JSON Schema

Source URL: https://zod.dev/json-schema

JSON Schema

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New — Zod 4 introduces a new feature: native JSON Schema conversion. JSON Schema is a standard for describing the structure of JSON (with JSON). It’s widely used in OpenAPI definitions and defining structured outputs for AI.

z.fromJSONSchema()

Experimental — The z.fromJSONSchema() function is experimental and is not considered part of Zod’s stable API. It is likely to undergo implementation changes in future releases.

Zod provides z.fromJSONSchema() to convert a JSON Schema into a Zod schema.

    import * as z from "zod";

    const jsonSchema = {
      type: "object",
      properties: {
        name: { type: "string" },
        age: { type: "number" },
      },
      required: ["name", "age"],
    };

    const zodSchema = z.fromJSONSchema(jsonSchema);

z.toJSONSchema()

To convert a Zod schema to JSON Schema, use the z.toJSONSchema() function.

    import * as z from "zod";

    const schema = z.object({
      name: z.string(),
      age: z.number(),
    });

    z.toJSONSchema(schema)
    // => {
    //   type: 'object',
    //   properties: { name: { type: 'string' }, age: { type: 'number' } },
    //   required: [ 'name', 'age' ],
    //   additionalProperties: false,
    // }

All schema & checks are converted to their closest JSON Schema equivalent. Some types have no analog and cannot be reasonably represented. See the unrepresentable section below for more information on handling these cases.

    z.bigint(); // ❌
    z.int64(); // ❌
    z.symbol(); // ❌
    z.undefined(); // ❌
    z.void(); // ❌
    z.date(); // ❌
    z.map(); // ❌
    z.set(); // ❌
    z.transform(); // ❌
    z.nan(); // ❌
    z.custom(); // ❌

A second argument can be used to customize the conversion logic.

    z.toJSONSchema(schema, {
      // ...params
    })

Below is a quick reference for each supported parameter. Each one is explained in more detail below.

    interface ToJSONSchemaParams {
      /** The JSON Schema version to target.
       * - `"draft-2020-12"` — Default. JSON Schema Draft 2020-12
       * - `"draft-07"` — JSON Schema Draft 7
       * - `"draft-04"` — JSON Schema Draft 4
       * - `"openapi-3.0"` — OpenAPI 3.0 Schema Object */
      target?:
        | "draft-04"
        | "draft-4"
        | "draft-07"
        | "draft-7"
        | "draft-2020-12"
        | "openapi-3.0"
        | ({} & string)
        | undefined;

      /** A registry used to look up metadata for each schema.
       * Any schema with an `id` property will be extracted as a $def. */
      metadata?: $ZodRegistry<Record<string, any>>;

      /** How to handle unrepresentable types.
       * - `"throw"` — Default. Unrepresentable types throw an error
       * - `"any"` — Unrepresentable types become `{}` */
      unrepresentable?: "throw" | "any";

      /** How to handle cycles.
       * - `"ref"` — Default. Cycles will be broken using $defs
       * - `"throw"` — Cycles will throw an error if encountered */
      cycles?: "ref" | "throw";

      /* How to handle reused schemas.
       * - `"inline"` — Default. Reused schemas will be inlined
       * - `"ref"` — Reused schemas will be extracted as $defs */
      reused?: "ref" | "inline";

      /** A function used to convert `id` values to URIs to be used in *external* $refs.
       *
       * Default is `(id) => id`.
       */
      uri?: (id: string) => string;
    }

• io

Some schema types have different input and output types, e.g. ZodPipe, ZodDefault, and coerced primitives. By default, the result of z.toJSONSchema represents the output type ; use "io": "input" to extract the input type instead.

    const mySchema = z.string().transform(val => val.length).pipe(z.number());
    // ZodPipe

    const jsonSchema = z.toJSONSchema(mySchema);
    // => { type: "number" }

    const jsonSchema = z.toJSONSchema(mySchema, { io: "input" });
    // => { type: "string" }

• target

To set the target JSON Schema version, use the target parameter. By default, Zod will target Draft 2020-12.

    z.toJSONSchema(schema, { target: "draft-07" });
    z.toJSONSchema(schema, { target: "draft-2020-12" });
    z.toJSONSchema(schema, { target: "draft-04" });
    z.toJSONSchema(schema, { target: "openapi-3.0" });

• metadata

If you haven’t already, read through the Metadata and registries page for context on storing metadata in Zod.

In Zod, metadata is stored in registries. Zod exports a global registry z.globalRegistry that can be used to store common metadata fields like id, title, description, and examples.

ZodZod Mini

    import * as z from "zod";

    // `.meta()` is a convenience method for registering a schema in `z.globalRegistry`
    const emailSchema = z.string().meta({
      title: "Email address",
      description: "Your email address",
    });

    z.toJSONSchema(emailSchema);
    // => { type: "string", title: "Email address", description: "Your email address", ... }

All metadata fields get copied into the resulting JSON Schema.

    const schema = z.string().meta({
      whatever: 1234
    });

    z.toJSONSchema(schema);
    // => { type: "string", whatever: 1234 }

• unrepresentable

The following APIs are not representable in JSON Schema. By default, Zod will throw an error if they are encountered. It is unsound to attempt a conversion to JSON Schema; you should modify your schemas as they have no equivalent in JSON. An error will be thrown if any of these are encountered.

    z.bigint(); // ❌
    z.int64(); // ❌
    z.symbol(); // ❌
    z.undefined(); // ❌
    z.void(); // ❌
    z.date(); // ❌
    z.map(); // ❌
    z.set(); // ❌
    z.transform(); // ❌
    z.nan(); // ❌
    z.custom(); // ❌

By default, Zod will throw an error if any of these are encountered.

    z.toJSONSchema(z.bigint());
    // => throws Error

You can change this behavior by setting the unrepresentable option to "any". This will convert any unrepresentable types to {} (the equivalent of unknown in JSON Schema).

    z.toJSONSchema(z.bigint(), { unrepresentable: "any" });
    // => {}

• cycles

How to handle cycles. If a cycle is encountered as z.toJSONSchema() traverses the schema, it will be represented using $ref.

    const User = z.object({
      name: z.string(),
      get friend() {
        return User;
      },
    });

    z.toJSONSchema(User);
    // => {
    //   type: 'object',
    //   properties: { name: { type: 'string' }, friend: { '$ref': '#' } },
    //   required: [ 'name', 'friend' ],
    //   additionalProperties: false,
    // }

If instead you want to throw an error, set the cycles option to "throw".

    z.toJSONSchema(User, { cycles: "throw" });
    // => throws Error

• reused

How to handle schemas that occur multiple times in the same schema. By default, Zod will inline these schemas.

    const name = z.string();
    const User = z.object({
      firstName: name,
      lastName: name,
    });

    z.toJSONSchema(User);
    // => {
    //   type: 'object',
    //   properties: {
    //     firstName: { type: 'string' },
    //     lastName: { type: 'string' }
    //   },
    //   required: [ 'firstName', 'lastName' ],
    //   additionalProperties: false,
    // }

Instead you can set the reused option to "ref" to extract these schemas into $defs.

    z.toJSONSchema(User, { reused: "ref" });
    // => {
    //   type: 'object',
    //   properties: {
    //     firstName: { '$ref': '#/$defs/__schema0' },
    //     lastName: { '$ref': '#/$defs/__schema0' }
    //   },
    //   required: [ 'firstName', 'lastName' ],
    //   additionalProperties: false,
    //   '$defs': { __schema0: { type: 'string' } }
    // }

• override

To define some custom override logic, use override. The provided callback has access to the original Zod schema and the default JSON Schema. This function should directly modifyctx.jsonSchema.

    const mySchema = /* ... */
    z.toJSONSchema(mySchema, {
      override: (ctx)=>{
        ctx.zodSchema; // the original Zod schema
        ctx.jsonSchema; // the default JSON Schema

        // directly modify
        ctx.jsonSchema.whatever = "sup";
      }
    });

Note that unrepresentable types will throw an Error before this function is called. If you are trying to define custom behavior for an unrepresentable type, you’ll need to set the unrepresentable: "any" alongside override.

    // support z.date() as ISO datetime strings
    const result = z.toJSONSchema(z.date(), {
      unrepresentable: "any",
      override: (ctx) => {
        const def = ctx.zodSchema._zod.def;
        if(def.type ==="date"){
          ctx.jsonSchema.type = "string";
          ctx.jsonSchema.format = "date-time";
        }
      },
    });

Conversion

Below are additional details regarding Zod’s JSON Schema conversion logic.

• String formats

Zod converts the following schema types to the equivalent JSON Schema format:

    // Supported via `format`
    z.email(); // => { type: "string", format: "email" }
    z.iso.datetime(); // => { type: "string", format: "date-time" }
    z.iso.date(); // => { type: "string", format: "date" }
    z.iso.time(); // => { type: "string", format: "time" }
    z.iso.duration(); // => { type: "string", format: "duration" }
    z.ipv4(); // => { type: "string", format: "ipv4" }
    z.ipv6(); // => { type: "string", format: "ipv6" }
    z.uuid(); // => { type: "string", format: "uuid" }
    z.guid(); // => { type: "string", format: "uuid" }
    z.url(); // => { type: "string", format: "uri" }

These schemas are supported via contentEncoding:

    z.base64(); // => { type: "string", contentEncoding: "base64" }

All other string formats are supported via pattern:

    z.base64url();
    z.cuid();
    z.emoji();
    z.nanoid();
    z.cuid2();
    z.ulid();
    z.cidrv4();
    z.cidrv6();
    z.mac();

• Numeric types

Zod converts the following numeric types to JSON Schema:

    // number
    z.number(); // => { type: "number" }
    z.float32(); // => { type: "number", exclusiveMinimum: ..., exclusiveMaximum: ... }
    z.float64(); // => { type: "number", exclusiveMinimum: ..., exclusiveMaximum: ... }

    // integer
    z.int(); // => { type: "integer" }
    z.int32(); // => { type: "integer", exclusiveMinimum: ..., exclusiveMaximum: ... }

• Object schemas

By default, z.object() schemas contain additionalProperties: "false". This is an accurate representation of Zod’s default behavior, as plain z.object() schema strip additional properties.

    import * as z from "zod";

    const schema = z.object({
      name: z.string(),
      age: z.number(),
    });

    z.toJSONSchema(schema)
    // => {
    //   type: 'object',
    //   properties: { name: { type: 'string' }, age: { type: 'number' } },
    //   required: [ 'name', 'age' ],
    //   additionalProperties: false,
    // }

When converting to JSON Schema in "input" mode, additionalProperties is not set. See the io docs for more information.

    import * as z from "zod";

    const schema = z.object({
      name: z.string(),
      age: z.number(),
    });

    z.toJSONSchema(schema, { io: "input" });
    // => {
    //   type: 'object',
    //   properties: { name: { type: 'string' }, age: { type: 'number' } },
    //   required: [ 'name', 'age' ],
    // }

By contrast:

• z.looseObject() will never set additionalProperties: false
• z.strictObject() will always set additionalProperties: false

• File schemas

Zod converts z.file() to the following OpenAPI-friendly schema:

    z.file();
    // => { type: "string", format: "binary", contentEncoding: "binary" }

Size and MIME checks are also represented:

    z.file().min(1).max(1024 * 1024).mime("image/png");
    // => {
    //   type: "string",
    //   format: "binary",
    //   contentEncoding: "binary",
    //   contentMediaType: "image/png",
    //   minLength: 1,
    //   maxLength: 1048576,
    // }

• Nullability

Zod converts z.null() to { type: "null" } in JSON Schema.

    z.null();
    // => { type: "null" }

Note that z.undefined() is unrepresentable in JSON Schema (see below).

Similarly, nullable is represented via a union with null:

    z.nullable(z.string());
    // => { oneOf: [{ type: "string" }, { type: "null" }] }

Optional schemas are represented as-is, though they are decorated with an optional annotation.

    z.optional(z.string());
    // => { type: "string" }

Registries

Passing a schema into z.toJSONSchema() will return a self-contained JSON Schema.

In other cases, you may have a set of Zod schemas you’d like to represent using multiple interlinked JSON Schemas, perhaps to write to .json files and serve from a web server.

    import * as z from "zod";

    const User = z.object({
      name: z.string(),
      get posts(){
        return z.array(Post);
      }
    });

    const Post = z.object({
      title: z.string(),
      content: z.string(),
      get author(){
        return User;
      }
    });

    z.globalRegistry.add(User, {id: "User"});
    z.globalRegistry.add(Post, {id: "Post"});

To achieve this, you can pass a registry into z.toJSONSchema().

Important — All schemas should have a registered id property in the registry! Any schemas without an id will be ignored.

    z.toJSONSchema(z.globalRegistry);
    // => {
    //   schemas: {
    //     User: {
    //       id: 'User',
    //       type: 'object',
    //       properties: {
    //         name: { type: 'string' },
    //         posts: { type: 'array', items: { '$ref': 'Post' } }
    //       },
    //       required: [ 'name', 'posts' ],
    //       additionalProperties: false,
    //     },
    //     Post: {
    //       id: 'Post',
    //       type: 'object',
    //       properties: {
    //         title: { type: 'string' },
    //         content: { type: 'string' },
    //         author: { '$ref': 'User' }
    //       },
    //       required: [ 'title', 'content', 'author' ],
    //       additionalProperties: false,
    //     }
    //   }
    // }

By default, the $ref URIs are simple relative paths like "User". To make these absolute URIs, use the uri option. This expects a function that converts an id to a fully-qualified URI.

    z.toJSONSchema(z.globalRegistry, {
      uri: (id) => `https://example.com/${id}.json`
    });
    // => {
    //   schemas: {
    //     User: {
    //       id: 'User',
    //       type: 'object',
    //       properties: {
    //         name: { type: 'string' },
    //         posts: {
    //           type: 'array',
    //           items: { '$ref': 'https://example.com/Post.json' }
    //         }
    //       },
    //       required: [ 'name', 'posts' ],
    //       additionalProperties: false,
    //     },
    //     Post: {
    //       id: 'Post',
    //       type: 'object',
    //       properties: {
    //         title: { type: 'string' },
    //         content: { type: 'string' },
    //         author: { '$ref': 'https://example.com/User.json' }
    //       },
    //       required: [ 'title', 'content', 'author' ],
    //       additionalProperties: false,
    //     }
    //   }
    // }