fix(mcp): search explicit exception links before __context__ when finding the upstream response

litellm/proxy/_experimental/mcp_server/faults/list_outcomes.py

@@ -63,7 +63,10 @@ class AggregateToolListing(NamedTuple):
 
 def _find_upstream_response(exc: BaseException) -> httpx.Response | None:
     """Walk the exception tree (``__cause__``/``__context__``/ExceptionGroup members) for an
-    ``httpx.Response``, mirroring how upstream failures surface through the MCP SDK's task groups."""
+    ``httpx.Response``, mirroring how upstream failures surface through the MCP SDK's task groups.
+    Explicit links are searched first: each node's ``raise ... from`` cause, then group members in
+    raise order, then the incidental ``__context__`` chain, so a response raised while handling the
+    real failure can never shadow the response on the explicit causal chain."""
     seen: set[int] = set()
     stack = [exc]
     while stack:
@@ -74,12 +77,13 @@ def _find_upstream_response(exc: BaseException) -> httpx.Response | None:
         response = getattr(current, "response", None)
         if isinstance(response, httpx.Response):
             return response
+        if current.__context__ is not None:
+            stack.append(current.__context__)
         exceptions = getattr(current, "exceptions", None)
         if isinstance(exceptions, tuple):
-            stack.extend(exceptions)
-        for link in (current.__cause__, current.__context__):
-            if link is not None:
-                stack.append(link)
+            stack.extend(reversed(exceptions))
+        if current.__cause__ is not None:
+            stack.append(current.__cause__)
     return None
 
 

tests/test_litellm/proxy/_experimental/mcp_server/faults/test_list_outcomes.py

@@ -49,6 +49,31 @@ def test_embedded_401_classifies_auth_required():
     assert classify_list_exception(exc).tag == "auth_required"
 
 
+def test_context_response_does_not_shadow_the_causal_chain_response():
+    real_response = httpx.Response(401, request=httpx.Request("POST", "https://mcp.example.com/mcp"))
+    real = httpx.HTTPStatusError("upstream rejected", request=real_response.request, response=real_response)
+    incidental_response = httpx.Response(500, request=httpx.Request("POST", "https://hooks.example.com/log"))
+    incidental = httpx.HTTPStatusError(
+        "logging hook failed", request=incidental_response.request, response=incidental_response
+    )
+    wrapper = RuntimeError("wrapper")
+    wrapper.__cause__ = real
+    wrapper.__context__ = incidental
+    fault = classify_list_exception(wrapper)
+    assert fault.tag == "auth_required"
+    assert fault.status_code == 401
+
+
+def test_exception_group_members_are_searched_in_raise_order():
+    first_response = httpx.Response(502, request=httpx.Request("POST", "https://mcp.example.com/mcp"))
+    first = httpx.HTTPStatusError("first", request=first_response.request, response=first_response)
+    second_response = httpx.Response(503, request=httpx.Request("POST", "https://mcp.example.com/mcp"))
+    second = httpx.HTTPStatusError("second", request=second_response.request, response=second_response)
+    fault = classify_list_exception(BaseExceptionGroup("task group", [first, second]))
+    assert fault.tag == "upstream_error"
+    assert fault.status_code == 502
+
+
 def test_unknown_exception_is_internal():
     assert classify_list_exception(ValueError("who knows")).tag == "internal"