feat(langchain): add v1 support

[hassiebp]

Important

Adds Langchain v1.0+ support with version detection and import updates, but has a critical dependency issue in pyproject.toml.

• Behavior:
  • Adds version detection in CallbackHandler.py to support Langchain v1.0+ and v0.x.
  • Updates imports for v1 from langchain_core.* instead of langchain.schema.* and langchain.callbacks.base.
  • Changes Serializable import path in serializer.py.
• Tests:
  • Updates test files to use v1 import paths and .invoke() API.
  • Removes deprecated chain-based tests.
• Dependencies:
  • Updates pyproject.toml to require langchain >= 1 and langgraph >= 1.
  • Bumps minimum Python version to 3.10+.
• Critical Issue:
  • langchain removed from optional dependencies but still referenced in extras, causing pip install langfuse[langchain] to fail.

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---

Disclaimer: Experimental PR review

Greptile Overview

Updated On: 2025-10-20 10:03:22 UTC

Summary

Adds Langchain v1.0+ support to the Langfuse Python SDK by implementing version detection and conditional imports in the CallbackHandler, maintaining backward compatibility with Langchain v0.x.

Key Changes

• Version Detection: Added langchain.__version__.startswith("1") check to conditionally import from appropriate modules
• Import Path Updates: For v1, imports from langchain_core.agents, langchain_core.callbacks, langchain_core.documents instead of langchain.schema.* and langchain.callbacks.base
• Serializer Update: Changed Serializable import from langchain.load.serializable to langchain_core.load.serializable
• Test Migration: Updated test files to use v1 import paths (langchain_core.prompts, langchain.messages) and modern API (.invoke() instead of direct call)
• Dependency Updates: Updated pyproject.toml to require langchain >= 1 and langgraph >= 1 in dev dependencies; bumped minimum Python version to 3.10+

Critical Issue Found

BLOCKING BUG: langchain was removed from optional dependencies in [tool.poetry.dependencies] but is still referenced in [tool.poetry.extras]. This will cause pip install langfuse[langchain] to fail. The dependency must be added back as langchain = { version = ">=1", optional = true }.

Confidence Score: 0/5

• This PR has a critical bug that will break package installation and must not be merged.
• Score is 0 because langchain dependency was removed from optional dependencies but still referenced in extras, causing pip install langfuse[langchain] to fail. While the code changes for v1 support are well-implemented, this packaging bug is a blocker that will break user installations.
• pyproject.toml requires immediate fix - langchain must be added back as optional dependency

Important Files Changed

File Analysis

Filename	Score	Overview
pyproject.toml	0/5	CRITICAL: langchain removed from optional dependencies but still referenced in extras, breaking pip installs. Python version bumped to 3.10+.
langfuse/langchain/CallbackHandler.py	4/5	Added version detection to support both Langchain v0 and v1 with appropriate imports. Clean implementation with backward compatibility.
tests/test_langchain.py	4/5	Updated imports for v1 compatibility. Removed deprecated chain-based tests (LLMChain, ConversationChain). Updated to use .invoke() API.

Sequence Diagram

sequenceDiagram
    participant User
    participant LangfuseSDK
    participant CallbackHandler
    participant Langchain
    participant LangchainCore
    
    User->>LangfuseSDK: pip install langfuse[langchain]
    LangfuseSDK->>User: Install with langchain dependency
    
    User->>CallbackHandler: Import CallbackHandler
    CallbackHandler->>Langchain: Check langchain.__version__
    
    alt Langchain v1.x
        Langchain-->>CallbackHandler: Version starts with "1"
        CallbackHandler->>LangchainCore: Import from langchain_core.agents
        CallbackHandler->>LangchainCore: Import from langchain_core.callbacks
        CallbackHandler->>LangchainCore: Import from langchain_core.documents
        CallbackHandler->>LangchainCore: Import from langchain_core.messages
        CallbackHandler->>LangchainCore: Import from langchain_core.outputs
    else Langchain v0.x
        Langchain-->>CallbackHandler: Version starts with "0"
        CallbackHandler->>Langchain: Import from langchain.callbacks.base
        CallbackHandler->>Langchain: Import from langchain.schema.agent
        CallbackHandler->>Langchain: Import from langchain.schema.document
        CallbackHandler->>LangchainCore: Import from langchain_core.messages
        CallbackHandler->>LangchainCore: Import from langchain_core.outputs
    end
    
    CallbackHandler-->>User: Handler ready
    
    User->>Langchain: chat.invoke(messages, callbacks=[handler])
    Langchain->>CallbackHandler: Trigger callback methods
    CallbackHandler->>LangfuseSDK: Create spans/generations via OTel
    LangfuseSDK-->>User: Track LLM operations

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[greptile-apps]

Additional Comments (1)

1. pyproject.toml, line 37-38 (link)

   logic: langchain dependency removed from [tool.poetry.dependencies] but [tool.poetry.extras] still references it. Users installing with pip install langfuse[langchain] will fail.

   Add langchain back as optional dependency:

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