**TL;DR:** In v1 tools are plain functions decorated with @tool from langchain.tools. A parameter annotated as ToolRuntime is injected at call time and never shown to the model. Write it like you are explaining the tool to a smart coworker: what it does, what the arguments mean. - Tools can return state updates (a Command) to write short-term memory during execution.

## The fix

1. `runtime` is reserved for the ToolRuntime parameter. Naming your own argument `config` or `runtime` breaks the injection; use the ToolRuntime annotation instead.

2. The docstring is the tool description the model sees. Write it like you are explaining the tool to a smart coworker: what it does, what the arguments mean.

3. Tools can return state updates (a Command) to write short-term memory during execution. See the agent-with-memory workflow skill.

## When to use this

- This covers exactly what the title says: Defining tools.
- You are setting this up for the first time, or auditing an existing setup.
- You want the key gotchas in one place before you start.

## When not to use this

- You are doing a different workflow with Defining; these steps are specific to the title above.
- You need the full reference docs; this is the short path, not the manual.

## Compatibility

- Not pinned to a specific version; follows current Defining behavior.