Using the `messages` in `prepareStep` in subsequent `generateText` sometimes throws `InvalidPromptError`

[zsiegel92]

Description

This doesn't happen every time (the worst kind of bug!) but I sometimes get this kind of error:

InvalidPromptError [AI_InvalidPromptError]: Invalid prompt: The messages must be a ModelMessage[]. If you have passed a UIMessage[], you can use convertToModelMessages to convert them.
    at standardizePrompt (/home/runner/_work/my-repo/node_modules/.pnpm/ai@5.0.106_zod@3.25.76/node_modules/ai/src/prompt/standardize-prompt.ts:72:11)
    at async generateText (/home/runner/_work/my-repo/node_modules/.pnpm/ai@5.0.106_zod@3.25.76/node_modules/ai/src/generate-text/generate-text.ts:272:25)

when I do

let modelMessages: ModelMessage[] = []
await generateText({
  prepareStep: async ({messages}) =>{
    modelMessages = messages
    return {}
  }
  // ...
})

// ...

await generateText({
 messages: modelMessages,
  // ...
})

AI SDK Version

"ai": "^5.0.106"
"@ai-sdk/openai": "^2.0.77"
"zod": "^3.25.76"

Code of Conduct

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