🫙 fix: Drop Blank Content Blocks From Promptless Sends (#15530)

* 🫙 fix: Drop Blank Content Blocks From Promptless Sends

* fix: Preserve the promptless turn instead of dropping it

api/app/clients/prompts/formatAgentMessages.spec.js

@@ -622,4 +622,59 @@ describe('formatAgentMessages', () => {
       expect(result[0].content).toEqual(media);
     });
   });
+
+  describe('promptless sends', () => {
+    const PLACEHOLDER = [{ type: 'text', text: '(no text)' }];
+
+    it('should stand in for a replayed promptless turn instead of a blank block', () => {
+      const result = formatAgentMessages([
+        { role: 'user', content: '' },
+        { role: 'assistant', content: 'Hi' },
+      ]);
+
+      expect(result).toHaveLength(2);
+      expect(result[0]).toBeInstanceOf(HumanMessage);
+      expect(result[0].content).toEqual(PLACEHOLDER);
+      expect(result[0].content).not.toContainEqual(
+        expect.objectContaining({ type: 'text', text: '' }),
+      );
+    });
+
+    it('should keep the turn so assistant messages never become adjacent', () => {
+      const result = formatAgentMessages([
+        { role: 'assistant', content: 'first' },
+        { role: 'user', content: '' },
+        { role: 'assistant', content: 'second' },
+      ]);
+
+      expect(result).toHaveLength(3);
+      expect(result[1]).toBeInstanceOf(HumanMessage);
+    });
+
+    it('should not emit a blank text block for a whitespace-only user message', () => {
+      const result = formatAgentMessages([{ role: 'user', content: '   ' }]);
+
+      expect(result).toHaveLength(1);
+      expect(result[0].content).toEqual(PLACEHOLDER);
+    });
+
+    it('should keep a promptless user message that still carries its images', () => {
+      const image_urls = [{ type: 'image_url', image_url: { url: 'data:image/png;base64,AAA' } }];
+      const result = formatAgentMessages([{ role: 'user', content: '', image_urls }]);
+
+      expect(result).toHaveLength(1);
+      expect(result[0].content).toEqual(image_urls);
+      expect(result[0].content).not.toContainEqual(
+        expect.objectContaining({ type: 'text', text: '' }),
+      );
+    });
+
+    it('should still format a user message that has text', () => {
+      const result = formatAgentMessages([{ role: 'user', content: 'hello' }]);
+
+      expect(result).toHaveLength(1);
+      expect(result[0]).toBeInstanceOf(HumanMessage);
+      expect(result[0].content).toEqual([{ type: 'text', text: 'hello' }]);
+    });
+  });
 });

api/app/clients/prompts/formatMessages.js

@@ -7,6 +7,12 @@ const {
   SystemMessage,
 } = require('@librechat/agents/langchain/messages');
 
+/**
+ * Stands in for a user turn that carries no text and whose attachments are no longer
+ * being resent, so the turn stays valid without inventing content it never had.
+ */
+const EMPTY_MESSAGE_PLACEHOLDER = '(no text)';
+
 /**
  * Formats a message to OpenAI Vision API payload format.
  *
@@ -163,10 +169,32 @@ const formatAgentMessages = (payload) => {
 
   for (const message of payload) {
     if (typeof message.content === 'string') {
-      message.content = [{ type: ContentTypes.TEXT, [ContentTypes.TEXT]: message.content }];
+      /** An empty string yields a blank text block, which strict providers (Bedrock,
+       *  Anthropic) reject outright for the whole request. `formatVisionMessage`
+       *  already guards this for image-bearing sends; history replay of a
+       *  promptless send reaches here with no `image_urls`, so guard it too. */
+      message.content = message.content.trim()
+        ? [{ type: ContentTypes.TEXT, [ContentTypes.TEXT]: message.content }]
+        : [];
     }
     if (message.role !== 'assistant') {
-      messages.push(formatMessage({ message, langChain: true }));
+      const formatted = formatMessage({ message, langChain: true });
+      /** A promptless send replayed from history can reduce to nothing once its
+       *  attachments are no longer resent. Providers reject a blank text block and
+       *  an empty content array alike, but dropping the turn is not safe either:
+       *  nothing merges the assistant turns it would leave adjacent, and the same
+       *  providers reject consecutive assistant messages. Keep the turn, and give
+       *  it the smallest honest stand-in for the content that is no longer there. */
+      const { content: formattedContent } = formatted;
+      const isEmpty = Array.isArray(formattedContent)
+        ? formattedContent.length === 0
+        : typeof formattedContent === 'string' && formattedContent.trim() === '';
+      if (isEmpty) {
+        formatted.content = [
+          { type: ContentTypes.TEXT, [ContentTypes.TEXT]: EMPTY_MESSAGE_PLACEHOLDER },
+        ];
+      }
+      messages.push(formatted);
       continue;
     }