Requisite variety frames agent capability as a matching problem: a system survives only if its internal response variety matches environmental variety, or if it attenuates the variety it accepts. This makes 'response option count' (tools, actions, retries, routing branches) a first-class design budget rather than an afterthought.
It gives a principled way to reason about why an agent fails on a task: either the environment presents more distinct situations than the agent can discriminate and respond to, or the system attenuated the input too aggressively and lost the signal needed to act. Both failure modes are diagnosable and fixable at different architectural layers.
For any agent runtime, enumerate the distinct input conditions the environment can produce and the distinct responses the harness can emit. If response variety < input variety, add response options (more tools, more autonomy, escalation paths) or add attenuation (classification, routing, filtering, compression) — but measure which side you changed.
Only variety can absorb variety.
To handle complexity from the environment, systems must have sufficient internal variety or attenuate external variety.
A goalkeeper facing multiple attackers needs adequate response options.