Consistency: One Feature, Every Surface, Same Answer

A coupon that works on the cart page but not in the cart drawer is not a code bug a type checker sees — it is a surface bug. Each surface either recomputes the value itself or was never told about the new field. Blast-radius tools find callers of a function; they cannot find a drawer that does its own maths. This skill is three habits that close that gap.

1. Map every surface before you change anything

Write the table first — in the plan (Superpowers writing-plans), or at the top of the PR.

Find them by data, not just symbols:

• graphify query "where is <value> computed and displayed" (or LSP find-references) for code paths.
• grep -rn the field names in every case form (discount_cents, discountCents, DISCOUNT), the UI copy ("Subtotal", "Discount", "You saved"), and the route/query keys.
• Pass-through files count: a serializer or email template that drops the new field is a surface.

Every empty "reads value from" cell is the bug you would have shipped.

2. One source of truth

Every surface reads the value from one server calculation (one function, one endpoint or query key). No surface does its own arithmetic — the client previews at most, the server is authoritative. When the map shows a surface computing its own total, fix that first: route it through the shared calculation instead of copying the new rule into it. Money stays in integer minor units end to end.

3. One spec that proves they agree

Add e2e/consistency-<feature>.spec.ts (template: templates/consistency.spec.ts). It drives the real app, applies the change, then reads the same values from every surface and asserts they are identical. Give each value the same data-testid on every surface (summary-subtotal, summary-discount, summary-total, …) so one helper reads them all. Read each surface inside its own region (drawer = getByRole("dialog"), page = main): a mini-cart is usually in the DOM on every page, so a page-wide read picks up the wrong copy.

Cover the edge cases, not just the happy path. For a coupon: valid · invalid · expired · below minimum spend · removed again · replaced by another code · stacked (if allowed) · quantity change after applying · reload / new session (persistence).

verify.sh already runs every Playwright spec, so the Stop hook blocks "done" the moment a later change breaks one surface.

Pitfalls

• Fixing the surface you were looking at. The bug report names one page; the map finds the other five.
• Asserting a hard-coded number per page. Assert surfaces equal each other and the server's answer — then a pricing change doesn't rewrite the spec.
• Mocking the server in this spec. The point is the real calculation reaching every surface.
• Forgetting async surfaces. Emails, webhooks and exports: assert the payload the job would send (test the render function), even if the browser can't see it.

Works with →

• Superpowers brainstorming → writing-plans — the surface map (step 1) is part of the plan.
• graphify / lsp-plugins — find code paths; this skill adds the data/copy search they miss.
• browser-verify — writes and runs the spec; the compare loop checks each surface looks right.
• verify-gate — the spec runs inside verify.sh; a disagreement blocks the turn.
• taste-code — one shared calculation beats N copies; no duplicated pricing logic.

Verification

• Break one surface on purpose (e.g. make the drawer ignore the discount) → the spec fails and names that surface.
• Remove the break → ./verify.sh passes.