diff --git a/tests/tools/test_send_message_tool.py b/tests/tools/test_send_message_tool.py
index 60f71af69d..3fc08b31e3 100644
--- a/tests/tools/test_send_message_tool.py
+++ b/tests/tools/test_send_message_tool.py
@@ -825,9 +825,13 @@ class TestParseTargetRefSlack:
     def test_dm_id_is_explicit(self):
         assert _parse_target_ref("slack", "D123ABCDEF")[2] is True
 
-    def test_user_id_is_explicit(self):
-        assert _parse_target_ref("slack", "U123ABCDEF")[2] is True
-        assert _parse_target_ref("slack", "W123ABCDEF")[2] is True
+    def test_user_id_is_not_explicit(self):
+        """Slack user IDs (U...) and workspace IDs (W...) are NOT explicit send
+        targets. chat.postMessage rejects them — a DM must be opened first via
+        conversations.open to obtain a D... conversation ID.
+        """
+        assert _parse_target_ref("slack", "U123ABCDEF")[2] is False
+        assert _parse_target_ref("slack", "W123ABCDEF")[2] is False
 
     def test_whitespace_is_stripped(self):
         chat_id, _, is_explicit = _parse_target_ref("slack", "  C0B0QV5434G  ")
diff --git a/tools/send_message_tool.py b/tools/send_message_tool.py
index cbf7e042e1..738cf6ca6f 100644
--- a/tools/send_message_tool.py
+++ b/tools/send_message_tool.py
@@ -20,10 +20,13 @@ logger = logging.getLogger(__name__)
 
 _TELEGRAM_TOPIC_TARGET_RE = re.compile(r"^\s*(-?\d+)(?::(\d+))?\s*$")
 _FEISHU_TARGET_RE = re.compile(r"^\s*((?:oc|ou|on|chat|open)_[-A-Za-z0-9]+)(?::([-A-Za-z0-9_]+))?\s*$")
-# Slack channel/user IDs: C (public), G (private/group), D (DM), U/W (user).
-# Always uppercase alphanumeric, 9+ chars. Without this, Slack IDs fall through
-# to channel-name resolution, which only matches by name and fails.
-_SLACK_TARGET_RE = re.compile(r"^\s*([CGDUW][A-Z0-9]{8,})\s*$")
+# Slack conversation IDs: C (public channel), G (private/group channel), D (DM).
+# Must be uppercase alphanumeric, 9+ chars. User IDs (U...) and workspace IDs
+# (W...) are NOT valid chat.postMessage channel values — posting to them fails
+# because the API requires a conversation ID. To DM a user you must first call
+# conversations.open to obtain a D... ID. Without this gate, Slack IDs fall
+# through to channel-name resolution, which only matches by name and fails.
+_SLACK_TARGET_RE = re.compile(r"^\s*([CGD][A-Z0-9]{8,})\s*$")
 _WEIXIN_TARGET_RE = re.compile(r"^\s*((?:wxid|gh|v\d+|wm|wb)_[A-Za-z0-9_-]+|[A-Za-z0-9._-]+@chatroom|filehelper)\s*$")
 # Discord snowflake IDs are numeric, same regex pattern as Telegram topic targets.
 _NUMERIC_TOPIC_RE = _TELEGRAM_TOPIC_TARGET_RE