diff --git a/.github/workflows/pr.yml b/.github/workflows/pr.yml
index 925c520ba4..7fe5fca58e 100644
--- a/.github/workflows/pr.yml
+++ b/.github/workflows/pr.yml
@@ -287,9 +287,11 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          # A successful PR run on 2026-08-04 (30876682788) spent 256s in
-          # serialized shard 2/4, making its 305s job the run's slowest check.
-          # Five shards reduce the measured 739s suite total to about 148s per
+          # A successful PR run on 2026-08-17 (32012408876) spent 291s in
+          # serialized shard 1/5 while its siblings ran 170-201s: round-robin
+          # clustered the heavy suites on one runner. Shards are now balanced
+          # by recorded duration (scripts/serialized-shard-durations.json),
+          # which levels the measured 968s suite total to about 194s per
           # runner before setup overhead.
           - shard_index: 0
             shard_count: 5
diff --git a/scripts/__tests__/run-vitest-stable-shard.test.mjs b/scripts/__tests__/run-vitest-stable-shard.test.mjs
index ea74a3de12..690de987a3 100644
--- a/scripts/__tests__/run-vitest-stable-shard.test.mjs
+++ b/scripts/__tests__/run-vitest-stable-shard.test.mjs
@@ -13,6 +13,11 @@ import {
 const repoRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "..", "..");
 const script = path.join(repoRoot, "scripts", "run-vitest-stable.mjs");
 const durationsManifest = path.join(repoRoot, "scripts", "general-server-shard-durations.json");
+const serializedDurationsManifest = path.join(
+  repoRoot,
+  "scripts",
+  "serialized-shard-durations.json",
+);
 
 function dryRun(args) {
   const result = spawnSync(process.execPath, [script, ...args, "--dry-run"], {
@@ -167,6 +172,39 @@ test("the checked-in manifest loads and covers most of the current suite set", (
   );
 });
 
+test("the checked-in serialized manifest loads and covers most of the current suite set", () => {
+  const durations = loadShardDurations(serializedDurationsManifest);
+  assert.ok(Object.keys(durations).length > 0, "manifest must parse to a non-empty duration map");
+
+  const shard = dryRunJson(["--mode", "serialized", "--shard-index", "0", "--shard-count", "1"]);
+  const currentFiles = shard.selectedSerializedSuites;
+  const known = currentFiles.filter((file) => durations[file] !== undefined).length;
+  assert.ok(
+    known / currentFiles.length >= 0.5,
+    `manifest is stale: only ${known} of ${currentFiles.length} suites have recorded durations — regenerate it from a recent PR run (see the manifest's $comment)`,
+  );
+});
+
+test("the real serialized shard partition is duration-balanced", () => {
+  const durations = loadShardDurations(serializedDurationsManifest);
+  const fallback = defaultSuiteWeight(durations);
+  const shards = Array.from({ length: SERIALIZED_SHARD_COUNT }, (_, index) =>
+    dryRunJson(["--mode", "serialized", "--shard-index", String(index), "--shard-count", String(SERIALIZED_SHARD_COUNT)]),
+  );
+
+  const totals = shards.map((shard) =>
+    shard.selectedSerializedSuites.reduce((sum, file) => sum + (durations[file] ?? fallback), 0),
+  );
+  const maxTotal = Math.max(...totals);
+  const minTotal = Math.min(...totals);
+  // LPT keeps the spread within the heaviest single suite; use that as the bound.
+  const heaviest = Math.max(...Object.values(durations));
+  assert.ok(
+    maxTotal - minTotal <= heaviest,
+    `serialized shard weight spread ${maxTotal - minTotal}ms exceeds heaviest suite ${heaviest}ms: ${totals.join(", ")}`,
+  );
+});
+
 test("the real shard partition is duration-balanced", () => {
   const durations = loadShardDurations(durationsManifest);
   const fallback = defaultSuiteWeight(durations);
diff --git a/scripts/run-vitest-stable.mjs b/scripts/run-vitest-stable.mjs
index b77c9d8a38..4f7af35bd4 100644
--- a/scripts/run-vitest-stable.mjs
+++ b/scripts/run-vitest-stable.mjs
@@ -11,6 +11,9 @@ const scriptsDir = path.dirname(fileURLToPath(import.meta.url));
 const generalServerShardDurations = loadShardDurations(
   path.join(scriptsDir, "general-server-shard-durations.json"),
 );
+const serializedShardDurations = loadShardDurations(
+  path.join(scriptsDir, "serialized-shard-durations.json"),
+);
 const serverRoot = path.join(repoRoot, "server");
 const serverSrcDir = path.join(repoRoot, "server", "src");
 const serverTestsDir = path.join(repoRoot, "server", "src", "__tests__");
@@ -251,7 +254,18 @@ function parseCliOptions(argv) {
 }
 
 function selectSerializedSuites(routeTests, shardIndex, shardCount) {
-  return routeTests.filter((_, index) => index % shardCount === shardIndex);
+  // Same duration-aware LPT partition as the general-server lane. Round-robin
+  // over the alphabetical list clustered the heavy heartbeat/issues suites on
+  // one shard (291s vs 170-201s test steps across the matrix in actions run
+  // 32012408876), which made that shard the whole PR run's slowest check.
+  const byRepoPath = new Map(routeTests.map((routeTest) => [routeTest.repoPath, routeTest]));
+  const shardFiles = selectGeneralServerShard(
+    routeTests.map((routeTest) => routeTest.repoPath),
+    shardIndex,
+    shardCount,
+    serializedShardDurations,
+  );
+  return shardFiles.map((file) => byRepoPath.get(file));
 }
 
 function runVitest(args, label) {
diff --git a/scripts/serialized-shard-durations.json b/scripts/serialized-shard-durations.json
new file mode 100644
index 0000000000..4e6fe71ee7
--- /dev/null
+++ b/scripts/serialized-shard-durations.json
@@ -0,0 +1,140 @@
+{
+  "$comment": "Per-suite wall-clock durations (ms) for the serialized route/authz vitest lane, used by scripts/run-vitest-stable.mjs to balance suites across the PR shard matrix. Sampled from a real PR run of .github/workflows/pr.yml (actions run 32012408876, 2026-08-17) by diffing consecutive '[test:run] <suite>' label timestamps in the 'Run serialized server test shard' logs - that captures each suite's true serial cost including the per-suite vitest spawn overhead, not just the vitest-reported test time. Suites missing here get the median weight, so the manifest only needs occasional refreshes.",
+  "unit": "ms",
+  "durations": {
+    "server/src/__tests__/access-routes-permissions-upgrade.test.ts": 11105,
+    "server/src/__tests__/activity-routes.test.ts": 3181,
+    "server/src/__tests__/adapter-model-refresh-routes.test.ts": 3934,
+    "server/src/__tests__/adapter-routes-authz.test.ts": 4391,
+    "server/src/__tests__/adapter-routes.test.ts": 5343,
+    "server/src/__tests__/agent-action-audit-routes.test.ts": 11458,
+    "server/src/__tests__/agent-adapter-validation-routes.test.ts": 5758,
+    "server/src/__tests__/agent-cross-tenant-authz-routes.test.ts": 3818,
+    "server/src/__tests__/agent-device-login-routes.test.ts": 6486,
+    "server/src/__tests__/agent-instructions-routes.test.ts": 5532,
+    "server/src/__tests__/agent-live-run-routes.test.ts": 5498,
+    "server/src/__tests__/agent-permissions-routes.test.ts": 8978,
+    "server/src/__tests__/agent-secrets-routes.test.ts": 11780,
+    "server/src/__tests__/agent-skills-routes.test.ts": 7404,
+    "server/src/__tests__/agent-test-environment-routes.test.ts": 5845,
+    "server/src/__tests__/approval-routes-idempotency.test.ts": 6142,
+    "server/src/__tests__/assets.test.ts": 3555,
+    "server/src/__tests__/auth-routes.test.ts": 2094,
+    "server/src/__tests__/auth-session-route.test.ts": 3070,
+    "server/src/__tests__/authz-company-access.test.ts": 2893,
+    "server/src/__tests__/authz-existence-oracle-guard.test.ts": 1121,
+    "server/src/__tests__/authz-secret-context.test.ts": 2398,
+    "server/src/__tests__/board-chat-route-feature-flag.test.ts": 1032,
+    "server/src/__tests__/bootstrap-claim-routes.test.ts": 4748,
+    "server/src/__tests__/built-in-agent-routes.test.ts": 4015,
+    "server/src/__tests__/cases-routes.test.ts": 12174,
+    "server/src/__tests__/cli-auth-routes.test.ts": 5228,
+    "server/src/__tests__/cloud-routes.test.ts": 2146,
+    "server/src/__tests__/companies-route-cross-company-authz.test.ts": 4314,
+    "server/src/__tests__/companies-route-path-guard.test.ts": 3004,
+    "server/src/__tests__/company-branding-route.test.ts": 3920,
+    "server/src/__tests__/company-import-transfer-routes.test.ts": 8905,
+    "server/src/__tests__/company-portability-routes.test.ts": 3034,
+    "server/src/__tests__/company-portability.test.ts": 4800,
+    "server/src/__tests__/company-search-extract-routes.test.ts": 7621,
+    "server/src/__tests__/company-search-rate-limit-routes.test.ts": 7587,
+    "server/src/__tests__/company-skill-policy-routes.test.ts": 6894,
+    "server/src/__tests__/company-skills-import-authz-routes.test.ts": 12068,
+    "server/src/__tests__/company-skills-routes.test.ts": 7484,
+    "server/src/__tests__/company-user-directory-route.test.ts": 4728,
+    "server/src/__tests__/costs-service.test.ts": 8319,
+    "server/src/__tests__/decision-queues-routes.test.ts": 6956,
+    "server/src/__tests__/document-annotation-routes.test.ts": 4375,
+    "server/src/__tests__/environment-custom-image-routes.test.ts": 4387,
+    "server/src/__tests__/environment-instance-routes.test.ts": 4527,
+    "server/src/__tests__/environment-routes.test.ts": 4780,
+    "server/src/__tests__/environment-selection-route-guards.test.ts": 4208,
+    "server/src/__tests__/execution-workspaces-routes.test.ts": 3450,
+    "server/src/__tests__/express5-auth-wildcard.test.ts": 1105,
+    "server/src/__tests__/external-object-routes.test.ts": 6231,
+    "server/src/__tests__/folders-routes.test.ts": 2802,
+    "server/src/__tests__/health-dev-server-token.test.ts": 2541,
+    "server/src/__tests__/health.test.ts": 2090,
+    "server/src/__tests__/heartbeat-dependency-scheduling.test.ts": 14289,
+    "server/src/__tests__/heartbeat-issue-liveness-escalation.test.ts": 13747,
+    "server/src/__tests__/heartbeat-process-recovery.test.ts": 55336,
+    "server/src/__tests__/inbox-agent-policy-routes.test.ts": 10057,
+    "server/src/__tests__/inbox-archive-routes.test.ts": 9644,
+    "server/src/__tests__/instance-database-backups-routes.test.ts": 2962,
+    "server/src/__tests__/instance-settings-routes.test.ts": 5356,
+    "server/src/__tests__/invite-accept-existing-member.test.ts": 4812,
+    "server/src/__tests__/invite-accept-gateway-defaults.test.ts": 10423,
+    "server/src/__tests__/invite-accept-replay.test.ts": 5116,
+    "server/src/__tests__/invite-create-route.test.ts": 4640,
+    "server/src/__tests__/invite-expiry.test.ts": 7291,
+    "server/src/__tests__/invite-join-manager.test.ts": 7124,
+    "server/src/__tests__/invite-list-route.test.ts": 7580,
+    "server/src/__tests__/invite-logo-route.test.ts": 5104,
+    "server/src/__tests__/invite-onboarding-text.test.ts": 7095,
+    "server/src/__tests__/invite-rate-limit-route.test.ts": 7446,
+    "server/src/__tests__/invite-summary-route.test.ts": 7598,
+    "server/src/__tests__/invite-test-resolution-route.test.ts": 6135,
+    "server/src/__tests__/invite-url-public-base-url.test.ts": 3482,
+    "server/src/__tests__/issue-activity-events-routes.test.ts": 6254,
+    "server/src/__tests__/issue-agent-mutation-ownership-routes.test.ts": 13283,
+    "server/src/__tests__/issue-assigned-backlog-contract-routes.test.ts": 5442,
+    "server/src/__tests__/issue-assignee-invokability-routes.test.ts": 4602,
+    "server/src/__tests__/issue-attachment-routes.test.ts": 5267,
+    "server/src/__tests__/issue-blocker-diagnostics-routes.test.ts": 10902,
+    "server/src/__tests__/issue-closed-workspace-routes.test.ts": 6589,
+    "server/src/__tests__/issue-comment-attribution-audit-routes.test.ts": 11053,
+    "server/src/__tests__/issue-comment-cancel-routes.test.ts": 4687,
+    "server/src/__tests__/issue-comment-reopen-routes.test.ts": 4009,
+    "server/src/__tests__/issue-create-deduplication-routes.test.ts": 11885,
+    "server/src/__tests__/issue-dependency-wakeups-routes.test.ts": 5701,
+    "server/src/__tests__/issue-document-restore-routes.test.ts": 5902,
+    "server/src/__tests__/issue-execution-policy-routes.test.ts": 6571,
+    "server/src/__tests__/issue-feedback-routes.test.ts": 3978,
+    "server/src/__tests__/issue-identifier-routes.test.ts": 11276,
+    "server/src/__tests__/issue-list-assignee-filter-routes.test.ts": 12518,
+    "server/src/__tests__/issue-list-updatedsince-filter-routes.test.ts": 10919,
+    "server/src/__tests__/issue-onboarding-first-task-routes.test.ts": 12493,
+    "server/src/__tests__/issue-scheduled-retry-routes.test.ts": 11559,
+    "server/src/__tests__/issue-stale-execution-lock-routes.test.ts": 11281,
+    "server/src/__tests__/issue-stalled-review-decision-routes.test.ts": 11437,
+    "server/src/__tests__/issue-subtree-diagnostics-routes.test.ts": 11043,
+    "server/src/__tests__/issue-telemetry-routes.test.ts": 4745,
+    "server/src/__tests__/issue-thread-interaction-routes.test.ts": 8825,
+    "server/src/__tests__/issue-tree-control-routes.test.ts": 2689,
+    "server/src/__tests__/issue-update-comment-wakeup-routes.test.ts": 6552,
+    "server/src/__tests__/issue-wake-diagnostics-routes.test.ts": 11098,
+    "server/src/__tests__/issue-watchdogs-routes.test.ts": 15909,
+    "server/src/__tests__/issue-workspace-command-authz.test.ts": 3660,
+    "server/src/__tests__/issues-checkout-wakeup.test.ts": 1053,
+    "server/src/__tests__/issues-goal-context-routes.test.ts": 5356,
+    "server/src/__tests__/issues-service.test.ts": 37910,
+    "server/src/__tests__/llms-routes.test.ts": 2370,
+    "server/src/__tests__/low-trust-red-team-routes.test.ts": 25333,
+    "server/src/__tests__/multilingual-issues-routes.test.ts": 11329,
+    "server/src/__tests__/onboarding-seed-route.test.ts": 8981,
+    "server/src/__tests__/openapi-routes.test.ts": 3234,
+    "server/src/__tests__/openclaw-invite-prompt-route.test.ts": 3938,
+    "server/src/__tests__/opencode-local-adapter-environment.test.ts": 1071,
+    "server/src/__tests__/permissions-upgrade-boundary-routes.test.ts": 11054,
+    "server/src/__tests__/pipelines-routes.test.ts": 13514,
+    "server/src/__tests__/plugin-install-route-security.test.ts": 10977,
+    "server/src/__tests__/plugin-routes-authz.test.ts": 3628,
+    "server/src/__tests__/plugin-scoped-api-routes.test.ts": 3535,
+    "server/src/__tests__/project-goal-telemetry-routes.test.ts": 4566,
+    "server/src/__tests__/project-routes-env.test.ts": 3141,
+    "server/src/__tests__/projects-list-archived-routes.test.ts": 10943,
+    "server/src/__tests__/redaction.test.ts": 973,
+    "server/src/__tests__/resource-memberships-routes.test.ts": 11206,
+    "server/src/__tests__/routine-document-annotation-routes.test.ts": 2824,
+    "server/src/__tests__/routines-e2e.test.ts": 12006,
+    "server/src/__tests__/routines-routes.test.ts": 4019,
+    "server/src/__tests__/secret-proposals-routes.test.ts": 15670,
+    "server/src/__tests__/secrets-routes.test.ts": 4757,
+    "server/src/__tests__/sidebar-preferences-routes.test.ts": 3080,
+    "server/src/__tests__/summary-slot-routes.test.ts": 3858,
+    "server/src/__tests__/teams-catalog-routes.test.ts": 3463,
+    "server/src/__tests__/user-profile-routes.test.ts": 6441,
+    "server/src/__tests__/workspace-runtime-routes-authz.test.ts": 3296,
+    "server/src/__tests__/workspace-runtime-service-authz.test.ts": 6251
+  }
+}