MCP SDK Path Handling Bug for Subpath Servers

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
74/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Quiet
Tech stack
typescript
Domain
api

Research direction

Start by reading src/client/sse.ts around line 146 and reproduce the connection failure with a server URL containing /subpath/sse. Verify that the message endpoint retains the server subpath rather than becoming /messages/, and confirm that Inspector or Cherry Studio can connect successfully through the subpath.

Written by the indexing model from the issue text.

Description

bug fix proposed P2 ready for work

Issue:
Clients (Inspector/Cherry Studio) fail to connect when MCP Server uses a subpath (e.g., http://domain.com/subpath/sse). Root paths (http://domain.com/) work normally.

Reproduce:

  1. Start MCP Server at http://domain.com/subpath/sse
  2. Client connection fails with error:
    Error POSTing to endpoint (HTTP 503): no healthy upstream

Cause:
In sse.ts#L146, new URL() discards the subpath from the server URL. Example:
Request to /messages/ becomes http://domain.com/messages/ instead of http://domain.com/subpath/messages/.

Dominant language
TypeScript
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Merged PRs (30d)
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