Fix: Handle error responses in Streamable HTTP SSE streams

[DePasqualeOrg]

While developing my Swift SDK for MCP, I encountered the following issue in the TypeScript SDK:

The StreamableHTTPClientTransport only checked for success responses (isJSONRPCResultResponse) when processing SSE stream messages. This caused two issues:

1. Unnecessary reconnection: Error responses didn't set receivedResponse, potentially triggering reconnection attempts
2. Missing ID remapping: Error response IDs weren't remapped during stream resumption

Solution

Handle both success and error responses when detecting stream completion and remapping IDs:

if (isJSONRPCResultResponse(message) || isJSONRPCErrorResponse(message)) {
    receivedResponse = true;
    if (replayMessageId !== undefined) {
        message.id = replayMessageId;
    }
}

This aligns with the MCP specification which states the SSE stream should include a "JSON-RPC response" (which per JSON-RPC 2.0 spec includes both success and error responses), and matches the Python SDK implementation.

Testing

Added test case verifying that error responses in SSE streams prevent unnecessary reconnection and are properly delivered to the message handler.

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[changeset-bot]

🦋 Changeset detected

Latest commit: 6b4426a

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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commit: 6b4426a