Agent-First API Design

An interface design philosophy where API endpoints are built primarily for machine callers (AI agents) rather than exclusively for human users through a UI. The user interface becomes one consumer among many, changing how operations are scoped, errors are communicated, and tool contracts are specified.

Traditional API design optimizes for human developers writing application code. Agent-first API design optimizes for AI agents that discover capabilities at runtime, compose operations dynamically, and handle errors through reasoning rather than hardcoded error handling.

Key differences include: operations are discrete and composable rather than bundled into complex flows, error messages are explanatory rather than coded, capabilities are self-describing so agents can discover what actions are available, and responses include context that helps agents decide their next step. The user interface still exists but consumes the same API that agents do, ensuring consistency between human and machine access paths.