diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 55457c9dbf..c449060c9d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -531,23 +531,19 @@ jobs:
       - name: CLI help smoke
         run: bun run src/cli/index.ts help
 
-  # macOS is focused on dev pushes and relevant pull requests, while main,
-  # preview, and dispatch retain the full unsharded control. This keeps Darwin
-  # process coverage on the fast path without dropping the release control.
+  # Dev pushes and relevant pull requests keep a focused Darwin/process lane.
+  # Release refs and main-targeting changes use the timing-balanced full-suite
+  # matrix below; nightly-macos retains the unsharded scheduler/load control.
   platform-macos:
     name: macos
     needs: changes
     if: >-
-      (github.event_name != 'pull_request' && github.event_name != 'merge_group') ||
+      (github.event_name == 'push' && github.ref == 'refs/heads/dev') ||
       (github.event_name == 'pull_request' && github.base_ref == 'main') ||
       (github.event_name == 'merge_group' && github.event.merge_group.base_ref == 'refs/heads/main') ||
-      needs.changes.outputs.macos == 'true'
+      ((github.event_name == 'pull_request' || github.event_name == 'merge_group') && needs.changes.outputs.macos == 'true')
     runs-on: macos-latest
-    # The unsharded control for the sharded Linux lane: the only place the whole
-    # suite runs in one pool, so it is the place that catches what sharding
-    # hides. The flakes it keeps surfacing are timing, not logic, and the fix
-    # is the tests, not a fourth lane.
-    timeout-minutes: 30
+    timeout-minutes: 15
     steps:
       - name: Checkout
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
@@ -619,15 +615,84 @@ jobs:
             tests/storage/storage-worker-teardown-isolate.test.ts \
             tests/ci-workflows/test-runner.test.ts
 
-      - name: Full macOS suite
-        if: >-
-          github.event_name == 'workflow_dispatch' ||
-          (github.event_name == 'pull_request' && github.base_ref == 'main') ||
-          (github.event_name == 'merge_group' &&
-           github.event.merge_group.base_ref == 'refs/heads/main') ||
-          (github.event_name == 'push' &&
-           (github.ref == 'refs/heads/main' || github.ref == 'refs/heads/preview'))
-        run: bash scripts/ci/run-bun-with-crash-retry.sh -- bun scripts/test.ts --timeout 60000
+      - name: CLI help smoke
+        run: bun run src/cli/index.ts help
+
+  # Full macOS coverage is split by measured file duration. Every test remains
+  # in exactly one platform lane; load-sensitive files run one per fresh Bun
+  # process, matching the full-suite planner. The daily nightly job deliberately
+  # stays unsharded so it can still expose whole-pool scheduling interactions.
+  platform-macos-full:
+    name: macos full ${{ matrix.shard }}/2
+    needs: changes
+    if: >-
+      github.event_name == 'workflow_dispatch' ||
+      (github.event_name == 'pull_request' && github.base_ref == 'main') ||
+      (github.event_name == 'merge_group' &&
+       github.event.merge_group.base_ref == 'refs/heads/main') ||
+      (github.event_name == 'push' &&
+       (github.ref == 'refs/heads/main' || github.ref == 'refs/heads/preview'))
+    runs-on: macos-latest
+    timeout-minutes: 20
+    strategy:
+      fail-fast: false
+      matrix:
+        shard: [1, 2]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
+        with:
+          persist-credentials: false
+          fetch-tags: true
+
+      - name: Setup project Bun
+        uses: ./.github/actions/setup-project-bun
+
+      - name: Install dependencies
+        run: |
+          bun install --frozen-lockfile
+          cd gui
+          bun install --frozen-lockfile
+
+      - name: Build GUI
+        run: |
+          cd gui
+          bun run build
+
+      - name: Restore canonical test timings
+        uses: actions/cache/restore@caa296126883cff596d87d8935842f9db880ef25 # v5.1.0
+        with:
+          path: .bun-timings.json
+          key: ocx-test-timings-dev-${{ github.sha }}
+          restore-keys: |
+            ocx-test-timings-dev-
+
+      - name: Validate restored timing data
+        if: hashFiles('.bun-timings.json') != ''
+        run: bun scripts/ci/validate-timings.ts .bun-timings.json --discard-invalid
+
+      - name: Full macOS shard with isolated load-sensitive files
+        shell: bash
+        env:
+          TEST_SHARD: ${{ matrix.shard }}/2
+        run: |
+          set -euo pipefail
+          main_list="$(mktemp -t ocx-macos-main.XXXXXX)"
+          serial_list="$(mktemp -t ocx-macos-serial.XXXXXX)"
+          cleanup() { rm -f -- "$main_list" "$serial_list"; }
+          trap cleanup EXIT
+          bun scripts/ci/test-lanes.ts --lane platform-main --timings .bun-timings.json --shard "$TEST_SHARD" > "$main_list"
+          bun scripts/ci/test-lanes.ts --lane platform-serial --timings .bun-timings.json --shard "$TEST_SHARD" > "$serial_list"
+          main_files=()
+          while IFS= read -r file; do main_files+=("$file"); done < "$main_list"
+          test "${#main_files[@]}" -gt 0
+          bash scripts/ci/run-bun-with-crash-retry.sh -- bun scripts/test.ts --isolate --timeout 60000 "${main_files[@]}"
+          serial_files=()
+          while IFS= read -r file; do serial_files+=("$file"); done < "$serial_list"
+          test "${#serial_files[@]}" -gt 0
+          for file in "${serial_files[@]}"; do
+            bash scripts/ci/run-bun-with-crash-retry.sh -- bun scripts/test.ts --isolate --parallel=1 --timeout 60000 "$file"
+          done
 
       - name: CLI help smoke
         run: bun run src/cli/index.ts help
@@ -944,7 +1009,7 @@ jobs:
     name: ci
     if: always()
     # Every producer, including the ones that only feed other jobs.
-    needs: [changes, test, storage-policy, api-usage, serial-load-sensitive, publish-test-timings, gates, platform-macos, platform-windows, keyring-smoke, npm-global-smoke]
+    needs: [changes, test, storage-policy, api-usage, serial-load-sensitive, publish-test-timings, gates, platform-macos, platform-macos-full, platform-windows, keyring-smoke, npm-global-smoke]
     runs-on: ubuntu-latest
     timeout-minutes: 5
     steps:
diff --git a/scripts/ci/test-lanes.ts b/scripts/ci/test-lanes.ts
index 47130594a7..68932b770e 100644
--- a/scripts/ci/test-lanes.ts
+++ b/scripts/ci/test-lanes.ts
@@ -113,7 +113,15 @@ export const DEDICATED_TEST_FILES = [
 export const STORAGE_TEST_FILES = DEDICATED_TEST_FILES.slice(0, 6);
 export const API_TEST_FILES = ["tests/server/api-usage.test.ts"] as const;
 
-const laneNames = ["general", "serial", "dedicated", "dedicated-storage", "dedicated-api"] as const;
+const laneNames = [
+  "general",
+  "serial",
+  "dedicated",
+  "dedicated-storage",
+  "dedicated-api",
+  "platform-main",
+  "platform-serial",
+] as const;
 export type TestLane = (typeof laneNames)[number];
 
 type DirectoryReader = (directory: string) => Dirent[];
@@ -159,7 +167,7 @@ export function validateLaneManifest(inventory: string[]): {
   const normalized = [...inventory].sort();
   if (new Set(normalized).size !== normalized.length) throw new Error("lane manifest contains duplicate paths");
   for (const path of normalized) {
-    if (!path.startsWith("tests/") || path.includes("..")) {
+    if (!/^tests\/[A-Za-z0-9._/-]+$/.test(path) || path.includes("..")) {
       throw new Error(`lane manifest contains path outside tests: ${path}`);
     }
   }
@@ -192,6 +200,10 @@ export function laneFiles(lane: TestLane, root = process.cwd()): string[] {
   const lanes = validateLaneManifest(discoverTestFiles(root));
   if (lane === "dedicated-storage") return [...STORAGE_TEST_FILES];
   if (lane === "dedicated-api") return [...API_TEST_FILES];
+  // Match the full-suite planner: both serial and dedicated files receive
+  // their own process on macOS, outside the general worker pool.
+  if (lane === "platform-main") return lanes.general;
+  if (lane === "platform-serial") return [...lanes.serial, ...lanes.dedicated];
   return lanes[lane];
 }
 
@@ -240,6 +252,6 @@ if (import.meta.main) {
       for (const path of allocateFilesByTiming(files, count, durations)[index - 1]!) console.log(path);
     } else for (const path of files) console.log(path);
   } else {
-    throw new Error("usage: --lane general|serial|dedicated|dedicated-storage|dedicated-api");
+    throw new Error(`usage: --lane ${laneNames.join("|")}`);
   }
 }
diff --git a/tests/ci-workflows/ci-workflows.test.ts b/tests/ci-workflows/ci-workflows.test.ts
index c0dc889fe9..76a92928b6 100644
--- a/tests/ci-workflows/ci-workflows.test.ts
+++ b/tests/ci-workflows/ci-workflows.test.ts
@@ -141,7 +141,8 @@ describe("GitHub Actions hardening", () => {
     expect(ci.jobs?.["select-windows-runner"]).toBeUndefined();
     expect(ci.jobs?.test?.["timeout-minutes"]).toBe(15);
     expect(ci.jobs?.gates?.["timeout-minutes"]).toBe(15);
-    expect(ci.jobs?.["platform-macos"]?.["timeout-minutes"]).toBe(30);
+    expect(ci.jobs?.["platform-macos"]?.["timeout-minutes"]).toBe(15);
+    expect(ci.jobs?.["platform-macos-full"]?.["timeout-minutes"]).toBe(20);
     expect(ci.jobs?.["macos-control"]).toBeUndefined();
     // Higher than the Linux shards on purpose: at 15 the Windows leg cancelled a
     // shard mid-suite, which reports as neither pass nor fail (#2152).
@@ -215,7 +216,7 @@ describe("GitHub Actions hardening", () => {
     // how the first cut of that test shipped, so pin the flag rather than trusting a
     // comment. Asserted per job so a future edit cannot drop it from one leg while
     // the other still carries it.
-    for (const jobName of ["test", "platform-macos", "platform-windows"]) {
+    for (const jobName of ["test", "platform-macos", "platform-macos-full", "platform-windows"]) {
       const steps = (ci.jobs?.[jobName] as { steps?: Array<{ uses?: string; with?: Record<string, unknown> }> })?.steps ?? [];
       const checkout = steps.find(step => typeof step.uses === "string" && step.uses.includes("actions/checkout"));
       expect(`${jobName}:${String(checkout?.with?.["fetch-tags"])}`).toBe(`${jobName}:true`);
@@ -275,7 +276,8 @@ describe("GitHub Actions hardening", () => {
     expect(hasExactShellCommand(gatesGuiRun, "cd gui && bun test --isolate tests")).toBe(true);
     expect(hasExactShellCommand(gatesGuiRun, "cd gui && bun test tests")).toBe(false);
 
-    // macOS is focused on dev and relevant PRs; main/preview keep full control.
+    // macOS is focused on dev and relevant PRs; promotion events use two
+    // timing-balanced full-suite runners while nightly keeps the pool control.
     const macosSteps = (ci.jobs?.["platform-macos"] as {
       steps?: { name?: string; if?: string; run?: string }[];
     })?.steps ?? [];
@@ -285,8 +287,23 @@ describe("GitHub Actions hardening", () => {
     expect(macosSteps.some(step => step.run?.includes("--shard"))).toBe(false);
     const focusedMacos = macosSteps.find(step => step.name === "Focused Darwin/process lifecycle tests");
     expect(focusedMacos?.run).toContain("tests/codex-integration/codex-prompt-text-probe.test.ts");
-    const fullMacos = macosSteps.find(step => step.name === "Full macOS suite");
-    expect(fullMacos?.if).toContain("github.event_name == 'pull_request' && github.base_ref == 'main'");
+    expect(macosSteps.find(step => step.name === "Full macOS suite")).toBeUndefined();
+    const macosFull = ci.jobs?.["platform-macos-full"] as {
+      if?: string;
+      name?: string;
+      strategy?: { matrix?: { shard?: number[] } };
+      steps?: Array<{ name?: string; env?: Record<string, string>; run?: string }>;
+    } | undefined;
+    expect(macosFull?.strategy?.matrix?.shard).toEqual([1, 2]);
+    expect(macosFull?.name).toBe("macos full ${{ matrix.shard }}/2");
+    expect(macosFull?.if).toContain("github.event_name == 'pull_request' && github.base_ref == 'main'");
+    const macosFullStep = macosFull?.steps?.find(step =>
+      step.name === "Full macOS shard with isolated load-sensitive files"
+    );
+    expect(macosFullStep?.env?.TEST_SHARD).toBe("${{ matrix.shard }}/2");
+    expect(macosFullStep?.run).toContain("--lane platform-main");
+    expect(macosFullStep?.run).toContain("--lane platform-serial");
+    expect(macosFullStep?.run).toContain("--parallel=1");
     const pathPolicy = Bun.YAML.parse(await readText(".github/policies/ci-paths.yml")) as {
       macos?: string[];
       swift?: string[];
@@ -335,7 +352,7 @@ describe("GitHub Actions hardening", () => {
     expect(crashRetry).not.toContain("while true");
     expect((ci.jobs?.["platform-macos"] as { needs?: string; if?: string })?.needs).toBe("changes");
     expect((ci.jobs?.["platform-macos"] as { if?: string })?.if)
-      .toBe("(github.event_name != 'pull_request' && github.event_name != 'merge_group') || (github.event_name == 'pull_request' && github.base_ref == 'main') || (github.event_name == 'merge_group' && github.event.merge_group.base_ref == 'refs/heads/main') || needs.changes.outputs.macos == 'true'");
+      .toBe("(github.event_name == 'push' && github.ref == 'refs/heads/dev') || (github.event_name == 'pull_request' && github.base_ref == 'main') || (github.event_name == 'merge_group' && github.event.merge_group.base_ref == 'refs/heads/main') || ((github.event_name == 'pull_request' || github.event_name == 'merge_group') && needs.changes.outputs.macos == 'true')");
 
     // Windows is required for every integration push and CI-relevant PR. The
     // changes dependency keeps documentation-only PRs cheap without letting a
@@ -436,7 +453,7 @@ describe("GitHub Actions hardening", () => {
     // accident, because the same job also ran the GUI build — splitting the suite
     // away from the gates removed that coincidence, and the shards went red on a
     // pull request before this pin existed.
-    for (const jobName of ["test", "platform-macos", "platform-windows"]) {
+    for (const jobName of ["test", "platform-macos", "platform-macos-full", "platform-windows"]) {
       const steps = (ci.jobs?.[jobName] as { steps?: { if?: string; run?: string }[] })?.steps ?? [];
       const build = steps.find(step => step.run?.includes("bun run build"));
       expect(`${jobName}:${build === undefined}`).toBe(`${jobName}:false`);
@@ -678,7 +695,12 @@ describe("GitHub Actions hardening", () => {
     }
     const macosJob = ci.jobs?.["platform-macos"] as { needs?: string; if?: string } | undefined;
     expect(`platform-macos:${macosJob?.needs}`).toBe("platform-macos:changes");
-    expect(`platform-macos:${macosJob?.if}`).toBe("platform-macos:(github.event_name != 'pull_request' && github.event_name != 'merge_group') || (github.event_name == 'pull_request' && github.base_ref == 'main') || (github.event_name == 'merge_group' && github.event.merge_group.base_ref == 'refs/heads/main') || needs.changes.outputs.macos == 'true'");
+    expect(`platform-macos:${macosJob?.if}`).toBe("platform-macos:(github.event_name == 'push' && github.ref == 'refs/heads/dev') || (github.event_name == 'pull_request' && github.base_ref == 'main') || (github.event_name == 'merge_group' && github.event.merge_group.base_ref == 'refs/heads/main') || ((github.event_name == 'pull_request' || github.event_name == 'merge_group') && needs.changes.outputs.macos == 'true')");
+    const macosFullJob = ci.jobs?.["platform-macos-full"] as { needs?: string; if?: string } | undefined;
+    expect(`platform-macos-full:${macosFullJob?.needs}`).toBe("platform-macos-full:changes");
+    expect(macosFullJob?.if).toContain("github.event_name == 'workflow_dispatch'");
+    expect(macosFullJob?.if).toContain("refs/heads/main");
+    expect(macosFullJob?.if).toContain("refs/heads/preview");
   });
 
   test("cross-platform CI keeps the GUI lint and build gates", async () => {
diff --git a/tests/ci-workflows/zz-ci-timing.test.ts b/tests/ci-workflows/zz-ci-timing.test.ts
index e1742ec402..4feaf84668 100644
--- a/tests/ci-workflows/zz-ci-timing.test.ts
+++ b/tests/ci-workflows/zz-ci-timing.test.ts
@@ -63,6 +63,19 @@ describe("CI lane manifest", () => {
   test("never accepts a lane path outside tests", () => {
     expect(() => validateLaneManifest(["tests/good.test.ts", "src/not-a-test.ts"]))
       .toThrow(/outside tests/);
+    expect(() => validateLaneManifest(["tests/good.test.ts\n--shard=1/1"]))
+      .toThrow(/outside tests/);
+  });
+
+  test("platform lanes preserve full-suite coverage and singleton isolation", () => {
+    const inventory = discoverTestFiles(process.cwd());
+    const main = laneFiles("platform-main", process.cwd());
+    const serial = laneFiles("platform-serial", process.cwd());
+    expect(new Set([...main, ...serial])).toEqual(new Set(inventory));
+    expect(main.length + serial.length).toBe(inventory.length);
+    expect(new Set(main).intersection(new Set(serial)).size).toBe(0);
+    expect(serial).toEqual([...SERIAL_TEST_FILES, ...DEDICATED_TEST_FILES]);
+    for (const dedicated of DEDICATED_TEST_FILES) expect(main).not.toContain(dedicated);
   });
 
   test("never drops a nested test that shares a quarantined basename", () => {
@@ -214,6 +227,13 @@ test("only trusted dev shards publish the canonical timing cache", async () => {
     step.uses === "actions/cache/save@caa296126883cff596d87d8935842f9db880ef25",
   )).toBe(true);
   expect(ci.jobs["platform-macos"].steps?.some(step => step.uses?.startsWith("actions/cache/save@"))).toBe(false);
+  const macosFull = ci.jobs["platform-macos-full"] as typeof ci.jobs[string] & {
+    strategy?: { matrix?: { shard?: number[] } };
+  };
+  expect(macosFull.strategy?.matrix?.shard).toEqual([1, 2]);
+  expect(macosFull.steps?.some(step => step.uses?.startsWith("actions/cache/save@"))).toBe(false);
+  expect(macosFull.steps?.some(step => step.run?.includes("--lane platform-main"))).toBe(true);
+  expect(macosFull.steps?.some(step => step.run?.includes("--lane platform-serial"))).toBe(true);
   const windows = ci.jobs["platform-windows"];
   const windowsRestore = windows.steps?.find(step => step.uses?.startsWith("actions/cache/restore@"));
   expect(windowsRestore?.with?.key).toContain("ocx-test-timings-dev-");
@@ -231,9 +251,7 @@ test("only trusted dev shards publish the canonical timing cache", async () => {
   expect(focused?.run).not.toContain("tests/adapters/google/aistudio-native-webkit.test.ts");
   expect(focused?.run).toContain("tests/ci-workflows/test-runner.test.ts");
   expect(focused?.run).toContain("tests/providers/cursor/cursor-native-exec-shell.test.ts");
-  const full = ci.jobs["platform-macos"].steps?.find(step => step.name === "Full macOS suite");
-  expect(full?.run).toContain("bun scripts/test.ts --timeout 60000");
-  expect(full?.run).not.toContain("--update-timings");
+  expect(ci.jobs["platform-macos"].steps?.find(step => step.name === "Full macOS suite")).toBeUndefined();
 });
 
 test("nightly macOS is timed at 08:17 UTC and names its timing file", async () => {