diff --git a/gateway/platforms/discord.py b/gateway/platforms/discord.py
index 0f2b0bbad6..c5b12e09c1 100644
--- a/gateway/platforms/discord.py
+++ b/gateway/platforms/discord.py
@@ -2305,7 +2305,16 @@ class DiscordAdapter(BasePlatformAdapter):
             return (True, None)
 
         user_id = str(user.id)
-        if not self._is_allowed_user(user_id, author=user):
+        # Pass guild + is_dm so role check is scoped to the originating
+        # guild and cross-guild DM bypass (#12136) can't land via the
+        # slash surface either.
+        interaction_guild = getattr(interaction, "guild", None)
+        if not self._is_allowed_user(
+            user_id,
+            author=user,
+            guild=interaction_guild,
+            is_dm=in_dm,
+        ):
             return (
                 False,
                 "user not in DISCORD_ALLOWED_USERS / DISCORD_ALLOWED_ROLES",
diff --git a/tests/gateway/test_discord_roles_dm_scope.py b/tests/gateway/test_discord_roles_dm_scope.py
index a8c8561164..604b4e0aab 100644
--- a/tests/gateway/test_discord_roles_dm_scope.py
+++ b/tests/gateway/test_discord_roles_dm_scope.py
@@ -252,3 +252,93 @@ def test_empty_allowlists_allow_everyone():
         adapter._is_allowed_user("42", author=None, guild=None, is_dm=True)
         is True
     )
+
+
+# ---------------------------------------------------------------------------
+# Slash-surface sibling site: _evaluate_slash_authorization must pass
+# guild/is_dm through so the cross-guild bypass can't land via slash either.
+# ---------------------------------------------------------------------------
+
+
+def test_slash_authorization_rejects_cross_guild_role_dm(monkeypatch):
+    """Slash interaction in a DM must not be authorized by a role held in
+    any mutual guild (parallel to the on_message cross-guild bypass)."""
+    import discord as _discord  # type: ignore
+    monkeypatch.delenv("DISCORD_DM_ROLE_AUTH_GUILD", raising=False)
+
+    public_guild, _ = _guild_with_member(
+        guild_id=111111,
+        member_id=42,
+        role_ids=[5555],
+    )
+    adapter = _make_adapter(
+        allowed_roles=[5555],
+        guilds=[public_guild],
+    )
+
+    # Fake a DM interaction: user is Member-like, channel is DMChannel,
+    # interaction.guild is None.
+    interaction = SimpleNamespace(
+        user=SimpleNamespace(id=42),
+        channel=MagicMock(spec=_discord.DMChannel),
+        channel_id=None,
+        guild=None,
+    )
+
+    allowed, reason = adapter._evaluate_slash_authorization(interaction)
+    assert allowed is False
+    assert "ALLOWED" in (reason or "")
+
+
+def test_slash_authorization_rejects_cross_guild_role_in_guild(monkeypatch):
+    """Slash in guild B must not be authorized by a role held in guild A."""
+    monkeypatch.delenv("DISCORD_DM_ROLE_AUTH_GUILD", raising=False)
+
+    public_guild, _ = _guild_with_member(
+        guild_id=111111,
+        member_id=42,
+        role_ids=[5555],
+    )
+    # Interaction arrives in trusted_guild where user 42 has no role
+    trusted_guild = SimpleNamespace(id=222222, get_member=lambda uid: None)
+    adapter = _make_adapter(
+        allowed_roles=[5555],
+        guilds=[public_guild, trusted_guild],
+    )
+
+    interaction = SimpleNamespace(
+        user=SimpleNamespace(id=42),
+        channel=SimpleNamespace(id=9999),  # not a DMChannel instance
+        channel_id=9999,
+        guild=trusted_guild,
+    )
+
+    allowed, reason = adapter._evaluate_slash_authorization(interaction)
+    assert allowed is False
+    assert "ALLOWED" in (reason or "")
+
+
+def test_slash_authorization_allows_in_scope_guild_role(monkeypatch):
+    """Positive control: slash in guild B, user has role in guild B → allowed."""
+    monkeypatch.delenv("DISCORD_DM_ROLE_AUTH_GUILD", raising=False)
+
+    trusted_guild, _ = _guild_with_member(
+        guild_id=222222,
+        member_id=42,
+        role_ids=[5555],
+    )
+    adapter = _make_adapter(
+        allowed_roles=[5555],
+        guilds=[trusted_guild],
+    )
+
+    interaction = SimpleNamespace(
+        user=SimpleNamespace(id=42),
+        channel=SimpleNamespace(id=9999),
+        channel_id=9999,
+        guild=trusted_guild,
+    )
+
+    allowed, reason = adapter._evaluate_slash_authorization(interaction)
+    assert allowed is True
+    assert reason is None
diff --git a/tests/gateway/test_discord_slash_auth.py b/tests/gateway/test_discord_slash_auth.py
index a52ee1fd7e..e51f240e3a 100644
--- a/tests/gateway/test_discord_slash_auth.py
+++ b/tests/gateway/test_discord_slash_auth.py
@@ -158,7 +158,11 @@ def _make_interaction(
 
     return SimpleNamespace(
         user=user_obj,
-        guild=SimpleNamespace(owner_id=999),
+        # `get_member` needed for the guild-scoped role fallback path in
+        # _is_allowed_user after the #12136 cross-guild fix. Fixture guild
+        # has no members by default — tests exercising positive role paths
+        # assign their own Member via user.roles + matching allowed_role_ids.
+        guild=SimpleNamespace(owner_id=999, id=guild_id, get_member=lambda uid: None),
         guild_id=guild_id,
         channel_id=channel_id,
         channel=channel,