Checked other resources

• This is a bug, not a usage question. For questions, please use the LangChain Forum (https://forum.langchain.com/).
• I added a clear and detailed title that summarizes the issue.
• I read what a minimal reproducible example is (https://stackoverflow.com/help/minimal-reproducible-example).
• I included a self-contained, minimal example that demonstrates the issue INCLUDING all the relevant imports. The code run AS IS to reproduce the issue.

Example Code

from langgraph.graph.message import add_messages
from langchain_core.messages import AIMessage, HumanMessage, BaseMessage


initial_messages: list[BaseMessage] = [
    AIMessage(content="Hello! How can I assist you?", name="Model"),
    HumanMessage(
        content="I'm looking for information on marine biology.", name="Lance"
    ),
]

# New message to add
new_message = AIMessage(
    content="Sure, I can help with that. What specifically are you interested in?",
    name="Model",
)

# Test
add_messages(initial_messages, new_message) # generate a type hint error

Error Message and Stack Trace (if applicable)

Argument of type "list[BaseMessage]" cannot be assigned to parameter of type "Messages"
  Type "list[BaseMessage]" is not assignable to type "Messages"
    "list[BaseMessage]" is not assignable to "list[MessageLikeRepresentation]"
      Type parameter "_T@list" is invariant, but "BaseMessage" is not the same as "MessageLikeRepresentation"
      Consider switching from "list" to "Sequence" which is covariant
    "list[BaseMessage]" is not assignable to "BaseMessage"
    "list[BaseMessage]" is not assignable to "list[str]"
      Type parameter "_T@list" is invariant, but "BaseMessage" is not the same as "str"
      Consider switching from "list" to "Sequence" which is covariant
  ...PylancereportArgumentType

Description

Static type checkers flag list[BaseMessage] as incompatible with list[MessageLikeRepresentation] due to list invariance.

Proposed fix
Change

Messages = Union[list[MessageLikeRepresentation], MessageLikeRepresentation]

to

from collections.abc import Sequence
Messages = Union[Sequence[MessageLikeRepresentation], MessageLikeRepresentation]

relevant code here

System Info

uv run python -m langchain_core.sys_info

System Information

OS: Linux
OS Version: #41~22.04.1-Ubuntu SMP Thu Sep 11 10:54:48 UTC 2025
Python Version: 3.11.13 (main, Sep 18 2025, 19:46:39) [Clang 20.1.4 ]

Package Information

langchain_core: 0.3.76
langchain: 0.3.27
langchain_community: 0.3.30
langsmith: 0.4.31
langchain_openai: 0.3.33
langchain_text_splitters: 0.3.11
langgraph_api: 0.4.28
langgraph_cli: 0.4.2
langgraph_license: Installed. No version info available.
langgraph_runtime: Installed. No version info available.
langgraph_runtime_inmem: 0.14.1
langgraph_sdk: 0.2.9

Optional packages not installed

langserve

Other Dependencies

aiohttp<4.0.0,>=3.8.3: Installed. No version info available.
async-timeout<5.0.0,>=4.0.0;: Installed. No version info available.
blockbuster<2.0.0,>=1.5.24: Installed. No version info available.
click>=8.1.7: Installed. No version info available.
cloudpickle>=3.0.0: Installed. No version info available.
cryptography<45.0,>=42.0.0: Installed. No version info available.
dataclasses-json<0.7.0,>=0.6.7: Installed. No version info available.
httpx-sse<1.0.0,>=0.4.0: Installed. No version info available.
httpx<1,>=0.23.0: Installed. No version info available.
httpx>=0.25.0: Installed. No version info available.
httpx>=0.25.2: Installed. No version info available.
jsonpatch<2.0,>=1.33: Installed. No version info available.
jsonschema-rs<0.30,>=0.20.0: 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,>=0.3.72: Installed. No version info available.
langchain-core<1.0.0,>=0.3.76: Installed. No version info available.
langchain-core<2.0.0,>=0.3.75: Installed. No version info available.
langchain-core>=0.3.64: 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,>=0.3.9: Installed. No version info available.
langchain-together;: Installed. No version info available.
langchain-xai;: Installed. No version info available.
langchain<2.0.0,>=0.3.27: Installed. No version info available.
langgraph-api<0.5.0,>=0.3;: Installed. No version info available.
langgraph-checkpoint>=2.0.23: Installed. No version info available.
langgraph-checkpoint>=2.0.25: Installed. No version info available.
langgraph-runtime-inmem<0.15.0,>=0.14.0: Installed. No version info available.
langgraph-runtime-inmem>=0.7;: Installed. No version info available.
langgraph-sdk>=0.1.0;: Installed. No version info available.
langgraph-sdk>=0.2.0: Installed. No version info available.
langgraph>=0.2: Installed. No version info available.
langgraph>=0.4.0: Installed. No version info available.
langsmith-pyo3>=0.1.0rc2;: Installed. No version info available.
langsmith<1.0.0,>=0.1.125: Installed. No version info available.
langsmith>=0.1.17: Installed. No version info available.
langsmith>=0.3.45: Installed. No version info available.
numpy>=1.26.2;: Installed. No version info available.
numpy>=2.1.0;: Installed. No version info available.
openai-agents>=0.0.3;: Installed. No version info available.
openai<2.0.0,>=1.104.2: Installed. No version info available.
opentelemetry-api>=1.30.0;: Installed. No version info available.
opentelemetry-exporter-otlp-proto-http>=1.30.0;: Installed. No version info available.
opentelemetry-sdk>=1.30.0;: Installed. No version info available.
orjson>=3.10.1: Installed. No version info available.
orjson>=3.9.14;: Installed. No version info available.
orjson>=3.9.7: Installed. No version info available.
packaging>=23.2: Installed. No version info available.
pydantic-settings<3.0.0,>=2.10.1: Installed. No version info available.
pydantic<3,>=1: 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.
pyjwt>=2.9.0: Installed. No version info available.
pytest>=7.0.0;: Installed. No version info available.
python-dotenv>=0.8.0;: Installed. No version info available.
PyYAML<7.0.0,>=5.3.0: Installed. No version info available.
PyYAML>=5.3: Installed. No version info available.
requests-toolbelt>=1.0.0: Installed. No version info available.
requests<3,>=2: Installed. No version info available.
requests<3.0.0,>=2.32.5: Installed. No version info available.
requests>=2.0.0: Installed. No version info available.
rich>=13.9.4;: Installed. No version info available.
SQLAlchemy<3,>=1.4: Installed. No version info available.
SQLAlchemy<3.0.0,>=1.4.0: Installed. No version info available.
sse-starlette<2.2.0,>=2.1.0: Installed. No version info available.
sse-starlette>=2: Installed. No version info available.
starlette>=0.37: Installed. No version info available.
starlette>=0.38.6: Installed. No version info available.
structlog<26,>=24.1.0: Installed. No version info available.
structlog>23: Installed. No version info available.
tenacity!=8.4.0,<10.0.0,>=8.1.0: Installed. No version info available.
tenacity>=8.0.0: Installed. No version info available.
tiktoken<1,>=0.7: Installed. No version info available.
truststore>=0.1: Installed. No version info available.
typing-extensions>=4.7: Installed. No version info available.
uvicorn>=0.26.0: Installed. No version info available.
vcrpy>=7.0.0;: Installed. No version info available.
watchfiles>=0.13: Installed. No version info available.
zstandard>=0.23.0: Installed. No version info available.