fix: preserve custom Accept headers instead of unconditionally overwriting#1697
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The SDK unconditionally overwrites Accept headers with headers.set(),
discarding any custom Accept values provided via requestInit.headers.
This breaks APIs that require specific Accept header values.
Now checks headers.has('Accept') before setting the default value,
preserving user-provided Accept headers in both Streamable HTTP and
SSE transports.
Fixes #1646
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Problem
The SDK unconditionally overwrites
Acceptheaders usingheaders.set(), discarding any custom values provided viarequestInit.headers. This breaks MCP servers behind APIs that require specific Accept header values.{ "mcpServers": { "example-api": { "url": "https://api.example.com/mcp", "headers": { "Accept": "application/vnd.example.v1+json" } } } }The custom Accept header is overwritten with
application/json, text/event-stream.Fix
Check
headers.has('Accept')before setting the default value. Custom Accept headers fromrequestInit.headersare preserved; the SDK default is only applied when no Accept header was provided.Fixed in both transports:
packages/client/src/client/streamableHttp.ts(GET SSE + POST)packages/client/src/client/sse.tsFixes #1646