diff --git a/.github/workflows/pr.yml b/.github/workflows/pr.yml
index 5c9f483db5..de0a70a6f7 100644
--- a/.github/workflows/pr.yml
+++ b/.github/workflows/pr.yml
@@ -270,18 +270,25 @@ 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
+          # runner before setup overhead.
           - shard_index: 0
-            shard_count: 4
-            shard_label: 1/4
+            shard_count: 5
+            shard_label: 1/5
           - shard_index: 1
-            shard_count: 4
-            shard_label: 2/4
+            shard_count: 5
+            shard_label: 2/5
           - shard_index: 2
-            shard_count: 4
-            shard_label: 3/4
+            shard_count: 5
+            shard_label: 3/5
           - shard_index: 3
-            shard_count: 4
-            shard_label: 4/4
+            shard_count: 5
+            shard_label: 4/5
+          - shard_index: 4
+            shard_count: 5
+            shard_label: 5/5
 
     steps:
       - name: Checkout repository
diff --git a/.github/workflows/release-verify.yml b/.github/workflows/release-verify.yml
index 890dd4aea4..cac52d75b5 100644
--- a/.github/workflows/release-verify.yml
+++ b/.github/workflows/release-verify.yml
@@ -109,17 +109,20 @@ jobs:
       matrix:
         include:
           - shard_index: 0
-            shard_count: 4
-            shard_label: 1/4
+            shard_count: 5
+            shard_label: 1/5
           - shard_index: 1
-            shard_count: 4
-            shard_label: 2/4
+            shard_count: 5
+            shard_label: 2/5
           - shard_index: 2
-            shard_count: 4
-            shard_label: 3/4
+            shard_count: 5
+            shard_label: 3/5
           - shard_index: 3
-            shard_count: 4
-            shard_label: 4/4
+            shard_count: 5
+            shard_label: 4/5
+          - shard_index: 4
+            shard_count: 5
+            shard_label: 5/5
 
     steps:
       - name: Checkout repository
diff --git a/scripts/__tests__/release-verify-workflow.test.mjs b/scripts/__tests__/release-verify-workflow.test.mjs
index 3bd0a616a8..b5911ab4e8 100644
--- a/scripts/__tests__/release-verify-workflow.test.mjs
+++ b/scripts/__tests__/release-verify-workflow.test.mjs
@@ -43,8 +43,8 @@ test("release verify workflow covers the same split test surface as stable PR ve
     );
   }
 
-  for (const shardIndex of [0, 1, 2, 3]) {
-    assert.match(verifyWorkflow, new RegExp(`shard_index: ${shardIndex}[\\s\\S]*?shard_count: 4`));
+  for (const shardIndex of [0, 1, 2, 3, 4]) {
+    assert.match(verifyWorkflow, new RegExp(`shard_index: ${shardIndex}[\\s\\S]*?shard_count: 5`));
   }
 
   assert.match(verifyWorkflow, /pnpm test:run:general -- --group/);
diff --git a/scripts/__tests__/run-vitest-stable-shard.test.mjs b/scripts/__tests__/run-vitest-stable-shard.test.mjs
index 9b69130527..252629bc06 100644
--- a/scripts/__tests__/run-vitest-stable-shard.test.mjs
+++ b/scripts/__tests__/run-vitest-stable-shard.test.mjs
@@ -29,6 +29,18 @@ function dryRunJson(args) {
 }
 
 const SHARD_COUNT = 4;
+const SERIALIZED_SHARD_COUNT = 5;
+
+test("the serialized shards form a complete, non-overlapping partition", () => {
+  const shards = Array.from({ length: SERIALIZED_SHARD_COUNT }, (_, index) =>
+    dryRunJson(["--mode", "serialized", "--shard-index", String(index), "--shard-count", String(SERIALIZED_SHARD_COUNT)]),
+  );
+
+  const total = shards[0].serializedSuiteCount;
+  const selected = shards.flatMap((shard) => shard.selectedSerializedSuites);
+  assert.equal(selected.length, total, "every serialized suite must be selected exactly once");
+  assert.equal(new Set(selected).size, total, "serialized shards must not overlap");
+});
 
 test("the general-server shards form a complete, non-overlapping partition", () => {
   const shards = Array.from({ length: SHARD_COUNT }, (_, index) =>