How to fix: model won’t call my tool / function calling not triggering
Last verified: June 2026· Tool use and agents
Where this shows up
Tool use and agents
The fix
- 1Rewrite the tool description to be prescriptive about *when* to call it, not just what it does (e.g. “Call this when the user asks about current prices or recent events”).
- 2Raise the effort/reasoning setting if your model supports it — higher effort generally increases appropriate tool use.
- 3Add explicit guidance in the system prompt for when the tool is required.
- 4Verify the tool schema is valid and the tool is actually included in the request.
- 5Avoid over-aggressive language (“CRITICAL: you MUST…”), which can cause modern models to over- or mis-trigger.
Prevent it
Treat tool descriptions as part of the prompt and validate tool-use behavior with evals across representative inputs.
Common variations and related errors
You'll usually hit this same root cause under a few different names. Same fix.
- "tool not called"
- "function calling failed"
- "agent stopped before tool"
- "no tool_use in response"
Frequently asked questions
What causes “model won’t call my tool / function calling not triggering”?
The tool description is vague, the model is being conservative, or the prompt doesn’t make clear when the tool should be used.
How do I prevent “model won’t call my tool / function calling not triggering” from recurring?
Treat tool descriptions as part of the prompt and validate tool-use behavior with evals across representative inputs.