UitbreidenOS Vertical Slice Planner
When to activate
- Planning a new feature or project before building starts
- User wants to break down a feature into work units before writing code
- Claude defaulted to a sequential "database → API → frontend" phased plan and you want cross-layer slices instead
- You need to order work by risk or value rather than by technical layer
- Feature scope is unclear and needs decomposition into independently shippable increments
When NOT to use
- Simple single-endpoint tasks or small bug fixes scoped to one layer
- Tasks that are already a single vertical unit (e.g., "add a new field to this form")
- Very small tasks under half a day of estimated work — merge them, don't slice them
- When the team has already committed to a specific phased delivery contract and cannot reorder work
Instructions
The problem with sequential phases:
AI models default to: Phase 1 = database schema, Phase 2 = API endpoints, Phase 3 = frontend. This delays end-to-end integration feedback until the last phase, where architectural problems surface too late to fix cheaply. You don't see a working path through the system until phase 3 is done.
Vertical slicing approach:
Each slice is a thin cut across all layers — database + API + frontend + acceptance criteria — that delivers a working, testable end-to-end capability. Every slice ships independently. A slice is done when a user can interact with it, not when a layer is done.
Step 1 — Identify the core user actions (not technical components)
Ask: "What can the user actually do?" — not "What tables do we need?"
Bad decomposition: users table → /users endpoint → UserList component
Good decomposition: user can search by name → user can filter by status → user can export results
List each distinct user action. These become your slice candidates.
Step 2 — Order slices by value and risk
Rank slices:
- Highest business value first — what unblocks the most downstream work or user testing?
- Highest integration risk first — what has the most unknowns across layers?
- Tracer bullet first in execution — the thinnest possible path that validates the architecture before building content
Step 3 — Define each slice
Use this template for every slice:
Slice: [Name]
User action: [What the user does — written as a user action, not a technical task]
Layers:
Database: [schema change, migration, or query involved]
API: [endpoint(s) — method, path, request/response shape]
Frontend: [UI component(s) affected]
Integration: [any external service calls, queues, or event emissions]
Acceptance criteria:
- [Specific, testable condition — observable behavior, not implementation detail]
- [Additional condition]
Definition of done: [How to verify this slice is fully complete and ready to merge]
Estimate: [days]
Step 4 — Size each slice
Target 1–3 days of work per slice. If a slice takes longer, split it. Signals a slice is too large:
- More than two API endpoints created in one slice
- Schema migration and business logic in the same slice
- Frontend and a new background job in the same slice
Slices shorter than half a day are too granular — merge them with an adjacent slice.
Step 5 — Start with a tracer bullet
The first slice to execute is the thinnest possible path across all layers — even if it returns empty results or stub data. This validates:
- The database can be queried from the API
- The API response reaches the frontend
- The frontend can render the shape of a real response
If the tracer bullet breaks, you've found your hardest problem on day one — when it's cheapest to fix.
Build content (real queries, real UI, real validation) in subsequent slices on top of a proven foundation.
Output format:
VERTICAL SLICE PLAN — [Feature Name]
User actions identified: [list]
Slice order rationale: [one sentence on why this order]
Slice 1 (Tracer): [name] — [one-line description]
Slice 2: [name] — [one-line description]
...
[Full slice definition for each slice using the template above]
Example
Input: "Plan the search feature for our product catalog. Break it into vertical slices ordered by risk."
Expected output:
VERTICAL SLICE PLAN — Product Catalog Search
User actions identified: search by keyword, filter by category/price, sort results, paginate results
Slice order rationale: tracer bullet first to validate search infrastructure, core search next, filters last (highest implementation complexity)
Slice 1 (Tracer): Basic search endpoint returns empty results
Slice 2: Keyword search returns matching products
Slice 3: Filter by category and price range
Slice 4: Pagination
---
Slice 1 (Tracer): Basic search endpoint returns empty results
User action: User submits a search query and receives a response (empty is acceptable)
Layers:
Database: Add products table with name, description, price, category columns; seed 10 records
API: GET /search?q= → returns { results: [], total: 0 }
Frontend: Search box + results list renders an empty state message
Integration: none
Acceptance criteria:
- Submitting any query returns HTTP 200 with the correct response shape
- Frontend renders without errors when results array is empty
Definition of done: End-to-end request from search box to database and back completes without errors
Estimate: 0.5 days
Slice 2: Keyword search returns matching products
User action: User searches by keyword and sees matching products
Layers:
Database: Full-text index on name and description columns
API: GET /search?q= → returns { results: [{ id, name, price, category }], total: N }
Frontend: Results list renders product cards with name and price
Integration: none
Acceptance criteria:
- Searching "laptop" returns products with "laptop" in name or description
- Results are ordered by relevance (name match ranked above description match)
- Empty query returns all products up to page size
Definition of done: Search returns accurate results for 10 test queries; result cards render correctly
Estimate: 2 days
Slice 3: Filter by category and price range
User action: User narrows results using category dropdown and price range inputs
Layers:
Database: Query updated to accept category and price_min/price_max parameters
API: GET /search?q=&category=&price_min=&price_max=
Frontend: Filter panel with category dropdown and price range inputs; results update on apply
Integration: none
Acceptance criteria:
- Applying a category filter returns only products in that category
- Price range filter returns only products within the range
- Filters combine correctly with keyword search
Definition of done: All filter combinations tested; filter state persists on page refresh
Estimate: 1.5 days
Slice 4: Pagination
User action: User navigates through multiple pages of results
Layers:
Database: LIMIT/OFFSET applied to query
API: GET /search?q=&page=&page_size= → adds { page, total_pages } to response
Frontend: Pagination controls render; page state updates URL
Integration: none
Acceptance criteria:
- Page 2 returns the correct offset of results
- Total pages reflects actual result count
- Navigating to a paginated URL directly returns the correct page
Definition of done: Pagination works across all filter and search combinations
Estimate: 1 day