Output parsers

note

The information here refers to parsers that take a text output from a model try to parse it into a more structured representation. More and more models are supporting function (or tool) calling, which handles this automatically. It is recommended to use function/tool calling rather than output parsing. See documentation for that here.

Output parsers are responsible for taking the output of a model and transforming it to a more suitable format for downstream tasks. Useful when you are using LLMs to generate structured data, or to normalize output from chat models and LLMs.

LangChain has lots of different types of output parsers. This is a list of output parsers LangChain supports. The table below has various pieces of information:

• Name: The name of the output parser
• Supports Streaming: Whether the output parser supports streaming.
• Has Format Instructions: Whether the output parser has format instructions. This is generally available except when (a) the desired schema is not specified in the prompt but rather in other parameters (like OpenAI function calling), or (b) when the OutputParser wraps another OutputParser.
• Calls LLM: Whether this output parser itself calls an LLM. This is usually only done by output parsers that attempt to correct misformatted output.
• Input Type: Expected input type. Most output parsers work on both strings and messages, but some (like OpenAI Functions) need a message with specific kwargs.
• Output Type: The output type of the object returned by the parser.
• Description: Our commentary on this output parser and when to use it.

Name	Supports Streaming	Has Format Instructions	Calls LLM	Input Type	Output Type	Description
JSON	✅	✅		string | Message	JSON object	Returns a JSON object as specified. Probably the most reliable output parser for getting structured data that does NOT use function calling.
XML	✅	✅		string | Message	object	Returns a object of tags. Use when XML output is needed. Use with models that are good at writing XML (like Anthropic's).
CSV	✅	✅		string | Message	Array<string>	Returns a list of comma separated values.
OutputFixing			✅	string | Message		Wraps another output parser. If that output parser errors, then this will pass the error message and the bad output to an LLM and ask it to fix the output.
Datetime		✅		string | Message	Date	Parses response into a datetime string.
Structured		✅		string | Message	Record<string, string>	An output parser that returns structured information. It is less powerful than other output parsers since it only allows for fields to be strings. This can be useful when you are working with smaller LLMs.

For specifics on how to use output parsers, see the relevant how-to guides here.