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api-and-interface-design

Use when asked to design or change a public API, route, CLI flag, or module boundary. Not for remote, credential, publish, deploy, or irreversible changes.

.devin/skills/api-and-interface-design/SKILL.md

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# API and interface design

## Contract

| Field | Bound contract |
|---|---|
| Trigger | Designing or changing a public API, route, CLI flag, or module boundary. |
| Authority | Reversible local: writes only named local interface definitions and contract docs, edits consumer files onto the new contract, and deletes legacy paths (old signatures, aliases, re-exports, deprecated entry points) during cutover; rollback is undo (discard the uncommitted draft) or version control. No remote mutation. No credential, paid, published, or deployed change, and no VCS history rewrite. |
| Side effect | Interface definitions and contract docs written before implementation; consumer files edited onto the new contract and legacy paths (old signatures, aliases, re-exports, deprecated entry points) deleted during cutover; no build, publish, or remote mutation. |
| Done | Contract is documented with semantics and errors, every consumer is migrated, and no legacy path remains. |

## Inputs

The interface being designed or changed: its name and kind (API endpoint, route, CLI flag, or module boundary), and whether it is new or a change to an existing interface. The current contract text, when changing an existing interface. The list of known consumers, found by search over the codebase. Optional: target language or runtime conventions for type and error spelling.