Forcing tool use

forcing-tool-use

Set model_settings=ModelSettings(tool_choice="required") to force the model to call at least one tool. Use tool_choice={"type": "function", "name": "my_tool"} to force a specific tool.

When to apply

When you need the agent to always use a tool on its first turn (e.g., always search before answering, always validate input). Prevents the model from skipping the tool and answering directly.

Core snippet

from agents import Agent, ModelSettings, function_tool

@function_tool
def search_knowledge_base(query: str) -> str:
    """Search the knowledge base for relevant information."""
    return f"Results for: {query}"

# Force any tool call
agent = Agent(
    name="Research assistant",
    instructions="Always search before answering.",
    tools=[search_knowledge_base],
    model_settings=ModelSettings(tool_choice="required"),
    reset_tool_choice=True,  # Default: reset after first tool call to avoid loops
)

# Force a specific tool
agent_specific = Agent(
    name="Search assistant",
    tools=[search_knowledge_base],
    model_settings=ModelSettings(
        tool_choice={"type": "function", "name": "search_knowledge_base"}
    ),
    reset_tool_choice=True,
)

Key notes

• tool_choice="required": model must call at least one tool
• tool_choice="auto": model decides whether to call a tool (default)
• tool_choice="none": model cannot call any tool
• reset_tool_choice=True (default): resets tool_choice to "auto" after the first tool call to avoid infinite tool loops
• Set reset_tool_choice=False if you want every turn to require a tool call