plan-to-slices

name	plan-to-slices
description	Decompose a PROJECT_BRIEF.md into vertical, TDD-ready implementation slices. Each slice is independently demoable. Use when user wants to break down a project into slices

Read PROJECT_BRIEF.md and produce SLICES.md — an ordered sequence of vertical, demoable slices, each with acceptance criteria a dev can turn into tests

If there's no brief, tell the user to run define-project first. Slicing without a brief just ships the ambiguity forward into dev turns

Process

1. Read the brief. Scope, MVP, technical approach, risks
2. Explore the codebase if one exists. Module boundaries, test patterns, what's already there. Slices land where seams already exist
3. Identify vertical slices. Slice 1 is the thinnest end-to-end path that proves the core approach
4. Order by dependency and risk. Uncertain integrations push forward so you learn early. User-facing value before internal polish. Clearly label dependencies so independent slices can run in parallel
5. Write acceptance criteria — 3–5 observable outcomes per slice

Slicing rules

• Every slice is vertical. No "set up the database" slice unless the project is pure infra. A horizontal slice can't be demoed, so you lose the feedback that makes slicing valuable
• O11y lives inside the slice that introduces the behavior
• Sizes within an order of magnitude. If one slice is 10x the others, split it; if one is 1/10th, fold it in. Agents complete well-sized vertical slices strongly — don't over-fragment out of caution

Acceptance criteria

Observable and verifiable. "POST /users returns 201 with user ID" — not "user creation works." Scoped to this slice; don't duplicate criteria earlier slices already own

Cover: happy path always; input validation where a new interface appears; edge cases when core to the slice; integration with prior slices from slice 2 onward. The implementer discovers more tests during red-green-refactor — you're defining the boundary of done, not the full test plan

Output — SLICES.md

# [Project Name] — Implementation Slices

> Generated from [PROJECT_BRIEF.md](./PROJECT_BRIEF.md)

## Slice 1: [Verb] [what] *(MVP)*

### What this delivers
One sentence — what a user/system can do once this ships

### Acceptance criteria
1. ...
2. ...

### Implementation notes
Key decisions, constraints, layers touched. For slice 2+: **Depends on**: Slice N

---

(repeat per slice)

Quality bar

• One SLICES.md, not scattered files
• Every slice is demoable and testable
• Dependencies between slices are explicit
• A dev reading SLICES.md knows what to build first, what done looks like, and what order to go in
• Slices are right sized for expert agents