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langchain: use langchain_core as version sentinel instead of umbrella #1656

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langchain: use langchain_core as version sentinel instead of umbrella #1656

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16 changes: 11 additions & 5 deletions langfuse/langchain/CallbackHandler.py
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Original file line number Diff line number Diff line change
Expand Up @@ -38,9 +38,15 @@
from langfuse.types import TraceContext

try:
import langchain

if langchain.__version__.startswith("1"):
# Use langchain_core as the version sentinel instead of the langchain
# umbrella package: langchain_core is the common runtime dependency of
# langchain, langgraph, and every langchain-* provider package, so it
# is present on every install that needs this handler. Importing the
# umbrella here previously forced langchain itself to be installed
# even for the v1 branch, which never uses any langchain.* import.
import langchain_core

if langchain_core.__version__.startswith("1"):
# Langchain v1
from langchain_core.agents import AgentAction, AgentFinish
from langchain_core.callbacks import (
Expand All @@ -59,7 +65,7 @@
from langchain_core.outputs import ChatGeneration, LLMResult

else:
# Langchain v0
# Langchain v0 — these symbols only exist in the umbrella package
from langchain.callbacks.base import ( # type: ignore
BaseCallbackHandler as LangchainBaseCallbackHandler,
)
Expand All @@ -81,7 +87,7 @@

except ImportError:
raise ModuleNotFoundError(
"Please install langchain to use the Langfuse langchain integration: 'pip install langchain'"
"Please install langchain_core to use the Langfuse langchain integration: 'pip install langchain-core'"
)
Comment on lines 88 to 91
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@greptile-apps greptile-apps Bot May 16, 2026

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P1 Misleading error message when langchain umbrella is missing on the v0 path

A user who has langchain_core 0.x installed (e.g. via LangGraph) but does not have the langchain umbrella will enter the v0 branch and immediately hit an ImportError on from langchain.callbacks.base import .... That error is caught here and re-raised telling them pip install langchain-core — but langchain-core is already present. The real fix is pip install langchain. This is exactly the class of user the PR description says will benefit from this change, so they are particularly likely to hit it on a 0.x core install.

Prompt To Fix With AI 
This is a comment left during a code review.
Path: langfuse/langchain/CallbackHandler.py
Line: 88-91

Comment:
**Misleading error message when `langchain` umbrella is missing on the v0 path**

A user who has `langchain_core` 0.x installed (e.g. via LangGraph) but does not have the `langchain` umbrella will enter the v0 branch and immediately hit an `ImportError` on `from langchain.callbacks.base import ...`. That error is caught here and re-raised telling them `pip install langchain-core` — but `langchain-core` is already present. The real fix is `pip install langchain`. This is exactly the class of user the PR description says will benefit from this change, so they are particularly likely to hit it on a 0.x core install.

How can I resolve this? If you propose a fix, please make it concise.


LANGSMITH_TAG_HIDDEN: str = "langsmith:hidden"
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