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fix(mcp): advertise a relative SSE endpoint so clients work behind the /server proxy - #1855
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The SSE endpoint event advertised the absolute path /mcp/messages, which drops the /server prefix when clients connect through the bundled nginx proxy on port 20211. Clients then POST to /mcp/messages, nginx serves it as a static file and returns 404, so MCP clients can't connect via the proxy. A relative path resolves against whichever SSE URL the client used, so it works both directly and behind the prefix.
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📝 Walkthrough
Priority: ➖ Normal Change: Bug fix · Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to
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| Check name | Status | Explanation |
|---|---|---|
| Description Check |  |
Check skipped - CodeRabbit’s high-level summary is enabled. |
| Title check | |
The title clearly and concisely describes the main change: advertising a relative MCP SSE endpoint to support clients behind the /server proxy. |
| Linked Issues check | |
The PR satisfies the coding requirements in [#1854]. server/api_server/mcp_endpoint.py now emits messages?session_id=... as a relative endpoint. URL resolution preserves /mcp/messages for direct… |
| Out of Scope Changes check | |
The changes stay within [#1854]. The PR changes only the MCP SSE endpoint payload and adds focused tests for direct and /server-prefixed URL resolution. No unrelated route, nginx, setting, or featur… |
| Docstring Coverage | |
Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 2 files. |
Full details: Linked Issues check
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Explanation
The PR satisfies the coding requirements in [
#1854].server/api_server/mcp_endpoint.pynow emitsmessages?session_id=...as a relative endpoint. URL resolution preserves/mcp/messagesfor direct connections and produces/server/mcp/messagesfor the bundled proxy path. The new test covers both resolutions and requires asession_idquery parameter.
Full details: Out of Scope Changes check
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Explanation
The changes stay within [
#1854]. The PR changes only the MCP SSE endpoint payload and adds focused tests for direct and/server-prefixed URL resolution. No unrelated route, nginx, setting, or feature changes are present.
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📌 Description
The MCP SSE stream advertises its message endpoint as the absolute path
/mcp/messages?session_id=.... MCP clients resolve that against the origin, so a client connected through the bundled nginx proxy (http://<host>:20211/server/mcp/sse) posts to/mcp/messagesinstead of/server/mcp/messages. nginx has no route for that path, falls through to static file serving (open() "/app/front/mcp/messages" failed), returns 404, and the client never finishes connecting.This change advertises a relative path (
messages?session_id=...) instead. Clients resolve it against whichever SSE URL they used::20212/mcp/sse(direct)/mcp/messages✅/mcp/messages✅:20211/server/mcp/sse(proxy)/mcp/messages❌ 404/server/mcp/messages✅No routes, nginx config or settings change.
🔍 Related Issues
Fixes #1854
📋 Type of Change
📷 Screenshots or Logs (if applicable)
New test before the fix:
After the fix:
🧪 Testing Steps
test/api_endpoints/test_mcp_sse_endpoint.py. It opens/mcp/sse, reads theendpointevent and resolves it withurljoin(as the Python MCP SDK does) against both a direct and a/server-prefixed SSE URL. Wrote it first and confirmed the proxy case failed, then applied the fix.pytest -m 'not (docker or compose or feature_complete)'): 1738 passed, 30 skipped, 0 failed.test/docker_testswas excluded locally because building the image through the Docker socket fails on Docker Desktop for macOS (build context not visible to the host daemon); unrelated to this change.flake8with the CI settings passes on the changed files./server/mcp/messages?session_id=...through port 20211 returns 202, the unprefixed path returns 404.✅ Checklist
🙋 Additional Notes
urljoin(url, sse.data)and the TypeScript SDK usesnew URL(data, this._url), both of which resolve relative paths. Both SDKs' same-origin checks still pass, since a relative path can't change the origin.origin + data(not per spec) would be affected; such a client already can't work behind the proxy./mcp/messagesroute, which is unchanged, and don't describe the event payload.Summary by CodeRabbit
Bug Fixes
Tests