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How to return structured data from a model

Prerequisites

This guide assumes familiarity with the following concepts:

It is often useful to have a model return output that matches a specific schema. One common use-case is extracting data from text to insert into a database or use with some other downstream system. This guide covers a few strategies for getting structured outputs from a model.

The .with_structured_output() method​

Supported models

You can find a list of models that support this method here.

This is the easiest and most reliable way to get structured outputs. with_structured_output() is implemented for models that provide native APIs for structuring outputs, like tool/function calling or JSON mode, and makes use of these capabilities under the hood.

This method takes a schema as input which specifies the names, types, and descriptions of the desired output attributes. The method returns a model-like Runnable, except that instead of outputting strings or Messages it outputs objects corresponding to the given schema. The schema can be specified as a JSON Schema or a Pydantic class. If JSON Schema is used then a dictionary will be returned by the Runnable, and if a Pydantic class is used then Pydantic objects will be returned.

As an example, let's get a model to generate a joke and separate the setup from the punchline:

pip install -qU langchain-openai
import getpass
import os

os.environ["OPENAI_API_KEY"] = getpass.getpass()

from langchain_openai import ChatOpenAI

llm = ChatOpenAI(model="gpt-3.5-turbo-0125")

If we want the model to return a Pydantic object, we just need to pass in the desired Pydantic class:

from typing import Optional

from langchain_core.pydantic_v1 import BaseModel, Field


class Joke(BaseModel):
    """Joke to tell user."""

    setup: str = Field(description="The setup of the joke")
    punchline: str = Field(description="The punchline to the joke")
    rating: Optional[int] = Field(description="How funny the joke is, from 1 to 10")


structured_llm = llm.with_structured_output(Joke)

structured_llm.invoke("Tell me a joke about cats")
Joke(setup='Why was the cat sitting on the computer?', punchline='To keep an eye on the mouse!', rating=None)
tip

Beyond just the structure of the Pydantic class, the name of the Pydantic class, the docstring, and the names and provided descriptions of parameters are very important. Most of the time with_structured_output is using a model's function/tool calling API, and you can effectively think of all of this information as being added to the model prompt.

We can also pass in a JSON Schema dict if you prefer not to use Pydantic. In this case, the response is also a dict:

json_schema = {
    "title": "joke",
    "description": "Joke to tell user.",
    "type": "object",
    "properties": {
        "setup": {
            "type": "string",
            "description": "The setup of the joke",
        },
        "punchline": {
            "type": "string",
            "description": "The punchline to the joke",
        },
        "rating": {
            "type": "integer",
            "description": "How funny the joke is, from 1 to 10",
        },
    },
    "required": ["setup", "punchline"],
}
structured_llm = llm.with_structured_output(json_schema)

structured_llm.invoke("Tell me a joke about cats")
{'setup': 'Why was the cat sitting on the computer?',
 'punchline': 'Because it wanted to keep an eye on the mouse!',
 'rating': 8}

Choosing between multiple schemas​

The simplest way to let the model choose from multiple schemas is to create a parent Pydantic class that has a Union-typed attribute:

from typing import Union


class ConversationalResponse(BaseModel):
    """Respond in a conversational manner. Be kind and helpful."""

    response: str = Field(description="A conversational response to the user's query")


class Response(BaseModel):
    output: Union[Joke, ConversationalResponse]


structured_llm = llm.with_structured_output(Response)

structured_llm.invoke("Tell me a joke about cats")
Response(output=Joke(setup='Why was the cat sitting on the computer?', punchline='To keep an eye on the mouse!', rating=8))
structured_llm.invoke("How are you today?")
Response(output=ConversationalResponse(response="I'm just a digital assistant, so I don't have feelings, but I'm here and ready to help you. How can I assist you today?"))

Alternatively, you can use tool calling directly to allow the model to choose between options, if your chosen model supports it. This involves a bit more parsing and setup but in some instances leads to better performance because you don't have to use nested schemas. See this how-to guide for more details.

Streaming​

We can stream outputs from our structured model when the output type is a dict (i.e., when the schema is specified as a JSON Schema dict).

info

Note that what's yielded is already aggregated chunks, not deltas.

structured_llm = llm.with_structured_output(json_schema)

for chunk in structured_llm.stream("Tell me a joke about cats"):
    print(chunk)
{}
{'setup': ''}
{'setup': 'Why'}
{'setup': 'Why was'}
{'setup': 'Why was the'}
{'setup': 'Why was the cat'}
{'setup': 'Why was the cat sitting'}
{'setup': 'Why was the cat sitting on'}
{'setup': 'Why was the cat sitting on the'}
{'setup': 'Why was the cat sitting on the computer'}
{'setup': 'Why was the cat sitting on the computer?'}
{'setup': 'Why was the cat sitting on the computer?', 'punchline': ''}
{'setup': 'Why was the cat sitting on the computer?', 'punchline': 'Because'}
{'setup': 'Why was the cat sitting on the computer?', 'punchline': 'Because it'}
{'setup': 'Why was the cat sitting on the computer?', 'punchline': 'Because it wanted'}
{'setup': 'Why was the cat sitting on the computer?', 'punchline': 'Because it wanted to'}
{'setup': 'Why was the cat sitting on the computer?', 'punchline': 'Because it wanted to keep'}
{'setup': 'Why was the cat sitting on the computer?', 'punchline': 'Because it wanted to keep an'}
{'setup': 'Why was the cat sitting on the computer?', 'punchline': 'Because it wanted to keep an eye'}
{'setup': 'Why was the cat sitting on the computer?', 'punchline': 'Because it wanted to keep an eye on'}
{'setup': 'Why was the cat sitting on the computer?', 'punchline': 'Because it wanted to keep an eye on the'}
{'setup': 'Why was the cat sitting on the computer?', 'punchline': 'Because it wanted to keep an eye on the mouse'}
{'setup': 'Why was the cat sitting on the computer?', 'punchline': 'Because it wanted to keep an eye on the mouse!'}
{'setup': 'Why was the cat sitting on the computer?', 'punchline': 'Because it wanted to keep an eye on the mouse!', 'rating': 8}

Few-shot prompting​

For more complex schemas it's very useful to add few-shot examples to the prompt. This can be done in a few ways.

The simplest and most universal way is to add examples to a system message in the prompt:

from langchain_core.prompts import ChatPromptTemplate

system = """You are a hilarious comedian. Your specialty is knock-knock jokes. \
Return a joke which has the setup (the response to "Who's there?") and the final punchline (the response to "<setup> who?").

Here are some examples of jokes:

example_user: Tell me a joke about planes
example_assistant: {{"setup": "Why don't planes ever get tired?", "punchline": "Because they have rest wings!", "rating": 2}}

example_user: Tell me another joke about planes
example_assistant: {{"setup": "Cargo", "punchline": "Cargo 'vroom vroom', but planes go 'zoom zoom'!", "rating": 10}}

example_user: Now about caterpillars
example_assistant: {{"setup": "Caterpillar", "punchline": "Caterpillar really slow, but watch me turn into a butterfly and steal the show!", "rating": 5}}"""

prompt = ChatPromptTemplate.from_messages([("system", system), ("human", "{input}")])

few_shot_structured_llm = prompt | structured_llm
few_shot_structured_llm.invoke("what's something funny about woodpeckers")
API Reference:ChatPromptTemplate
{'setup': 'Woodpecker',
 'punchline': "Woodpecker goes 'knock knock', but don't worry, they never expect you to answer the door!",
 'rating': 8}

When the underlying method for structuring outputs is tool calling, we can pass in our examples as explicit tool calls. You can check if the model you're using makes use of tool calling in its API reference.

from langchain_core.messages import AIMessage, HumanMessage, ToolMessage

examples = [
    HumanMessage("Tell me a joke about planes", name="example_user"),
    AIMessage(
        "",
        name="example_assistant",
        tool_calls=[
            {
                "name": "joke",
                "args": {
                    "setup": "Why don't planes ever get tired?",
                    "punchline": "Because they have rest wings!",
                    "rating": 2,
                },
                "id": "1",
            }
        ],
    ),
    # Most tool-calling models expect a ToolMessage(s) to follow an AIMessage with tool calls.
    ToolMessage("", tool_call_id="1"),
    # Some models also expect an AIMessage to follow any ToolMessages,
    # so you may need to add an AIMessage here.
    HumanMessage("Tell me another joke about planes", name="example_user"),
    AIMessage(
        "",
        name="example_assistant",
        tool_calls=[
            {
                "name": "joke",
                "args": {
                    "setup": "Cargo",
                    "punchline": "Cargo 'vroom vroom', but planes go 'zoom zoom'!",
                    "rating": 10,
                },
                "id": "2",
            }
        ],
    ),
    ToolMessage("", tool_call_id="2"),
    HumanMessage("Now about caterpillars", name="example_user"),
    AIMessage(
        "",
        tool_calls=[
            {
                "name": "joke",
                "args": {
                    "setup": "Caterpillar",
                    "punchline": "Caterpillar really slow, but watch me turn into a butterfly and steal the show!",
                    "rating": 5,
                },
                "id": "3",
            }
        ],
    ),
    ToolMessage("", tool_call_id="3"),
]
system = """You are a hilarious comedian. Your specialty is knock-knock jokes. \
Return a joke which has the setup (the response to "Who's there?") \
and the final punchline (the response to "<setup> who?")."""

prompt = ChatPromptTemplate.from_messages(
    [("system", system), ("placeholder", "{examples}"), ("human", "{input}")]
)
few_shot_structured_llm = prompt | structured_llm
few_shot_structured_llm.invoke({"input": "crocodiles", "examples": examples})
{'setup': 'Crocodile',
 'punchline': "Crocodile 'see you later', but in a while, it becomes an alligator!",
 'rating': 7}

For more on few shot prompting when using tool calling, see here.

(Advanced) Specifying the method for structuring outputs​

For models that support more than one means of structuring outputs (i.e., they support both tool calling and JSON mode), you can specify which method to use with the method= argument.

JSON mode

If using JSON mode you'll have to still specify the desired schema in the model prompt. The schema you pass to with_structured_output will only be used for parsing the model outputs, it will not be passed to the model the way it is with tool calling.

To see if the model you're using supports JSON mode, check its entry in the API reference.

structured_llm = llm.with_structured_output(Joke, method="json_mode")

structured_llm.invoke(
    "Tell me a joke about cats, respond in JSON with `setup` and `punchline` keys"
)
Joke(setup='Why was the cat sitting on the computer?', punchline='Because it wanted to keep an eye on the mouse!', rating=None)

Prompting and parsing model directly​

Not all models support .with_structured_output(), since not all models have tool calling or JSON mode support. For such models you'll need to directly prompt the model to use a specific format, and use an output parser to extract the structured response from the raw model output.

Using PydanticOutputParser​

The following example uses the built-in PydanticOutputParser to parse the output of a chat model prompted to match the given Pydantic schema. Note that we are adding format_instructions directly to the prompt from a method on the parser:

from typing import List

from langchain_core.output_parsers import PydanticOutputParser
from langchain_core.prompts import ChatPromptTemplate
from langchain_core.pydantic_v1 import BaseModel, Field


class Person(BaseModel):
    """Information about a person."""

    name: str = Field(..., description="The name of the person")
    height_in_meters: float = Field(
        ..., description="The height of the person expressed in meters."
    )


class People(BaseModel):
    """Identifying information about all people in a text."""

    people: List[Person]


# Set up a parser
parser = PydanticOutputParser(pydantic_object=People)

# Prompt
prompt = ChatPromptTemplate.from_messages(
    [
        (
            "system",
            "Answer the user query. Wrap the output in `json` tags\n{format_instructions}",
        ),
        ("human", "{query}"),
    ]
).partial(format_instructions=parser.get_format_instructions())

Let’s take a look at what information is sent to the model:

query = "Anna is 23 years old and she is 6 feet tall"

print(prompt.invoke(query).to_string())
System: Answer the user query. Wrap the output in `json` tags
The output should be formatted as a JSON instance that conforms to the JSON schema below.

As an example, for the schema {"properties": {"foo": {"title": "Foo", "description": "a list of strings", "type": "array", "items": {"type": "string"}}}, "required": ["foo"]}
the object {"foo": ["bar", "baz"]} is a well-formatted instance of the schema. The object {"properties": {"foo": ["bar", "baz"]}} is not well-formatted.

Here is the output schema:

{"description": "Identifying information about all people in a text.", "properties": {"people": {"title": "People", "type": "array", "items": {"$ref": "#/definitions/Person"}}}, "required": ["people"], "definitions": {"Person": {"title": "Person", "description": "Information about a person.", "type": "object", "properties": {"name": {"title": "Name", "description": "The name of the person", "type": "string"}, "height_in_meters": {"title": "Height In Meters", "description": "The height of the person expressed in meters.", "type": "number"}}, "required": ["name", "height_in_meters"]}}}

Human: Anna is 23 years old and she is 6 feet tall

And now let's invoke it:

chain = prompt | llm | parser

chain.invoke({"query": query})
People(people=[Person(name='Anna', height_in_meters=1.8288)])

For a deeper dive into using output parsers with prompting techniques for structured output, see this guide.

Custom Parsing​

You can also create a custom prompt and parser with LangChain Expression Language (LCEL), using a plain function to parse the output from the model:

import json
import re
from typing import List

from langchain_core.messages import AIMessage
from langchain_core.prompts import ChatPromptTemplate
from langchain_core.pydantic_v1 import BaseModel, Field


class Person(BaseModel):
    """Information about a person."""

    name: str = Field(..., description="The name of the person")
    height_in_meters: float = Field(
        ..., description="The height of the person expressed in meters."
    )


class People(BaseModel):
    """Identifying information about all people in a text."""

    people: List[Person]


# Prompt
prompt = ChatPromptTemplate.from_messages(
    [
        (
            "system",
            "Answer the user query. Output your answer as JSON that  "
            "matches the given schema: ```json\n{schema}\n```. "
            "Make sure to wrap the answer in ```json and ``` tags",
        ),
        ("human", "{query}"),
    ]
).partial(schema=People.schema())


# Custom parser
def extract_json(message: AIMessage) -> List[dict]:
    """Extracts JSON content from a string where JSON is embedded between ```json and ``` tags.

    Parameters:
        text (str): The text containing the JSON content.

    Returns:
        list: A list of extracted JSON strings.
    """
    text = message.content
    # Define the regular expression pattern to match JSON blocks
    pattern = r"```json(.*?)```"

    # Find all non-overlapping matches of the pattern in the string
    matches = re.findall(pattern, text, re.DOTALL)

    # Return the list of matched JSON strings, stripping any leading or trailing whitespace
    try:
        return [json.loads(match.strip()) for match in matches]
    except Exception:
        raise ValueError(f"Failed to parse: {message}")

Here is the prompt sent to the model:

query = "Anna is 23 years old and she is 6 feet tall"

print(prompt.format_prompt(query=query).to_string())
System: Answer the user query. Output your answer as JSON that  matches the given schema: ```json
{'title': 'People', 'description': 'Identifying information about all people in a text.', 'type': 'object', 'properties': {'people': {'title': 'People', 'type': 'array', 'items': {'$ref': '#/definitions/Person'}}}, 'required': ['people'], 'definitions': {'Person': {'title': 'Person', 'description': 'Information about a person.', 'type': 'object', 'properties': {'name': {'title': 'Name', 'description': 'The name of the person', 'type': 'string'}, 'height_in_meters': {'title': 'Height In Meters', 'description': 'The height of the person expressed in meters.', 'type': 'number'}}, 'required': ['name', 'height_in_meters']}}}
```. Make sure to wrap the answer in ```json and ``` tags
Human: Anna is 23 years old and she is 6 feet tall

And here's what it looks like when we invoke it:

chain = prompt | llm | extract_json

chain.invoke({"query": query})
[{'people': [{'name': 'Anna', 'height_in_meters': 1.8288}]}]

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