An Eval Harness for Tool-Use Agents: 90 Lines, 3 Judges, $3 Per Run

📰 Dev.to · Gabriel Anhaia

Learn to build an eval harness for tool-use agents in 90 lines of Python to prevent silent failures

intermediate Published 29 Apr 2026

Action Steps

Build a Python script using 90 lines of code to create an eval harness
Configure 3 judges in a ladder to evaluate tool-use agents
Run the eval harness on a small golden set to test for silent failures
Apply the eval harness to your existing tool-use agents to improve reliability
Test and compare the performance of your tool-use agents with and without the eval harness

Who Needs to Know This

AI engineers and researchers can benefit from this technique to improve the reliability of their tool-use agents, while product managers can use this to ensure more robust AI-powered products

Key Insight

💡 A simple eval harness can significantly improve the reliability of tool-use agents by detecting silent failures