abildtoft/kramme:code:api-design

API and Interface Design

Design stable APIs and module boundaries with a contract-first workflow. Decide the shape, validation points, error cases, and naming before writing the handler, so the interface does not have to be rediscovered or reshaped once callers exist — the contract is surface area you can never take back.

When to use

• Adding an HTTP endpoint, GraphQL mutation, or RPC surface that external or cross-team callers will hit.
• Exporting a new public module, package, or SDK function from a shared library.
• Changing the shape of an existing response, error, or input payload.
• Introducing a new resource, pagination scheme, or list endpoint.
• Self-check while implementing: if you catch yourself sketching a handler before sketching the contract, stop and run this skill on the contract first.

Hyrum's Law

"With a sufficient number of users of an API, all observable behaviors of your system will be depended on by somebody, regardless of what you promise in the contract."

Implication: every observable behavior is part of the contract — including bugs, timing quirks, ordering of fields, and undocumented side effects. You cannot "fix" an observable behavior later without breaking at least one caller. Design the contract now as if every incidental detail is permanent, because it is.

The Rules

Rule 0 — Simplicity First

Before sketching any type or route, emit a SIMPLICITY CHECK marker stating the smallest coherent interface that satisfies the requirement. Only expand beyond that if a concrete requirement forces it.

SIMPLICITY CHECK: <the smallest interface that meets the requirement>

If the interface you end up designing is not the smallest version, write a second line explaining what forced the expansion. If there is no forcing requirement, ship the smaller surface.

Rule 1 — Contract first

Design the contract before the handler. The contract is: input type, output type, error cases, HTTP status mapping, and naming. Write it down — as a TypeScript type, an OpenAPI stub, a comment block, whatever the project reads — before writing implementation code.

Signs the contract is not first:

• You are typing a handler body and still deciding what fields go on the response.
• You are choosing a status code after observing what the code happens to throw.
• Input validation is being sprinkled in as you hit edge cases.

If any of these are happening, stop. Write the contract, then continue.

Rule 2 — Validate at boundaries, not between internals

Validation belongs at the points where untrusted data enters your trusted code — once — and nowhere else; internal functions trust their inputs. The full trust-boundary doctrine (what counts as a boundary, the validate-once pattern, third-party responses) lives in /kramme:code:harden-security.

Convention detection (before Rules 3–5)

Before applying Rules 3–5, scan the project's existing API surfaces for an established error shape, naming style, or pagination envelope. If a consistent convention exists, match it — consistency with existing callers beats the canonical shapes below (Hyrum's Law has already locked the existing shape in). The canonical shapes in Rules 3–5 apply only to greenfield surfaces with no established convention.

Rule 3 — Consistent error shape

Every error returned from the API uses the same shape: { code, message, details? } (the APIError type). HTTP status comes from a fixed mapping: 400 invalid data, 401 not authenticated, 403 not authorized, 404 not found, 409 conflict, 422 validation failed, 500 server error (never expose internals).

Never mix error shapes across endpoints — callers build parsing logic against the first shape they see, and Hyrum's Law nails that shape in place. The canonical APIError type, full status mapping, worked examples per status code, and the 500-disclosure rule live in references/error-shapes.md.

Rule 4 — Naming conventions

Quick reference for REST/JSON surfaces:

• Endpoints — plural nouns, no verbs (/tasks, not /createTask).
• Query params and response fields — camelCase.
• Booleans — prefix with is, has, or can (isArchived, hasPermission).
• Enum values — UPPER_SNAKE (STATUS_ACTIVE, not active or Active).

Full table plus worked good/bad examples and per-ecosystem deviations (Python snake_case, Go PascalCase for exported fields) live in references/naming-conventions.md.

Rule 5 — Pagination shape

List endpoints paginate. Always. Even the ones that "only return a few rows today." Fixed shape:

type PaginatedResponse<T> = {
  data: T[];
  pagination: {
    page: number;
    pageSize: number;
    totalItems: number;
    totalPages: number;
  };
};

Callers build pagination UI and prefetching logic against this shape. Adding pagination later is a breaking change; adding it now is free.

Rule 6 — TypeScript patterns

Three patterns keep TypeScript interfaces stable under Hyrum's Law:

• Discriminated unions for variants — tag each shape with a literal field so callers narrow on the tag, not on field presence.
• Input / Output type separation — CreateTaskInput is not Task. Do not reuse the read type as the write payload.
• Branded types for IDs — give each ID kind a phantom-branded string so the compiler refuses cross-resource swaps.

Code examples, factory patterns, and the per-pattern rationale live in references/typescript-patterns.md. Non-TS projects can skip this rule, but the underlying intent (don't conflate read and write shapes; don't let raw strings flow where a typed ID is expected) applies everywhere.

Rule 7 — Noticed but not touching

When you notice an adjacent endpoint with an inconsistent shape, a sibling that uses a different error format, or a neighboring module that missed pagination, emit a NOTICED BUT NOT TOUCHING marker and move on. Do not silently reshape the adjacent surface during this design.

NOTICED BUT NOT TOUCHING: <what you saw>
Why skipping: <out-of-scope / unrelated / deferred>

The reason: reshaping adjacent contracts is itself a breaking change, and it deserves its own design pass — not a drive-by edit during work on a different surface.

REST patterns

• Resource design — nouns, hierarchy via paths (/teams/:teamId/tasks, not /getTasksForTeam).
• Filtering — via query params (/tasks?status=OPEN&assignee=abc).
• Partial update — PATCH with only the changed fields.
• Full replacement — PUT with the complete resource.

Design It Twice mode

The first interface that comes to mind is rarely the best. Design It Twice drafts radically different shapes for the same problem in parallel, then compares them in the open before any single design is locked in.

Platform requirement — the parallel form requires a multi-agent capability (Claude Code's Agent tool). On platforms without it, fall back to the sequential variant: draft each design in turn under the same constraint slate, taking care not to read prior designs while drafting the next, then compare them with the same rubric. references/design-it-twice.md covers both forms.

When this mode applies — opt-in only. Trigger on --design-twice, on the user phrase "design it twice" or "show me alternatives". For high-leverage surfaces (SDK, cross-team contract) where the obvious shape feels suspiciously obvious, ask before entering this mode. Do not enter it by default; the cost of multiplication is wasted on low-stakes interfaces.

How it works — frame the problem (not the interface), produce 3+ designs each pinned to a different constraint (minimize methods / maximize flexibility / optimize common case / ports & adapters), present each in full sequentially, then compare in prose by depth, locality, and seam placement. End with a single recommendation: pick one, hybridize and explain the borrow, or redesign because the framing was wrong. See references/design-it-twice.md for the full process, prompt template, and comparison rubric.

Relationship to the rules — Design It Twice is upstream of Rule 0–7. Once a design is picked, every rule above applies to that design. The mode does not loosen any rule; it raises the chance the picked design is worth applying them to.

Integration with other skills

• Upstream: kramme:siw:generate-phases — when a planned phase introduces a new interface, run this skill first to lock the contract before slicing begins.
• Companion: kramme:code:incremental — once the contract is locked, the implementation is sliced through the incremental loop. Each slice conforms to the contract rather than rediscovering it.
• Downstream review: the kramme:injection-reviewer and kramme:auth-reviewer agents verify the validation and authorization boundaries set here. A contract that declares its validation boundary makes these reviews mechanical.

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Common Rationalizations

These are the lies you will tell yourself to justify shipping an unstable contract:

• "I'll validate in the service layer, it's cleaner." → No. Validate at the boundary. The service layer is internal; pushing validation there hides the real edge of trust.
• "This endpoint is internal, no pagination needed." → Internal today, public tomorrow. Add pagination now; adding it later is a breaking change.
• "The client doesn't care about the error shape, they just check the status code." → Until one of them doesn't. Then the inconsistent shape becomes load-bearing for exactly one caller, and you cannot fix it.
• "I'll rename this field before anyone uses it." → There is no "before anyone uses it." The first caller is faster than you think.
• "These two endpoints are similar enough to share the same response type." → Reusing a read type as a write payload is the most common source of accidentally public fields. Split them.
• "We can tighten the validation later." → Loosening is cheap; tightening is a breaking change. Start strict.
• "I'll add the status code to the error body so clients can read it there." → Redundant state. The status code is on the response already. Two sources of truth drift.

Red Flags

If you notice any of these in your own design, stop and redesign:

• Verbs in REST URLs (/api/createTask, /deleteUser).
• Mixed error formats across endpoints in the same service.
• List endpoints without pagination.
• Third-party API responses flowing into business logic without validation.
• The response type is the exact same type used as the input — no Input/Output separation.
• Internal field names leaking into the response (user_internal_id, debug_payload).
• Status code and response shape inconsistent — 200 with an error field, 400 with no error at all.
• A field whose meaning depends on another field's value, without a discriminated-union tag to make it safe.
• Adjacent endpoints in the same module use different shape conventions, and this design adds a third.

Verification

Before declaring the contract done, self-check every item:

• Does the endpoint follow the resource-noun naming rule (plural, no verbs)?
• Do all error responses use the APIError shape with a correct HTTP status from the Rule 3 mapping (full table in references/error-shapes.md)?
• Do list endpoints return the pagination envelope?
• Are all untrusted inputs validated exactly at the boundary — and no inputs validated redundantly between internals?
• Do public types have separate Input and Output variants where the read shape differs from the write shape?
• For any ID type used in multiple modules, is it branded (TS) or otherwise distinct from a raw string?
• Is there a NOTICED BUT NOT TOUCHING entry for every inconsistency in adjacent code that this design did not fix?

If any answer is no, close the gap before handing off to implementation.