A MUSCLE (*.muscle.json) declares reusable behavior around the SKEL lifecycle: import, validation, prompt assembly, generation routing, render completion, and export.
MUSCLEs never mutate a story directly. They return proposed patches; the host checks approved capabilities, protects locked content, applies atomically, re-validates, and rolls back failures.
- Observe — audit without mutation.
- Transform — enforce house style, normalize prompts, or process outputs.
- Veto — stop continuity breaks or invalid delivery before they ship.