# ADR-013: Use-Case Input/Output Unification + Presenter Pattern + Feature-Scoped Error Mapping

**Status:** Accepted
**Date:** 2026-05-06
**Supersedes:** none — extends ADR-008 (per-feature DI), ADR-011 (TDD foundation), ADR-012 (feature conventions)

## Context

ADR-012 established factory-function use cases and one-controller-
per-use-case. But the input contract was still defined three times — once
in the tRPC procedure's `.input(z.object({...}))`, once in the controller's
local `const inputSchema`, and once implicitly in the use case's TypeScript
parameter type. The three definitions drifted: the controller's
`z.string().min(3).max(31)` was stricter than the tRPC version's
`z.string()`. Output validation was TypeScript-only — repositories could
return malformed values and use cases happily passed them through.

There was also no consistent error-translation between domain errors
(`ArticleNotFoundError`, `AuthenticationError`, …) and `TRPCError`,
meaning the wire response code was unpredictable per feature.

Per-feature public-API surfaces conflated UI artifacts (query builders
imported React Query) with pure contracts (entity types) on the same
top-level export, making "what does this package expose to whom" muddy.

## Decision

Adopt four interlocking patterns, codified as 30 RFC-2119 rules in the
spec:

1. **Use-case file is the single source of truth for input AND output
   contracts.** Every use case exports `xInputSchema` (always a
   `z.ZodObject`, even for void inputs via `z.object({}).strict()`) and
   — for non-void use cases — `xOutputSchema`. The use-case body ends
   with `xOutputSchema.parse(result)` before returning. Type aliases
   `XInput`/`XOutput`/`IXUseCase` are exported alongside.

2. **Controllers consume the use-case schema; output passes through a
   co-located `function presenter`.** Controllers receive `unknown`,
   `safeParse` against `xInputSchema`, throw `InputParseError` on
   failure, then call the use case and pass the result through a
   top-level `function presenter(value: XOutput)` defined in the same
   file. The controller's return type is `Promise<ReturnType<typeof
presenter>>`. Identity presenters are permitted and expected for
   pass-through cases — the function form must always exist (R11) so
   adding a transform is a one-line edit. Void-output controllers
   (e.g., `signOutController`, `deleteMediaController`) skip the
   presenter and return `Promise<void>` (R12).

3. **Feature-scoped error→TRPCError middleware.** Each feature's
   `integrations/api/procedures.ts` exports an `xProcedure` built from
   `t.procedure.use(defineErrorMiddleware([[ErrorCtor, "TRPC_CODE"],
...]))`. The factory `defineErrorMiddleware` lives in
   `core-shared/trpc/`; it discriminates by `instanceof` and preserves
   the original error as `TRPCError.cause`. **`core-shared` never
   enumerates feature-specific error classes** — each feature passes its
   own constructors in via its own `procedures.ts`. Routers use the
   feature's `xProcedure` instead of bare `publicProcedure` and
   `.input(xInputSchema)` instead of redefining input shapes.

4. **Per-feature public surface split.** Feature root `.` exports only
   contracts: domain types, domain errors, schemas, `IXUseCase`/`IXController`
   aliases, router type, constants. UI artifacts (query builders,
   future React components) move to a new `./ui` subpath
   (`src/ui/index.ts`). Apps that need queries import from
   `@repo/<feature>/ui`; apps that need the type-only contract import
   from `@repo/<feature>`.

## Consequences

### Positive

- **Single source of truth for I/O contracts.** Schema drift is no
  longer possible — there's one definition, imported by everyone.
- **Runtime-validated outputs.** `xOutputSchema.parse(...)` catches
  "repo returned malformed data" bugs at the layer that owns the
  contract, instead of silently flowing wrong shapes downstream.
- **Predictable error responses.** Every domain error maps to a known
  `TRPCError.code` via the per-feature middleware; clients can rely on
  status codes.
- **Discoverable transforms via presenter.** When a view needs to drop
  fields, rename them, or serialize dates, the presenter function is
  already there — change one function body. No structural refactor.
- **Clean public surface.** Feature root packages no longer pretend to
  be UI packages; apps make explicit choices about what they need.
- **Frontend gets schemas for free.** Forms can `import { signInInputSchema
} from "@repo/auth"` and feed it into `react-hook-form` + `zodResolver`
  with the same constraints the backend enforces.

### Negative

- **More code.** Every use case grows by ~10 lines (input + output
  schema + parse). Every controller grows by ~5 lines (presenter,
  even if identity). Acceptable cost for the consistency.
- **Per-feature `procedures.ts` boilerplate.** Five new files (~10
  lines each) — one per feature. Maintaining the error map is one of
  the few feature-level chores; new error classes need adding to the
  map.
- **Schemas run twice on the tRPC path** (tRPC's `.input()` parse +
  controller's `safeParse`). Negligible cost; zero behavioral risk
  because both use the same schema. Defense-in-depth value when the
  controller is invoked from non-tRPC entry points.
- **Apps with existing imports may need updating** — `articleBySlugQuery`,
  `pageBySlugQuery`, etc. now live behind `@repo/<feature>/ui`.
  (At the time of this ADR, no apps consume these yet, so the cost is
  forward-only.)

## Alternatives considered

- **Keep schemas in controllers.** The reference pattern has only one
  validation layer (server actions skip `.input()`), so one schema is
  sufficient. Our entry point is tRPC, which insists on a schema for
  type inference — putting the canonical schema in the controller and
  exporting it for the router was considered. Rejected because the use
  case is the contract owner; schemas describe the _operation_, not the
  _transport_.

- **Centralized error-name → code map in `core-shared`.** Considered
  using `error.name` discrimination with a small global registry.
  Rejected because it violates feature ownership — `core-shared` would
  need to know about every feature's error classes. The
  `defineErrorMiddleware` factory cleanly inverts the dependency:
  `core-shared` provides the plumbing, features pass their own
  constructors.

- **Validate outputs only in tests.** Considered using TypeScript
  types alone for output, deferring runtime validation to
  contract-suite tests. Rejected because the cost of `.parse()` on
  return is trivial and the bug-catching value at runtime is real
  (Payload integrations have surprised us before).

- **Presenters only when reshaping.** Considered limiting presenters to
  cases with actual transforms. Rejected because the discoverable hook
  for future shaping is worth the trivial identity-function boilerplate.

- **Presenters in a separate `presenters/` folder.** Considered as a
  concession to "controllers = thin orchestration". Rejected because
  co-locating the presenter with its controller keeps the contract
  visible in one file.

- **Shared `./schemas` subpath.** Considered exposing schemas only via
  a dedicated subpath instead of the feature root. Rejected because
  schemas ARE feature contracts — they belong with the other contracts
  (types, errors). Adding a fourth subpath felt like ceremony.

## Acceptance criteria

- Tests: 360 total. Coverage: every acceptance rule represented.
- `pnpm typecheck && pnpm lint && pnpm test && pnpm turbo boundaries
&& pnpm build` green.
- Five feature-level router error-mapping tests demonstrate domain
  error → `TRPCError.code` translation works end-to-end.

## References

- Prior ADRs: ADR-008 (per-feature DI), ADR-011 (TDD foundation), ADR-012 (feature conventions)
