LangChain 1.0+ compatibility: Update imports to use langchain_core

[andrewm4894]

Bug Description

The PostHog LangChain CallbackHandler is incompatible with LangChain 1.0+ due to deprecated import paths.

Current Behavior

When attempting to use the CallbackHandler with LangChain 1.0+:

from posthog.ai.langchain import CallbackHandler as PostHogCallbackHandler

This raises:

ModuleNotFoundError: No module named 'langchain.callbacks'

Root Cause

The issue is in posthog/ai/langchain/callbacks.py lines 23-24:

from langchain.callbacks.base import BaseCallbackHandler
from langchain.schema.agent import AgentAction, AgentFinish

LangChain 1.0 (released September 2024) restructured the codebase and moved these modules to langchain_core.

Environment

• PostHog SDK version: 6.7.11
• LangChain version: 1.0.3
• Python: 3.x

Suggested Fix

Update the imports in posthog/ai/langchain/callbacks.py lines 23-24:

# Current (incompatible with LangChain 1.0+)
from langchain.callbacks.base import BaseCallbackHandler
from langchain.schema.agent import AgentAction, AgentFinish

# Proposed fix
from langchain_core.callbacks.base import BaseCallbackHandler
from langchain_core.agents import AgentAction, AgentFinish

This change maintains backward compatibility since langchain_core exists in both LangChain 0.x and 1.0+.

Additional Context

LangChain 1.0 removed the langchain.callbacks and langchain.schema modules as part of the core refactoring. All core functionality was moved to the langchain_core package, which is the proper import path for both old and new versions.

[andrewm4894]

Backward Compatibility Consideration

Looking at pyproject.toml, I see the package requires langchain-core>=0.3.65. The file already successfully imports from langchain_core (lines 25-35), so we know it's available.

Recommended Backward-Compatible Fix

Use a try/except pattern to support both LangChain 0.x and 1.0+:

# Lines 23-24 in posthog/ai/langchain/callbacks.py
try:
    # LangChain 1.0+ (and modern 0.x with langchain-core)
    from langchain_core.callbacks.base import BaseCallbackHandler
    from langchain_core.agents import AgentAction, AgentFinish
except (ImportError, ModuleNotFoundError):
    # Fallback for older LangChain versions
    from langchain.callbacks.base import BaseCallbackHandler
    from langchain.schema.agent import AgentAction, AgentFinish

This approach:

• ✅ Works with LangChain 1.0+ (where langchain.callbacks is removed)
• ✅ Works with LangChain 0.2+ (which has both langchain_core and legacy langchain modules)
• ✅ Maintains backward compatibility with older versions
• ✅ Consistent with existing imports in the same file (lines 25-35 already use langchain_core)

The rest of the file already uses langchain_core for Document, BaseMessage, LLMResult, etc., so this change brings consistency to all imports.