feat: Add strict_mode option to function_schema and function_tool (#60)

This PR introduces a `strict_mode: bool = True` option to
`@function_tool`, allowing optional parameters when set to False. This
change enables more flexibility while maintaining strict JSON schema
validation by default.

resolves #43 

## Changes:

- Added `strict_mode` parameter to `@function_tool` and passed it to
`function_schema` and `FunctionTool`.
- Updated `function_schema.py` to respect `strict_mode` and allow
optional parameters when set to False.
- Added unit tests to verify optional parameters work correctly,
including multiple optional params with different types.

## Tests:

- Verified function calls with missing optional parameters behave as
expected.
- Added async tests to validate behavior under different configurations.

src/agents/function_schema.py

@@ -33,6 +33,9 @@ class FuncSchema:
     """The signature of the function."""
     takes_context: bool = False
     """Whether the function takes a RunContextWrapper argument (must be the first argument)."""
+    strict_json_schema: bool = True
+    """Whether the JSON schema is in strict mode. We **strongly** recommend setting this to True,
+    as it increases the likelihood of correct JSON input."""
 
     def to_call_args(self, data: BaseModel) -> tuple[list[Any], dict[str, Any]]:
         """
@@ -337,4 +340,5 @@ def function_schema(
         params_json_schema=json_schema,
         signature=sig,
         takes_context=takes_context,
+        strict_json_schema=strict_json_schema,
     )

src/agents/tool.py

@@ -137,6 +137,7 @@ def function_tool(
     docstring_style: DocstringStyle | None = None,
     use_docstring_info: bool = True,
     failure_error_function: ToolErrorFunction | None = None,
+    strict_mode: bool = True,
 ) -> FunctionTool:
     """Overload for usage as @function_tool (no parentheses)."""
     ...
@@ -150,6 +151,7 @@ def function_tool(
     docstring_style: DocstringStyle | None = None,
     use_docstring_info: bool = True,
     failure_error_function: ToolErrorFunction | None = None,
+    strict_mode: bool = True,
 ) -> Callable[[ToolFunction[...]], FunctionTool]:
     """Overload for usage as @function_tool(...)."""
     ...
@@ -163,6 +165,7 @@ def function_tool(
     docstring_style: DocstringStyle | None = None,
     use_docstring_info: bool = True,
     failure_error_function: ToolErrorFunction | None = default_tool_error_function,
+    strict_mode: bool = True,
 ) -> FunctionTool | Callable[[ToolFunction[...]], FunctionTool]:
     """
     Decorator to create a FunctionTool from a function. By default, we will:
@@ -186,6 +189,8 @@ def function_tool(
         failure_error_function: If provided, use this function to generate an error message when
             the tool call fails. The error message is sent to the LLM. If you pass None, then no
             error message will be sent and instead an Exception will be raised.
+        strict_mode: If False, parameters with default values become optional in the
+            function schema.
     """
 
     def _create_function_tool(the_func: ToolFunction[...]) -> FunctionTool:
@@ -195,6 +200,7 @@ def function_tool(
             description_override=description_override,
             docstring_style=docstring_style,
             use_docstring_info=use_docstring_info,
+            strict_json_schema=strict_mode,
         )
 
         async def _on_invoke_tool_impl(ctx: RunContextWrapper[Any], input: str) -> str:
@@ -273,6 +279,7 @@ def function_tool(
             description=schema.description or "",
             params_json_schema=schema.params_json_schema,
             on_invoke_tool=_on_invoke_tool,
+            strict_json_schema=strict_mode,
         )
 
     # If func is actually a callable, we were used as @function_tool with no parentheses

tests/test_function_tool_decorator.py

@@ -1,6 +1,6 @@
 import asyncio
 import json
-from typing import Any
+from typing import Any, Optional
 
 import pytest
 
@@ -142,3 +142,52 @@ async def test_no_error_on_invalid_json_async():
     tool = will_not_fail_on_bad_json_async
     result = await tool.on_invoke_tool(ctx_wrapper(), "{not valid json}")
     assert result == "error_ModelBehaviorError"
+
+
+@function_tool(strict_mode=False)
+def optional_param_function(a: int, b: Optional[int] = None) -> str:
+    if b is None:
+        return f"{a}_no_b"
+    return f"{a}_{b}"
+
+
+@pytest.mark.asyncio
+async def test_optional_param_function():
+    tool = optional_param_function
+
+    input_data = {"a": 5}
+    output = await tool.on_invoke_tool(ctx_wrapper(), json.dumps(input_data))
+    assert output == "5_no_b"
+
+    input_data = {"a": 5, "b": 10}
+    output = await tool.on_invoke_tool(ctx_wrapper(), json.dumps(input_data))
+    assert output == "5_10"
+
+
+@function_tool(strict_mode=False)
+def multiple_optional_params_function(
+    x: int = 42,
+    y: str = "hello",
+    z: Optional[int] = None,
+) -> str:
+    if z is None:
+        return f"{x}_{y}_no_z"
+    return f"{x}_{y}_{z}"
+
+
+
+@pytest.mark.asyncio
+async def test_multiple_optional_params_function():
+    tool = multiple_optional_params_function
+
+    input_data: dict[str,Any] = {}
+    output = await tool.on_invoke_tool(ctx_wrapper(), json.dumps(input_data))
+    assert output == "42_hello_no_z"
+
+    input_data = {"x": 10, "y": "world"}
+    output = await tool.on_invoke_tool(ctx_wrapper(), json.dumps(input_data))
+    assert output == "10_world_no_z"
+
+    input_data = {"x": 10, "y": "world", "z": 99}
+    output = await tool.on_invoke_tool(ctx_wrapper(), json.dumps(input_data))
+    assert output == "10_world_99"