From d223409c889b5ad8a96d3c3758862cfebb441bef Mon Sep 17 00:00:00 2001
From: Peter Steinberger <peter@steipete.me>
Date: Wed, 15 Jul 2026 04:16:53 -0700
Subject: [PATCH] fix(ci): run all compact test bins serially (#108266)

---
 scripts/lib/ci-node-test-plan.mjs      | 12 ++++++------
 test/scripts/ci-node-test-plan.test.ts | 13 ++++---------
 2 files changed, 10 insertions(+), 15 deletions(-)

diff --git a/scripts/lib/ci-node-test-plan.mjs b/scripts/lib/ci-node-test-plan.mjs
index be726474a939..f4a2ecc3e642 100644
--- a/scripts/lib/ci-node-test-plan.mjs
+++ b/scripts/lib/ci-node-test-plan.mjs
@@ -1381,12 +1381,12 @@ function createCompactNodeTestShardBundles(options = {}) {
         ...(bin.hasWholeConfigGroup
           ? { timeoutMinutes: COMPACT_WHOLE_NODE_TEST_TIMEOUT_MINUTES }
           : {}),
-        // 4 vCPU bins run their plans serially: overlapping two Vitest runs
-        // there starves process-spawning tests (worker-startup timeouts in
-        // core-runtime-infra-process, tooling process-group waits), and the
-        // packed weights are contention-inflated so serializing is roughly
-        // wall-neutral. Exclusive spawn/signal bins stay serial everywhere.
-        ...(bin.exclusive || runnerClass === "small" ? { planConcurrency: 1 } : {}),
+        // Every compact bin runs its plans serially. Overlapping two Vitest
+        // runs on one runner starves timing-sensitive tests on both runner
+        // classes (worker-startup timeouts on 4 vCPU, UI-animation and
+        // lock-timing flakes on 8 vCPU), and the packed weights are
+        // contention-inflated so serializing is roughly wall-neutral.
+        planConcurrency: 1,
       });
     }
   }
diff --git a/test/scripts/ci-node-test-plan.test.ts b/test/scripts/ci-node-test-plan.test.ts
index 0e17cc7fe91c..6e61af38cba7 100644
--- a/test/scripts/ci-node-test-plan.test.ts
+++ b/test/scripts/ci-node-test-plan.test.ts
@@ -203,10 +203,9 @@ describe("scripts/lib/ci-node-test-plan.mjs", () => {
     expect(jobOf("agentic-agents-core-runner-embedded")).toBeGreaterThanOrEqual(0);
     expect(jobOf("core-unit-fast")).toBeGreaterThanOrEqual(0);
     expect(jobOf("agentic-agents-core-runner-embedded")).not.toBe(jobOf("core-unit-fast"));
-    // Spawn/signal-timing suites flake next to a concurrent sibling Vitest
-    // run; their bins never mix with regular groups, and every 4 vCPU bin
-    // runs serially because overlapping Vitest runs starve process-spawning
-    // tests on that runner class.
+    // Spawn/signal-timing suites never mix with regular groups, and every
+    // compact bin runs serially: overlapping Vitest runs flake timing-
+    // sensitive tests on both runner classes.
     const exclusiveGroupRe = /^core-tooling(?:-\d+|-isolated|-docker)?$|^core-runtime-tui-pty$/u;
     for (const shard of compact) {
       const exclusiveCount = shard.groups.filter((group) =>
@@ -215,11 +214,7 @@ describe("scripts/lib/ci-node-test-plan.mjs", () => {
       if (exclusiveCount > 0) {
         expect(exclusiveCount).toBe(shard.groups.length);
       }
-      if (exclusiveCount > 0 || !shard.runner.includes("-8vcpu-")) {
-        expect(shard.planConcurrency).toBe(1);
-      } else {
-        expect(shard.planConcurrency).toBeUndefined();
-      }
+      expect(shard.planConcurrency).toBe(1);
     }
     expect(
       compact.filter((shard) =>