fix(client): preserve custom Accept headers in StreamableHTTPClientTransport

[nielskaspers]

Summary

StreamableHTTPClientTransport unconditionally overwrites custom Accept headers provided via requestInit.headers with SDK defaults. This change only sets the default Accept header when the user has not already provided one.

Issue

Fixes #1646

Changes

• In send() (POST requests): only set accept: application/json, text/event-stream if no Accept header exists
• In _startOrAuthSse() (GET SSE requests): only set accept: text/event-stream if no Accept header exists

Testing

• Added test: custom Accept header is preserved on POST requests
• Added test: custom Accept header is preserved on GET SSE requests
• Added test: default Accept header is set when none provided
• All 260 existing tests continue to pass
• Lint passes

[changeset-bot]

🦋 Changeset detected

Latest commit: acb3d5c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@modelcontextprotocol/client	Patch

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[felixweinberger]

The Accept header on MCP transport requests is spec-mandated. POST needs application/json, text/event-stream and GET SSE needs text/event-stream for the transport to work. The current fix lets users omit these entirely, which would break the connection (406, or unparseable responses).

If the goal is to add a media type (e.g. for a gateway that routes on application/vnd.example.v1+json), the right approach is to append rather than replace:

const userAccept = headers.get('accept');
headers.set('accept', userAccept
    ? `${userAccept}, application/json, text/event-stream`
    : 'application/json, text/event-stream');

This keeps the required types present while letting the user's value through. The C# SDK does something similar via Headers.Accept.Add().

Before reworking though, could you say more on #1646 about what's requiring the custom Accept? Want to make sure append actually solves it.

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commit: acb3d5c

[felixweinberger]

@claude review