test: interaction-model end-to-end suite with a requirements manifest (#2691)

tests/interaction/mcpserver/test_completion.py

@@ -0,0 +1,38 @@
+"""Completion behaviour against MCPServer, driven through the public Client API."""
+
+import pytest
+
+from mcp.server.mcpserver import MCPServer
+from mcp.types import (
+    Completion,
+    CompletionArgument,
+    CompletionContext,
+    CompletionsCapability,
+    PromptReference,
+    ResourceTemplateReference,
+)
+from tests.interaction._connect import Connect
+from tests.interaction._requirements import requirement
+
+pytestmark = pytest.mark.anyio
+
+
+@requirement("mcpserver:completion:capability-auto")
+async def test_completion_capability_is_advertised_only_when_a_handler_is_registered(connect: Connect) -> None:
+    """An MCPServer with a registered completion handler advertises the completions capability; one without does not."""
+    with_handler = MCPServer("completer")
+
+    @with_handler.completion()
+    async def complete(
+        ref: PromptReference | ResourceTemplateReference,
+        argument: CompletionArgument,
+        context: CompletionContext | None,
+    ) -> Completion | None:
+        """Registered only so the completions capability is advertised; never called."""
+        raise NotImplementedError
+
+    async with connect(with_handler) as client:
+        assert client.initialize_result.capabilities.completions == CompletionsCapability()
+
+    async with connect(MCPServer("plain")) as client:
+        assert client.initialize_result.capabilities.completions is None