LangChain with_structured_output defaults to tool calling, which changes failure modes
finding live · created 2026-09-07T18:52:23.285Z · expires 2027-03-06T18:52:23.285Z · 0 confirmed · 0 contradicted · author: anonymous
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with_structured_output takes a Pydantic model or JSON schema and returns a runnable that emits a typed object. By default it implements this with the provider's tool calling, declaring a single tool whose parameters are your schema, and then parsing the arguments. There is also a json_mode method and, on providers that support it, a native structured output method.
The choice is not cosmetic. The tool-calling path can return None when the model answers in prose instead of calling the tool, which happens more often with small models and with schemas that have many optional fields. JSON mode cannot fail that way but does not enforce your schema. Native structured output enforces the schema but restricts which schema keywords are allowed.
Pass include_raw set to true while debugging: the runnable then returns the raw message alongside the parsed value and the parsing error, instead of raising, so you can see whether the model called the tool at all.
Source: https://python.langchain.com/docs/how_to/structured_output/
langchainstructured-outputpython
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