fix: include handoffs when deciding to inject JSON

 prompt

The JSON prompt injection was only triggered when tools list was non-empty,
but handoffs are converted to function tools and added separately. This meant
that agents using only handoffs with output_schema would not get the prompt
injection even when enable_structured_output_with_tools=True, causing Gemini
to error with 'Function calling with response mime type application/json is
unsupported.'

Changes:
- Combine tools and handoffs before checking if JSON prompt should be injected
- Add test case for handoffs-only scenario
- Update inline comment to clarify why handoffs must be included

This ensures the opt-in flag works correctly for multi-agent scenarios where
an agent might use handoffs without regular tools.

Author: devv-shayan

src/agents/extensions/models/litellm_model.py

@@ -292,8 +292,12 @@ async def _fetch_response(
 
         # Check if we need to inject JSON output prompt for models that don't support
         # tools + structured output simultaneously (like Gemini)
+        # Note: handoffs are converted to function tools, so we need to include them in the check
+        tools_and_handoffs = list(tools) if tools else []
+        if handoffs:
+            tools_and_handoffs.extend(handoffs)
         inject_json_prompt = should_inject_json_prompt(
-            output_schema, tools, self.enable_structured_output_with_tools
+            output_schema, tools_and_handoffs, self.enable_structured_output_with_tools
         )
         if inject_json_prompt and output_schema:
             json_prompt = get_json_output_prompt(output_schema)

tests/utils/test_prompts.py

@@ -105,3 +105,14 @@ def test_should_inject_json_prompt_multiple_tools():
     ]
     result = should_inject_json_prompt(schema, tools, enable_structured_output_with_tools=True)
     assert result is True
+
+
+def test_should_inject_json_prompt_with_handoffs_as_tools():
+    """Test that handoffs (passed as tools) trigger injection when enabled."""
+    schema = AgentOutputSchema(SimpleModel)
+    # Simulate handoffs being passed in the tools list
+    handoffs_as_tools = [{"type": "function", "name": "handoff_to_agent"}]
+    result = should_inject_json_prompt(
+        schema, handoffs_as_tools, enable_structured_output_with_tools=True
+    )
+    assert result is True