gan

Overview

Run a planner → generator → evaluator loop with a score-gated stop condition. The planner expands the brief into acceptance criteria and a rubric. The generator implements against the criteria. The evaluator scores the result, returns feedback. The loop iterates until the score crosses the threshold or the iteration cap is hit.

When to use

The output has a measurable quality signal, Playwright test outcome, lint pass, custom rubric.
The first attempt is unlikely to be the last attempt.
The cost of running a feedback loop is justified by the quality lift.

When NOT to use

One-shot implementations where the spec is unambiguous.
Recurring schedules. Use the /loop built-in.
Visual design exploration. Use /design variants.
Trivial fixes. Direct edit is faster.

Arguments

/gan "<brief>" is the canonical form. The brief is one or two sentences describing what to build.

Environment variables:

GAN_MAX_ITERATIONS (default 8). Hard cap on planner-generator-evaluator cycles.
GAN_PASS_THRESHOLD (default 8.0). Weighted score 0-10 to consider passing.
GAN_EVAL_MODE (default playwright). One of playwright, screenshot, code-only. Determines which evaluator surface runs.
GAN_DEV_SERVER_PORT (default 3000). Port the running app is reachable on.
GAN_DEV_SERVER_CMD (default pnpm dev). Command to start the dev server when not already running.

Subagents

The skill orchestrates three Claude Code agents shipped alongside it:

agents/gan-planner.md: expands the brief into acceptance criteria, file list, and a 5-row scoring rubric.
agents/gan-generator.md: implements the plan; returns proposed diffs or full file contents, NOT direct writes.
agents/gan-evaluator.md: scores the implementation against the rubric, returns per-row scores and a verdict.

The orchestrator, this skill, is the sole filesystem writer. Same single-writer invariant as /plan multi-execute.

Process

Pre-flight.
- Verify playwright is installed when GAN_EVAL_MODE=playwright.
- Verify the dev server is reachable on GAN_DEV_SERVER_PORT, start it if not.
- Initialize gan-harness/ workspace in the spec folder: feedback/, screenshots/, iterations/.
Plan once. Spawn gan-planner with the brief. Receive:
- Acceptance criteria as a checklist.
- File list, paths the implementation will touch.
- Rubric: 5 rows, each weighted 1-10, total weights sum to 10. Each row names a measurable quality.
Iterate, cap GAN_MAX_ITERATIONS:

a. Generate. Spawn gan-generator with the plan, current state, and last iteration's feedback, if any. Generator returns proposed changes as unified diffs.

b. Apply. Orchestrator parses the diffs and writes via Edit / Write / MultiEdit. On apply failure, treat as a generator failure: feed the error back to the generator, retry once, then move on.

c. Evaluate. Spawn gan-evaluator against the running app. Evaluator returns: - Per-row score 0-10. - Weighted total 0-10. - Verdict: ship, iterate, or abandon. - Concrete feedback per low-scoring row. d. Check stop. Stop when: - Weighted total >= GAN_PASS_THRESHOLD. Status: passed. - Verdict is abandon. Status: aborted by evaluator. - Iterations >= GAN_MAX_ITERATIONS. Status: budget exhausted. - Score has not improved across the last 2 iterations. Status: plateau.
Close. Write gan-harness/result.md with: final score, iteration count, verdict, list of changed files, and a one-paragraph next-step suggestion.

Single-Writer Invariant (NON-NEGOTIABLE)

gan-generator and gan-evaluator are briefed: "Return your output as text. Do NOT call Edit, Write, or MultiEdit. Do NOT modify any files." The orchestrator applies. If a subagent ignores the briefing, discard its response and respawn.

Rules

Planner runs once. Re-planning mid-loop indicates the rubric was wrong; abort and start a fresh /gan instead.
Apply costs are real. Cap GAN_MAX_ITERATIONS at 8 by default; loops past that have diminishing returns.
Always check GAN_PASS_THRESHOLD against the rubric's weighted total, not against any single row.
The evaluator must run the same evidence path each iteration. Switching from screenshot to Playwright mid-loop invalidates the score history.

Common Rationalizations

"Letting the generator write directly saves a step": two writers corrupt the iteration history. Discard any response that edited files and respawn.
"The rubric is off, so re-plan mid-loop": a new rubric makes earlier scores incomparable. Abort and start a fresh /gan.
"Playwright is slow, switch to screenshots for this round": changing the evidence path invalidates the score history. Keep GAN_EVAL_MODE fixed for the run.
"One row scored 9, so it passes": the threshold applies to the weighted total.
"One more iteration will clear the plateau": two iterations without improvement is the stop condition. Close with status plateau.

Red Flags

Stop and discard the response when gan-generator or gan-evaluator called Edit, Write, or MultiEdit.
Stop and report when the dev server is unreachable on GAN_DEV_SERVER_PORT after running GAN_DEV_SERVER_CMD.
Stop and ask for a corrected rubric when the planner's 5 weights do not sum to 10.
Stop the loop when the weighted total has not improved across the last 2 iterations.

Verification

gan-harness/result.md exists with the final score, iteration count, verdict, changed files, and a next-step paragraph.
The stop status is one of passed, aborted by evaluator, budget exhausted, or plateau, and matches the last evaluator output.
The iteration count is at most GAN_MAX_ITERATIONS.
gan-harness/iterations/ holds one record per iteration, all scored with the same GAN_EVAL_MODE.
git status lists only files from the planner's file list plus gan-harness/.

Related skills

/plan. Use first to scope the brief. The plan can be the input to /gan.
/plan multi-execute. Different parallelism shape: multi-execute splits one task across model tiers; /gan iterates one task through a feedback loop.
/test. The evaluator may call /test under the hood when GAN_EVAL_MODE=code-only.
/loop. The Claude Code built-in for recurring or self-paced execution. /gan is a single-task iteration; /loop is a cadence.

SKILL.mdskills/gan/SKILL.md