[next-auth][error][CLIENT_FETCH_ERROR], *DESPITE* Environmental Variables Being Present

[machineghost]

Environment

System:
OS: Linux 5.15 Linux Mint 21.1 (Vera)
CPU: (8) x64 Intel(R) Core(TM) i7-4810MQ CPU @ 2.80GHz
Memory: 8.78 GB / 23.38 GB
Container: Yes
Shell: 5.1.16 - /bin/bash
Binaries:
Node: 20.3.0 - /usr/local/bin/node
npm: 9.6.7 - /usr/local/bin/npm
Browsers:
Chrome: 114.0.5735.198

Reproduction URL

None; if I can't get the starting instructions to even work, I certainly won't deploy it live :)

Describe the issue

There seems to be several variations (see #3560), but the core idea is that when you get the CLIENT_FETCH_ERROR:

client.js:1 [next-auth][error][CLIENT_FETCH_ERROR]
https://next-auth.js.org/errors#client_fetch_error Unexpected token '<', "<!DOCTYPE "... is not valid JSON {error: {…}, url: '/api/auth/session', message: Unexpected token '<', "<!DOCTYPE "... is not valid JSON}

it directs you to a URL telling you to set an environmental variable ... but that variable isn't the issue.

How to reproduce

I was just following the tutorial and I ran into it. Presumably, any number of things could go wrong with setting up the route, including forgetting to set it up entirely.

Expected behavior

Instead of getting an error message with a URL that directs the user to set the environmental variable, the URL should explain that the route handler is not working, and suggest several things to check for what could be making it not work.

[marquez138]

getting a similar error with the fauna adapter

[github-actions]

We cannot recreate the issue with the provided information. Please add a reproduction in order for us to be able to investigate.

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Useful Resources

• How to create a Minimal, Complete, and Verifiable example
• Reporting a NextAuth.js bug
• How to Contribute to Open Source (Next.js)

[machineghost]

Repro is very simple: follow the basic starting instructions on your website, get anything wrong with the server-side route (eg. forget to make the [...nextauth]/route.js file, put it in the wrong place, have a typo in it, etc.), and get the error:

client.js:1 [next-auth][error][CLIENT_FETCH_ERROR]
https://next-auth.js.org/errors#client_fetch_error Unexpected token '<', "<!DOCTYPE "... is not valid JSON {error: {…}, url: '/api/auth/session', message: Unexpected token '<', "<!DOCTYPE "... is not valid JSON}

That error links to https://next-auth.js.org/errors#client_fetch_error, which falsely suggests that the problem is an environmental variable ... when the actual problem has nothing to do with any environment variable (the problem is the route setup).

[balazsorban44]

Clarified in the docs: dcb11da

[machineghost]

While the new language is a lot better, it still doesn't clarify that the fundamental problem is that the /api/auth/session endpoint doesn't exist :(

Why not just explain that in the message?