Zod 3 to Json Schema

 

_Looking for the exact opposite? Check out json-schema-to-zod_

Summary

Does what it says on the tin; converts Zod schemas into JSON schemas!

- Supports all relevant schema types, basic string, number and array length validations and string patterns.
- Resolves recursive and recurring schemas with internal $refs.
- Supports targeting legacy Open API 3.0 specification (3.1 supports regular Json Schema).
- Supports Open AI strict mode schemas (Optional object properties are replaced with required but nullable ones).

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Usage

$3

``typescript
import { z } from "zod/v3";
import { zodToJsonSchema } from "zod-v3-to-json-schema";

const mySchema = z
.object({
myString: z.string().min(5),
myUnion: z.union([z.number(), z.boolean()]),
})
.describe("My neat object schema");

const jsonSchema = zodToJsonSchema(mySchema, "mySchema");
`

#### Expected output

`json
{
"$schema": "http://json-schema.org/draft-07/schema#",
"$ref": "#/definitions/mySchema",
"definitions": {
"mySchema": {
"description": "My neat object schema",
"type": "object",
"properties": {
"myString": {
"type": "string",
"minLength": 5
},
"myUnion": {
"type": ["number", "boolean"]
}
},
"additionalProperties": false,
"required": ["myString", "myUnion"]
}
}
}
`

Options

$3

You can pass a string as the second parameter of the main zodToJsonSchema function. If you do, your schema will end up inside a definitions object property on the root and referenced from there. Alternatively, you can pass the name as the name property of the options object (see below).

$3

Instead of the schema name (or nothing), you can pass an options object as the second parameter. The following options are available:

| Option | Effect |
| ---------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| name?: _string_ | As described above. |
| nameStrategy?: "ref" \| "title" | Adds name as "title" meta instead of as a ref as described above |
| basePath?: string[] | The base path of the root reference builder. Defaults to ["#"]. |
| $refStrategy?: "root" \| "relative" \| "seen" \| "none" | The reference builder strategy;

• "root" resolves $refs from the root up, ie: "#/definitions/mySchema".
• "relative" uses relative JSON pointers. _See known issues!_
• "seen" reuses the output of any "seen" Zod schema. In theory it's a more performant version of "none", but in practice this behaviour can cause issues with nested schemas and has now gotten its own option.
• "none" ignores referencing all together, creating a new schema branch even on "seen" schemas. Recursive references defaults to "any", ie {}.

Defaults to "root". |
| effectStrategy?: "input" \| "any" | The effects output strategy. Defaults to "input". _See known issues!_ |
| dateStrategy?: "format:date" \| "format:date-time" \| "string" \| "integer" | Date strategy, integer allow to specify in unix-time min and max values. "format:date" creates a string schema with format: "date". "format:date-time" creates a string schema with format: "date-time". "string" is intepreted as "format:date-time". "integer" creates an integer schema with format "unix-time" (unless target "openApi3" is used min max checks are also respected) |
| |
| emailStrategy?: "format:email" \| "format:idn-email" \| "pattern:zod" | Choose how to handle the email string check. Defaults to "format:email". |
| base64Strategy?: "format:binary" \| "contentEnconding:base64" \| "pattern:zod" | Choose how to handle the base64 string check. Defaults to "contentEncoding:base64" as described here. Note that "format:binary" is not represented in the output type as it's not part of the JSON Schema spec and only intended to be used when targeting OpenAPI 3.0. Later versions of OpenAPI support contentEncoding. |
| definitionPath?: "definitions" \| "$defs" | The name of the definitions property when name is passed. Defaults to "definitions". |
| target?: "jsonSchema7" \| "jsonSchema2019-09" \| "openApi3" \| "openAi" | Which spec to target. Defaults to "jsonSchema7" |
| strictUnions?: boolean | Scrubs unions of any-like json schemas, like {} or true. Multiple zod types may result in these out of necessity, such as z.instanceof() |
| definitions?: Record | See separate section below |
| errorMessages?: boolean | Include custom error messages created via chained function checks for supported zod types. See section below |
| markdownDescription?: boolean | Copies the description meta to markdownDescription |
| patternStrategy?: "escape" \| "preserve" | The Zod string validations .includes(), .startsWith(), and .endsWith() must be converted to regex to be compatible with JSON Schema's pattern. For safety, all non-alphanumeric characters are escaped by default (consider z.string().includes(".")), but this can occasionally cause problems with Unicode-flagged regex parsers. Use preserve to prevent this escaping behaviour and preserve the exact string written, even if it results in an inaccurate regex. |
| applyRegexFlags?: boolean | JSON Schema's pattern doesn't support RegExp flags, but Zod's z.string().regex() does. When this option is true (default false), a best-effort is made to transform regexes into a flag-independent form (e.g. /x/i => /[xX]/ ). Supported flags: i (basic Latin only), m, s. |
| pipeStrategy?: "all" \| "input" \| "output" | Decide which types should be included when using z.pipe, for example z.string().pipe(z.number()) would return both string and number by default, only string for "input" and only number for "output". |
| removeAdditionalStrategy?: "passthrough" \| "strict" | Decide when additionalProperties should be allowed. See the section on additional properties for details. |
| allowedAdditionalProperties?: true \| undefined | What value to give additionalProperties when allowed. See the section on additional properties for details. |
| rejectedAdditionalProperties?: false \| undefined | What value to give additionalProperties when rejected. See the section on additional properties for details. |
| override?: callback | See section |
| postProcess?: callback | See section |
| openAiAnyTypeName?: string | Decides the name of a Json schema used to allow semi-arbitrary values in Open AI structured output. If any value in the Zod-schema resolves to any "any"-type schema it will reference a definition of this name. If no such definition is provided a default Json schema will be used. Defaults to "OpenAiAnyType" |

$3

The definitions option lets you manually add recurring schemas into definitions for cleaner outputs. It's fully compatible with named schemas, changed definitions path and base path. Here's a simple example:

`typescript
const myRecurringSchema = z.string();
const myObjectSchema = z.object({ a: myRecurringSchema, b: myRecurringSchema });

const myJsonSchema = zodToJsonSchema(myObjectSchema, {
definitions: { myRecurringSchema },
});
`

#### Result

`json
{
"type": "object",
"properties": {
"a": {
"$ref": "#/definitions/myRecurringSchema"
},
"b": {
"$ref": "#/definitions/myRecurringSchema"
}
},
"definitions": {
"myRecurringSchema": {
"type": "string"
}
}
}
`

$3

This feature allows optionally including error messages created via chained function calls for supported zod types:

`ts
// string schema with additional chained function call checks
const EmailSchema = z.string().email("Invalid email").min(5, "Too short");

const jsonSchema = zodToJsonSchema(EmailSchema, { errorMessages: true });
`

#### Result

`json
{
"$schema": "http://json-schema.org/draft-07/schema#",
"type": "string",
"format": "email",
"minLength": 5,
"errorMessage": {
"format": "Invalid email",
"minLength": "Too short"
}
}
`

This allows for field specific, validation step specific error messages which can be useful for building forms and such. This format is accepted by react-hook-form's ajv resolver (and therefor ajv-errors which it uses under the hood). Note that if using AJV with this format will require enabling ajv-errors as vanilla AJV does not accept this format by default.

#### Custom Error Message Support

- ZodString
- regex
- min, max
- email, cuid, uuid, url
- endsWith, startsWith
- ZodNumber
- min, max, lt, lte, gt, gte,
- int
- multipleOf
- ZodSet
- min, max
- ZodArray
- min, max

$3

By default, Zod removes undeclared properties when parsing object schemas. In order to replicate the expected output of this behaviour, the default for behaviour of zodToJsonSchema is to set "additionalProperties" to false (although the correctness of this can be debated). If you wish to allow undeclared properties you can either:

- Set removeAdditionalStrategy to "strict". This will allow additional properties for any object schema that is not declared with .strict().
- Leave removeAdditionalStrategy set to its default value of "passthrough", and add .passtrough() to your object schema.

#### Removing the additionalProperties keyword using the allowedAdditionalProperties and/or rejectedAdditionalProperties options.

Some schema definitions (like Googles Gen AI API for instance) does not allow the additionalProperties keyword at all. Luckily the JSON Schema spec allows for this: leaving the keyword undefined _should_ have the same effect as setting it to true (as per usual YMMV). To enable this behaviour, set the option allowedAdditionalProperties to undefined.

To exclude the keyword even when additional properties are _not_ allowed, set the rejectedAdditionalProperties to undefined as well.

_Heads up ⚠️: Both of these options will be ignored if your schema is declared with .catchall(...) as the provided schema will be used instead (if valid)._

#### Expected outputs

| z.object({}) + option | "additionalProperties" value |
| ------------------------- | ----------------------------------------------------------- |
| .strip() (default) | false if strategy is "passtrough", true if "strict" |
| .passtrough() | true |
| .strict() | false |
| .catchall(z.string()) | { "type": "string" } |
| .catchall(z.function()) | undefined (function schemas are not currently parseable) |

Substitute true and false for undefined according to allowedAdditionalProperties and/or rejectedAdditionalProperties respectively.

$3

This options takes a callback receiving a Zod schema definition, the current reference object (containing the current ref path and other options), an argument containing inforation about wether or not the schema has been encountered before, and a forceResolution argument.

Important: if you don't want to override the current item you have to return the ignoreOverride symbol exported from the index. This is because undefined is a valid option to return when you want the property to be excluded from the resulting JSON schema.

`typescript
import zodToJsonSchema, { ignoreOverride } from "zod-v3-to-json-schema";

zodToJsonSchema(
z.object({
ignoreThis: z.string(),
overrideThis: z.string(),
removeThis: z.string(),
}),
{
override: (def, refs) => {
const path = refs.currentPath.join("/");

if (path === "#/properties/overrideThis") {
return {
type: "integer",
};
}

if (path === "#/properties/removeThis") {
return undefined;
}

// Important! Do not return undefined or void unless you want to remove the property from the resulting schema completely.
return ignoreOverride;
},
},
);
`

Expected output:

`json
{
"type": "object",
"required": ["ignoreThis", "overrideThis"],
"properties": {
"ignoreThis": {
"type": "string"
},
"overrideThis": {
"type": "integer"
}
},
"additionalProperties": false
}
`

$3

Besided receiving all arguments of the override callback, the postProcess callback also receives the generated schema. It should always return a JSON Schema, or undefined if you wish to filter it out. Unlike the override callback you do not have to return ignoreOverride if you are happy with the produced schema; simply return it unchanged.

`typescript
import zodToJsonSchema, { PostProcessCallback } from "zod-v3-to-json-schema";

// Define the callback to be used to process the output using the PostProcessCallback type:
const postProcess: PostProcessCallback = (
// The original output produced by the package itself:
jsonSchema,
// The ZodSchema def used to produce the original schema:
def,
// The refs object containing the current path, passed options, etc.
refs,
) => {
if (!jsonSchema) {
return jsonSchema;
}

// Try to expand description as JSON meta:
if (jsonSchema.description) {
try {
jsonSchema = {
...jsonSchema,
...JSON.parse(jsonSchema.description),
};
} catch {}
}

// Make all numbers nullable:
if ("type" in jsonSchema! && jsonSchema.type === "number") {
jsonSchema.type = ["number", "null"];
}

// Add the refs path, just because
(jsonSchema as any).path = refs.currentPath;

return jsonSchema;
};

const jsonSchema = zodToJsonSchema(zodSchema, { postProcess });
`

#### Using postProcess for including examples and other meta

Adding support for examples and other JSON Schema meta keys are among the most commonly requested features for this project. Unfortunately the current Zod major (3) has pretty anemic support for this, so some userland hacking is required. Since this is such a common usecase I've included a helper function that simply tries to parse any description as JSON and expand it into the resulting schema.

Simply stringify whatever you want added to the output schema as the description, then import and use jsonDescription as the postProcess option:

`typescript
import zodToJsonSchema, { jsonDescription } from "zod-v3-to-json-schema";

const zodSchema = z.string().describe(
JSON.stringify({
title: "My string",
description: "My description",
examples: ["Foo", "Bar"],
whatever: 123,
}),
);

const jsonSchema = zodToJsonSchema(zodSchema, {
postProcess: jsonDescription,
});
`

Expected output:

`json
{
"$schema": "http://json-schema.org/draft-07/schema#",
"type": "string",
"title": "My string",
"description": "My description",
"examples": ["Foo", "Bar"],
"whatever": 123
}
`

Known issues

- The OpenAI target should be considered experimental for now, as some combination of options may break the compatibility.
- When using .transform, the return type is inferred from the supplied function. In other words, there is no schema for the return type, and there is no way to convert it in runtime. Currently the JSON schema will therefore reflect the input side of the Zod schema and not necessarily the output (the latter aka. z.infer). If this causes problems with your schema, consider using the effectStrategy "any", which will allow any type of output.
- JSON Schemas does not support any other key type than strings for objects. When using z.record with any other key type, this will be ignored. An exception to this rule is z.enum as is supported since 3.11.3
- Relative JSON pointers, while published alongside JSON schema draft 2020-12, is not technically a part of it. Currently, most resolvers do not handle them at all.
- Since v3, the Object parser uses .isOptional() to check if a property should be included in required or not. This has the potentially dangerous behavior of calling .safeParse with undefined. To work around this, make sure your preprocess and other effects callbacks are pure and not liable to throw errors. An issue has been logged in the Zod repo and can be tracked here.
- JSON Schema version 2020-12 is not yet officially supported. However, you should be able to use this library to obtain a compatible schema for the 2020-12 format just by changing the returned schema's $schema` field to: "https://json-schema.org/draft/2020-12/schema#"

Versioning

This package _does not_ follow semantic versioning. The major and minor versions of this package instead reflects feature parity with the Zod package.

I will do my best to keep API-breaking changes to an absolute minimum, but new features may appear as "patches", such as introducing the options pattern in 3.9.1.

Changelog

https://github.com/StefanTerdell/zod-v3-to-json-schema/blob/master/changelog.md