ADK code unit design

A unit design documents a code unit as implemented, the way a unit test exercises it as implemented. Nothing proposed or aspirational belongs in one. The reader is deciding what they may safely change, so answer "what breaks if I touch this?" — not "how do I call this?".

Inputs

Require the source file, or a class or method named inside it. Also read, when they exist: the base classes and interfaces the unit implements, its unit tests (the best evidence of intended behaviour), and example usage.

Analyse before writing

Answer each of these from the source. Anything the code does not show stays out of the document — do not infer a design intent from a name.

Purpose and intended use of the unit.
Execution flow, and the data that flows in and out.
Upstream dependencies, and which classes depend on this unit.
Extension surfaces: abstract methods, hooks, callbacks, configurable fields.
Constraints — what a subclass or caller must not change, and why.
Operational limitations.

Where the document goes

Mirror the source path under docs/design/, one directory per unit, document named index.md. Drop the leading underscore of a private module:

Source	Design document
`src/google/adk/workflow/_function_node.py`	`docs/design/workflow/function_node/index.md`
`src/google/adk/events/event.py`	`docs/design/events/event/index.md`

If a document already exists at that path, update it in place and keep the existing wording wherever the code has not changed, so the diff shows only what the change actually altered.

docs/design/ does not exist in this repository yet — the first design document creates it. docs/guides/ is the established sibling tree; match its directory shape.

Link to GitHub, not to local paths

These documents render on GitHub, where a local filesystem path is dead. Rewrite {repo_root}/src/google/adk/{topic}/{unit}.py#L93 as https://github.com/google/adk-python/blob/main/src/google/adk/{topic}/{unit}.py#L93.

Structure

Follow references/design-template.md section by section.