HTTP transport swallows non-2xx status codes causing client to hang

Summary

When an MCP server returns non-2xx HTTP status codes (401/403/404/5xx), the Streamable HTTP and SSE transports do not reliably propagate the error to the caller. In the Streamable HTTP transport, post_writer catches exceptions and logs them, but the caller blocks indefinitely waiting for a response on the read stream. HTTP 404 responses are also converted to JSONRPCError(code=32600), losing the original HTTP status information.

Expected Behavior

• Non-2xx HTTP responses should be surfaced as exceptions to the caller, not silently logged
• The original HTTP status code should be preserved and accessible
• Auth-related errors (401/403) should be distinguishable from other failures

Current Behavior

• post_writer in Streamable HTTP transport catches and logs errors without forwarding them through the read stream
• Callers hang indefinitely waiting for a response
• HTTP 404 is converted to JSONRPCError(code=32600), destroying HTTP-level context

Affected Code

• src/mcp/client/streamable_http.py (post_writer)
• src/mcp/client/sse.py (sse_reader)

Related

• #1742 — broader typed error classes design

まず、src/mcp/client/streamable_http.py、特に post_writer と、src/mcp/client/sse.py、特に sse_reader のエラーハンドリングを追跡します。型付きエラーの設計について、関連する issue #1742 を確認します。非2xxレスポンスが例外として呼び出し元に到達し、HTTPステータスを保持し、401/403を区別し、呼び出し元が無期限に待ち続けることがなくなれば完了です。