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Structured output lets you define a Pydantic model and have the LLM return data that conforms exactly to that schema. No more parsing free-text responses.

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Basic Usage

Define a Pydantic model and pass it as response_format: 
from pydantic import BaseModel
from definable.models import OpenAIChat

class MovieRecommendation(BaseModel):
    title: str
    year: int
    genre: str
    reason: str

model = OpenAIChat(id="gpt-4o")
response = model.invoke(
    messages=[{"role": "user", "content": "Recommend a great sci-fi movie."}],
    response_format=MovieRecommendation,
)

movie = response.parsed  # MovieRecommendation instance
print(f"{movie.title} ({movie.year}) - {movie.genre}")
print(f"Why: {movie.reason}")
The response.parsed field contains a fully validated Pydantic model instance. The raw JSON is still available in response.content.

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Complex Schemas

Structured output handles nested models, lists, enums, and optional fields: 
from typing import List, Optional
from enum import Enum
from pydantic import BaseModel, Field

class Priority(str, Enum):
    low = "low"
    medium = "medium"
    high = "high"

class Task(BaseModel):
    title: str = Field(description="Short task title")
    description: str = Field(description="Detailed description")
    priority: Priority
    estimated_hours: float
    subtasks: Optional[List[str]] = None

class ProjectPlan(BaseModel):
    project_name: str
    tasks: List[Task]
    total_estimated_hours: float

response = model.invoke(
    messages=[{"role": "user", "content": "Create a plan for building a REST API."}],
    response_format=ProjectPlan,
)

plan = response.parsed
for task in plan.tasks:
    print(f"[{task.priority.value}] {task.title} ({task.estimated_hours}h)")

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Provider Support

ProviderNative JSON SchemaPrompt-Based Fallback
OpenAIYes—
DeepSeekNoYes
MoonshotNoYes
xAINoYes
When a provider does not support native structured outputs, Definable automatically includes the JSON Schema in the system prompt and instructs the model to respond in the correct format.

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Strict Mode

OpenAI’s strict mode ensures the output adheres exactly to the schema. It is enabled by default: 
model = OpenAIChat(id="gpt-4o", strict_output=True)  # default
Set strict_output=False if you encounter schema compatibility issues with strict mode.

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Using with Agents

Pass output_schema to an agent run to get structured output from the final response: 
from definable.agents import Agent

agent = Agent(model=model, tools=[search_web])

output = agent.run(
    "Find the top 3 Python web frameworks and compare them.",
    output_schema=FrameworkComparison,
)
print(output.content)  # Parsed FrameworkComparison

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Async

response = await model.ainvoke(
    messages=[{"role": "user", "content": "Recommend a sci-fi movie."}],
    response_format=MovieRecommendation,
)
movie = response.parsed