colinhacks/zod · error · Error

Function types cannot be represented in JSON Schema

Error message

Function types cannot be represented in JSON Schema

What it means

Functions are not serializable, so `functionProcessor` (json-schema-processors.ts:250) refuses `z.function()` when `ctx.unrepresentable === "throw"` (default). JSON Schema has no concept of a callable value.

Source

Thrown at packages/zod/src/v4/core/json-schema-processors.ts:252

    }
  } else {
    Object.assign(_json, file);
  }
};

export const successProcessor: Processor<schemas.$ZodSuccess> = (_schema, _ctx, json, _params) => {
  (json as JSONSchema.BooleanSchema).type = "boolean";
};

export const customProcessor: Processor<schemas.$ZodCustom> = (_schema, ctx, _json, _params) => {
  if (ctx.unrepresentable === "throw") {
    throw new Error("Custom types cannot be represented in JSON Schema");
  }
};

export const functionProcessor: Processor<schemas.$ZodFunction> = (_schema, ctx, _json, _params) => {
  if (ctx.unrepresentable === "throw") {
    throw new Error("Function types cannot be represented in JSON Schema");
  }
};

export const transformProcessor: Processor<schemas.$ZodTransform> = (_schema, ctx, _json, _params) => {
  if (ctx.unrepresentable === "throw") {
    throw new Error("Transforms cannot be represented in JSON Schema");
  }
};

export const mapProcessor: Processor<schemas.$ZodMap> = (_schema, ctx, _json, _params) => {
  if (ctx.unrepresentable === "throw") {
    throw new Error("Map cannot be represented in JSON Schema");
  }
};

export const setProcessor: Processor<schemas.$ZodSet> = (_schema, ctx, _json, _params) => {
  if (ctx.unrepresentable === "throw") {
    throw new Error("Set cannot be represented in JSON Schema");

View on GitHub (pinned to 912f0f51b0)

Solutions

1. Pass `{ unrepresentable: "any" }` to skip the function node.
2. Remove the function field from the converted shape; model RPC/callable surfaces with separate request/response schemas instead.
3. Use `.omit({ handler: true })` on the object before conversion.

Example fix

// before
z.toJSONSchema(z.object({ onClick: z.function() })); // throws
// after
z.toJSONSchema(z.object({ onClick: z.function() }).omit({ onClick: true }), { unrepresentable: "any" });

// Omit function fields before conversion.
const contractSchema = schema.omit({ onClick: true });
const json = z.toJSONSchema(contractSchema, { unrepresentable: "any" });

function usesFunction(schema: z.ZodType): boolean {
  return schema._zod.traits.has("$ZodFunction");
}

try {
  return z.toJSONSchema(schema);
} catch (e) {
  if (e instanceof Error && /cannot be represented in JSON Schema/.test(e.message)) {
    return z.toJSONSchema(schema, { unrepresentable: "any" });
  }
  throw e;
}

When it happens

Trigger: `z.toJSONSchema()` over a schema tree that includes `z.function(args, ret)`, with default options.

Common situations: Documenting a schema that carries callbacks or RPC handles; converting a meta-schema that wraps function types.

Related errors

• BigInt cannot be represented in JSON Schema
• Symbols cannot be represented in JSON Schema
• Undefined cannot be represented in JSON Schema
• Void cannot be represented in JSON Schema
• Date cannot be represented in JSON Schema

AI-assisted analysis of colinhacks/zod@912f0f51b0 (2026-08-03). Data as JSON: /data/errors/40d2cba7e29d7e68.json. Report an issue: GitHub.