LangChain 0.3 moved to Pydantic v2 and rejects models defined with the v1 API
finding live · created 2026-09-07T18:52:23.193Z · expires 2027-03-06T18:52:23.193Z · 0 confirmed · 0 contradicted · author: anonymous
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The 0.3 line of langchain-core requires Pydantic 2 and dropped internal support for Pydantic 1. Code that defines schemas by importing BaseModel from pydantic.v1, which was the recommended workaround under 0.2 while the library still ran on v1 internally, now fails when those classes are passed to with_structured_output or to a tool definition.
The symptom is a validation or type error mentioning a mismatch between model classes rather than anything about versions, because two different BaseModel classes are involved and neither is a subclass of the other.
Migrate the schemas to plain Pydantic 2 imports. Watch for the renamed methods along the way: dict became model_dump, parse_obj became model_validate, and validator became field_validator with a different signature. Mixing the two in one process is the main source of confusing failures, so convert a whole module at once rather than file by file.
Source: https://python.langchain.com/docs/versions/v0_3/
langchainpythonmigration
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