Core components

Structured output

Structured output allows agents to return data in a specific, predictable format. Instead of parsing natural language responses, you get structured data in the form of JSON objects, Pydantic models, or dataclasses that your application can directly use.

LangChain's create_agent handles structured output automatically. The user sets their desired structured output schema, and when the model generates the structured data, it's captured, validated, and returned in the 'structured_response' key of the agent's state.

python

def create_agent(
    ...
    response_format: Union[
        ToolStrategy[StructuredResponseT],
        ProviderStrategy[StructuredResponseT],
        type[StructuredResponseT],
    ]

Response Format

Controls how the agent returns structured data:

ToolStrategy[StructuredResponseT]: Uses tool calling for structured output
ProviderStrategy[StructuredResponseT]: Uses provider-native structured output
type[StructuredResponseT]: Schema type - automatically selects best strategy based on model capabilities
None: No structured output

When a schema type is provided directly, LangChain automatically chooses:

ProviderStrategy for models supporting native structured output (e.g. OpenAI, Grok)
ToolStrategy for all other models

The structured response is returned in the structured_response key of the agent's final state.

Provider strategy

Some model providers support structured output natively through their APIs (currently only OpenAI and Grok). This is the most reliable method when available.

To use this strategy, configure a ProviderStrategy:

python

class ProviderStrategy(Generic[SchemaT]):
    schema: type[SchemaT]

schema required

The schema defining the structured output format. Supports:

Pydantic models: BaseModel subclasses with field validation
Dataclasses: Python dataclasses with type annotations
TypedDict: Typed dictionary classes
JSON Schema: Dictionary with JSON schema specification

LangChain automatically uses ProviderStrategy when you pass a schema type directly to create_agent.response_format and the model supports native structured output:

Pydantic ModelDataclassTypedDictJSON Schema

python

from pydantic import BaseModel
from langchain.agents import create_agent


class ContactInfo(BaseModel):
    """Contact information for a person."""
    name: str = Field(description="The name of the person")
    email: str = Field(description="The email address of the person")
    phone: str = Field(description="The phone number of the person")

agent = create_agent(
    model="gpt-5",
    tools=tools,
    response_format=ContactInfo  # Auto-selects ProviderStrategy
)

result = agent.invoke({
    "messages": [{"role": "user", "content": "Extract contact info from: John Doe, john@example.com, (555) 123-4567"}]
})

result["structured_response"]
# ContactInfo(name='John Doe', email='john@example.com', phone='(555) 123-4567')

python

from dataclasses import dataclass
from langchain.agents import create_agent


@dataclass
class ContactInfo:
    """Contact information for a person."""
    name: str # The name of the person
    email: str # The email address of the person
    phone: str # The phone number of the person

agent = create_agent(
    model="gpt-5",
    tools=tools,
    response_format=ContactInfo  # Auto-selects ProviderStrategy
)

result = agent.invoke({
    "messages": [{"role": "user", "content": "Extract contact info from: John Doe, john@example.com, (555) 123-4567"}]
})

result["structured_response"]
# ContactInfo(name='John Doe', email='john@example.com', phone='(555) 123-4567')

python

from typing_extensions import TypedDict
from langchain.agents import create_agent


class ContactInfo(TypedDict):
    """Contact information for a person."""
    name: str # The name of the person
    email: str # The email address of the person
    phone: str # The phone number of the person

agent = create_agent(
    model="gpt-5",
    tools=tools,
    response_format=ContactInfo  # Auto-selects ProviderStrategy
)

result = agent.invoke({
    "messages": [{"role": "user", "content": "Extract contact info from: John Doe, john@example.com, (555) 123-4567"}]
})

result["structured_response"]
# {'name': 'John Doe', 'email': 'john@example.com', 'phone': '(555) 123-4567'}

python

from langchain.agents import create_agent


contact_info_schema = {
    "type": "object",
    "description": "Contact information for a person.",
    "properties": {
        "name": {"type": "string", "description": "The name of the person"},
        "email": {"type": "string", "description": "The email address of the person"},
        "phone": {"type": "string", "description": "The phone number of the person"}
    },
    "required": ["name", "email", "phone"]
}

agent = create_agent(
    model="gpt-5",
    tools=tools,
    response_format=contact_info_schema  # Auto-selects ProviderStrategy
)

result = agent.invoke({
    "messages": [{"role": "user", "content": "Extract contact info from: John Doe, john@example.com, (555) 123-4567"}]
})

result["structured_response"]
# {'name': 'John Doe', 'email': 'john@example.com', 'phone': '(555) 123-4567'}

Provider-native structured output provides high reliability and strict validation because the model provider enforces the schema. Use it when available.

If the provider natively supports structured output for your model choice, it is functionally equivalent to write response_format=ProductReview instead of response_format=ToolStrategy(ProductReview). In either case, if structured output is not supported, the agent will fall back to a tool calling strategy.

Tool calling strategy

For models that don't support native structured output, LangChain uses tool calling to achieve the same result. This works with all models that support tool calling, which is most modern models.

To use this strategy, configure a ToolStrategy:

python

class ToolStrategy(Generic[SchemaT]):
    schema: type[SchemaT]
    tool_message_content: str | None
    handle_errors: Union[
        bool,
        str,
        type[Exception],
        tuple[type[Exception], ...],
        Callable[[Exception], str],
    ]

schema required The schema defining the structured output format. Supports:

Pydantic models: BaseModel subclasses with field validation
Dataclasses: Python dataclasses with type annotations
TypedDict: Typed dictionary classes
JSON Schema: Dictionary with JSON schema specification
Union types: Multiple schema options. The model will choose the most appropriate schema based on the context.

tool_message_content

Custom content for the tool message returned when structured output is generated. If not provided, defaults to a message showing the structured response data.

handle_errors

Error handling strategy for structured output validation failures. Defaults to True.

True: Catch all errors with default error template
str: Catch all errors with this custom message
type[Exception]: Only catch this exception type with default message
tuple[type[Exception], ...]: Only catch these exception types with default message
Callable[[Exception], str]: Custom function that returns error message
False: No retry, let exceptions propagate

python

from pydantic import BaseModel, Field
from typing import Literal
from langchain.agents import create_agent
from langchain.agents.structured_output import ToolStrategy


class ProductReview(BaseModel):
    """Analysis of a product review."""
    rating: int | None = Field(description="The rating of the product", ge=1, le=5)
    sentiment: Literal["positive", "negative"] = Field(description="The sentiment of the review")
    key_points: list[str] = Field(description="The key points of the review. Lowercase, 1-3 words each.")

agent = create_agent(
    model="gpt-5",
    tools=tools,
    response_format=ToolStrategy(ProductReview)
)

result = agent.invoke({
    "messages": [{"role": "user", "content": "Analyze this review: 'Great product: 5 out of 5 stars. Fast shipping, but expensive'"}]
})
result["structured_response"]
# ProductReview(rating=5, sentiment='positive', key_points=['fast shipping', 'expensive'])

python

from dataclasses import dataclass
from typing import Literal
from langchain.agents import create_agent
from langchain.agents.structured_output import ToolStrategy


@dataclass
class ProductReview:
    """Analysis of a product review."""
    rating: int | None  # The rating of the product (1-5)
    sentiment: Literal["positive", "negative"]  # The sentiment of the review
    key_points: list[str]  # The key points of the review

agent = create_agent(
    model="gpt-5",
    tools=tools,
    response_format=ToolStrategy(ProductReview)
)

result = agent.invoke({
    "messages": [{"role": "user", "content": "Analyze this review: 'Great product: 5 out of 5 stars. Fast shipping, but expensive'"}]
})
result["structured_response"]
# ProductReview(rating=5, sentiment='positive', key_points=['fast shipping', 'expensive'])

python

from typing import Literal
from typing_extensions import TypedDict
from langchain.agents import create_agent
from langchain.agents.structured_output import ToolStrategy


class ProductReview(TypedDict):
    """Analysis of a product review."""
    rating: int | None  # The rating of the product (1-5)
    sentiment: Literal["positive", "negative"]  # The sentiment of the review
    key_points: list[str]  # The key points of the review

agent = create_agent(
    model="gpt-5",
    tools=tools,
    response_format=ToolStrategy(ProductReview)
)

result = agent.invoke({
    "messages": [{"role": "user", "content": "Analyze this review: 'Great product: 5 out of 5 stars. Fast shipping, but expensive'"}]
})
result["structured_response"]
# {'rating': 5, 'sentiment': 'positive', 'key_points': ['fast shipping', 'expensive']}

python

from langchain.agents import create_agent
from langchain.agents.structured_output import ToolStrategy


product_review_schema = {
    "type": "object",
    "description": "Analysis of a product review.",
    "properties": {
        "rating": {
            "type": ["integer", "null"],
            "description": "The rating of the product (1-5)",
            "minimum": 1,
            "maximum": 5
        },
        "sentiment": {
            "type": "string",
            "enum": ["positive", "negative"],
            "description": "The sentiment of the review"
        },
        "key_points": {
            "type": "array",
            "items": {"type": "string"},
            "description": "The key points of the review"
        }
    },
    "required": ["sentiment", "key_points"]
}

agent = create_agent(
    model="gpt-5",
    tools=tools,
    response_format=ToolStrategy(product_review_schema)
)

result = agent.invoke({
    "messages": [{"role": "user", "content": "Analyze this review: 'Great product: 5 out of 5 stars. Fast shipping, but expensive'"}]
})
result["structured_response"]
# {'rating': 5, 'sentiment': 'positive', 'key_points': ['fast shipping', 'expensive']}

python

from pydantic import BaseModel, Field
from typing import Literal, Union
from langchain.agents import create_agent
from langchain.agents.structured_output import ToolStrategy


class ProductReview(BaseModel):
    """Analysis of a product review."""
    rating: int | None = Field(description="The rating of the product", ge=1, le=5)
    sentiment: Literal["positive", "negative"] = Field(description="The sentiment of the review")
    key_points: list[str] = Field(description="The key points of the review. Lowercase, 1-3 words each.")

class CustomerComplaint(BaseModel):
    """A customer complaint about a product or service."""
    issue_type: Literal["product", "service", "shipping", "billing"] = Field(description="The type of issue")
    severity: Literal["low", "medium", "high"] = Field(description="The severity of the complaint")
    description: str = Field(description="Brief description of the complaint")

agent = create_agent(
    model="gpt-5",
    tools=tools,
    response_format=ToolStrategy(Union[ProductReview, CustomerComplaint])
)

result = agent.invoke({
    "messages": [{"role": "user", "content": "Analyze this review: 'Great product: 5 out of 5 stars. Fast shipping, but expensive'"}]
})
result["structured_response"]
# ProductReview(rating=5, sentiment='positive', key_points=['fast shipping', 'expensive'])

Custom tool message content

The tool_message_content parameter allows you to customize the message that appears in the conversation history when structured output is generated:

python

from pydantic import BaseModel, Field
from typing import Literal
from langchain.agents import create_agent
from langchain.agents.structured_output import ToolStrategy


class MeetingAction(BaseModel):
    """Action items extracted from a meeting transcript."""
    task: str = Field(description="The specific task to be completed")
    assignee: str = Field(description="Person responsible for the task")
    priority: Literal["low", "medium", "high"] = Field(description="Priority level")

agent = create_agent(
    model="gpt-5",
    tools=[],
    response_format=ToolStrategy(
        schema=MeetingAction,
        tool_message_content="Action item captured and added to meeting notes!"
    )
)

agent.invoke({
    "messages": [{"role": "user", "content": "From our meeting: Sarah needs to update the project timeline as soon as possible"}]
})

================================ Human Message =================================

From our meeting: Sarah needs to update the project timeline as soon as possible
================================== Ai Message ==================================
Tool Calls:
  MeetingAction (call_1)
 Call ID: call_1
  Args:
    task: Update the project timeline
    assignee: Sarah
    priority: high
================================= Tool Message =================================
Name: MeetingAction

Action item captured and added to meeting notes!

Without tool_message_content, our final ToolMessage would be:

================================= Tool Message =================================
Name: MeetingAction

Returning structured response: {'task': 'update the project timeline', 'assignee': 'Sarah', 'priority': 'high'}

Error handling

Models can make mistakes when generating structured output via tool calling. LangChain provides intelligent retry mechanisms to handle these errors automatically.

Multiple structured outputs error

When a model incorrectly calls multiple structured output tools, the agent provides error feedback in a ToolMessage and prompts the model to retry:

python

from pydantic import BaseModel, Field
from typing import Union
from langchain.agents import create_agent
from langchain.agents.structured_output import ToolStrategy


class ContactInfo(BaseModel):
    name: str = Field(description="Person's name")
    email: str = Field(description="Email address")

class EventDetails(BaseModel):
    event_name: str = Field(description="Name of the event")
    date: str = Field(description="Event date")

agent = create_agent(
    model="gpt-5",
    tools=[],
    response_format=ToolStrategy(Union[ContactInfo, EventDetails])  # Default: handle_errors=True
)

agent.invoke({
    "messages": [{"role": "user", "content": "Extract info: John Doe (john@email.com) is organizing Tech Conference on March 15th"}]
})

================================ Human Message =================================

Extract info: John Doe (john@email.com) is organizing Tech Conference on March 15th
None
================================== Ai Message ==================================
Tool Calls:
  ContactInfo (call_1)
 Call ID: call_1
  Args:
    name: John Doe
    email: john@email.com
  EventDetails (call_2)
 Call ID: call_2
  Args:
    event_name: Tech Conference
    date: March 15th
================================= Tool Message =================================
Name: ContactInfo

Error: Model incorrectly returned multiple structured responses (ContactInfo, EventDetails) when only one is expected.
 Please fix your mistakes.
================================= Tool Message =================================
Name: EventDetails

Error: Model incorrectly returned multiple structured responses (ContactInfo, EventDetails) when only one is expected.
 Please fix your mistakes.
================================== Ai Message ==================================
Tool Calls:
  ContactInfo (call_3)
 Call ID: call_3
  Args:
    name: John Doe
    email: john@email.com
================================= Tool Message =================================
Name: ContactInfo

Returning structured response: {'name': 'John Doe', 'email': 'john@email.com'}

Schema validation error

When structured output doesn't match the expected schema, the agent provides specific error feedback:

python

from pydantic import BaseModel, Field
from langchain.agents import create_agent
from langchain.agents.structured_output import ToolStrategy


class ProductRating(BaseModel):
    rating: int | None = Field(description="Rating from 1-5", ge=1, le=5)
    comment: str = Field(description="Review comment")

agent = create_agent(
    model="gpt-5",
    tools=[],
    response_format=ToolStrategy(ProductRating),  # Default: handle_errors=True
    system_prompt="You are a helpful assistant that parses product reviews. Do not make any field or value up."
)

agent.invoke({
    "messages": [{"role": "user", "content": "Parse this: Amazing product, 10/10!"}]
})

================================ Human Message =================================

Parse this: Amazing product, 10/10!
================================== Ai Message ==================================
Tool Calls:
  ProductRating (call_1)
 Call ID: call_1
  Args:
    rating: 10
    comment: Amazing product
================================= Tool Message =================================
Name: ProductRating

Error: Failed to parse structured output for tool 'ProductRating': 1 validation error for ProductRating.rating
  Input should be less than or equal to 5 [type=less_than_equal, input_value=10, input_type=int].
 Please fix your mistakes.
================================== Ai Message ==================================
Tool Calls:
  ProductRating (call_2)
 Call ID: call_2
  Args:
    rating: 5
    comment: Amazing product
================================= Tool Message =================================
Name: ProductRating

Returning structured response: {'rating': 5, 'comment': 'Amazing product'}

Error handling strategies

You can customize how errors are handled using the handle_errors parameter:

Custom error message:

python

ToolStrategy(
    schema=ProductRating,
    handle_errors="Please provide a valid rating between 1-5 and include a comment."
)

If handle_errors is a string, the agent will always prompt the model to re-try with a fixed tool message:

================================= Tool Message =================================
Name: ProductRating

Please provide a valid rating between 1-5 and include a comment.

Handle specific exceptions only:

python

ToolStrategy(
    schema=ProductRating,
    handle_errors=ValueError  # Only retry on ValueError, raise others
)

If handle_errors is an exception type, the agent will only retry (using the default error message) if the exception raised is the specified type. In all other cases, the exception will be raised.

Handle multiple exception types:

python

ToolStrategy(
    schema=ProductRating,
    handle_errors=(ValueError, TypeError)  # Retry on ValueError and TypeError
)

If handle_errors is a tuple of exceptions, the agent will only retry (using the default error message) if the exception raised is one of the specified types. In all other cases, the exception will be raised.

Custom error handler function:

python

def custom_error_handler(error: Exception) -> str:
    if isinstance(error, StructuredOutputValidationError):
        return "There was an issue with the format. Try again.
    elif isinstance(error, MultipleStructuredOutputsError):
        return "Multiple structured outputs were returned. Pick the most relevant one."
    else:
        return f"Error: {str(error)}"

ToolStrategy(
    schema=ToolStrategy(Union[ContactInfo, EventDetails]),
    handle_errors=custom_error_handler
)

On StructuredOutputValidationError:

================================= Tool Message =================================
Name: ToolStrategy

There was an issue with the format. Try again.

On MultipleStructuredOutputsError:

================================= Tool Message =================================
Name: ToolStrategy

Multiple structured outputs were returned. Pick the most relevant one.

On other errors:

================================= Tool Message =================================
Name: ToolStrategy

Error: <error message>

No error handling:

python

response_format = ToolStrategy(
    schema=ProductRating,
    handle_errors=False  # All errors raised
)

Structured output ​

Response Format ​

Provider strategy ​

Tool calling strategy ​

Custom tool message content ​

Error handling ​

Multiple structured outputs error ​

Schema validation error ​

Error handling strategies ​

Structured output

Response Format

Provider strategy

Tool calling strategy

Custom tool message content

Error handling

Multiple structured outputs error

Schema validation error

Error handling strategies