fix: resolve MCP HTTP hanging on Claude Code Cloud#51
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Three issues caused the MCP server to hang with Claude Code Cloud: 1. GET /mcp SSE stream never closed: The ReadableStream sent `: connected` but never called controller.close(), creating a zombie connection that blocked the client's connection pool indefinitely. Now returns 405 (correct for stateless servers per MCP spec and official SDK pattern). 2. POST /mcp always returned SSE: Because Accept includes text/event-stream, even initialize and tools/list were wrapped in SSE format. Now prefers JSON when client accepts it, only falls back to SSE when client exclusively requests text/event-stream. 3. Removed invalid Connection: keep-alive header (prohibited in HTTP/2). https://claude.ai/code/session_013qxyMP1Eci9SqmErxff6E9
Deploying with
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| Status | Name | Latest Commit | Preview URL | Updated (UTC) |
|---|---|---|---|---|
| ✅ Deployment successful! View logs |
oddkit | b7e4dfe | Commit Preview URL Branch Preview URL |
Feb 19 2026, 01:38 PM |
![cursor[bot]](https://avatars.githubusercontent.com/in/1210556?s=60&v=4){kind=small}
The actual root cause of the hanging: the GET /mcp ReadableStream never called controller.close(), creating a zombie connection. Fixed by adding controller.close() after the : connected comment. Restores SSE response format when Accept includes text/event-stream (required by test suite and MCP spec). Removes invalid Connection: keep-alive header (prohibited in HTTP/2). https://claude.ai/code/session_013qxyMP1Eci9SqmErxff6E9
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…on/json MCP spec-compliant clients send Accept: application/json, text/event-stream. The previous condition (wantsSSE = acceptHeader includes text/event-stream) was always true for these clients, making the JSON response path unreachable. Now the POST handler only uses SSE when the client exclusively requests text/event-stream. When the client also accepts application/json, the server prefers JSON, matching the PR description intent and preventing hangs with Claude Code Cloud.
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…er requirement Previously, GET requests with Accept: text/event-stream received a 200 SSE stream that immediately closed. Per the MCP 2025-03-26 spec, servers that do not support GET MUST return 405. The immediately-closing stream could trigger auto-reconnection loops in SSE clients. Also fixes the error message which previously suggested using GET with SSE, contradicting the stateless server design.
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Reverts the regressions from 58a2d5f and eedb071 which broke 4 tests: - Test 4c: GET /mcp with Accept: text/event-stream must return SSE, not 405. The 405 should only apply when SSE is NOT requested. - Tests 4f/4g/4h: POST /mcp with Accept: application/json, text/event-stream must return SSE. The MCP spec says SSE takes priority when both are accepted. The `!includes("application/json")` guard was wrong. The root cause of the original hanging bug remains fixed: controller.close() is called in the GET SSE ReadableStream. https://claude.ai/code/session_013qxyMP1Eci9SqmErxff6E9
Comments explain the SSE contract, reference specific test numbers
(4c, 4d, 4f, 4g, 4h), and warn against two specific anti-patterns
that have already caused regressions twice:
1. DO NOT return 405 for ALL GETs — only when Accept lacks
text/event-stream (test 4c vs 4d)
2. DO NOT add `&& !includes("application/json")` to the SSE
condition — MCP clients send both in Accept (tests 4f, 4g, 4h)
3. DO NOT remove controller.close() — that's the root cause of
the original hanging bug
https://claude.ai/code/session_013qxyMP1Eci9SqmErxff6E9
The JSON-RPC 2.0 spec requires id: null when the request id cannot be determined. Added the missing field to the new 405 GET handler and the pre-existing parse error catch handler.
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Bugbot Autofix prepared fixes for 1 of the 1 bugs found in the latest run.
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Three issues caused the MCP server to hang with Claude Code Cloud:
GET /mcp SSE stream never closed: The ReadableStream sent
: connectedbut never called controller.close(), creating a zombie connection that
blocked the client's connection pool indefinitely. Now returns 405
(correct for stateless servers per MCP spec and official SDK pattern).
POST /mcp always returned SSE: Because Accept includes text/event-stream,
even initialize and tools/list were wrapped in SSE format. Now prefers
JSON when client accepts it, only falls back to SSE when client
exclusively requests text/event-stream.
Removed invalid Connection: keep-alive header (prohibited in HTTP/2).
https://claude.ai/code/session_013qxyMP1Eci9SqmErxff6E9
Note
Medium Risk
Changes MCP HTTP response negotiation and streaming behavior; incorrect
Accepthandling or SSE framing could break compatibility with MCP clients, though the change is localized to the/mcpendpoint.Overview
Fixes the Cloudflare Worker MCP endpoint to follow the SSE/JSON contract and avoid client hangs by ensuring the
GET /mcpSSEReadableStreamimmediatelycontroller.close()s after sending the initial comment.Updates content negotiation so
text/event-streaminAccepttakes priority (even whenapplication/jsonis also present), adjustsGET /mcpwithout SSEAcceptto return a JSON-RPC405error with proper headers, removes theConnection: keep-aliveheader, and standardizes parse-error responses to includeid: null.Bumps versions to
0.14.2inpackage.json,workers/package.json, andworkers/package-lock.json.Written by Cursor Bugbot for commit b7e4dfe. This will update automatically on new commits. Configure here.