Symptom
SSE responses from a Streamable HTTP MCP server hang indefinitely when several requests are in flight or a response has several parts.
Evidence what you'll see
- Client waits forever on call_tool
- Happens with concurrent requests or responses with 3 or more items
Cause
Each request's stream had no buffer, so one slow reader blocked the shared message router and every other request behind it.
Fix steps
0 of 1 done
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Verify the fix worked
- Send several concurrent tool calls and confirm all of them complete.
Sources and credit
Summarized in our own words from these public reports. Credit goes to their authors.
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