Vertical Slice

Overview

Build a feature as a sequence of thin vertical slices. Each slice cuts through the full stack — UI → logic → data — and delivers exactly one observable behavior, kept behind a feature flag until it is complete and tested.

Core principle: thin vertical slice (defined in ${CLAUDE_PLUGIN_ROOT}/references/engineering-principles.md). The opposite is horizontal building — finishing the whole data layer, then the whole logic layer, then the whole UI. Horizontal work integrates last, which means every integration risk surfaces at the end, all at once, when there is no budget left to absorb it. Vertical slices integrate first and continuously: each slice proves the stack connects before you build the next one.

A slice is the unit of progress. Finish one — flagged, end-to-end, tested, committed — before starting the next. Cutting a feature into sliced tasks happens during planning ([[create-tasks]]); this skill governs building each slice. See [[work-on]] for driving the work item end-to-end and [[test-driven-development]] for the inner loop of each slice.

When to Use

• A feature that touches more than one layer (endpoint + service + table; component + handler + store)
• Any work big enough to tempt "I'll build all the models first, then wire them up"
• Work that must ship incrementally or be safe to pause and resume

Skip for a change confined to one layer, or a fix small enough that there is nothing to slice.

Process

1. Cut the feature into thin slices

List slices, each delivering one behavior a user or caller can observe. "User sees their order total" is a slice; "build the orders table" is not — it is a layer. If a slice can't be stated as an observable behavior, it is horizontal; re-cut it. Prefer the thinnest slice that still demonstrates the stack end-to-end (a single happy path through every layer beats a complete-but-disconnected layer).

2. Put the flag in first

Add the feature flag before building the slice, defaulting off. The slice's new path lives behind it. This keeps main shippable at every commit and lets unfinished slices merge without exposing half-built behavior.

3. Build the slice end-to-end with TDD

Implement the one behavior through every layer it touches, driven by [[test-driven-development]]. A slice is done only when the behavior works from its entry point (UI / API) all the way to persistence — not when the layer it added is "complete."

4. Test, commit, then next slice

Commit the slice with its tests green before starting the next. Each slice is a clean revert point. Repeat until the feature is whole, then remove the flag.

Rationalizations

Red Flags - STOP

• A "slice" described as a layer ("the data model", "the API layer") rather than a behavior
• Building a complete layer before anything connects to it end-to-end
• New code path not behind a flag, leaving main un-shippable mid-feature
• Slices that can only be merged or reverted together
• Deferring all UI (or all persistence) to a final "wire it up" phase

Verification

A slice is done only when every box is checked:

• Behind a feature flag, defaulting off until the feature is complete
• End-to-end: works from entry point through to data, not just one layer
• Delivers one observable behavior, stated as such
• Tested green via [[test-driven-development]] and committed
• Independently revertible — reverting this slice breaks no other slice

Can't check every box? It is a horizontal layer, not a vertical slice. Re-cut it.