Reusing existing types

Payload types often already exist, generated from the server's OpenAPI schema for its REST endpoints. Reference them instead of generating duplicates:

npx @chanx-js/codegen <schema> -o src/generated \
  --reuse-from 'src/types/backend/**/*.ts' \
  --alias 'src/=@/'

Any type in the schema whose name matches an exported declaration in those files is imported rather than generated:

import type { PostPayload } from '@/types/backend/rooms';

The scan reads the TypeScript AST, so export type, interface and indented declarations are all found. Only exported declarations count, since importing anything else would not compile, and files inside the output directory are ignored.

With --validation zod, a reused type is not regenerated, so its validator is permissive. A reused message still checks its action.

If your existing types are global scripts (bare type X = ... files with no imports or exports), add --ambient. Matching types are simply not emitted, and the global declaration wins.

Checking reused types still match

A matching name does not guarantee a matching shape: two schema generators can disagree on nullability, date formats, or required fields. --reuse-strict emits a reuse-check file that fails to compile when they drift:

type MutuallyAssignable<A, B> = [A] extends [B]
  ? [B] extends [A]
    ? true
    : false
  : false;
type Assert<T extends true> = T;

export type CheckPostPayload = Assert<
  MutuallyAssignable<PostPayload, GeneratedPostPayload>
>;

It is purely type-level, so it works as a .d.ts under --emit js too, and runs in the typecheck you already have.