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`ChatMistralAI` now supports `stop` sequences. Previously, a `stop` value passed to the model was silently discarded: the code carried a stale "not yet supported" note, dropped the parameter before the request, and logged a warning. Mistral's chat completions API does accept `stop` (a string or list of strings, up to 4 sequences), so anyone setting `stop` and expecting generation to halt was getting no effect. Now `stop` is a first-class parameter. It can be set on the constructor (`ChatMistralAI(stop=[...])`) or per call (`model.invoke(prompt, stop=[...])`) and is forwarded to the API. A per-call value overrides the instance default, and an empty list is treated as "no stop sequences" — omitted from the request rather than sent as an empty array (which the API rejects). Verified against the live Mistral API: with `stop=["5"]`, "Count from 1 to 10" returns `1 2 3 4 ` instead of the full sequence. The 422 `extra_forbidden` response the API returns for genuinely unknown fields confirms `stop` is a real schema field, not silently ignored. This PR also folds in some test hygiene: the base-URL env test uses `monkeypatch.setenv` so `MISTRAL_BASE_URL=boo` no longer leaks into later serialization tests, and `test_extra_kwargs` asserts the intentional unknown-kwarg warning with `pytest.warns`. ## Review notes - Behavior change worth a careful look: `stop` now reaches the API instead of being dropped. This changes request payloads for anyone previously passing `stop`. It is the intended fix, but flagging it explicitly. - Coverage: `test_stop_sequence` (integration) exercises the end-to-end behavior; unit tests cover parameter wiring, per-call-vs-instance precedence, and the empty-list case.
The agent engineering platform.
LangChain is a framework for building agents and LLM-powered applications. It helps you chain together interoperable components and third-party integrations to simplify AI application development — all while future-proofing decisions as the underlying technology evolves.
Tip
Just getting started? Check out Deep Agents — a higher-level package built on LangChain for agents that have built-in capabilites for common usage patterns such as planning, subagents, file system usage, and more.
Quickstart
pip install langchain
# or
uv add langchain
from langchain.chat_models import init_chat_model
model = init_chat_model("openai:gpt-5.4")
result = model.invoke("Hello, world!")
If you're looking for more advanced customization or agent orchestration, check out LangGraph, our framework for building controllable agent workflows.
For an equivalent JS/TS library, check out LangChain.js.
Tip
For developing, debugging, and deploying AI agents and LLM applications, see LangSmith.
LangChain ecosystem
While the LangChain framework can be used standalone, it also integrates seamlessly with any LangChain product, giving developers a full suite of tools when building LLM applications.
- Deep Agents — Build agents that can plan, use subagents, and leverage file systems for complex tasks
- LangGraph — Build agents that can reliably handle complex tasks with our low-level agent orchestration framework
- Integrations — Chat & embedding models, tools & toolkits, and more
- LangSmith — Agent evals, observability, and debugging for LLM apps
- LangSmith Deployment — Deploy and scale agents with a purpose-built platform for long-running, stateful workflows
Why use LangChain?
LangChain helps developers build applications powered by LLMs through a standard interface for models, embeddings, vector stores, and more.
- Real-time data augmentation — Easily connect LLMs to diverse data sources and external/internal systems, drawing from LangChain's vast library of integrations with model providers, tools, vector stores, retrievers, and more
- Model interoperability — Swap models in and out as your engineering team experiments to find the best choice for your application's needs. As the industry frontier evolves, adapt quickly — LangChain's abstractions keep you moving without losing momentum
- Rapid prototyping — Quickly build and iterate on LLM applications with LangChain's modular, component-based architecture. Test different approaches and workflows without rebuilding from scratch, accelerating your development cycle
- Production-ready features — Deploy reliable applications with built-in support for monitoring, evaluation, and debugging through integrations like LangSmith. Scale with confidence using battle-tested patterns and best practices
- Vibrant community and ecosystem — Leverage a rich ecosystem of integrations, templates, and community-contributed components. Benefit from continuous improvements and stay up-to-date with the latest AI developments through an active open-source community
- Flexible abstraction layers — Work at the level of abstraction that suits your needs — from high-level chains for quick starts to low-level components for fine-grained control. LangChain grows with your application's complexity
Documentation
- docs.langchain.com – Comprehensive documentation, including conceptual overviews and guides
- reference.langchain.com/python – API reference docs for LangChain packages
- Chat LangChain – Chat with the LangChain documentation and get answers to your questions
Discussions: Visit the LangChain Forum to connect with the community and share all of your technical questions, ideas, and feedback.
Additional resources
- Contributing Guide – Learn how to contribute to LangChain projects and find good first issues.
- Code of Conduct – Our community guidelines and standards for participation.
- LangChain Academy – Comprehensive, free courses on LangChain libraries and products, made by the LangChain team.