Streamable HTTP server accepts invalid media types by Accept-header substring

Summary

WebStandardStreamableHTTPServerTransport validates the Accept header with raw substring checks. As a result, invalid media types such as application/jsonx and text/event-stream-bogus satisfy the transport requirements and are accepted as though the client had listed application/json and text/event-stream.

This affects both POST negotiation and the GET SSE check.

Reproduction

On current main (24be4040), send an initialize request with:

Accept: application/jsonx, text/event-stream-bogus
Content-Type: application/json

Equivalent unit-level reproduction:

const request = new Request('http://localhost/mcp', {
  method: 'POST',
  headers: {
    Accept: 'application/jsonx, text/event-stream-bogus',
    'Content-Type': 'application/json',
  },
  body: JSON.stringify(initializeRequest),
});

const response = await transport.handleRequest(request);
console.log(response.status); // currently 200; expected 406

For GET, Accept: text/event-stream-bogus likewise passes the current check instead of returning 406.

Root cause

packages/server/src/server/streamableHttp.ts currently uses:

acceptHeader?.includes('text/event-stream')
acceptHeader?.includes('application/json')

Those checks match substrings inside other type/subtype tokens and parameters; they do not parse the comma-separated media ranges.

This is the same class of bug recently fixed for Content-Type in #2441 / #2444, where the server switched from substring matching to parsed media-type comparison. The Accept checks still use the older substring behavior.

Expected behavior

The transport should require the exact concrete media types named by the MCP Streamable HTTP requirements:

• GET: text/event-stream
• POST: both application/json and text/event-stream

Parameters and case variants should remain valid, for example:

Accept: Application/JSON; q=0.9, text/event-stream; charset=utf-8

Values that only contain the required strings as substrings should return 406.

Suggested fix

Parse the comma-separated Accept values and compare each media-type essence (type/subtype, case-insensitive, without parameters) to the required concrete values. Add regression coverage for POST and GET substring false positives plus valid parameterized media types.

Start in packages/server/src/server/streamableHttp.ts and trace the Accept-header checks used by POST negotiation and the GET SSE response. Compare the existing Content-Type parsing fix referenced in #2441 and #2444, then add regression coverage for substring false positives and valid parameterized, case-insensitive media types. Done means invalid values return 406 while valid concrete media types continue to negotiate successfully.