fix: run Nitro in-process in Start WebContainers

[AlemTuzlak]

After the Rolldown WASI load and the standalone example tsconfig, Vite starts. The first request then fails with Vite environment "nitro" is unavailable.

Nitro's default runner is node-worker. In WebContainer that worker never finishes loading the nitro entry. This PR runs Nitro in-process (self) for the Start sandbox profile only. Query, Table, Charts, and other esbuild sandboxes stay unchanged.

Testing

Commands run:

1. Isolated Basic Chat with the sandbox Vite wrapper (vite --config .tanstack/vite.config.mjs) returned HTTP 200 locally.
2. The same boot with NITRO_DEV_RUNNER=self returned HTTP 200.
3. pnpm exec tsx --test tests/example-webcontainer-start.test.ts passed.
4. Pre-commit pnpm run test: 525 passed, 3 skipped.

Manual test after deploy:

1. Open https://tanstack.com/ai/latest/docs/framework/react/examples/basic-chat
2. Wait for Vite to start.
3. Confirm the preview loads the chat UI.
4. Confirm there is no Vite environment "nitro" is unavailable error.

Risk / rollback

Start docs sandboxes get the same in-process Nitro runner. Revert this PR to restore node-worker.

Summary by CodeRabbit

• Bug Fixes

  • Improved TanStack Start development environments by configuring the Nitro development server to run in the WebContainer context.
  • Added the required environment setting for consistent WebContainer startup behavior.

• Tests

  • Added coverage verifying the WebContainer environment and generated development-server configuration.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: 50f27d4e-cc50-4e2f-82a3-00f608fca105

📥 Commits

Reviewing files that changed from the base of the PR and between 9a00f44 and 5d0c3c1.

📒 Files selected for processing (2)

• src/utils/example-webcontainer-start.ts
• tests/example-webcontainer-start.test.ts

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📝 Walkthrough

Walkthrough

The async-context WebContainer setup now selects Nitro’s self runner in its environment and generated Vite configuration. Tests verify both settings.

Changes

WebContainer Nitro runner

Layer / File(s)	Summary
Configure and validate the Nitro self runner src/utils/example-webcontainer-start.ts, tests/example-webcontainer-start.test.ts	The WebContainer environment sets NITRO_DEV_RUNNER to self. The generated Vite configuration sets nitro.devServer.runner to self. Tests verify both values.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 5d0c3

The change is narrowly scoped to Nitro runner configuration, with no established merge-blocking risk.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: running Nitro in-process in Start WebContainers.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches 💡 1

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🧪 Generate unit tests (beta)

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