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You should build a custom harness when a workflow needs the same setup and produces the same outcomes every time, combining deterministic steps with non-deterministic AI judgment.

Claire explains the trigger for building a harness: a job that's partly fixed process and partly AI judgment, using her Sentry bug-triage harness as the qualifying example. ✦ AI generated

Claire · Lenny's Newsletter · 2026-07-13 · original ↗

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The trigger is recognizing a job that is partly deterministic (defined steps, defined tools) and partly non-deterministic (the AI figures out root causes and writes the report). Sentry bug triage qualified because every investigation follows the same evidence-gathering process and ends with the same artifact bundle.

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Evidence · 1

The system surrounding an AI model — the harness that manages workflows, context, permissions, evaluation, persistent state and continuous improvement — has become just as important as the agent itself, marking a shift from prompting models to engineering reliable systems around them.Lilian Weng · Latent Space · conf 60%

Context · 1

A harness is simply code you write to make an AI agent more effective at a specific job.Claire · Lenny's Newsletter · conf 75%

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extends → A harness is simply code you write to make an AI agent more effective at a specific job.Claire · Lenny's Newsletter extends → The system surrounding an AI model — the harness that manages workflows, context, permissions, evaluation, persistent state and continuous improvement — has become just as important as the agent itself, marking a shift from prompting models to engineering reliable systems around them.Lilian Weng · Latent Space