fix: preserve custom paths in SSE endpoint URLs

[bhosmer-ant]

Summary

When an SSE client connects with a custom path (e.g., /api/v1/custom/sse), ensure the endpoint URL maintains the same base path structure but with /messages instead of /sse. This fixes issues where custom endpoints were getting collapsed to the root path.

Problem

The SSE client was not preserving custom paths when constructing endpoint URLs. When a client connected to a server at a custom path like /api/v1/custom/sse, the endpoint URL would be constructed without preserving the custom path, resulting in POST requests being sent to /messages instead of /api/v1/custom/messages.

Solution

Modified the endpoint event handler in the SSE client to:

1. Check if the original connection URL contains a custom path
2. Extract the base path (removing the /sse suffix)
3. Apply the base path to the endpoint URL with /messages suffix

Testing

This PR makes the failing tests from #439 pass. The tests verify that:

• Custom paths are preserved when constructing endpoint URLs
• Multiple levels of paths are handled correctly
• Query parameters are handled appropriately

Related Issues

• Should fixIncorrect URL in sse.js when providing a custom path inspector#313
• Should fixRoutes other than root ("/") for remote MCP servers don't seem to be supported. inspector#296
• Tests demonstrating the issue were added in Add failing tests for custom endpoints #439

Test Plan

• All existing tests pass
• New tests from Add failing tests for custom endpoints #439 now pass
• No breaking changes to existing functionality

🤖 Generated with Claude Code

[ihrpr]

should we include those filing tests in this PR

[bhosmer-ant]

@ihrpr brought the tests over - note, dropped the change to package-lock.json

[ihrpr]

LGTM

[onlysyz]

@ihrpr do you have a plan for this release?

[chrisdickinson]

As a heads up, this breaks SSE integrations with mcp.run – we were sending the full SSE endpoint down to support the python-sdk and now the endpoint pathname is being rewritten to /messages.

[bhosmer-ant]

As a heads up, this breaks SSE integrations with mcp.run – we were sending the full SSE endpoint down to support the python-sdk and now the endpoint pathname is being rewritten to /messages.

@chrisdickinson thanks for catching - #504 should fix, though it couldn't hurt to verify

[edit: not that simple, reverted path preservation and will revisit]