Intercepted requests in `msw/node` are missing `Content-Length` header

[brianhelba]

Prerequisites

• I confirm my issue is not in the opened issues
• I confirm the Frequently Asked Questions didn't contain the answer to my issue

Environment check

• I'm using the latest msw version
• I'm using Node.js version 20 or higher

Node.js version

v25.6.1

Reproduction repository

https://github.com/brianhelba/msw-content-length-repro

Reproduction steps

npm install && npm test

Current behavior

request.headers.get('Content-Length') is null in the handler. The Content-Length header is normally calculated by the HTTP transport layer when sending the request, but @mswjs/interceptors intercepts before that point, so the header is never set.

This breaks server-side libraries that validate Content-Length before processing POST bodies.

Expected behavior

When using msw/node, intercepted requests should include a calculated Content-Length header, since handlers simulate server-side processing where this header would always be present. The Content-Length for a string body is straightforward to compute.

Note that MSW already calculates Content-Length on responses (added in v2.2.0 via #1996). The same treatment for request headers would be consistent.

[kettanaito]

Hi, @brianhelba. Thanks for reporting this.

This is going to be gone after mswjs/interceptors#770. Meanwhile, it would be great to observe the native behavior in Node.js regarding the headers. Simply constructing a request, for example, does not add content-length:

new Request('https://foo.com', { method: 'POST', body: 'hello world' }).headers.get('content-length')
// null

But as I said, regardless of the actual Node.js behaviors, interception on the socket level will run everything Node.js does. This is unfeasible to address on the current architecture so I will close this and recommend you monitor when the linked pull request gets merged and propagated to MSW.