SvelteKit Node Adapter Returns an "Invalid request body" HTTP Error that Can Be Misleading and Hard to Debug

A minimal reproducible example.

It simply constists in the skeleton app one gets with npm create svelte@latest (with the "latest" at the time of writing, see package-lock.json), using the Node adapter. To reproduce, simply run the following in your shell:

npm install
npm run build
ORIGIN=localhost:3000 node build # note the invalid value for ORIGIN

Then go to http://localhost:3000/: you should get an "Invalid request body" response with a HTTP 400 Bad Request status.

The error message is misleading because your request obviously does not have an invalid body (it does not even have a body since it is a GET request). Furthermore it is quite hard to debug because the error is not caught by the handleError hook in src/hooks.server.ts.

The cause of the error is that the ORIGIN environment variable is invalid, it should be http://localhost:3000 (or just localhost:3000; see also the MDN page about "origin"). This causes an error in build/handler.js that happens too early to be caught by handleError. The code that handles this error (which does seem like it could be instrumented at the moment) mistakenly assumes that an error here can only be caused by an invalid body, hence the error message: https://github.com/sveltejs/kit/blob/d4043863e5aa6b5fb003ebafef2a6e9098255b45/packages/adapter-node/src/handler.js#L90

    } catch (err) {
        res.statusCode = err.status || 400;
        res.end('Invalid request body');
        return;
    }

If we add a console.error(err); in this catch block we get the following error log:

TypeError: Failed to parse URL from localhost/
    at new Request (file:///home/cedricvr/tinkering/sveltekit-invalid-body/build/shims.js:12760:16)
    at getRequest (file:///home/cedricvr/tinkering/sveltekit-invalid-body/build/handler.js:1031:9)
    ... 4 lines matching cause stack trace ...
    at handle (file:///home/cedricvr/tinkering/sveltekit-invalid-body/build/handler.js:1257:23)
    at file:///home/cedricvr/tinkering/sveltekit-invalid-body/build/handler.js:1257:40
    at Array.<anonymous> (file:///home/cedricvr/tinkering/sveltekit-invalid-body/build/handler.js:653:28)
    at handle (file:///home/cedricvr/tinkering/sveltekit-invalid-body/build/handler.js:1257:23) {
  [cause]: TypeError [ERR_INVALID_URL]: Invalid URL
      at new NodeError (node:internal/errors:387:5)
      at URL.onParseError (node:internal/url:565:9)
      at new URL (node:internal/url:641:5)
      at new Request (file:///home/cedricvr/tinkering/sveltekit-invalid-body/build/shims.js:12758:22)
      at getRequest (file:///home/cedricvr/tinkering/sveltekit-invalid-body/build/handler.js:1031:9)
      at Array.ssr (file:///home/cedricvr/tinkering/sveltekit-invalid-body/build/handler.js:1186:19)
      at handle (file:///home/cedricvr/tinkering/sveltekit-invalid-body/build/handler.js:1257:23)
      at file:///home/cedricvr/tinkering/sveltekit-invalid-body/build/handler.js:1257:40
      at Array.<anonymous> (file:///home/cedricvr/tinkering/sveltekit-invalid-body/build/handler.js:1175:4)
      at handle (file:///home/cedricvr/tinkering/sveltekit-invalid-body/build/handler.js:1257:23) {
    input: 'localhost/',
    code: 'ERR_INVALID_URL'
  }
}

We then see that the error comes from NodeJS builting new URL(...) constructor, due to the invalid origin. We can easily reproduce it on NodeJS itself:

$ docker run -it node:18 node
Welcome to Node.js v18.16.0.
Type ".help" for more information.
> new URL('localhost/');
Uncaught TypeError [ERR_INVALID_URL]: Invalid URL
    at __node_internal_captureLargerStackTrace (node:internal/errors:490:5)
    at new NodeError (node:internal/errors:399:5)
    at new URL (node:internal/url:560:13) {
  input: 'localhost/',
  code: 'ERR_INVALID_URL'
}