diff --git a/agent/anthropic_adapter.py b/agent/anthropic_adapter.py
index 4c41c823..695f4ac9 100644
--- a/agent/anthropic_adapter.py
+++ b/agent/anthropic_adapter.py
@@ -706,14 +706,21 @@ def convert_messages_to_anthropic(
                 result.append({"role": "user", "content": [tool_result]})
             continue
 
-        # Regular user message
+        # Regular user message — validate non-empty content (Anthropic rejects empty)
         if isinstance(content, list):
             converted_blocks = _convert_content_to_anthropic(content)
-            result.append({
-                "role": "user",
-                "content": converted_blocks or [{"type": "text", "text": ""}],
-            })
+            # Check if all text blocks are empty
+            if not converted_blocks or all(
+                b.get("text", "").strip() == ""
+                for b in converted_blocks
+                if isinstance(b, dict) and b.get("type") == "text"
+            ):
+                converted_blocks = [{"type": "text", "text": "(empty message)"}]
+            result.append({"role": "user", "content": converted_blocks})
         else:
+            # Validate string content is non-empty
+            if not content or (isinstance(content, str) and not content.strip()):
+                content = "(empty message)"
             result.append({"role": "user", "content": content})
 
     # Strip orphaned tool_use blocks (no matching tool_result follows)
diff --git a/gateway/platforms/discord.py b/gateway/platforms/discord.py
index cb5bab1f..7ee1d3d7 100644
--- a/gateway/platforms/discord.py
+++ b/gateway/platforms/discord.py
@@ -2096,6 +2096,11 @@ class DiscordAdapter(BasePlatformAdapter):
         if pending_text_injection:
             event_text = f"{pending_text_injection}\n\n{event_text}" if event_text else pending_text_injection
 
+        # Defense-in-depth: prevent empty user messages from entering session
+        # (can happen when user sends @mention-only with no other text)
+        if not event_text or not event_text.strip():
+            event_text = "(The user sent a message with no text content)"
+
         event = MessageEvent(
             text=event_text,
             message_type=msg_type,
diff --git a/tests/test_anthropic_adapter.py b/tests/test_anthropic_adapter.py
index 71638f0d..00f78098 100644
--- a/tests/test_anthropic_adapter.py
+++ b/tests/test_anthropic_adapter.py
@@ -801,6 +801,48 @@ class TestConvertMessages:
         assert all(not (b.get("type") == "text" and b.get("text") == "") for b in assistant_blocks)
         assert any(b.get("type") == "tool_use" for b in assistant_blocks)
 
+    def test_empty_user_message_string_gets_placeholder(self):
+        """Empty user message strings should get '(empty message)' placeholder.
+
+        Anthropic rejects requests with empty user message content.
+        Regression test for #3143 — Discord @mention-only messages.
+        """
+        messages = [
+            {"role": "user", "content": ""},
+        ]
+        _, result = convert_messages_to_anthropic(messages)
+        assert result[0]["role"] == "user"
+        assert result[0]["content"] == "(empty message)"
+
+    def test_whitespace_only_user_message_gets_placeholder(self):
+        """Whitespace-only user messages should also get placeholder."""
+        messages = [
+            {"role": "user", "content": "   \n\t  "},
+        ]
+        _, result = convert_messages_to_anthropic(messages)
+        assert result[0]["content"] == "(empty message)"
+
+    def test_empty_user_message_list_gets_placeholder(self):
+        """Empty content list for user messages should get placeholder block."""
+        messages = [
+            {"role": "user", "content": []},
+        ]
+        _, result = convert_messages_to_anthropic(messages)
+        assert result[0]["role"] == "user"
+        assert isinstance(result[0]["content"], list)
+        assert len(result[0]["content"]) == 1
+        assert result[0]["content"][0] == {"type": "text", "text": "(empty message)"}
+
+    def test_user_message_with_empty_text_blocks_gets_placeholder(self):
+        """User message with only empty text blocks should get placeholder."""
+        messages = [
+            {"role": "user", "content": [{"type": "text", "text": ""}, {"type": "text", "text": "  "}]},
+        ]
+        _, result = convert_messages_to_anthropic(messages)
+        assert result[0]["role"] == "user"
+        assert isinstance(result[0]["content"], list)
+        assert result[0]["content"] == [{"type": "text", "text": "(empty message)"}]
+
 
 # ---------------------------------------------------------------------------
 # Build kwargs