fix: Expose completions capability (#865)

tests/server/test_session.py

@@ -11,8 +11,13 @@ from mcp.shared.message import SessionMessage
 from mcp.shared.session import RequestResponder
 from mcp.types import (
     ClientNotification,
+    Completion,
+    CompletionArgument,
+    CompletionsCapability,
     InitializedNotification,
+    PromptReference,
     PromptsCapability,
+    ResourceReference,
     ResourcesCapability,
     ServerCapabilities,
 )
@@ -80,6 +85,7 @@ async def test_server_capabilities():
     caps = server.get_capabilities(notification_options, experimental_capabilities)
     assert caps.prompts is None
     assert caps.resources is None
+    assert caps.completions is None
 
     # Add a prompts handler
     @server.list_prompts()
@@ -89,6 +95,7 @@ async def test_server_capabilities():
     caps = server.get_capabilities(notification_options, experimental_capabilities)
     assert caps.prompts == PromptsCapability(listChanged=False)
     assert caps.resources is None
+    assert caps.completions is None
 
     # Add a resources handler
     @server.list_resources()
@@ -98,6 +105,19 @@ async def test_server_capabilities():
     caps = server.get_capabilities(notification_options, experimental_capabilities)
     assert caps.prompts == PromptsCapability(listChanged=False)
     assert caps.resources == ResourcesCapability(subscribe=False, listChanged=False)
+    assert caps.completions is None
+
+    # Add a complete handler
+    @server.completion()
+    async def complete(ref: PromptReference | ResourceReference, argument: CompletionArgument):
+        return Completion(
+            values=["completion1", "completion2"],
+        )
+
+    caps = server.get_capabilities(notification_options, experimental_capabilities)
+    assert caps.prompts == PromptsCapability(listChanged=False)
+    assert caps.resources == ResourcesCapability(subscribe=False, listChanged=False)
+    assert caps.completions == CompletionsCapability()
 
 
 @pytest.mark.anyio