AI Agent Evaluations: The Complete 2025-2026 Guide to Bulletproof Testing

How to Build Reliable Evaluations for AI Agents: A Complete Practical Guide (2025–2026 Edition) If you’re building, shipping, or scaling AI agents in 2025 or 2026, you’ve probably already discovered one hard truth: The same autonomy, tool use, long-horizon reasoning, and adaptability that make powerful agents incredibly valuable… also make them extremely difficult to test and improve reliably. Without a solid evaluation system, teams usually fall into the same reactive cycle: users complain → engineers reproduce the bug manually → a fix is shipped → something else quietly regresses → repeat. Good evaluations break this loop. They turn vague feelings …