Next.js "invariant expected layout router to be mounted" error after restarting dev — no changes made

[hendrixejegi]

Body

I’m working on a Next.js project using the App Router. Everything was working fine — I made changes, committed the code, shut down my machine.

Came back the next day, ran npm run dev, and immediately got this error:

Unhandled Runtime Error
Error: invariant expected layout router to be mounted

No changes were made between when it was working and when it broke.

Has anyone else experienced this after a clean shutdown/restart? How did you resolve it?

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[Leorev01]

Hye @hendrixejegi,
Look at this stack overflow: this may help you solve your problem: https://stackoverflow.com/questions/74515807/uncaught-error-invariant-expected-app-router-to-be-mounted/74601107#74601107

[hendrixejegi]

@Leorev01 Thank you for the suggestions. However, I have not found a solution yet. The issue seems to persist across different projects. Even when I create a new Next.js project, I encounter the same error. I did notice that the problem didn't occur when I scaffolded a new project using version 14, and even after I upgraded to version 15 it wasn't there. I also shut down my PC for a while to determine if the problem starts after a reboot.

[Daniel-tech-cmd]

did you find a solution to this?
i am facing same issue currently

[Gecko51]

Start by clearing the cache:

rm -rf .next
npm run dev

If that doesn't fix it, the fact that it works on a fresh Next 14 scaffold but breaks after upgrading to 15 is the real clue. Next.js 15 ships with React 19 by default, and if your package.json still pins React 18, you can get runtime mismatches that surface after a restart when the module graph resolves slightly differently.

Check what's actually installed:

node -e "const p = require('./node_modules/react/package.json'); console.log(p.version)"
node -e "const p = require('./node_modules/react-dom/package.json'); console.log(p.version)"

If they're mismatched, upgrade both:

npm install react@latest react-dom@latest

Or if you want to stay on React 18:

npm install react@18 react-dom@18 --legacy-peer-deps

One more thing to check: if you're using nvm, your default Node version might be changing between sessions. A different Node version can cause a slightly different module graph on startup, which is the kind of thing that explains "worked yesterday, broken today, no code changes." Run node -v and compare it to what you were on before.