yup

@seampact/codegen-yup writes a yup schema for each schema of your contract. NestJS, Express, Fastify, AdonisJS and Hono can use them to refuse a bad request before your handler runs.

Used by: any Node server that takes the validators link: NestJS, Express, Hono, Fastify and AdonisJS. The default is zod.

Set up

Install yup in your app, then point the validators entry at this package.

# seampact.config.yaml, under targets
  - id: validators
    use: "@seampact/codegen-yup"
    apis: [notes]

Link it from your server entry, next to types:

    link:
      types: types
      validators: validators

Run npx seampact generate. The files land in validators/<api>/ under your outDir, with the same layout as zod plus a yup-support.ts.

What you get

For the CreateTask schema of the overview, trimmed from schemas/input.ts:

export const CreateTaskSchema = yup
  .object({
    title: yup
      .string()
      .strict()
      .min(1, "Expected at least 1 character.")
      .max(200, "Expected at most 200 characters.")
      .defined(),
    effort: yup
      .number()
      .strict()
      .integer()
      .test({
        name: "minimum",
        message: "Expected at least -9007199254740991.",
        test: (value) =>
          value === undefined ||
          !Number.isInteger(value) ||
          value >= -Number.MAX_SAFE_INTEGER,
      })
      .test({
        name: "maximum",
        message: "Expected at most 9007199254740991.",
        test: (value) =>
          value === undefined ||
          !Number.isInteger(value) ||
          value <= Number.MAX_SAFE_INTEGER,
      })
      .min(0, "Expected at least 0.")
      .optional(),
  })
  .strict()
  .default(undefined)
  .test(unknownKeysTest);

The schema is strict: it refuses a value of the wrong type instead of converting it, and unknownKeysTest refuses a key the contract does not declare. The operation schemas in operations/input.ts check path, query, headers and body together.

Use them in your code

import { inputSchemas } from "./__generated__/validators/notes/index.js";

const isValid = inputSchemas.CreateTaskSchema.isValidSync({ title: "" });

When a check fails

Each yup error becomes one issue of the 400 body. The generated checks carry the contract rule (minimum, unknownKeys) in their params, and the server reads that rule, never yup’s own message.

Native or consistent rules

With the default rules: native, the schemas use yup’s own rules, which differ from the contract in a few places.

Lengths count UTF-16 units: an emoji counts 2, so maxLength: 1 refuses “😀”.
uuid also checks the version and variant digits, and refuses version 7 and the max UUID ffffffff-ffff-ffff-ffff-ffffffffffff.
date-time checks only the digit layout: it accepts an empty string, month 13, hour 99, second 60, 2026-02-30, an offset like +01 or +0100, and more than 9 fraction digits. It refuses a lowercase t or z.
A number with a wrong type also reports its range issues.
multipleOf has no yup rule: it stays the contract’s check.

Set rules: seampact-consistent for the contract’s exact checks (Switch the validator).

Limits

The generator refuses by name a oneOf, a property default, a nullable value other than an opaque one and the other forms in Generate fails. No client guide uses these schemas.

Switch the validator.
The 400 body: When a check fails.

yup