fix: Expose completions capability (#865)

src/mcp/server/lowlevel/server.py

@@ -190,6 +190,7 @@ class Server(Generic[LifespanResultT, RequestT]):
         resources_capability = None
         tools_capability = None
         logging_capability = None
+        completions_capability = None
 
         # Set prompt capabilities if handler exists
         if types.ListPromptsRequest in self.request_handlers:
@@ -209,12 +210,17 @@ class Server(Generic[LifespanResultT, RequestT]):
         if types.SetLevelRequest in self.request_handlers:
             logging_capability = types.LoggingCapability()
 
+        # Set completions capabilities if handler exists
+        if types.CompleteRequest in self.request_handlers:
+            completions_capability = types.CompletionsCapability()
+
         return types.ServerCapabilities(
             prompts=prompts_capability,
             resources=resources_capability,
             tools=tools_capability,
             logging=logging_capability,
             experimental=experimental_capabilities,
+            completions=completions_capability,
         )
 
     @property

src/mcp/types.py

@@ -286,6 +286,12 @@ class LoggingCapability(BaseModel):
     model_config = ConfigDict(extra="allow")
 
 
+class CompletionsCapability(BaseModel):
+    """Capability for completions operations."""
+
+    model_config = ConfigDict(extra="allow")
+
+
 class ServerCapabilities(BaseModel):
     """Capabilities that a server may support."""
 
@@ -299,6 +305,8 @@ class ServerCapabilities(BaseModel):
     """Present if the server offers any resources to read."""
     tools: ToolsCapability | None = None
     """Present if the server offers any tools to call."""
+    completions: CompletionsCapability | None = None
+    """Present if the server offers autocompletion suggestions for prompts and resources."""
     model_config = ConfigDict(extra="allow")