Checked other resources

• I added a very descriptive title to this issue.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangChain rather than my code.
• The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package).
• I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS.

Example Code

from langchain_core.callbacks.manager import BaseCallbackManager

h1 = object()
h2 = object()
ih1 = object()
ih2 = object()

m1 = BaseCallbackManager(handlers=[h1], inheritable_handlers=[ih1])
m2 = BaseCallbackManager(handlers=[h2], inheritable_handlers=[ih2])

merged = m1.merge(m2)

print("merged.handlers =", len(merged.handlers)) # 4
print("merged.inheritable_handlers =", len(merged.inheritable_handlers)) # 4
print(set(merged.handlers) == set(merged.inheritable_handlers)) # True

Error Message and Stack Trace (if applicable)

No response

Description

When I attempted to use BaseCallbackManager's merge() method, I noticed that its behavior regarding handlers and inheritable_handlers was somewhat unexpected.

I initially assumed that merge() would simply combine each list independently — merging handlers into handlers, and inheritable_handlers into inheritable_handlers. However, upon inspection, it appears that all handlers and inheritable_handlers from both managers are merged together and assigned to both handlers and inheritable_handlers in the resulting manager.

Given that handlers and inheritable_handlers serve different semantic purposes, would it be possible to revisit this logic to preserve the intended distinction between these two fields?

System Info

System Information

OS: Linux
OS Version: #148-Ubuntu SMP Fri Mar 14 19:05:48 UTC 2025
Python Version: 3.10.16 (main, Dec 11 2024, 16:24:50) [GCC 11.2.0]

Package Information

langchain_core: 0.3.65
langchain: 0.3.25
langchain_community: 0.3.25
langsmith: 0.3.45
langchain_anthropic: 0.3.1
langchain_aws: 0.2.2
langchain_chroma: 0.2.0
langchain_cohere: 0.3.5
langchain_experimental: 0.3.2
langchain_fireworks: 0.2.6
langchain_google_vertexai: 2.0.5
langchain_groq: 0.2.2
langchain_mistralai: 0.2.4
langchain_openai: 0.2.14
langchain_text_splitters: 0.3.8
langchain_together: 0.2.0
langchain_unstructured: 0.1.5
langgraph_sdk: 0.1.34

Optional packages not installed

langserve

Other Dependencies

aiohttp: 3.12.12
aiohttp<4.0.0,>=3.8.3: Installed. No version info available.
anthropic: 0.40.0
anthropic[vertexai]: Installed. No version info available.
async-timeout<5.0.0,>=4.0.0;: Installed. No version info available.
boto3: 1.35.42
chromadb: 0.5.23
cohere: 5.15.0
dataclasses-json<0.7,>=0.5.7: Installed. No version info available.
defusedxml: 0.7.1
fastapi: 0.115.9
fireworks-ai: 0.15.7
google-cloud-aiplatform: 1.70.0
google-cloud-storage: 2.18.2
groq: 0.11.0
httpx: 0.28.1
httpx-sse: 0.4.0
httpx-sse<1.0.0,>=0.4.0: Installed. No version info available.
jsonpatch<2.0,>=1.33: Installed. No version info available.
langchain-anthropic;: Installed. No version info available.
langchain-aws;: Installed. No version info available.
langchain-azure-ai;: Installed. No version info available.
langchain-cohere;: Installed. No version info available.
langchain-community;: Installed. No version info available.
langchain-core<1.0.0,>=0.3.51: Installed. No version info available.
langchain-core<1.0.0,>=0.3.58: Installed. No version info available.
langchain-core<1.0.0,>=0.3.65: Installed. No version info available.
langchain-deepseek;: Installed. No version info available.
langchain-fireworks;: Installed. No version info available.
langchain-google-genai;: Installed. No version info available.
langchain-google-vertexai;: Installed. No version info available.
langchain-groq;: Installed. No version info available.
langchain-huggingface;: Installed. No version info available.
langchain-mistralai;: Installed. No version info available.
langchain-ollama;: Installed. No version info available.
langchain-openai;: Installed. No version info available.
langchain-perplexity;: Installed. No version info available.
langchain-text-splitters<1.0.0,>=0.3.8: Installed. No version info available.
langchain-together;: Installed. No version info available.
langchain-xai;: Installed. No version info available.
langchain<1.0.0,>=0.3.25: Installed. No version info available.
langsmith-pyo3: Installed. No version info available.
langsmith<0.4,>=0.1.125: Installed. No version info available.
langsmith<0.4,>=0.1.17: Installed. No version info available.
langsmith<0.4,>=0.3.45: Installed. No version info available.
numpy: 1.26.4
numpy>=1.26.2;: Installed. No version info available.
numpy>=2.1.0;: Installed. No version info available.
openai: 1.86.0
openai-agents: Installed. No version info available.
opentelemetry-api: 1.34.1
opentelemetry-exporter-otlp-proto-http: 1.34.1
opentelemetry-sdk: 1.34.1
orjson: 3.10.16
packaging: 25.0
packaging<25,>=23.2: Installed. No version info available.
pandas: 2.2.3
pydantic: 2.9.2
pydantic-settings<3.0.0,>=2.4.0: Installed. No version info available.
pydantic<3.0.0,>=2.7.4: Installed. No version info available.
pydantic>=2.7.4: Installed. No version info available.
pytest: 8.4.1
PyYAML>=5.3: Installed. No version info available.
requests: 2.32.3
requests-toolbelt: 1.0.0
requests<3,>=2: Installed. No version info available.
rich: 14.0.0
SQLAlchemy<3,>=1.4: Installed. No version info available.
tabulate: 0.9.0
tenacity!=8.4.0,<10,>=8.1.0: Installed. No version info available.
tenacity!=8.4.0,<10.0.0,>=8.1.0: Installed. No version info available.
tiktoken: 0.8.0
tokenizers: 0.21.1
typing-extensions>=4.7: Installed. No version info available.
unstructured-client: 0.25.9
unstructured[all-docs]: Installed. No version info available.
zstandard: 0.23.0