Add ToolAnnotations support in FastMCP and lowlevel servers (#482)

src/mcp/server/fastmcp/server.py

@@ -41,6 +41,7 @@ from mcp.types import (
     GetPromptResult,
     ImageContent,
     TextContent,
+    ToolAnnotations,
 )
 from mcp.types import Prompt as MCPPrompt
 from mcp.types import PromptArgument as MCPPromptArgument
@@ -176,6 +177,7 @@ class FastMCP:
                 name=info.name,
                 description=info.description,
                 inputSchema=info.parameters,
+                annotations=info.annotations,
             )
             for info in tools
         ]
@@ -244,6 +246,7 @@ class FastMCP:
         fn: AnyFunction,
         name: str | None = None,
         description: str | None = None,
+        annotations: ToolAnnotations | None = None,
     ) -> None:
         """Add a tool to the server.
 
@@ -254,11 +257,17 @@ class FastMCP:
             fn: The function to register as a tool
             name: Optional name for the tool (defaults to function name)
             description: Optional description of what the tool does
+            annotations: Optional ToolAnnotations providing additional tool information
         """
-        self._tool_manager.add_tool(fn, name=name, description=description)
+        self._tool_manager.add_tool(
+            fn, name=name, description=description, annotations=annotations
+        )
 
     def tool(
-        self, name: str | None = None, description: str | None = None
+        self,
+        name: str | None = None,
+        description: str | None = None,
+        annotations: ToolAnnotations | None = None,
     ) -> Callable[[AnyFunction], AnyFunction]:
         """Decorator to register a tool.
 
@@ -269,6 +278,7 @@ class FastMCP:
         Args:
             name: Optional name for the tool (defaults to function name)
             description: Optional description of what the tool does
+            annotations: Optional ToolAnnotations providing additional tool information
 
         Example:
             @server.tool()
@@ -293,7 +303,9 @@ class FastMCP:
             )
 
         def decorator(fn: AnyFunction) -> AnyFunction:
-            self.add_tool(fn, name=name, description=description)
+            self.add_tool(
+                fn, name=name, description=description, annotations=annotations
+            )
             return fn
 
         return decorator

src/mcp/server/fastmcp/tools/base.py

@@ -8,6 +8,7 @@ from pydantic import BaseModel, Field
 
 from mcp.server.fastmcp.exceptions import ToolError
 from mcp.server.fastmcp.utilities.func_metadata import FuncMetadata, func_metadata
+from mcp.types import ToolAnnotations
 
 if TYPE_CHECKING:
     from mcp.server.fastmcp.server import Context
@@ -30,6 +31,9 @@ class Tool(BaseModel):
     context_kwarg: str | None = Field(
         None, description="Name of the kwarg that should receive context"
     )
+    annotations: ToolAnnotations | None = Field(
+        None, description="Optional annotations for the tool"
+    )
 
     @classmethod
     def from_function(
@@ -38,9 +42,10 @@ class Tool(BaseModel):
         name: str | None = None,
         description: str | None = None,
         context_kwarg: str | None = None,
+        annotations: ToolAnnotations | None = None,
     ) -> Tool:
         """Create a Tool from a function."""
-        from mcp.server.fastmcp import Context
+        from mcp.server.fastmcp.server import Context
 
         func_name = name or fn.__name__
 
@@ -73,6 +78,7 @@ class Tool(BaseModel):
             fn_metadata=func_arg_metadata,
             is_async=is_async,
             context_kwarg=context_kwarg,
+            annotations=annotations,
         )
 
     async def run(

src/mcp/server/fastmcp/tools/tool_manager.py

@@ -7,6 +7,7 @@ from mcp.server.fastmcp.exceptions import ToolError
 from mcp.server.fastmcp.tools.base import Tool
 from mcp.server.fastmcp.utilities.logging import get_logger
 from mcp.shared.context import LifespanContextT
+from mcp.types import ToolAnnotations
 
 if TYPE_CHECKING:
     from mcp.server.fastmcp.server import Context
@@ -35,9 +36,12 @@ class ToolManager:
         fn: Callable[..., Any],
         name: str | None = None,
         description: str | None = None,
+        annotations: ToolAnnotations | None = None,
     ) -> Tool:
         """Add a tool to the server."""
-        tool = Tool.from_function(fn, name=name, description=description)
+        tool = Tool.from_function(
+            fn, name=name, description=description, annotations=annotations
+        )
         existing = self._tools.get(tool.name)
         if existing:
             if self.warn_on_duplicate_tools:

src/mcp/types.py

@@ -705,6 +705,54 @@ class ListToolsRequest(PaginatedRequest[RequestParams | None, Literal["tools/lis
     params: RequestParams | None = None
 
 
+class ToolAnnotations(BaseModel):
+    """
+    Additional properties describing a Tool to clients.
+
+    NOTE: all properties in ToolAnnotations are **hints**.
+    They are not guaranteed to provide a faithful description of
+    tool behavior (including descriptive properties like `title`).
+
+    Clients should never make tool use decisions based on ToolAnnotations
+    received from untrusted servers.
+    """
+
+    title: str | None = None
+    """A human-readable title for the tool."""
+
+    readOnlyHint: bool | None = None
+    """
+    If true, the tool does not modify its environment.
+    Default: false
+    """
+
+    destructiveHint: bool | None = None
+    """
+    If true, the tool may perform destructive updates to its environment.
+    If false, the tool performs only additive updates.
+    (This property is meaningful only when `readOnlyHint == false`)
+    Default: true
+    """
+
+    idempotentHint: bool | None = None
+    """
+    If true, calling the tool repeatedly with the same arguments 
+    will have no additional effect on the its environment.
+    (This property is meaningful only when `readOnlyHint == false`)
+    Default: false
+    """
+
+    openWorldHint: bool | None = None
+    """
+    If true, this tool may interact with an "open world" of external
+    entities. If false, the tool's domain of interaction is closed.
+    For example, the world of a web search tool is open, whereas that
+    of a memory tool is not.
+    Default: true
+    """
+    model_config = ConfigDict(extra="allow")
+
+
 class Tool(BaseModel):
     """Definition for a tool the client can call."""
 
@@ -714,6 +762,8 @@ class Tool(BaseModel):
     """A human-readable description of the tool."""
     inputSchema: dict[str, Any]
     """A JSON Schema object defining the expected parameters for the tool."""
+    annotations: ToolAnnotations | None = None
+    """Optional additional tool information."""
     model_config = ConfigDict(extra="allow")