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Test PydanticAI agents using TestModel, FunctionModel, VCR cassettes, and inline snapshots. Use when writing unit tests, mocking LLM responses, or recording API interactions.
npx skill4agent add existential-birds/beagle pydantic-ai-testingTestModelimport pytest
from pydantic_ai import Agent
from pydantic_ai.models.test import TestModel
def test_agent_basic():
agent = Agent('openai:gpt-4o')
# Override with TestModel for testing
result = agent.run_sync('Hello', model=TestModel())
# TestModel generates deterministic output based on output_type
assert isinstance(result.output, str)from pydantic_ai.models.test import TestModel
# Custom text output
model = TestModel(custom_output_text='Custom response')
result = agent.run_sync('Hello', model=model)
assert result.output == 'Custom response'
# Custom structured output (for output_type agents)
from pydantic import BaseModel
class Response(BaseModel):
message: str
score: int
agent = Agent('openai:gpt-4o', output_type=Response)
model = TestModel(custom_output_args={'message': 'Test', 'score': 42})
result = agent.run_sync('Hello', model=model)
assert result.output.message == 'Test'
# Seed for reproducible random output
model = TestModel(seed=42)
# Force tool calls
model = TestModel(call_tools=['my_tool', 'another_tool'])from pydantic_ai import Agent
from pydantic_ai.models.test import TestModel
agent = Agent('openai:gpt-4o', deps_type=MyDeps)
def test_with_override():
mock_deps = MyDeps(db=MockDB())
with agent.override(model=TestModel(), deps=mock_deps):
# All runs use TestModel and mock_deps
result = agent.run_sync('Hello')
assert result.outputfrom pydantic_ai import Agent, ModelMessage, ModelResponse, TextPart
from pydantic_ai.models.function import AgentInfo, FunctionModel
def custom_model(
messages: list[ModelMessage],
info: AgentInfo
) -> ModelResponse:
"""Custom model that inspects messages and returns response."""
# Access the last user message
last_msg = messages[-1]
# Return custom response
return ModelResponse(parts=[TextPart('Custom response')])
agent = Agent(FunctionModel(custom_model))
result = agent.run_sync('Hello')from pydantic_ai import ToolCallPart, ModelResponse
from pydantic_ai.models.function import AgentInfo, FunctionModel
def model_with_tools(
messages: list[ModelMessage],
info: AgentInfo
) -> ModelResponse:
# First request: call a tool
if len(messages) == 1:
return ModelResponse(parts=[
ToolCallPart(
tool_name='get_data',
args='{"id": 123}'
)
])
# After tool response: return final result
return ModelResponse(parts=[TextPart('Done with tool result')])
agent = Agent(FunctionModel(model_with_tools))
@agent.tool_plain
def get_data(id: int) -> str:
return f"Data for {id}"
result = agent.run_sync('Get data')import pytest
@pytest.mark.vcr
def test_with_recorded_response():
"""Uses recorded cassette from tests/cassettes/"""
agent = Agent('openai:gpt-4o')
result = agent.run_sync('Hello')
assert 'hello' in result.output.lower()
# To record/update cassettes:
# uv run pytest --record-mode=rewrite tests/test_file.pytests/cassettes/from inline_snapshot import snapshot
def test_agent_output():
result = agent.run_sync('Hello', model=TestModel())
# First run: creates snapshot
# Subsequent runs: asserts against it
assert result.output == snapshot('expected output here')
# Update snapshots:
# uv run pytest --inline-snapshot=fixfrom pydantic_ai import Agent, RunContext
from pydantic_ai.models.test import TestModel
def test_tool_is_called():
agent = Agent('openai:gpt-4o')
tool_called = False
@agent.tool_plain
def my_tool(x: int) -> str:
nonlocal tool_called
tool_called = True
return f"Result: {x}"
# Force TestModel to call the tool
result = agent.run_sync(
'Use my_tool',
model=TestModel(call_tools=['my_tool'])
)
assert tool_calledfrom dataclasses import dataclass
from unittest.mock import AsyncMock
@dataclass
class Deps:
api: ApiClient
def test_tool_with_deps():
# Create mock dependency
mock_api = AsyncMock()
mock_api.fetch.return_value = {'data': 'test'}
agent = Agent('openai:gpt-4o', deps_type=Deps)
@agent.tool
async def fetch_data(ctx: RunContext[Deps]) -> dict:
return await ctx.deps.api.fetch()
with agent.override(
model=TestModel(call_tools=['fetch_data']),
deps=Deps(api=mock_api)
):
result = agent.run_sync('Fetch data')
mock_api.fetch.assert_called_once()from pydantic_ai import Agent, capture_run_messages
agent = Agent('openai:gpt-4o')
with capture_run_messages() as messages:
result = agent.run_sync('Hello', model=TestModel())
# Inspect captured messages
for msg in messages:
print(msg)| Scenario | Approach |
|---|---|
| Unit tests without API | |
| Custom model logic | |
| Recorded real responses | |
| Assert output structure | |
| Test tools are called | |
| Mock dependencies | |
pyproject.toml[tool.pytest.ini_options]
testpaths = ["tests"]
asyncio_mode = "auto" # For async testsuv run pytest tests/test_agent.py -v
uv run pytest --inline-snapshot=fix # Update snapshots