| name | decompose-into-slices |
| description | Break a plan or milestone brief into independently-grabbable vertical slices (tracer bullets), written to `M###-ROADMAP.md` by default (GitHub issues only on explicit confirmation). Prefers many thin slices over few thick ones and marks dependency order. Use when asked to "break this into slices", "decompose the plan", "vertical slices", "break into issues", or when a plan needs task-level decomposition. |
Decompose an approved plan into the smallest useful vertical slices that each cut end-to-end through every relevant layer. Primary output is the `Slices` section of `M###-ROADMAP.md` (matching the template at `src/resources/extensions/gsd/templates/roadmap.md`). Secondary output, only with explicit confirmation, is a set of GitHub issues with blocked-by relationships wired up.
This skill runs after the brief is stable — `M###-CONTEXT.md` exists and the user has signed off on scope. It's the bridge from "we know what we're building" to "we know in what order and chunks." The vertical-slice discipline (tracer bullets) is non-negotiable here — it's the core of what makes GSD slices demoable and parallel-safe.
Typical invocation points:
- After
write-milestone-brief (or after a discuss phase that produced a brief)
- When a roadmap exists but slices are too thick, too few, or poorly ordered
- When exporting the plan for external collaborators (GitHub issues)
<core_principle>
VERTICAL, NOT HORIZONTAL. A slice that adds "schema + API + UI + tests for feature X on one narrow path" is vertical. A slice that adds "all schemas for all features" is horizontal and is wrong. Horizontal slices destroy the demoability property that makes slice completion meaningful.
MANY THIN, NOT FEW THICK. If a slice could be split into two demoable pieces, split it. Thin slices retire risk earlier, parallelize better, and give the user faster feedback on whether the direction is right.
DEPENDENCY GRAPH, NOT LINEAR LIST. Slices that don't depend on each other should be marked that way — means the slice can start immediately. The GSD engine uses this to parallelize.
</core_principle>
