diff --git a/horizon/tests/test_route_auth_audit.py b/horizon/tests/test_route_auth_audit.py
index 0291f5a4..20966ac6 100644
--- a/horizon/tests/test_route_auth_audit.py
+++ b/horizon/tests/test_route_auth_audit.py
@@ -19,8 +19,10 @@
     concern (see ``_warn_if_opal_verifier_disabled`` in horizon/pdp.py).
 """
 
+from typing import Annotated
+
 import pytest
-from fastapi import Depends, FastAPI
+from fastapi import APIRouter, Depends, FastAPI
 from fastapi.dependencies.utils import get_flat_dependant
 from fastapi.routing import APIRoute
 from horizon.authentication import PUBLIC_ROUTE_PATHS, enforce_pdp_token
@@ -134,3 +136,94 @@ def test_audit_flags_a_mounted_subapp():
     app = FastAPI()
     app.mount("/sub", FastAPI())
     assert any("/sub" in row for row in _find_unprotected_routes(app))
+
+
+# PUBLIC_ROUTE_PATHS entries the audit-built app legitimately does not mount. ``/scalar`` is
+# registered in ``PermitPDP.__init__`` *after* ``_configure_api_routes`` (see the comment
+# above PUBLIC_ROUTE_PATHS in horizon/authentication.py), so ``MockPermitPDP`` - which stops
+# at ``_configure_api_routes`` - never sees it, yet it is a real public route in production.
+#
+# TRADE-OFF (accepted): an entry listed here is exempted from the dead-entry check below, so
+# if its real route is ever deleted while the PUBLIC_ROUTE_PATHS entry stays, that now-dead
+# entry will NOT be flagged - and a later ungated route re-claiming the path would read as
+# public. Keep this set as small as possible. The exemption-free alternative is to make
+# MockPermitPDP mirror the post-_configure_api_routes registration so the path is really
+# mounted; we keep the exemption for now to stay test-only and not reshape production wiring.
+ALLOWLIST_ENTRIES_NOT_IN_AUDIT_APP: frozenset[str] = frozenset({"/scalar"})
+
+
+def test_allowlist_has_no_dead_entries():
+    """Every PUBLIC_ROUTE_PATHS entry must match a mounted route.
+
+    A dead allowlist entry is a pre-authorised hole: it exempts a path from the audit today
+    and silently waves through whatever route later claims that path. Matching by set
+    membership over ``getattr(route, "path", ...)`` covers the framework Swagger/OpenAPI
+    routes (plain starlette ``Route``s, not ``APIRoute``s) and paths shared across methods.
+    """
+    mounted = {getattr(route, "path", None) for route in _sidecar._app.routes}
+    dead = PUBLIC_ROUTE_PATHS - mounted - ALLOWLIST_ENTRIES_NOT_IN_AUDIT_APP
+    assert dead == set(), (
+        f"PUBLIC_ROUTE_PATHS entries that match no route on the audit-built app: {sorted(dead)}. "
+        "Either the entry is dead (remove it, or fix the path if a route was renamed), or it is a "
+        "real route registered in PermitPDP.__init__ after _configure_api_routes (like /scalar) "
+        "that MockPermitPDP never reaches - in which case add it to ALLOWLIST_ENTRIES_NOT_IN_AUDIT_APP."
+    )
+    # Keep the exemption honest: if ``/scalar`` ever becomes visible to the audit-built app,
+    # it no longer needs the special case and must be dropped from the set above.
+    assert ALLOWLIST_ENTRIES_NOT_IN_AUDIT_APP & mounted == set(), (
+        "An ALLOWLIST_ENTRIES_NOT_IN_AUDIT_APP entry is now mounted on the audit app; drop "
+        "it from that set so it is covered by the dead-entry check like every other path."
+    )
+
+
+def test_router_level_dependencies_surface_in_flat_dependant():
+    """Empirical FastAPI contract the whole audit rests on (proven on 0.125.0; pin is loose).
+
+    The audit detects gates by walking ``get_flat_dependant(route.dependant)``. That only
+    works if a dependency attached at ``include_router(dependencies=[...])`` propagates into
+    each child route's dependant, and if nested sub-dependencies (e.g. OPAL's
+    ``require_listener_token`` wrapping the authenticator) are flattened. If a future FastAPI
+    bump changes that, router-gated routes would read as unprotected and every real run would
+    fail for the wrong reason - so pin the assumption here, where the failure is legible.
+    """
+
+    def fake_gate():  # router-level gate
+        pass
+
+    def inner_gate():  # reachable ONLY as wrapper's sub-dependency - see the assertion below
+        pass
+
+    # Annotated-Depends (the repo's own convention, see horizon/authentication.py) keeps the
+    # sub-dependency in the annotation rather than the argument default - idiomatic FastAPI and
+    # B008-clean. inner_gate is nested one level under wrapper and attached nowhere else, so the
+    # nested-gated assertion genuinely exercises get_flat_dependant's recursion instead of
+    # passing on a directly-attached copy.
+    def wrapper(_: Annotated[None, Depends(inner_gate)] = None):
+        pass
+
+    router = APIRouter()
+
+    @router.get("/router-gated")
+    async def _router_gated():
+        return {}
+
+    @router.get("/nested-gated", dependencies=[Depends(wrapper)])
+    async def _nested_gated():
+        return {}
+
+    app = FastAPI()
+    app.include_router(router, dependencies=[Depends(fake_gate)])
+    by_path = {route.path: route for route in app.routes if isinstance(route, APIRoute)}
+
+    assert "fake_gate" in _route_auth_gates(by_path["/router-gated"]), (
+        "include_router(dependencies=...) no longer surfaces in get_flat_dependant - the "
+        "route audit's gate detection is broken for router-level gates; review it before "
+        "trusting a green run on this FastAPI version."
+    )
+    # inner_gate reaches this route ONLY through wrapper (wrapper is the route-level dep;
+    # fake_gate is router-level). If get_flat_dependant stops recursing into sub-dependencies,
+    # inner_gate drops out and this fails - the exact regression the assertion exists to pin.
+    assert {"wrapper", "inner_gate"} <= _route_auth_gates(by_path["/nested-gated"]), (
+        "nested Depends() is no longer flattened by get_flat_dependant - closure-wrapped "
+        "gates (e.g. OPAL's require_listener_token) would go undetected by the audit."
+    )