mastra-ai/mastra · error

Unsupported schema type: function (schema libraries should i

Error message

Unsupported schema type: function (schema libraries should implement StandardSchemaWithJSON)

What it means

After toStandardSchema() rules out null/primitives, it checks functions: a bare function is not a recognized schema representation. Only StandardSchemaWithJSON functions (which expose a '~standard' and 'jsonSchema' interface) are accepted. A plain function such as a bare zod schema factory, a class constructor, or a schema-producing callback is rejected with this message telling library authors to implement StandardSchemaWithJSON.

Source

Thrown at packages/schema-compat/src/standard-schema/standard-schema.ts:179

  // Zod v4 schemas are also instanceof z3.ZodType due to prototype compatibility
  if (isZodV3(schema)) {
    return toStandardSchemaZodV3(schema as ZodType);
  }

  // Check for AI SDK Schema objects (Vercel's jsonSchema wrapper)
  if (isVercelSchema(schema)) {
    return toStandardSchemaAiSdk(schema as Schema<T>);
  }

  // At this point, assume it's a plain JSON Schema object
  // JSON Schema objects are plain objects with properties like 'type', 'properties', etc.
  if (schema === null || (typeof schema !== 'object' && typeof schema !== 'function')) {
    throw new Error(`Unsupported schema type: ${typeof schema}`);
  }

  // If it's a function that's not StandardSchemaWithJSON, it's not supported
  if (typeof schema === 'function') {
    throw new Error(`Unsupported schema type: function (schema libraries should implement StandardSchemaWithJSON)`);
  }

  return toStandardSchemaJsonSchema(schema as JSONSchema7);
}

/**
 * Type guard to check if a value implements the StandardSchemaV1 interface.
 *
 * @param value - The value to check
 * @returns True if the value implements StandardSchemaV1
 *
 * @example
 * ```typescript
 * import { isStandardSchema } from '@mastra/schema-compat';
 *
 * if (isStandardSchema(someValue)) {
 *   const result = someValue['~standard'].validate(input);
 * }

View on GitHub (pinned to 75dd419e61)

Solutions

1. Call the factory function to get the schema instance: () => z.object({...}) becomes z.object({...}).
2. If using a schema library, upgrade to a version implementing the Standard Schema spec (Zod 3.24+/v4, Valibot, ArkType).
3. Wrap the function's returned schema in a StandardSchemaWithJSON adapter before passing it.
4. Convert the schema to plain JSON Schema (JSONSchema7 object) and pass that instead.

Example fix

// before
agent({ schema: MyZodObjectSchemaFactory }) // function passed
// after
agent({ schema: MyZodObjectSchemaFactory() }) // call it to get the zod schema instance

if (typeof schema === 'function' && !('~standard' in (schema as any))) {
  throw new TypeError('Function schemas must implement StandardSchemaWithJSON; call the factory or pass its result.');
}

const isStandardSchemaWithJSONFn = (s: unknown): s is { '~standard': unknown; jsonSchema: unknown } =>
  typeof s === 'function' && '~standard' in s;

let normalized;
try {
  normalized = toStandardSchema(schema);
} catch (e) {
  if (/Unsupported schema type: function/.test(e.message)) {
    normalized = toStandardSchema((schema as () => unknown)()); // call factory
  } else throw e;
}

When it happens

Trigger: Passing a plain function (e.g. () => z.object(...), a class, or a legacy schema-library constructor) to toStandardSchema via wrapped/result/rewrapped, when the function does not implement StandardSchemaWithJSON.

Common situations: Passing a Zod v2/legacy schema class; passing a function from an older schema library that never implemented the Standard Schema spec; accidentally passing the schema factory instead of calling it (z.object({...}) vs z.object); custom schema wrappers that are callables.

Related errors

• Your schema is async, which is not supported. Please use a s
• Unsupported schema type: ${typeof schema}
• StandardSchemaWithJSON is not supported for applyCompatLayer
• ${label} contains an unsupported field.
• SCHEMA_UNAVAILABLE

AI-assisted analysis of mastra-ai/mastra@75dd419e61 (2026-08-30). Data as JSON: /api/errors/a8484d7fdb247069. Report an issue: GitHub.