fix: filter out messages with empty content arrays for Bedrock API (#31)

* fix: filter out messages with empty content arrays for Bedrock API

The convertToModelMessages function from AI SDK can produce messages with
empty content arrays when assistant messages have only tool call parts or
when tool results aren't properly converted. Bedrock API rejects these with
400 errors. This fix filters out invalid messages before sending to the API.

* fix: add diagnostic logging for empty message content

Added logging to capture the original UI message structure when empty content
is detected after conversion. This helps debug the root cause while the
filter provides a safety net for Bedrock API compatibility.

app/api/chat/route.ts

@@ -90,7 +90,30 @@ ${lastMessageText}
 
     // Convert UIMessages to ModelMessages and add system message
     const modelMessages = convertToModelMessages(messages);
-    let enhancedMessages = [...modelMessages];
+
+    // Log messages with empty content for debugging (helps identify root cause)
+    const emptyMessages = modelMessages.filter((msg: any) =>
+      !msg.content || !Array.isArray(msg.content) || msg.content.length === 0
+    );
+    if (emptyMessages.length > 0) {
+      console.warn('[Chat API] Messages with empty content detected:',
+        JSON.stringify(emptyMessages.map((m: any) => ({ role: m.role, contentLength: m.content?.length })))
+      );
+      console.warn('[Chat API] Original UI messages structure:',
+        JSON.stringify(messages.map((m: any) => ({
+          id: m.id,
+          role: m.role,
+          partsCount: m.parts?.length,
+          partTypes: m.parts?.map((p: any) => p.type)
+        })))
+      );
+    }
+
+    // Filter out messages with empty content arrays (Bedrock API rejects these)
+    // This is a safety measure - ideally convertToModelMessages should handle all cases
+    let enhancedMessages = modelMessages.filter((msg: any) =>
+      msg.content && Array.isArray(msg.content) && msg.content.length > 0
+    );
 
     // Update the last message with formatted content if it's a user message
     if (enhancedMessages.length >= 1) {