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When an Agent is created without a model, Agno sets OpenAIResponses(id="gpt-5.4") as the default. You’ll see this log line on the first run: 
INFO Setting default model to OpenAI Responses
The default requires the openai package and the OPENAI_API_KEY environment variable. If the package is missing you’ll get an ImportError, and if the key is missing the run fails.

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Set a Model Explicitly

Pass any model class to the model parameter: 
from agno.agent import Agent
from agno.models.anthropic import Claude

agent = Agent(model=Claude(id="claude-sonnet-4-5"))
agent.print_response("What is the capital of France?")
Set the model explicitly in production. The default exists for quick experiments, and relying on it means your agent’s behavior changes when Agno updates the default ID.

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Next Steps