Calling `flatRoutes()` from app code results in an invariant Error

[leondmello]

I'm using React Router as a...

framework

Reproduction

Go to https://stackblitz.com/edit/github-a7kuzc3f?file=app%2Fentry.server.tsx and run:

npm run build && npm run start

System Info

npx envinfo --system --npmPackages '{vite,react-router,@react-router/*}' --binaries --browsers
  System:
    OS: macOS 15.3
    CPU: (10) arm64 Apple M1 Pro
    Memory: 106.83 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.13.1 - ~/.nodenv/versions/22.13.1/bin/node
    npm: 10.9.2 - ~/.nodenv/versions/22.13.1/bin/npm
  Browsers:
    Chrome: 133.0.6943.127
    Safari: 18.3
  npmPackages:
    @react-router/dev: ^7.1.1 => 7.1.3 
    @react-router/fs-routes: ^7.1.1 => 7.1.3 
    @react-router/node: ^7.1.1 => 7.1.3 
    @react-router/serve: ^7.1.1 => 7.1.3 
    react-router: ^7.1.1 => 7.1.5 
    vite: ^5.4.11 => 5.4.14

Used Package Manager

npm

Expected Behavior

App should start

Actual Behavior

Fails to start

> start
> react-router-serve ./build/server/index.js

[react-router-serve] http://localhost:3000 (http://192.168.1.104:3000)
The following error is a bug in React Router; please open an issue! https://github.com/remix-run/react-router/issues/new/choose

/home/projects/llqiwvkkxr.github/node_modules/@react-router/dev/dist/routes.js:63
    throw new Error(message);
          ^
Error: 
    at invariant (/home/projects/llqiwvkkxr.github/node_modules/@react-router/dev/dist/routes.js:63:11)
    at getAppDirectory (/home/projects/llqiwvkkxr.github/node_modules/@react-router/dev/dist/routes.js:69:3)
    at Module.flatRoutes2 (/home/projects/llqiwvkkxr.github/node_modules/@react-router/fs-routes/dist/index.js:464:56)
    at onShellReady (file:///home/projects/llqiwvkkxr.github/build/server/index.js:47:53)
    at completeShell (/home/projects/llqiwvkkxr.github/node_modules/react-dom/cjs/react-dom-server.node.production.js:5235:3)
    at finishedTask (/home/projects/llqiwvkkxr.github/node_modules/react-dom/cjs/react-dom-server.node.production.js:5271:39)
    at performWork (/home/projects/llqiwvkkxr.github/node_modules/react-dom/cjs/react-dom-server.node.production.js:5404:15)
    at AsyncLocalStorage.run (node:async_hooks:18:3866)
    at eval (/home/projects/llqiwvkkxr.github/node_modules/react-dom/cjs/react-dom-server.node.production.js:5878:27)
    at node:internal/process/task_queues:192:1778

[leondmello]

For some background, we have an express middleware where we want to publish metrics about response time.
In these metrics we would like to include the current route identifier. So we were trying something like the following:

async function extractRouteId(url: string) {
  const parsedPath = parsePath(new URL(url).pathname);
  const allRoutes = await flatRoutes();
  const matchedRoutes = matchRoutes(
    allRoutes as AgnosticRouteObject[],
    parsedPath
  );

  return matchedRoutes?.at(-1)?.route?.id;
}

The flatRoutes() function call works when the application is running via the vite dev server

[jrestall]

flatRoutes() is only designed to be called during development from routes.ts, not from entry.server.tsx in production.

If you want a list of the app's routes you can get them by importing the ServerBuild and doing build.routes.

[leondmello]

Ok, maybe worthwhile to add that into the docs.
The data structure from the build routes cannot be passed into the match routes function as is though. Ideally, that transformation function was exposed, but looks like it is not.