Amplify Gen 2 Bedrock AI generation task returning ValidationException (mapping error)

The exact code in the docs here: https://docs.amplify.aws/react-native/ai/generation/ for AI generation tasks in Amplify Gen 2 produces the following response:

{
  "data": {
    "generateRecipe": null
  },
  "errors": [
    {
      "path": [
        "generateRecipe"
      ],
      "data": null,
      "errorType": "ValidationException:http://internal.amazon.com/coral/com.amazon.bedrock/",
      "errorInfo": null,
      "locations": [
        {
          "line": 2,
          "column": 3,
          "sourceName": null
        }
      ],
      "message": "A custom error was thrown from a mapping template."
    }
  ]
}

Exact code used to reproduce this in ./amplify/data/resource.ts:

const schema = a.schema({
...
  generateRecipe: a.generation({
    aiModel: a.ai.model('Amazon Nova Pro'),
    systemPrompt: 'You are a helpful assistant that generates recipes.',
  })
    .arguments({ description: a.string() })
    .returns(
      a.customType({
        name: a.string(),
        ingredients: a.string().array(),
        instructions: a.string(),
      })
    )
    .authorization((allow) => allow.authenticated()),
...
}).authorization((allow) => [
// just some lambda functions here
]);
export type Schema = ClientSchema<typeof schema>;

export const data = defineData({
  schema,
  authorizationModes: {
    defaultAuthorizationMode: "userPool",
    apiKeyAuthorizationMode: {
      expiresInDays: 30,
    },
  },
});

And the client side code to test is:

import { generateClient } from "aws-amplify/api";
import { createAIHooks } from "@aws-amplify/ui-react-ai";
import { Schema } from "../amplify/data/resource";
const client = generateClient<Schema>({ authMode: "userPool" });
const { useAIGeneration } = createAIHooks(client);
...
const [{ data, isLoading }, generateRecipe] = useAIGeneration("generateRecipe");

const generateSummary = async () => {
  await generateRecipe({
    description: 'I would like to bake a birthday cake for my friend. She has celiac disease and loves chocolate.',
  });
};

useEffect(() => console.log(isLoading, data), [isLoading, data]);
...
      <button onClick={() => generateSummary()}>click me!</button>

But this exact same error is produced when using the API playground off graphql queries like so:

query MyQuery {
  generateRecipe(description: "Chocolate cake") {
    ingredients
    instructions
    name
  }
}

I've attempted this with Claude Sonnet 3.5, Claude Sonnet 3.5 v2, Claude Haiku 3.5, DeepSeek-R1, and Amazon Nova Pro. Users are logged in, and this follows the exact code + guidelines from that aforementioned docs page. Any ideas?