diff --git a/plugins/platforms/slack/adapter.py b/plugins/platforms/slack/adapter.py
index 58ba0d91f10d..fe4b3091b7c5 100644
--- a/plugins/platforms/slack/adapter.py
+++ b/plugins/platforms/slack/adapter.py
@@ -4419,8 +4419,8 @@ class SlackAdapter(BasePlatformAdapter):
                 if stripped_blocks and stripped_blocks not in text.strip():
                     logger.debug(
                         "Slack: extracted additional text from blocks "
-                        "(likely quoted/forwarded content): %s",
-                        stripped_blocks[:300],
+                        "(likely quoted/forwarded content; chars=%d)",
+                        len(stripped_blocks),
                     )
                     text = (text.strip() + "\n" + stripped_blocks).strip()
 
diff --git a/tests/gateway/test_slack_mention.py b/tests/gateway/test_slack_mention.py
index eefc54f6d74e..3faaf7fdd3f2 100644
--- a/tests/gateway/test_slack_mention.py
+++ b/tests/gateway/test_slack_mention.py
@@ -6,8 +6,11 @@ Follows the same pattern as test_whatsapp_group_gating.py.
 
 import sys
 import inspect
+import logging
 from unittest.mock import MagicMock
 
+import pytest
+
 from gateway.config import Platform, PlatformConfig
 
 
@@ -802,6 +805,49 @@ def test_allowed_channels_env_var_blocks_channel(monkeypatch):
     assert _would_process(adapter, channel_id=CHANNEL_ID, mentioned=True) is True
 
 
+@pytest.mark.asyncio
+async def test_block_extraction_debug_log_does_not_include_message_preview(caplog):
+    secret_block_text = "private incident token: customer-id-12345"
+    adapter = _make_adapter(allowed_channels=[CHANNEL_ID])
+    adapter._dedup = MagicMock(is_duplicate=MagicMock(return_value=False))
+    adapter._lookup_assistant_thread_metadata = MagicMock(return_value={})
+    adapter._channel_team = {}
+
+    event = {
+        "channel": OTHER_CHANNEL_ID,
+        "channel_type": "channel",
+        "ts": "1710000000.000100",
+        "team": "T1",
+        "user": "U_USER",
+        "text": "<@U_BOT_123> see quoted message",
+        "blocks": [
+            {
+                "type": "rich_text",
+                "elements": [
+                    {
+                        "type": "rich_text_quote",
+                        "elements": [
+                            {
+                                "type": "rich_text_section",
+                                "elements": [
+                                    {"type": "text", "text": secret_block_text}
+                                ],
+                            }
+                        ],
+                    }
+                ],
+            }
+        ],
+    }
+
+    with caplog.at_level(logging.DEBUG, logger="plugins.platforms.slack.adapter"):
+        await adapter._handle_slack_message(event)
+
+    assert "extracted additional text from blocks" in caplog.text
+    assert "chars=" in caplog.text
+    assert secret_block_text not in caplog.text
+
+
 # ---------------------------------------------------------------------------
 # Tests: config bridging for allowed_channels
 # ---------------------------------------------------------------------------