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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"streamable_http_client hangs indefinitely when server sends POST-only endpoints without SSE fallback","articleBody":"## Problem\n\nWhen an MCP server implements only POST-based endpoints (no SSE support), the `streamable_http_client` in `mcp/client/sse.py` hangs indefinitely during connection setup. The client assumes SSE is available and blocks waiting for server-sent events that never arrive.\n\n## Root Cause\n\nThe `sse_client()` function attempts to establish an SSE connection via GET request. If the server only exposes POST endpoints (as seen with GitHub MCP and other stateless implementations), the GET request either:\n1. Returns a 405 Method Not Allowed that goes unhandled\n2. Hangs waiting for an SSE stream that the POST-only server never sends\n\nThe client lacks:\n- Timeout detection for failed SSE handshake\n- Fallback logic to detect POST-only servers\n- Clear error messaging distinguishing \"server error\" from \"transport mismatch\"\n\n## Impact\n\nUsers cannot connect to valid MCP servers that use POST-only transport, forcing them to use alternative clients or work around the limitation. The hang is silent—no exception is raised, making debugging difficult.\n\n## Expected Behavior\n\nOne of:\n1. **Auto-detection**: Client detects POST-only server and switches to stateless HTTP mode automatically\n2. **Explicit timeout**: GET request to SSE endpoint times out within 5-10 seconds with a clear error: \"Server does not support SSE transport. Use stateless HTTP mode instead.\"\n3. **Transport negotiation**: Client includes `Accept: application/json` header in GET request; if server responds with 406 or 415, client falls back to POST\n\n## Reproduction Steps\n\n1. Start an MCP server that only implements POST endpoints (e.g., `github-mcp` or `simple-stateless-http` example)\n2. Connect via:\n   ```python\n   async with sse_client(\"http://localhost:3000\") as client:\n       await client.initialize()\n   ```\n3. Observe: Connection hangs indefinitely; no timeout; no error message\n\n## References\n\n- Related issues: #157 (streamable_http_client hangs with GitHub MCP), #401 (HTTP errors not handled properly)\n- Related: `mcp/client/sse.py` lines 100-150 (SSE handshake logic)\n- Similar pattern in TypeScript SDK: handles POST-only servers explicitly\n\n\n---\n*Contributed by [Klement Gunndu](https://github.com/KlementMultiverse)*","author":{"url":"https://github.com/KlementMultiverse","@type":"Person","name":"KlementMultiverse"},"datePublished":"2026-03-06T12:42:00.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/2232/python-sdk/issues/2232"}