feat (ai): verify that system messages have string content (#2206)

Author: lgrammel

.changeset/late-penguins-fold.md

@@ -0,0 +1,5 @@
+---
+'ai': patch
+---
+
+feat (ai): verify that system messages have string content

packages/core/core/prompt/get-validated-prompt.test.ts

@@ -0,0 +1,19 @@
+import { InvalidPromptError } from '@ai-sdk/provider';
+import { getValidatedPrompt } from './get-validated-prompt';
+
+describe('getValidatedPrompt', () => {
+  describe('messages', () => {
+    it('should throw InvalidPromptError when system message has parts', () => {
+      expect(() => {
+        getValidatedPrompt({
+          messages: [
+            {
+              role: 'system',
+              content: [{ type: 'text', text: 'test' }] as any,
+            },
+          ],
+        });
+      }).toThrow(InvalidPromptError);
+    });
+  });
+});

packages/core/core/prompt/get-validated-prompt.ts

@@ -31,6 +31,17 @@ export function getValidatedPrompt(prompt: Prompt): ValidatedPrompt {
     });
   }
 
+  if (prompt.messages != null) {
+    for (const message of prompt.messages) {
+      if (message.role === 'system' && typeof message.content !== 'string') {
+        throw new InvalidPromptError({
+          prompt,
+          message: 'system message content must be a string',
+        });
+      }
+    }
+  }
+
   return prompt.prompt != null
     ? {
         type: 'prompt',