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.

mcp/client/streamable_http.py の DELETE セッション終了レスポンス処理から始めます。202 レスポンスが警告なしで成功として扱われ、405 は引き続き debug ケースとして扱われ、その他の失敗ステータスでは引き続き警告が出ることを確認してください。利用可能であれば、該当する client test suite を実行してください。