Planner-Executor-Verifier (PEV)
Triadic specialization where a planner produces the plan, an executor runs it, and a separate verifier checks each step's effects against the original goal.
Problem
Plan-and-execute without independent verification cannot detect that 'step succeeded' is not the same as 'plan progressed toward goal'. A tool can return success while the world state diverges from what the plan assumed. By the time the plan completes, drift has accumulated. Distinct from plan-and-execute by mandating the third independent verifier role.
Solution
Three components, possibly three model calls per step: Planner (one-shot or incremental), Executor (executes step, gets tool result), Verifier (compares post-step state against goal expectation). On verifier reject, trigger replan with the observed drift as context. Distinct from plan-and-execute (which has no verifier) and from evaluator-optimizer (which is per-output not per-step). Pair with replan-on-failure, mental-model-in-the-loop-simulator, stochastic-deterministic-boundary.
When to use
- Multi-step plans where goal-drift between steps is costly.
- Step effects can be observed and compared against goal expectation.
- Latency budget allows per-step verification.
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