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Cannot add Content-Length header to a multipart part

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@alexzargarian23 is already working on this.

Since Oct 24, 2026.

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
72/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
kotlin
Domain
backend-api-design

Research direction

Start with MultipartBody.Part.create and the Content-Length require check quoted in the issue, then inspect nearby multipart tests. Confirm how manually supplied part headers are handled for multipart types such as multipart/mixed. Done means the factory accepts the header where permitted, with tests covering the behavior.

Written by the indexing model from the issue text.

Description

bug

In #7319 the Content-Length header ceased being auto-included in every multipart part. That is fine, but manually adding that header to a part throws an exception:

MultipartBody.Part.create(
    headers = Headers.Builder().add("Content-Length", TEST_DATA.size.toString()).build(),
    body = TEST_DATA.toRequestBody(),
) // crash - "Unexpected header: Content-Length"

The crash is due to this line in the MultipartBody.Part.create method:

require(headers?.get("Content-Length") == null) { "Unexpected header: Content-Length" }

I think that require was added because that header was being auto-included at the time. Since that is no longer the case but the require still exists, it's impossible to add a Content-Length header using the factory method.

I believe including Content-Length on a multipart part is out-of-spec if the request is of type multipart/form-data, but is within RFC specs for other types like multipart/mixed.

Tests showing it's not possible to manually include Content-Length: https://gist.github.com/jasonrobinson-gpsw/830a56b81be94a704fad9409d8326865

Dominant language
Kotlin
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Merged PRs (30d)
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