convertToModelMessages produces invalid ModelMessage[] when providerMetadata is present in UI parts

[marinoska]

Description

When using convertToModelMessages from the Vercel AI SDK, the function copies providerMetadata into providerOptions on ui parts, causing the resulting output to be rejected by generateText() or streamText() with:

InvalidPromptError: The messages must be a ModelMessage[]. If you have passed a UIMessage[], you can use convertToModelMessages to convert them.

Steps to Reproduce

import { convertToModelMessages, generateText } from 'ai';

const uiMessages = [
  {
    role: 'assistant',
    parts: [
      {
        type: 'text',
        text: 'Hello!',
        providerMetadata: {}, // <-- triggers the issue
      },
    ],
  },
];

const modelMessages = convertToModelMessages(uiMessages);

await generateText({
  model: 'gpt-4o-mini',
  messages: modelMessages,
});

Actual behavior

The generated ModelMessage[] includes:

{
  "type": "text",
  "text": "Hello!",
  "providerOptions": {}
}

generateText() fails with AI_InvalidPromptError due to the invalid extra key.

Expected behavior

• convertToModelMessages should not add providerOptions for messages content.
• providerMetadata should remain untouched (consistent with how it’s used in UIMessage and text parts).

Proposed fix

In convertToModelMessages
→
fix in this spread:

...(part.providerMetadata != null
  ? { providerOptions: part.providerMetadata }
  : {}),

to

...(part.providerMetadata != null
  ? { providerMetadata: part.providerMetadata }
  : {}),

Or gate it only for parts that actually support providerOptions (e.g., tool-result, not text).

AI SDK Version

• "ai": "^5.0.76",

Code of Conduct

• I agree to follow this project's Code of Conduct

[tobiasbueschel]

Hi @marinoska 👋 I've drafted a PR to solve this here: #9756 let me know if that works for you?