adk-unit-design

# 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](references/design-template.md) section by section.