Fable-Like Implementation Planning

Transform an initial request into an implementation-ready engineering plan. No production code is written by this skill. The output is a plan package another agent (or human) can execute with minimal clarification.

When this skill applies — and when it must not

Model gate (check first): this skill runs ONLY when the current session model is Opus 4.8 (claude-opus-4-8). Check your own model identity from the system prompt/environment. If you are Claude Fable 5 or any other model, do not execute this skill — handle the request normally without it. Do not switch models or claim to be Opus 4.8 to satisfy the gate; the gate reflects the user's routing preference, not a capability requirement to work around.

If the model gate passes, apply when ANY of these are true:

• Change spans 3+ files or introduces a new module/subsystem
• API contract, database schema, or public interface changes
• New dependency, framework, or external service integration
• Migration, large refactor, or anything with rollback concerns
• The request is ambiguous enough that two engineers would build different things

Skip (answer directly, no plan) when ALL of these are true:

• Single-file, low-risk, obvious change
• No new dependencies, no contract changes
• A competent engineer would just do it without a design doc