Which project does this relate to?

Router

Describe the bug

After upgrading @tanstack/router-plugin from 1.166.12 to 1.167.9, all vitest tests that import the generated routeTree.gen.ts fail with:

TypeError: Cannot set properties of undefined (setting 'tsr-split-component:component')
 ❯ src/routes/login.tsx:1:57
 ❯ src/routeTree.gen.ts:12:1

The HMR state storage mechanism introduced in PR #7000 / #7005 (react-stable-hmr-split-route-components.ts) writes to import.meta.hot.data['tsr-split-component:component'], but in vitest import.meta.hot.data is undefined.

This affects any project using autoCodeSplitting: true with vitest.

Your Example Website or App

https://github.com/FredrikErikJohansson/tanstack-router-vitest-repro

Steps to Reproduce the Bug or Issue

1. Clone the repro: git clone https://github.com/FredrikErikJohansson/tanstack-router-vitest-repro
2. npm install
3. npm test → fails with TypeError: Cannot set properties of undefined (setting 'tsr-split-component:component')
4. Downgrade: npm install @tanstack/router-plugin@1.166.12 @tanstack/react-router@1.167.3
5. npm test → passes

Expected behavior

Tests should pass. The HMR code injected by react-stable-hmr-split-route-components.ts should guard against import.meta.hot.data being undefined in test environments.

Screenshots or Videos

No response

Platform

• Router Version: @tanstack/router-plugin 1.167.9, @tanstack/react-router 1.168.8
• OS: macOS
• Bundler: Vite 6.4.1 (also reproduced with 8.0.3)
• Vitest: 3.2.x / 4.1.x

Additional context

Root cause: PR #7000 and #7005 introduced tsr-split-component HMR state caching for React Fast Refresh stability. The injected code accesses import.meta.hot.data without a null guard. In vitest, import.meta.hot exists but .data is undefined.

Suggested fix: Add optional chaining in react-stable-hmr-split-route-components.ts:

import.meta.hot?.data?.['tsr-split-component:component']

Workaround: Pin to @tanstack/router-plugin@1.166.12 / @tanstack/react-router@1.167.3.