BUG REPORT: HTTP errors are being silently swallowed in the StreamableHTTP client

[Kulaxyz]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

When an MCP server returns HTTP 5xx errors (e.g., when the server is down) after initial connection, these errors are not being propagated to the caller. This makes it impossible for client applications to detect and handle server failures appropriately. More than that, clients which are waiting for response stream, like Strands SDK, are hanging forever since no response is returned

Example Code

Python & MCP Python SDK

Any

[Kulaxyz]

Prepared a PR that fixes this issue

[maxisbey]

Related report in #1835 includes a working patch that:

1. Catches httpx.HTTPStatusError in _handle_post_request
2. Creates a JSONRPCError response with the HTTP error details
3. Sends it to the read stream so clients receive the error instead of hanging

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