fix (ai/core): validate that messages are not empty (#5374)

Author: lgrammel

Diff for: .changeset/lazy-paws-divide.md

@@ -0,0 +1,5 @@
+---
+'ai': patch
+---
+
+fix (ai/core): validate that messages are not empty

Diff for: packages/ai/core/prompt/convert-to-language-model-prompt.ts

@@ -17,7 +17,6 @@ import {
 import { InvalidMessageRoleError } from './invalid-message-role-error';
 import { splitDataUrl } from './split-data-url';
 import { StandardizedPrompt } from './standardize-prompt';
-import { convertUint8ArrayToBase64 } from '@ai-sdk/provider-utils';
 
 export async function convertToLanguageModelPrompt({
   prompt,

Diff for: packages/ai/core/prompt/standardize-prompt.test.ts

@@ -17,4 +17,15 @@ describe('message prompt', () => {
       });
     }).toThrow(InvalidPromptError);
   });
+
+  it('should throw InvalidPromptError when messages array is empty', () => {
+    expect(() => {
+      standardizePrompt({
+        prompt: {
+          messages: [],
+        },
+        tools: undefined,
+      });
+    }).toThrow(InvalidPromptError);
+  });
 });

Diff for: packages/ai/core/prompt/standardize-prompt.ts

@@ -95,6 +95,13 @@ export function standardizePrompt<TOOLS extends ToolSet>({
           })
         : (prompt.messages as CoreMessage[]);
 
+    if (messages.length === 0) {
+      throw new InvalidPromptError({
+        prompt,
+        message: 'messages must not be empty',
+      });
+    }
+
     const validationResult = safeValidateTypes({
       value: messages,
       schema: z.array(coreMessageSchema),

Diff for: packages/ai/rsc/stream-ui/stream-ui.ui.test.tsx

@@ -192,7 +192,7 @@ describe('rsc - streamUI() onFinish callback', () => {
   beforeEach(async () => {
     const ui = await streamUI({
       model: mockToolModel,
-      messages: [],
+      prompt: '',
       tools: {
         tool1: {
           description: 'test tool 1',