diff --git a/gateway/run_adapters.py b/gateway/run_adapters.py
index 3ea22ad640..c8d8211fff 100644
--- a/gateway/run_adapters.py
+++ b/gateway/run_adapters.py
@@ -1304,10 +1304,15 @@ class GatewayAdapterLifecycleMixin:
         return _handler
 
     def _make_profile_busy_session_handler(self, profile_name: str):
-        """Stamp an owning adapter's profile before resolving busy policy."""
+        """Stamp an owning adapter's profile, then resolve busy policy under the profile scope
+        (auth runs against the profile's own allowlist, same as the cold-path message handler)."""
+        from gateway.run import _async_profile_runtime_scope
+        profile_home = self._profile_home_or_none(profile_name)
+
         async def _handler(event, _session_key):
             self._stamp_event_profile(event, profile_name)
-            return await self._handle_active_session_busy_message(event, self._session_key_for_source(event.source))
+            async with self._scope_or_null(_async_profile_runtime_scope, profile_home):
+                return await self._handle_active_session_busy_message(event, self._session_key_for_source(event.source))
 
         return _handler
 
diff --git a/tests/gateway/test_busy_session_profile_scope.py b/tests/gateway/test_busy_session_profile_scope.py
new file mode 100644
index 0000000000..46ef148dc4
--- /dev/null
+++ b/tests/gateway/test_busy_session_profile_scope.py
@@ -0,0 +1,88 @@
+"""Regression for #103717: a secondary multiplex profile's busy-session
+follow-ups must authorize against THAT profile's allowlist, not whatever
+``os.environ`` happens to hold (the primary profile's first-writer-bridged
+value under multiplex).
+
+``_make_profile_message_handler`` (the cold path) wraps ``_handle_message``
+in ``_async_profile_runtime_scope`` before authorization runs. Until this
+fix, ``_make_profile_busy_session_handler`` (the busy path) stamped
+``source.profile`` but never entered that scope, so ``_is_user_authorized``
+fell through to ``os.environ`` and read the wrong profile's
+``FEISHU_ALLOWED_USERS``.
+"""
+from pathlib import Path
+
+import pytest
+
+from gateway.config import GatewayConfig, Platform, PlatformConfig
+from gateway.pairing import PairingStore
+from gateway.session import SessionSource
+
+
+@pytest.fixture
+def mux_home(tmp_path, monkeypatch):
+    from agent import secret_scope
+
+    home = tmp_path / "hh"
+    (home / "profiles" / "secondary").mkdir(parents=True)
+    (home / ".env").write_text("FEISHU_ALLOWED_USERS=primary-user\n")
+    (home / "profiles" / "secondary" / ".env").write_text(
+        "FEISHU_ALLOWED_USERS=secondary-user\n"
+    )
+    monkeypatch.setenv("HERMES_HOME", str(home))
+    for key in ("FEISHU_ALLOWED_USERS", "GATEWAY_ALLOW_ALL_USERS", "GATEWAY_ALLOWED_USERS"):
+        monkeypatch.delenv(key, raising=False)
+    prev = secret_scope.is_multiplex_active()
+    secret_scope.set_multiplex_active(True)
+    yield home
+    secret_scope.set_multiplex_active(prev)
+
+
+def _runner(home):
+    from gateway.run import GatewayRunner
+
+    runner = object.__new__(GatewayRunner)
+    runner.config = GatewayConfig(multiplex_profiles=True)
+    runner.config.platforms = {Platform.FEISHU: PlatformConfig(enabled=True, extra={})}
+    runner.pairing_store = PairingStore(profile="default")
+    runner.pairing_stores = {"default": runner.pairing_store}
+    runner._profile_adapters = {"secondary": {}}
+    runner.adapters = {}
+    return runner
+
+
+def _feishu_event(user_id):
+    from gateway.platforms.base import MessageEvent, MessageType
+
+    source = SessionSource(
+        platform=Platform.FEISHU,
+        user_id=user_id,
+        user_name=user_id,
+        chat_id="oc_dm",
+        chat_type="dm",
+        profile=None,
+    )
+    return MessageEvent(text="follow-up", message_type=MessageType.TEXT, source=source, message_id="m1")
+
+
+@pytest.mark.asyncio
+async def test_busy_handler_authorizes_against_secondary_profile_allowlist(mux_home):
+    """Secondary profile owner's follow-up while their session is busy must be
+    authorized against the SECONDARY profile's own allowlist."""
+    runner = _runner(mux_home)
+
+    # Isolate exactly the authorization decision the busy path gates on
+    # (see gateway/run_busy.py::_handle_active_session_busy_message's first
+    # check) without pulling in queueing/steer machinery unrelated to the bug.
+    async def _stub_busy_message(event, session_key):
+        return runner._is_user_authorized(event.source)
+
+    runner._handle_active_session_busy_message = _stub_busy_message
+
+    handler = runner._make_profile_busy_session_handler("secondary")
+
+    authorized_event = _feishu_event("secondary-user")
+    assert await handler(authorized_event, "sk") is True
+
+    intruder_event = _feishu_event("primary-user")
+    assert await handler(intruder_event, "sk") is False