State Machine skill

State machine

This skill takes one component and renders it on a throwaway page in every state it can be in. Mock data feeds each state, a switcher flips between them and the page is deleted in one step when you are done.

It is a workbench, not a review. The page stays up while the user works on the visuals, one state on screen and every other a keypress away. Stacking hostile scenarios on one page for a single look is break, exploring alternative designs is variant and reviewing finished work is interface-review.

1. Scope one component

One component per run: the audit log table, the network panel, the billing card. "The settings page" spans several, so list them and ask which one.

Restate it in one sentence: what the component shows, where it renders in production and where its data comes from.

2. Find the states in the code

The states are the branches the component already has. Read it and its data hooks for every one:

Kind	States
Data	Loading, empty, error, one item, a typical set, a set long enough to scroll or paginate, some requests resolved and others pending, stale data refetching
Account	Plan tier, role and permission checks, an owner versus a member
Feature	Flags, trials, limits reached, a disabled or locked feature

A state the code cannot reach is not a state. Do not add a branch to render one. Where the design shows a state the code lacks, list it as missing and leave it out.

Write the set down before building, one line each, named the way the product talks about it: empty, enterprise, member-no-access. Group them under headings in the product's words, by what changes. Use Data and Account or narrower ones such as Connection or Access. The switcher shows these groups. Say which kinds you dropped and why in one line. Combine kinds only where the code branches on the combination, such as member-empty, never as a cross product.

3. Build the throwaway page

A scratch route inside the app holds the real component, imported from the project and untouched. The route inherits the app's layout, fonts, global styles and providers for free.

Where the framework splits server from client components, the page is client code, "use client" in Next. Otherwise fixture data can silently vanish crossing that boundary, and the page renders empty.

Render one instance in a container as wide as the component is in production. The container, the fixtures and the switcher are everything the page adds, with no fonts, styles or themes of its own. Keep the page, its fixtures and its switcher in one folder, such as /states/audit-log, so removal is one delete.

4. Feed each state at the data boundary

Supply the data where the component receives it, from the page, without editing the component. In order of preference:

1. Props, where the component takes its data as props.
2. The app's own data seam: a seeded query cache, a mock provider or the project's request mocks.

A fixture threaded through props five levels down tests a path production never takes. Where the only way in is editing the component, say so and ask before adding a seam.

Make the data look like the product. Real-shaped names, timestamps, amounts and the item counts users actually have. Three rows of "Test item" make every state look fine.

Loading and pending states hold still. Keep them pending until the switcher moves, rather than resolving after a timeout.

5. Add the switcher

A grouped list beside the component flips the __state search param, so every state is a link. Where there is no room beside the component, it moves above it as a select with the same groups. The double underscore keeps it clear of any param the app's layout reads. switcher.md holds the spec.

6. Confirm every state renders, then hand over

Load the page once in a browser already at hand and flip through every state. The component shows the mock data, not the real data and not a blank region. Mock data that never appears is the common failure here. Live sync, a cache that refetches over the seed or a server boundary dropping the fixture all cause it. Fix the plumbing before handing over.

With no browser at hand, say so and hand the URL over for the user to check.

Then hand over the URL, the state list and the switcher keys. Stop there. Changing how a state looks is the user's next request, not part of this one.

Re-check every state after each change

After a visual change, flip through every state again, since a fix for one state often breaks another.

Remove it in one step, on the user's word

Delete the page folder and any seam the user approved. Then search the codebase for the route and fixture names, and check that the diff touches nothing else of the setup.

Before you finish

Mistake	Fix
The component edited to accept fixtures	Feed data from the page through props or the app's data seam; ask before adding a seam
A rebuilt lookalike component on the page	Import the real component from the project
The page restyles or re-themes the component	The app's layout, fonts and tokens as they are
Fixtures passed as props deep in the tree	Supply the data where the component receives it
A branch added to the component to show a state	Leave out states the code cannot reach, and list missing ones
"Item 1", "Test user", three rows	Product-shaped data in real quantities
Loading resolves after a timeout	Hold it until the switcher moves
Handed over without loading each state	Load every state; a blank or real-data state means broken plumbing
Switcher styled with the project's tokens	Keep it visibly outside the design system
One long row of tabs, one per state	A grouped list beside the component that scrolls on its own; a select above it where the list does not fit
Visuals changed unasked	Hand over and wait
Page deleted in the same turn it was built	Remove it only on the user's word
Route or fixture names left behind	Search for both after removal