🐛 fix(model-runtime): handle array content in anthropic assistant messages (#11206)

When assistant messages have array content (e.g., containing thinking
blocks) but no tool_calls, the code incorrectly tried to call .trim()
on the array, causing "TypeError: content?.trim is not a function".

Changes:
- Add check for array content type before processing
- Use buildArrayContent() to properly handle array content
- Return undefined for empty array content (consistent with empty string)
- Add 2 test cases for array content scenarios

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

packages/model-runtime/src/core/contextBuilders/anthropic.test.ts

@@ -216,6 +216,33 @@ describe('anthropicHelpers', () => {
         role: 'assistant',
       });
     });
+
+    it('should correctly convert assistant message with array content but no tool_calls', async () => {
+      const message: OpenAIChatMessage = {
+        content: [
+          { thinking: 'Let me think about this...', type: 'thinking', signature: 'sig123' },
+          { type: 'text', text: 'Here is my response.' },
+        ],
+        role: 'assistant',
+      };
+      const result = await buildAnthropicMessage(message);
+      expect(result).toEqual({
+        content: [
+          { thinking: 'Let me think about this...', type: 'thinking', signature: 'sig123' },
+          { type: 'text', text: 'Here is my response.' },
+        ],
+        role: 'assistant',
+      });
+    });
+
+    it('should return undefined for assistant message with empty array content', async () => {
+      const message: OpenAIChatMessage = {
+        content: [],
+        role: 'assistant',
+      };
+      const result = await buildAnthropicMessage(message);
+      expect(result).toBeUndefined();
+    });
   });
 
   describe('buildAnthropicMessages', () => {

packages/model-runtime/src/core/contextBuilders/anthropic.ts

@@ -118,8 +118,15 @@ export const buildAnthropicMessage = async (
       }
 
       // or it's a plain assistant message
+      // Handle array content (e.g., content with thinking blocks)
+      if (Array.isArray(content)) {
+        const messageContent = await buildArrayContent(content);
+        if (messageContent.length === 0) return undefined;
+        return { content: messageContent, role: 'assistant' };
+      }
+
       // Anthropic API requires non-empty content, filter out empty/whitespace-only content
-      const textContent = (content as string)?.trim();
+      const textContent = content?.trim();
       if (!textContent) return undefined;
       return { content: textContent, role: 'assistant' };
     }