plan-feature

name	plan-feature
description	Use when the user wants a plan or breakdown for building a feature (or significantly reshaping one) before any code is written. Turns a feature request into a written implementation plan of self-contained, independently testable vertical slices. Do NOT use for bugfixes, refactors with no behavior change, or trivial one-step tasks.

Plan feature

Turn a feature request into a written implementation plan composed of self-contained, independently testable vertical slices. The plan is saved as a markdown file in the repo so it can be reviewed, tracked, and executed task-by-task.

This skill plans only. It does not implement the feature. After writing the plan, hand control back to the user.

Core rules

1. Never assume. Read the user's request literally. Any gap, ambiguity, or unstated decision is a blocker, not a thing to guess at.
2. Block until ambiguities are resolved. Do not write a plan while open questions remain. Use the question tool to ask, wait for answers, and only then proceed. If new ambiguities surface mid-planning, stop and ask again.
3. Every sub-task is a vertical slice. Each task delivers an end-to-end, user-or-API-observable piece of behavior (e.g. route + load + component + test), not a horizontal layer (e.g. "write all the types").
4. Every sub-task is independently testable. Each task names the concrete test(s) that prove it works and the command to run them. A task is not done until its tests pass. If in doubt of what it is the required outcome for a given task, ask.

Workflow

1. Confirm scope

Confirm the request is a new feature or a meaningful reshape of one. If it's a bugfix, a refactor with no behavior change, or a trivial one-step task, say so and stop — no plan is needed.

2. Interrogate until unambiguous

Read the request and list everything you do not know for certain. Ask the user about all of it using the question tool before planning. Cover at least:

• Goal & scope — what's explicitly in scope, and what's out of scope.
• User stories / acceptance criteria — what observable behavior defines "done"?
• Inputs & data — data sources, shapes, validation, edge cases, empty/error states.
• UI & UX — routes, pages, components, states (loading/empty/error), responsive/a11y expectations — only if the feature has a UI.
• API & contracts — endpoints, request/response shapes, status codes, auth.
• Server vs client — SSR vs CSR, where logic runs, what's in src/lib/server/.
• Auth & permissions — who can use it, what gates access.
• Dependencies & constraints — new packages, external services, existing code to reuse or avoid touching.
• Non-functional — performance, accessibility, i18n, telemetry, if relevant.
• Testing expectations — what must be covered and at what level (see Testing below).

Do not ask about things the user already specified. Ask only real, decision- changing questions. Keep asking until you could implement the feature without guessing.

3. Decompose into vertical slices

Break the feature into the smallest sequence of slices where each slice:

• Delivers an end-to-end, observable behavior on its own.
• Builds on prior slices but does not depend on later ones.
• Can be implemented, tested, and merged independently.
• Has a clear, verifiable acceptance check backed by tests.

Prefer a "walking skeleton" first slice (thinnest end-to-end path), then layer behavior in subsequent slices.

Make sure the created slices (each corresponds to a sub-task) follow a DAG (Direct Acyclic Graph) based on the dependencies of each task.

4. Write the plan file

Save to plan.md. Use the template below. Do not start implementing.

5. Hand off

Show the user the path to the plan and a one-line summary of the slices. Let them review and decide when to start. Do not begin coding unless they ask.

Testing (project conventions)

Reference these in each slice's test section so tasks are runnable in isolation:

• Component tests (browser/chromium project): name files *.svelte.spec.ts. Run one with npx vitest --project=client src/path/to/File.svelte.spec.ts.
• Server/unit tests (node project): any other *.spec.ts. Run one with npx vitest --project=server src/path/to/file.spec.ts.
• Full suite: npm test. Type check: npm run svelte-check. Lint/format: npm run check.
• expect.requireAssertions is on — every test must assert at least once.

Each slice should specify the exact test file path(s) and the exact command to verify that slice alone.

Plan file template

# Plan: <Feature name>

## Summary

<2–4 sentences: what the feature is and the observable outcome.>

## Scope

**In scope:** <bullets>
**Out of scope:** <bullets>

## Acceptance criteria

- <Observable, testable criterion>
- <...>

## Open questions

<Resolved decisions captured from the Q&A, or "None — all resolved." There must
be no unresolved blockers here before implementation starts.>

## Slices

### Slice 1 — <thin end-to-end behavior>

- **Delivers:** <observable behavior this slice adds>
- **Changes:** <files/areas to create or edit, with paths>
- **Tests:** <test file path(s)> — <what they assert>
- **Verify:** `<exact command to run this slice's tests>`
- **Depends on:** <none | earlier slice numbers>

### Slice 2 — <next behavior>

- **Delivers:** ...
- **Changes:** ...
- **Tests:** ...
- **Verify:** ...
- **Depends on:** Slice 1

<...more slices...>

Anti-patterns to avoid

• Writing a plan while any requirement is still assumed or unclear.
• Horizontal tasks ("define all types", "build all components", "write all tests") — these aren't independently testable.
• Slices with no concrete test or no run command.
• A slice that can't be verified until a later slice exists.
• Starting implementation. This skill stops at the written plan.