fix: ServerRunner error shapes match existing server (METHOD_NOT_FOUND, pre-init)

METHOD_NOT_FOUND message is now bare "Method not found" (no method
suffix); the interaction suite pins that.

The pre-init gate now returns the generic INVALID_PARAMS / "Invalid
request parameters" / data="" shape. The existing server has no
dedicated pre-init check; the request dies in ClientRequest validation,
so clients see this shape. TODO(maxisbey) marked.

Also: loosen the gap-8 _meta wire test to be tracer-agnostic (it was
order-dependent on the SpanCapture fixture).

Author: maxisbey

src/mcp/server/runner.py

@@ -34,7 +34,7 @@
 from mcp.shared.exceptions import MCPError
 from mcp.shared.transport_context import TransportContext
 from mcp.types import (
-    INVALID_REQUEST,
+    INVALID_PARAMS,
     LATEST_PROTOCOL_VERSION,
     METHOD_NOT_FOUND,
     Implementation,
@@ -164,13 +164,13 @@ async def _on_request(
         if method == "initialize":
             return self._handle_initialize(params)
         if not self._initialized and method not in _INIT_EXEMPT:
-            raise MCPError(
-                code=INVALID_REQUEST,
-                message=f"Received {method!r} before initialization was complete",
-            )
+            # TODO(maxisbey): pinned compat. The existing server has no
+            # dedicated pre-init check; the request dies in ClientRequest
+            # validation, so the client sees the generic invalid-params shape.
+            raise MCPError(code=INVALID_PARAMS, message="Invalid request parameters", data="")
         entry = self.server.get_request_handler(method)
         if entry is None:
-            raise MCPError(code=METHOD_NOT_FOUND, message=f"Method not found: {method}")
+            raise MCPError(code=METHOD_NOT_FOUND, message="Method not found")
         # ValidationError propagates; the dispatcher's exception boundary maps
         # it to INVALID_PARAMS.
         typed_params = entry.params_type.model_validate(params or {})

tests/server/test_runner.py

@@ -24,11 +24,12 @@
 from mcp.shared.exceptions import MCPError
 from mcp.types import (
     INTERNAL_ERROR,
-    INVALID_REQUEST,
+    INVALID_PARAMS,
     LATEST_PROTOCOL_VERSION,
     METHOD_NOT_FOUND,
     CallToolRequestParams,
     ClientCapabilities,
+    ErrorData,
     Implementation,
     InitializeRequestParams,
     ListToolsResult,
@@ -142,7 +143,7 @@ async def test_runner_gates_requests_before_initialize(server: SrvT):
     async with connected_runner(server, initialized=False) as (client, _):
         with pytest.raises(MCPError) as exc:
             await client.send_raw_request("tools/list", None)
-        assert exc.value.error.code == INVALID_REQUEST
+        assert exc.value.error == ErrorData(code=INVALID_PARAMS, message="Invalid request parameters", data="")
         # ping is exempt from the gate
         assert await client.send_raw_request("ping", None) == {}
 
@@ -361,7 +362,7 @@ async def test_otel_middleware_records_error_status_on_mcp_error(server: SrvT, s
         assert exc.value.error.code == METHOD_NOT_FOUND
     [span] = spans.finished()
     assert span.status.status_code == StatusCode.ERROR
-    assert span.status.description == "Method not found: nonexistent/method"
+    assert span.status.description == "Method not found"
     # MCPError is a protocol-level response, not a crash - no traceback event.
     assert not [e for e in span.events if e.name == "exception"]
 

tests/shared/test_jsonrpc_dispatcher.py

@@ -419,11 +419,15 @@ async def noop_progress(progress: float, total: float | None, message: str | Non
         with anyio.fail_after(5):
             await client.send_raw_request("a", None)
             await client.send_raw_request("b", {"x": 1, "_meta": {"k": "v"}}, opts)
-    assert seen[0] == {"_meta": {}}
-    assert seen[1] is not None
-    assert seen[1]["x"] == 1
+    # `_meta` is always present. Its contents depend on the active otel
+    # tracer (traceparent/tracestate may be injected), so assert presence
+    # and that anything beyond W3C keys is exactly what we expect.
+    w3c = {"traceparent", "tracestate"}
+    assert seen[0] is not None and seen[0].keys() == {"_meta"}
+    assert set(seen[0]["_meta"].keys()) <= w3c
+    assert seen[1] is not None and seen[1]["x"] == 1
+    assert set(seen[1]["_meta"].keys()) - w3c == {"k", "progressToken"}
     assert seen[1]["_meta"]["k"] == "v"
-    assert "progressToken" in seen[1]["_meta"]
 
 
 @pytest.mark.anyio