workers is not enough.
+const FUZZ_WORKER_TESTS: readonly string[] = [
+  // Replays the fuzz corpus through the worker watchdog, waiting its per-case budget (#1414).
   'scripts/fuzz/corpus-replay.test.ts',
 ];
+/**
+ * Everything the serialized projects own, which is what the fast lane must not also collect.
+ * The two lists above stay module-local: this union is the whole cross-file surface, and the
+ * mutation lane wants exactly it — every test the root config declines to run in parallel.
+ */
+export const SERIALIZED_TESTS: readonly string[] = [...SUBPROCESS_STUB_TESTS, ...FUZZ_WORKER_TESTS];
 
 // Imported by vitest.mutation.config.ts so the two lanes cannot drift: a guard
 // added here must reach the Stryker sandbox too.
@@ -126,7 +164,7 @@ export default defineConfig({
             // The Maestro conformance oracle runs via `node --test` in its own CI
             // job (scripts/maestro-conformance), like the layering guard.
           ],
-          exclude: [...SUBPROCESS_STUB_TESTS],
+          exclude: [...SERIALIZED_TESTS],
           setupFiles: SETUP_FILES,
         },
       },
@@ -140,6 +178,19 @@ export default defineConfig({
           maxWorkers: 1,
         },
       },
+      {
+        test: {
+          // Same serialization as its sibling above, for the same contention reason: the
+          // per-case watchdog budget is real wall clock. The project exists so the coverage
+          // run can leave it out, not to run it differently.
+          name: 'fuzz-worker',
+          include: [...FUZZ_WORKER_TESTS],
+          setupFiles: SETUP_FILES,
+          fileParallelism: false,
+          isolate: true,
+          maxWorkers: 1,
+        },
+      },
       {
         test: {
           name: 'provider-integration',
 
@@ -3,7 +3,7 @@ import { fileURLToPath } from 'node:url';
 import { defineConfig } from 'vitest/config';
 import { readTestScope, threadHostileTestFiles } from './scripts/mutation/test-scope.ts';
 import { workspaceSourceAliases } from './scripts/mutation/workspace-aliases.ts';
-import { SETUP_FILES, SUBPROCESS_STUB_TESTS } from './vitest.config.ts';
+import { SERIALIZED_TESTS, SETUP_FILES } from './vitest.config.ts';
 
 const repoRoot = path.dirname(fileURLToPath(import.meta.url));
 
@@ -24,7 +24,7 @@ const workspaceAliases = workspaceSourceAliases(repoRoot);
 // (`vitest related` over the mutated files) and hands it over through
 // AGENT_DEVICE_MUTATION_TEST_FILES; the fallback is the deterministic unit suite,
 // which keeps `pnpm exec stryker run` usable by hand. Excluded either way: the
-// subprocess-stub group and the CLI-capture tests — see
+// serialized groups (subprocess-stub and fuzz-worker) and the CLI-capture tests — see
 // scripts/mutation/test-scope.ts for why, and why excluding them cannot hide a
 // surviving mutant.
 const scope = readTestScope();
@@ -35,7 +35,7 @@ export default defineConfig({
   },
   test: {
     include: scope ?? ['src/**/*.test.ts', 'packages/*/src/**/*.test.ts'],
-    exclude: [...SUBPROCESS_STUB_TESTS, ...threadHostileTestFiles(repoRoot), '**/node_modules/**'],
+    exclude: [...SERIALIZED_TESTS, ...threadHostileTestFiles(repoRoot), '**/node_modules/**'],
     setupFiles: [...SETUP_FILES],
   },
 });