Streamable HTTP client logs a WARNING for valid 202 Accepted on session termination (DELETE)

Problem

mcp/client/streamable_http.py treats any DELETE session-termination response outside {200, 204} as a failure and logs:

WARNING mcp.client.streamable_http: Session termination failed: 202

But 202 Accepted is a valid, spec-compliant response for an asynchronous delete — the server accepted the request and will process it later. The current code path:

if response.status_code == 405:
    logger.debug("Server does not allow session termination")
elif response.status_code not in (200, 204):
    logger.warning(f"Session termination failed: {response.status_code}")

Evidence

A long-running Hermes Agent (NousResearch) deployment with several Streamable HTTP MCP servers accumulated 72 Session termination failed: 202 warnings in ~36 hours in errors.log — pure noise from servers that correctly answer 202 to asynchronous termination. Real failures (connection errors) also surface through this same line, so the constant false positives bury the signal.

Suggested fix

Treat 2xx as success on termination, e.g.:

if response.status_code == 405:
    logger.debug("Server does not allow session termination")
elif 200 <= response.status_code < 300:
    pass  # 200/202/204 all acceptable
else:
    logger.warning(f"Session termination failed: {response.status_code}")

Environment

• mcp Python SDK (client, streamable HTTP transport), observed on recent Hermes 0.21.x installs
• Servers: multiple Streamable HTTP MCP endpoints (remote + local)

Happy to send a PR if the direction is agreed.

从 mcp/client/streamable_http.py 中 DELETE 会话终止响应的处理部分开始。验证 202 响应会被视为成功且不会发出警告，同时 405 仍作为 debug 情况处理，其他不成功的状态仍会发出警告；如果相关 client 测试套件可用，请运行它。