Why does the exclusiveMaximum union type in JSON Schema include boolean?

[aiktb]

According to the https://json-schema.org/understanding-json-schema/reference/numeric documentation, boolean seems to be an older Draft 4 convention. Is there a way in Zod to remove boolean from it?

[dosubot[bot]]

Hey! I'm Dosu, a bot trained on Zod's codebase and issues. I can help shed light on this.

The exclusiveMaximum type is number | boolean because Zod's JSON Schema types need to accommodate multiple JSON Schema draft versions [1]:

• Draft 4 (and OpenAPI 3.0) uses boolean flags paired with maximum/minimum, e.g. { "maximum": 10, "exclusiveMaximum": true }
• Draft 7+ uses numeric values directly, e.g. { "exclusiveMaximum": 10 }

You're right that the boolean form is the older Draft 4 convention. The union type exists so the same TypeScript interface can represent output for any target draft.

If you want to ensure you only get the modern numeric form, you can set the target option when calling toJSONSchema() [2]:

import { z } from "zod/v4";

z.toJSONSchema(schema, {
  target: "draft-2020-12" // or "draft-07"
});

With "draft-07" or "draft-2020-12" as the target, the generated output will use numeric exclusiveMaximum/exclusiveMinimum values — never booleans [3]. The boolean form is only produced when target is "draft-04" or "openapi-3.0".

As for the TypeScript type definition itself (number | boolean), that union is intentional since the type needs to cover all possible output formats. If you're consuming the output in your own code and know your target is Draft 7+, you could narrow the type on your end, but the underlying Zod type will keep the union to remain draft-agnostic.

Feel free to close this discussion if that answers your question!

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