Planner Executor Verifier

Build me a planner-executor-verifier agent loop. Architecture: three-stage pipeline. Planner decomposes task into steps with success criteria. Executor runs each step and produces evidence. Verifier checks executor output against criteria and either approves or requests revision. Error handling: if verifier rejects 3 times, escalate to human. Edge cases: handle plans with no clear success criteria, executor producing no evidence. Best practices: log each stage's input/output for traceability. Testing: verify that verifier correctly catches executor errors.