`validateUIMessages` doesn't catch empty arrays

[rijkvanzanten]

Description

The validateUIMessages and safeValidateUIMessages functions don't properly check for arrays being not-empty, for example:

await safeValidateUIMessages({ messages: [] });
// ^ => { success: true, data: [] }

but empty arrays aren't valid input for streamText and other operations. Passing those as messages for streamText for example will throw

InvalidPromptError [AI_InvalidPromptError]: Invalid prompt: messages must not be empty

AI SDK Version

• ai: 5.0.86

Code of Conduct

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[rijkvanzanten]

Same goes for empty-arrays down the object, for example:

await safeValidateUIMessages({ messages: [
  {
    id: '1',
    role: 'user',
    parts: [],
  }
]});

will succeed, even though it is an invalid input for other functions