# Planner-Generator-Evaluator Harness

> Decompose a long-running coding or creative job into three role-isolated agents — a Planner that emits a structured feature list, a Generator that builds one chunk per fresh context, and an Evaluator that grades the artefact against a fixed rubric without seeing the Generator's reasoning trace.

- **Category**: Agentic AI
- **Subcategory**: Planning & Control Flow
- **Canonical URL**: https://designpattern.fyi/patterns/planner_generator_evaluator_harness/

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## Description
**Intent**: Decompose a long-running coding or creative job into three role-isolated agents — a Planner that emits a structured feature list, a Generator that builds one chunk per fresh context, and an Evaluator that grades the artefact against a fixed rubric without seeing the Generator's reasoning trace.
**Context**: A team runs a coding-agent harness on multi-day creative work — building a new feature across a large application, conducting a large refactor, drafting a long design document. The job is too big to fit into a single model context window, so it must be split across many runs. There is a clear external artefact that can be evaluated on its own merits without inspecting how it was produced.
**Solution**: - The Planner runs once (or rarely) and emits a structured feature-list artefact: ordered chunks, acceptance criteria, dependencies. - The Generator is invoked per-chunk in a fresh context containing only the feature list, current artefact state, and the chunk to build; it produces a new artefact revision and exits. - The Evaluator is invoked in its own fresh context with only the artefact and fixed rubric; it returns pass/fail plus structured findings, never seeing the Generator's chain of thought. - A small driver loop routes between the three: failed evaluation re-invokes the Generator with the findings as input.


## Use Cases
- A single agent run cannot fit the job into one context window.
- There is a clear external artefact that can be evaluated without inspecting how it was produced.
- A stable rubric exists or can be authored.





## Trade-offs


### Advantages

- Each role''s context stays small and bounded.

- Evaluator isolation makes scores harder to game from inside the Generator.

- Fresh-context generation per chunk avoids long-trace attention rot.

- Plans are durable artefacts that survive crashes and resumption.




### Considerations & Drawbacks

- Three-agent orchestration adds significant harness complexity over single-agent loops.

- Inter-role hand-offs through files add latency.

- A weak or mis-specified rubric makes the Evaluator useless or actively harmful.

- Planner errors propagate through the whole run because the Generator trusts the plan.







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**Reference**: [Original Source](https://www.agentpatternscatalog.org/patterns/planner-generator-evaluator-harness/)