| name | langchain-middleware-skill |
| description | LangChain uses Middleware as a way to more tightly control what happens inside the agent. There are many built-in middleware components supplied by LangChain for common needs, and custom middleware can be created. Use when this capability is needed. |
| metadata | {"author":"chicagopeabodydev-sudo"} |
LangChain Middleware
Middleware lets you intercept and control what happens inside the agent loop — before/after model calls, before/after tool calls, and at the start/end of the full agent run. Configure it via the middleware parameter of create_agent.
In this project, middleware is the mechanism for tracking off-topic inputs and terminating the conversation after too many are received. It is also used to manage the @wrap_model_call decorator for dynamic model selection (see langchain-skill for context, and Example 5 in examples.md for @wrap_model_call usage).
Common Middleware Uses
- Tracking agent behavior with logging, analytics, and debugging.
- Transforming prompts, tool selection, and output formatting.
- Adding retries, fallbacks, and early termination logic.
- Applying rate limits, guardrails, and PII detection.
Hook Types
The core agent loop calls a model, lets the model choose tools, and finishes when no more tools are called. Middleware exposes hooks before and after each step. There are two styles:
Node-Style Hooks
Run sequentially at specific execution points. Use for logging, validation, and state updates.
| Hook | When it runs | Good for |
|---|
before_agent | Once, before the first model call | Initializing custom state |
before_model | Before each model call | Prompt injection, context trimming |
after_model | After each model call | Logging responses, updating counters |
after_agent | Once, after the last tool call | Order summary finalization, cleanup |
Wrap-Style Hooks
Intercept execution and control when the inner handler is called. Use for retries, caching, and transformation.
| Hook | Wraps | Good for |
|---|
wrap_model_call | Each model call | Retries, dynamic model selection, token tracking |
wrap_tool_call | Each tool call | Error handling, caching, rate limiting |
Best Practices
- Keep middleware focused — each component should do one thing well.
- Handle errors gracefully — don’t let middleware errors crash the agent.
- Use appropriate hook types:
- Node-style for sequential logic (logging, validation)
- Wrap-style for control flow (retry, fallback, caching)
- Clearly document any custom state properties.
- Unit test middleware independently before integrating.
- Consider execution order — place critical middleware first in the list.
- Use built-in middleware when possible.
Additional Resources
Source: chicagopeabodydev-sudo/minimal-llm-usage-agent — distributed by TomeVault.
