fable-plan — durable decision plan before execution (hard multi-step only)

This skill turns a measured result into an action. In a controlled A/B (eval/technique-ab/run-plan-first.mjs, GPT-5.5-judged, n=12 on deliberately hard 5-part tasks), externalizing a numbered plan before executing beat diving straight to the deliverable 9–1 of 10 decided (90%, p=0.0215). On hard multi-step work, the plan-first deliverable was better organized and more correct. The cost is one short planning pass — so this is only worth it when the task is genuinely hard.

Enter this flow only when fable-plan is explicitly pulled for genuinely hard multi-part work. Never enter it from a default or always-on path.

The artifact is plans/<lowercase-kebab-slug>.md. It is a durable decision/criteria snapshot, not a second execution-state store. It may record the outcome, scope, acceptance criteria, dependency order, risky assumptions, and non-goals. It must never record progress, status, completed steps, retries, verification debt, or task checkboxes — those mutable facts belong only in the run ledger.

When NOT to use this (this bound is the whole point)

• A single-file change, a one-step fix, a question, or anything you can do correctly in one pass → just do it. A plan here is pure overhead — the A/B win was on hard 5-part tasks, and an A/B on routing found always-planning adds length without quality on simple work.
• You haven't read the relevant code yet → read enough to plan against reality, not a guess.

Optional clarify gate (never a default stage)

After reading the available context, enter this gate only if both conditions hold: the task remains genuinely ambiguous, and a wrong assumption would be costly or hard to reverse. Ask at most one clarifying question, aimed only at this task's acceptance criteria. If either condition is false — or the available context is already enough — ask no question and act when you have enough. A second question is not authorized; never turn this into an interview or follow-up round.

If that one answer adds task-specific acceptance criteria, capture them before plan/contract creation:

• add each criterion's id and description to contract.criteria;
• put the same descriptions under the plan's ## Criteria; and
• place this exact block in that section and pass it as optional task_criteria to fable_check:

<!-- fable-task-criteria:v1 -->
- [task.mobile-layout] Include a mobile layout.
- [task.desktop-layout] Include a desktop layout.
<!-- /fable-task-criteria -->

Use one line per criterion. Keep the IDs stable and record only criteria established for this task; do not invent extra requirements.

Procedure

1. Confirm the explicit trigger and complexity floor. Roughly four or more distinct parts, multiple files, or real ordering/dependency. Otherwise skip the entire plan flow.
2. Read enough reality to decide. Inspect the relevant code, constraints, and existing run contract before writing; do not plan from guesses.
3. During the plan-writing phase, write only plans/<slug>.md. Do not touch product code. Use the exact template below and replace every comment with decision content.
4. Lint before execution. Every required section and both Scope subsections must be non-empty; Ordered dependencies must be numbered. The repository/runtime linter is node orchestration/lib/plan-artifact.mjs lint plans/<slug>.md when that path is available.
5. Hash-bind the plan to the run contract. Record its exact SHA-256 as contract.planHash with the absolute contract.planPath; contract.created preserves the initial binding in the append-only ledger.
6. Execute against the decisions. Record progress, check results, blockers, and debt only as typed run-ledger events. Never update the plan merely because work advanced.
7. Handle material steering explicitly. A decision change may revise the relevant plan section, but it must be re-linted and followed by either a typed plan.rebound event to the new hash or a hash-specific plan.deviation.recorded event with a reason. Silent hash divergence is invalid.

Required template

# Plan: <short decision title>

<!-- Decision/criteria snapshot only. Progress and debt belong in the run ledger. -->

## Outcome

<!-- State the observable outcome this work must produce. -->

## Scope

### In

<!-- List what is in scope. -->

### Out

<!-- List what is explicitly out of scope. -->

## Criteria

<!-- List acceptance criteria. Do not use progress checkboxes. -->

## Ordered dependencies

<!-- Number dependencies/steps in the order decisions require them. -->

## Risky assumptions

<!-- List assumptions whose failure would change the decision. -->

## Non-goals

<!-- List outcomes this plan deliberately does not pursue. -->

Expected output

A lint-clean plans/<slug>.md, its contract-bound SHA-256, then the deliverable built against those decisions. Mutable progress and debt remain solely in the run ledger.

Honest bound

The A/B measured a forced-choice quality preference on hard 5-part tasks; it does not show plan-first helps on simple work (it doesn't — that's why this skill is trigger-gated), and it is a one-shot quality result, not a multi-session or productivity number. Use it where the task is actually hard.