diff --git a/.github/workflows/e2e.yml b/.github/workflows/e2e.yml
index ff63d7db458..59fb1e58b33 100644
--- a/.github/workflows/e2e.yml
+++ b/.github/workflows/e2e.yml
@@ -127,13 +127,10 @@ jobs:
         sandbox:
           - 'sandbox:none'
           - 'sandbox:docker'
-        # The suite is ~16min of wall clock on one runner, dominated by a long
-        # tail of sdk-typescript files. vitest assigns files to shards by path
-        # hash, so those spread out instead of clustering in one shard.
+        # Keep three-way test concurrency inside one runner so each sandbox
+        # shares setup work instead of occupying three pool runners.
         shard:
-          - '1/3'
-          - '2/3'
-          - '3/3'
+          - '1/1'
         node-version:
           - '22.x'
     steps:
@@ -362,10 +359,10 @@ jobs:
           # test:integration:sandbox:docker: that script would rebuild the image
           # the step above just built.
           if [[ "${{ matrix.sandbox }}" == "sandbox:docker" ]]; then
-            npx cross-env QWEN_E2E_RENDERER=ink QWEN_SANDBOX=docker vitest run --root ./integration-tests --exclude '**/interactive/cron-interactive.test.ts' --exclude '**/channel-plugin.test.ts' --exclude '**/chat-transcript-document.test.ts' --shard='${{ matrix.shard }}' 9>&-
+            npx cross-env QWEN_E2E_RENDERER=ink QWEN_SANDBOX=docker vitest run --root ./integration-tests --exclude '**/interactive/cron-interactive.test.ts' --exclude '**/channel-plugin.test.ts' --exclude '**/chat-transcript-document.test.ts' --poolOptions.forks.maxForks=3 --shard='${{ matrix.shard }}' 9>&-
           else
             run_shard() {
-              QWEN_E2E_RENDERER=ink npm run test:integration:sandbox:none -- --exclude '**/interactive/cron-interactive.test.ts' --exclude '**/channel-plugin.test.ts' --exclude '**/chat-transcript-document.test.ts' --shard='${{ matrix.shard }}'
+              QWEN_E2E_RENDERER=ink npm run test:integration:sandbox:none -- --exclude '**/interactive/cron-interactive.test.ts' --exclude '**/channel-plugin.test.ts' --exclude '**/chat-transcript-document.test.ts' --poolOptions.forks.maxForks=3 --shard='${{ matrix.shard }}'
             }
             # One bounded retry: pool runners' sandbox:none shards die under
             # shared-host pressure with every test green and no vitest FAIL
diff --git a/scripts/tests/e2e-workflow.test.js b/scripts/tests/e2e-workflow.test.js
index 12d1131d561..d6a6fe19964 100644
--- a/scripts/tests/e2e-workflow.test.js
+++ b/scripts/tests/e2e-workflow.test.js
@@ -34,6 +34,18 @@ describe('e2e workflow', () => {
     expect(group).toContain('github.head_ref || github.ref_name');
   });
 
+  it('runs three Vitest forks on one Linux runner per sandbox', () => {
+    const linuxJob = yml.jobs['e2e-test-linux'];
+    const runStep = linuxJob.steps.find(
+      (step) => step.name === 'Run E2E tests',
+    );
+
+    expect(linuxJob.strategy.matrix.shard).toEqual(['1/1']);
+    expect(runStep.run.match(/--poolOptions\.forks\.maxForks=3/g)).toHaveLength(
+      2,
+    );
+  });
+
   describe('sandbox image preparation', () => {
     const steps = yml.jobs['e2e-test-linux'].steps;
     const setupStep = steps.find((step) => step.name === 'Set up Docker');
@@ -193,7 +205,7 @@ describe('e2e workflow', () => {
       // shard and exclude coverage lives only in this argument list. The
       // excludes are shared verbatim with the docker leg above.
       expect(runStep.run).toContain(
-        "npm run test:integration:sandbox:none -- --exclude '**/interactive/cron-interactive.test.ts' --exclude '**/channel-plugin.test.ts' --exclude '**/chat-transcript-document.test.ts' --shard='${{ matrix.shard }}'",
+        "npm run test:integration:sandbox:none -- --exclude '**/interactive/cron-interactive.test.ts' --exclude '**/channel-plugin.test.ts' --exclude '**/chat-transcript-document.test.ts' --poolOptions.forks.maxForks=3 --shard='${{ matrix.shard }}'",
       );
     });