Instructions for using LangChain with Python

# LangChain Python Instructions

These instructions guide GitHub Copilot in generating code and documentation for LangChain applications in Python. Focus on LangChain-specific patterns, APIs, and best practices.

## Runnable Interface (LangChain-specific)

LangChain's `Runnable` interface is the foundation for composing and executing chains, chat models, output parsers, retrievers, and LangGraph graphs. It provides a unified API for invoking, batching, streaming, inspecting, and composing components.

**Key LangChain-specific features:**

- All major LangChain components (chat models, output parsers, retrievers, graphs) implement the Runnable interface.
- Supports synchronous (`invoke`, `batch`, `stream`) and asynchronous (`ainvoke`, `abatch`, `astream`) execution.
- Batching (`batch`, `batch_as_completed`) is optimized for parallel API calls; set `max_concurrency` in `RunnableConfig` to control parallelism.
- Streaming APIs (`stream`, `astream`, `astream_events`) yield outputs as they are produced, critical for responsive LLM apps.
- Input/output types are component-specific (e.g., chat models accept messages, retrievers accept strings, output parsers accept model outputs).
- Inspect schemas with `get_input_schema`, `get_output_schema`, and their JSONSchema variants for validation and OpenAPI generation.
- Use `with_types` to override inferred input/output types for complex LCEL chains.
- Compose Runnables declaratively with LCEL: `chain = prompt | chat_model | output_parser`.
- Propagate `RunnableConfig` (tags, metadata, callbacks, concurrency) automatically in Python 3.11+; manually in async code for Python 3.9/3.10.
- Create custom runnables with `RunnableLambda` (simple transforms) or `RunnableGenerator` (streaming transforms); avoid subclassing directly.
- Configure runtime attributes and alternatives with `configurable_fields` and `configurable_alternatives` for dynamic chains and LangServe deployments.

**LangChain best practices:**

- Use batching for parallel API calls to LLMs or retrievers; set `max_concurrency` to avoid rate limits.