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

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")