mid_run_fallback.py
"""
Fallback Models — Mid-Run Failure
====================================
Tests what happens when the primary model fails AFTER a tool call (and or within a run).
Flow:
1. gpt-4o receives the request and makes a tool call
2. The tool mutates the model instance's id to something invalid
3. The next API call inside Model.response()'s while-loop fails
4. The error bubbles up to call_model_with_fallback
5. Fallback (Claude) is tried
"""
from agno.agent import Agent
from agno.models.anthropic import Claude
from agno.models.openai import OpenAIChat
def break_model(agent: Agent) -> str:
"""Tool that corrupts the executing model mid-run."""
# Point the model at an unreachable server and clear the cached client
# so the next API call in the response() loop fails with a connection error.
agent.model.base_url = "http://localhost:1/v1" # type: ignore[union-attr]
agent.model.client = None # type: ignore[union-attr]
return "Tool executed successfully."
agent = Agent(
model=OpenAIChat(id="gpt-4o"),
tools=[break_model],
fallback_models=[Claude(id="claude-sonnet-4-20250514")],
)
if __name__ == "__main__":
agent.print_response("Call the break_model tool", stream=True)
Run the Example
1
Set up your virtual environment
uv venv --python 3.12
source .venv/bin/activate
uv venv --python 3.12
.venv\Scripts\activate
2
Install dependencies
uv pip install -U agno anthropic openai
3
Export your API keys
export ANTHROPIC_API_KEY="your_anthropic_api_key_here"
export OPENAI_API_KEY="your_openai_api_key_here"
$Env:ANTHROPIC_API_KEY="your_anthropic_api_key_here"
$Env:OPENAI_API_KEY="your_openai_api_key_here"
4
Run the example
Save the code above as
mid_run_fallback.py, then run:python mid_run_fallback.py