fix(slack): expose shared-thread author mention target

In shared Slack threads the model saw only [sender name] prefixes, with
no verifiable current-author Slack user ID — so 'mention me again'
requests could bind to a stale or unrelated <@U...> pulled from names,
memory, or prior history (#17916).

Two cooperating changes:
- The shared-session sender prefix on Slack now carries the current
  author's ID from the event envelope:
  '[Alice | Slack user <@U123>] ...' — per-turn data, so it does not
  touch the cached system prompt.
- The Slack platform notes gain a shared-thread instruction to use the
  current turn's sender prefix as the only verified mention target and
  never guess or reuse historical mentions.

Fixes #17916.

Salvaged from #18711 by @LeonSGP43 (asdigitos), rebased over the
sender-name neutralization added on main (the ID is appended after
neutralizing the display name; the ID itself comes from the Slack
event, not user-editable text).

gateway/run.py

@@ -11818,6 +11818,16 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
             # (mirrors the same field's treatment in
             # build_session_context_prompt via _format_untrusted_prompt_value).
             _safe_user_name = neutralize_untrusted_inline_text(source.user_name)
+            # On Slack, expose the current author's verifiable user ID next to
+            # the display name (#17916): "mention me again" requests need a
+            # trusted `<@U...>` target for the CURRENT speaker — display names
+            # are ambiguous and historical mentions may point at someone else.
+            # The user_id comes from the Slack event envelope (not
+            # user-editable text), so it does not need neutralization.
+            if source.platform == Platform.SLACK and source.user_id:
+                _safe_user_name = (
+                    f"{_safe_user_name} | Slack user <@{source.user_id}>"
+                )
             message_text = f"[{_safe_user_name}] {message_text}"
 
         # Prepend channel context from history backfill (if any).  This

gateway/session.py

@@ -526,6 +526,13 @@ def build_session_context_prompt(
             "current message's Slack block/attachment payload when available, but "
             "you still cannot call Slack APIs yourself."
         )
+        if context.shared_multi_user_session:
+            lines.append(
+                "In shared Slack threads, use the current turn's sender prefix "
+                "as the only verified current-author mention target. Do not "
+                "guess or reuse `<@U...>` mentions from names, memory, or prior "
+                "conversation history."
+            )
     elif context.source.platform == Platform.DISCORD:
         # Inject the Discord IDs block only when the agent actually has
         # Discord tools loaded this session — i.e. the user opted into

tests/gateway/test_session.py

@@ -299,6 +299,49 @@ class TestBuildSessionContextPrompt:
         assert "pin" in prompt.lower()
         assert "current message's slack block/attachment payload" in prompt.lower()
 
+    def test_shared_slack_prompt_warns_against_guessed_self_mentions(self):
+        """Shared Slack threads must instruct the agent to bind mention
+        targets to the current turn's sender prefix (#17916)."""
+        config = GatewayConfig(
+            platforms={
+                Platform.SLACK: PlatformConfig(enabled=True, token="fake"),
+            },
+        )
+        source = SessionSource(
+            platform=Platform.SLACK,
+            chat_id="C123",
+            chat_name="team-channel",
+            chat_type="group",
+            user_id="U123",
+            user_name="Alice",
+            thread_id="171.000",
+        )
+        ctx = build_session_context(source, config)
+        prompt = build_session_context_prompt(ctx)
+
+        assert "current turn's sender prefix" in prompt
+        assert "Do not guess or reuse `<@U...>` mentions" in prompt
+
+    def test_non_shared_slack_prompt_omits_self_mention_guidance(self):
+        """1:1 Slack DMs are single-user: the shared-thread mention guidance
+        must not appear."""
+        config = GatewayConfig(
+            platforms={
+                Platform.SLACK: PlatformConfig(enabled=True, token="fake"),
+            },
+        )
+        source = SessionSource(
+            platform=Platform.SLACK,
+            chat_id="D123",
+            chat_type="dm",
+            user_id="U123",
+            user_name="Alice",
+        )
+        ctx = build_session_context(source, config)
+        prompt = build_session_context_prompt(ctx)
+
+        assert "current turn's sender prefix" not in prompt
+
     def test_discord_prompt_with_channel_topic(self):
         """Channel topic should appear in the session context prompt."""
         config = GatewayConfig(

tests/gateway/test_shared_group_sender_prefix.py

@@ -68,3 +68,64 @@ async def test_preprocess_keeps_plain_text_for_default_group_sessions():
     )
 
     assert result == "hello"
+
+
+@pytest.mark.asyncio
+async def test_preprocess_includes_slack_author_mention_for_shared_thread():
+    """Shared Slack threads expose the current author's verifiable user ID
+    next to the display name so 'mention me again' requests can bind the
+    mention to the CURRENT speaker (#17916)."""
+    runner = _make_runner(
+        GatewayConfig(
+            platforms={
+                Platform.SLACK: PlatformConfig(enabled=True, token="fake"),
+            },
+        )
+    )
+    source = SessionSource(
+        platform=Platform.SLACK,
+        chat_id="C123",
+        chat_name="team-channel",
+        chat_type="group",
+        user_id="U123",
+        user_name="Alice",
+        thread_id="171.000",
+    )
+    event = MessageEvent(text="mention me again", source=source)
+
+    result = await runner._prepare_inbound_message_text(
+        event=event,
+        source=source,
+        history=[],
+    )
+
+    assert result == "[Alice | Slack user <@U123>] mention me again"
+
+
+@pytest.mark.asyncio
+async def test_preprocess_slack_shared_thread_without_user_id_keeps_name_only():
+    """No user_id on the source → fall back to the plain name prefix."""
+    runner = _make_runner(
+        GatewayConfig(
+            platforms={
+                Platform.SLACK: PlatformConfig(enabled=True, token="fake"),
+            },
+        )
+    )
+    source = SessionSource(
+        platform=Platform.SLACK,
+        chat_id="C123",
+        chat_name="team-channel",
+        chat_type="group",
+        user_name="Alice",
+        thread_id="171.000",
+    )
+    event = MessageEvent(text="hello", source=source)
+
+    result = await runner._prepare_inbound_message_text(
+        event=event,
+        source=source,
+        history=[],
+    )
+
+    assert result == "[Alice] hello"