Python: Add documentation for declaration-only tools and middleware ordering (#3774)

* added explanation doctrings

* copilot comments

python/packages/core/agent_framework/_tools.py

@@ -640,16 +640,25 @@ class FunctionTool(BaseTool, Generic[ArgsT, ReturnT]):
             name: The name of the function.
             description: A description of the function.
             approval_mode: Whether or not approval is required to run this tool.
-                Default is that approval is required.
+                Default is that approval is NOT required (``"never_require"``).
             max_invocations: The maximum number of times this function can be invoked.
                 If None, there is no limit. Should be at least 1.
             max_invocation_exceptions: The maximum number of exceptions allowed during invocations.
                 If None, there is no limit. Should be at least 1.
             additional_properties: Additional properties to set on the function.
-            func: The function to wrap.
+            func: The function to wrap. When ``None``, creates a declaration-only tool
+                that has no implementation. Declaration-only tools are useful when you want
+                the agent to reason about tool usage without executing them, or when the
+                actual implementation exists elsewhere (e.g., client-side rendering).
             input_model: The Pydantic model that defines the input parameters for the function.
                 This can also be a JSON schema dictionary.
-                If not provided, it will be inferred from the function signature.
+                If not provided and ``func`` is not ``None``, it will be inferred from
+                the function signature. When ``func`` is ``None`` and ``input_model`` is
+                not provided, the tool will use an empty input model (no parameters) in
+                its JSON schema. For declaration-only tools that should declare
+                parameters, explicitly provide ``input_model`` (either a Pydantic
+                ``BaseModel`` or a JSON schema dictionary) so the model can reason about
+                the expected arguments.
             **kwargs: Additional keyword arguments.
         """
         super().__init__(
@@ -1286,7 +1295,11 @@ def tool(
     to bypass automatic inference from the function signature.
 
     Args:
-        func: The function to decorate.
+        func: The function to decorate. This parameter enables the decorator to be used
+            both with and without parentheses: ``@tool`` directly decorates the function,
+            while ``@tool()`` or ``@tool(name="custom")`` returns a decorator. For
+            declaration-only tools (no implementation), use :class:`FunctionTool` directly
+            with ``func=None``—see the example below.
 
     Keyword Args:
         name: The name of the function. If not provided, the function's ``__name__``
@@ -1301,7 +1314,7 @@ def tool(
             When provided, the schema is used instead of inferring one from the
             function's signature. Defaults to ``None`` (infer from signature).
         approval_mode: Whether or not approval is required to run this tool.
-            Default is that approval is required.
+            Default is that approval is NOT required (``"never_require"``).
         max_invocations: The maximum number of times this function can be invoked.
             If None, there is no limit, should be at least 1.
         max_invocation_exceptions: The maximum number of exceptions allowed during invocations.
@@ -1369,6 +1382,20 @@ def tool(
                 '''Get weather for a location.'''
                 return f"Weather in {location}: 22 {unit}"
 
+
+            # Declaration-only tool (no implementation)
+            # Use FunctionTool directly when you need a tool declaration without
+            # an executable function. The agent can request this tool, but it won't
+            # be executed automatically. Useful for testing agent reasoning or when
+            # the implementation is handled externally (e.g., client-side rendering).
+            from agent_framework import FunctionTool
+
+            declaration_only_tool = FunctionTool(
+                name="get_current_time",
+                description="Get the current time in ISO 8601 format.",
+                func=None,  # Explicitly no implementation - makes declaration_only=True
+            )
+
     """
 
     def decorator(func: Callable[..., ReturnT | Awaitable[ReturnT]]) -> FunctionTool[Any, ReturnT]: