Simon Obstbaum & Rob Willoughby - Why evals are hard and how we're solving it - AI Native DevCon Jun
This session introduces advanced methods for evaluating AI agents, arguing that relying solely on 'output evals' (what came out) is insufficient. The focus must shift to 'trajectory evals,' which measure whether the agent followed the correct steps and utilized the right tools. By instrumenting agent behavior—specifically through structured skills and context—teams can significantly improve metrics like PR throughput, decrease cognitive complexity, and ensure adherence to unique organizational conventions (e.g., internal API choices or security policies).
Key takeaways
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Shift from Output Evals to Trajectory Evals
29:56
Evaluating agents requires measuring not just the final output, but also whether the agent activated the correct skills and followed the intended workflow (trajectory) [0:35:46]. Separating activation, trajectory, and outcome is essential for optimizing performance.
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Structured Context Improves Code Quality
5:46
The analysis shows that moving from unstructured (L1) to structured context (L3) significantly improves code quality metrics. Specifically, L2 and L3 teams show increased PR throughput, decreased revert rates, and lower cognitive complexity compared to L1 [0:58:46].
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Instruction Following is the Key Differentiator
6:32
While task completion may remain high regardless of structure, 'instruction following' (grounded in skills) measures adherence to unique organizational rules. This metric shows the biggest lift and represents the value of encoding proprietary business IP into the agent's context [1:03:52].
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The System, Not Just the Model, Matters
10:52
Performance is highly dependent on the entire system stack. Testing must account for the specific model harness (e.g., Opus 4 8 in Claude Code vs. OpenHands), as changing the harness can move scores by up to 100% [1:09:52].