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fix: use oneOf for discriminated unions in JSON Schema #5453

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colinhacks merged 1 commit into from
Nov 18, 2025
+57 −2
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fix: use oneOf for discriminated unions in JSON Schema #5453

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48 changes: 48 additions & 0 deletions packages/zod/src/v4/classic/tests/to-json-schema.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -657,6 +657,54 @@ describe("toJSONSchema", () => {
`);
});

test("discriminated unions", () => {
const schema = z.discriminatedUnion("type", [
z.object({ type: z.literal("success"), data: z.string() }),
z.object({ type: z.literal("error"), message: z.string() }),
]);
expect(z.toJSONSchema(schema)).toMatchInlineSnapshot(`
{
"$schema": "https://json-schema.org/draft/2020-12/schema",
"oneOf": [
{
"additionalProperties": false,
"properties": {
"data": {
"type": "string",
},
"type": {
"const": "success",
"type": "string",
},
},
"required": [
"type",
"data",
],
"type": "object",
},
{
"additionalProperties": false,
"properties": {
"message": {
"type": "string",
},
"type": {
"const": "error",
"type": "string",
},
},
"required": [
"type",
"message",
],
"type": "object",
},
],
}
`);
});

test("intersections", () => {
const schema = z.intersection(z.object({ name: z.string() }), z.object({ age: z.number() }));

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11 changes: 9 additions & 2 deletions packages/zod/src/v4/core/to-json-schema.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -330,13 +330,20 @@ export class JSONSchemaGenerator {
}
case "union": {
const json: JSONSchema.BaseSchema = _json as any;
// Discriminated unions use oneOf (exactly one match) instead of anyOf (one or more matches)
// because the discriminator field ensures mutual exclusivity between options in JSON Schema
const isDiscriminated = (def as any).discriminator !== undefined;
const options = def.options.map((x, i) =>
this.process(x, {
...params,
path: [...params.path, "anyOf", i],
path: [...params.path, isDiscriminated ? "oneOf" : "anyOf", i],
})
);
json.anyOf = options;
if (isDiscriminated) {
json.oneOf = options;
} else {
json.anyOf = options;
}
break;
}
case "intersection": {
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