Teknium
a565e2d493
Relocated onto the decomposed module layout and hardened: - Trigger covers the rejection CLASS, not just literal 400: SSE-only servers' load balancers answer the chunked Streamable HTTP initialize POST with 400/405/406/411, and the mcp>=2.0 SDK surfaces many such rejections as an opaque -32603 'Server returned an error response' (error class per #104363 by @RohithPariki). Timeouts and 5xx never trigger the fallback: they are not transport mismatches. - Reconnect exclusion via _ever_connected instead of _ready: run() clears _ready before re-entering the transport, so the original guard also fired on reconnects after a proven session. - Successful fallback latches _sse_fallback so reconnects go straight to SSE, and logs a warning suggesting the user pin transport: sse. - Both transports failing raises a ConnectionError naming both errors and suggesting transport: sse / checking the URL. - No fallback with strict_redirect_headers (SSE cannot enforce that boundary) or when transport is explicitly configured. - Tests trimmed to 3 invariant contracts (proven red on base): fallback connects + latches; no fallback on reconnect/timeout/5xx; both-fail error is actionable. The extracted SSE path reuses _sse_transport/_serve_transport from main, preserving the bounded handshake timeout and reconnect-retry semantics. Fixes #53676
87 lines
3.6 KiB
Python
87 lines
3.6 KiB
Python
"""Invariant tests: automatic Streamable HTTP -> SSE transport fallback (#53676, #104343).
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An SSE-only MCP server rejects the Streamable HTTP ``initialize`` POST (400-family status,
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or the SDK's opaque -32603 "Server returned an error response"); the client must retry over
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SSE on the initial connect only — never on reconnect after a proven session, never on
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timeout, and a both-transports failure must say so actionably.
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"""
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import asyncio
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import httpx
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import pytest
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from tools.mcp_tool import MCPServerTask
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def _http_400(status=400):
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request = httpx.Request("POST", "http://127.0.0.1:1/mcp")
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return httpx.HTTPStatusError("Bad Request", request=request,
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response=httpx.Response(status, request=request))
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class _SdkInternalError(Exception):
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"""Shape of mcp.shared.exceptions.MCPError for an opaque initialize rejection."""
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def __init__(self):
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super().__init__("Server returned an error response")
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self.error = type("E", (), {"code": -32603})()
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def _task(monkeypatch, http_exc, sse_result="shutdown", sse_exc=None):
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"""MCPServerTask whose transports are recorded fakes: HTTP raises, SSE serves or raises."""
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task = MCPServerTask("t")
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task._config = {}
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calls = []
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async def fake_serve(self, cm, label, timeout):
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calls.append(label)
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if label != "SSE":
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raise http_exc
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if sse_exc is not None:
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raise sse_exc
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self._ever_connected = True
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return sse_result
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monkeypatch.setattr(MCPServerTask, "_serve_transport", fake_serve)
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monkeypatch.setattr(MCPServerTask, "_streamable_http_transport", lambda self, *a, **k: object())
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monkeypatch.setattr(MCPServerTask, "_sse_transport", lambda self, *a, **k: object())
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monkeypatch.setattr(MCPServerTask, "_build_oauth_auth", lambda self, *a: None)
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return task, calls
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_CONFIG = {"url": "http://127.0.0.1:1/mcp", "connect_timeout": 1}
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@pytest.mark.parametrize("exc", [_http_400(), _http_400(405),
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ExceptionGroup("g", [_SdkInternalError()])])
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def test_sse_only_server_connects_via_fallback(monkeypatch, exc):
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"""Initial connect: a Streamable HTTP rejection falls back to SSE and serves; the latch
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routes subsequent reconnects straight to SSE without re-trying Streamable HTTP."""
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task, calls = _task(monkeypatch, exc)
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assert asyncio.run(task._run_http(dict(_CONFIG))) == "shutdown"
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assert calls[-1] == "SSE" and len(calls) == 2
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assert asyncio.run(task._run_http(dict(_CONFIG))) == "shutdown" # reconnect after latch
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assert calls[2:] == ["SSE"]
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@pytest.mark.parametrize("exc,ever_connected", [
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(_http_400(), True), # reconnect after a proven session: never mask the 400
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(asyncio.TimeoutError(), False), # timeout is not a transport mismatch
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(_http_400(500), False), # 5xx is a broken server, not SSE-only
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])
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def test_no_fallback_on_reconnect_timeout_or_server_error(monkeypatch, exc, ever_connected):
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task, calls = _task(monkeypatch, exc)
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task._ever_connected = ever_connected
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with pytest.raises(type(exc)):
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asyncio.run(task._run_http(dict(_CONFIG)))
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assert "SSE" not in calls
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def test_both_transports_failing_names_both_and_suggests_config(monkeypatch):
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task, calls = _task(monkeypatch, _http_400(), sse_exc=ConnectionRefusedError("no sse"))
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with pytest.raises(ConnectionError, match="both Streamable HTTP and SSE.*transport: sse"):
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asyncio.run(task._run_http(dict(_CONFIG)))
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assert calls == ["HTTP", "SSE"] or calls == ["legacy HTTP", "SSE"]
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assert task._sse_fallback is False # failed fallback must not latch
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