diff --git a/contextual_orchestrator/server.py b/contextual_orchestrator/server.py
index a76cdd427..66d6d0a0a 100644
--- a/contextual_orchestrator/server.py
+++ b/contextual_orchestrator/server.py
@@ -5965,10 +5965,21 @@ def _authorize(
             self._authorized_purpose = effective_purpose
 
         def _cache_partition(self) -> str:
-            """Return a non-secret cache partition for the authenticated bearer."""
+            """Return a non-secret cache partition for the authenticated principal.
+
+            Bearer-authenticated callers partition by their bearer token.
+            Browser admin sessions authenticate without a bearer header, so an
+            active opaque session id partitions those requests instead — the
+            session id is random per login, so cross-session cache reuse stays
+            impossible while cookie-authenticated operators still get hits
+            within their own session.
+            """
             raw = self.headers.get("authorization", "")
             token = raw.split(" ", 1)[1].strip() if raw.lower().startswith("bearer ") else ""
-            if not token:  # pragma: no cover - _authorize rejects this first
+            if not token:
+                session_id = security._extract_admin_session_cookie(self.headers)
+                if session_id and security._admin_session_is_active(session_id):
+                    return hashlib.sha256(f"admin-session:{session_id}".encode("utf-8")).hexdigest()
                 raise RequestError(401, "unauthorized", "bearer token is required")
             return hashlib.sha256(token.encode("utf-8")).hexdigest()
 
diff --git a/tests/test_security_hardening.py b/tests/test_security_hardening.py
index a60217abf..cfb219004 100644
--- a/tests/test_security_hardening.py
+++ b/tests/test_security_hardening.py
@@ -141,6 +141,51 @@ def test_admin_session_is_opaque_scoped_and_revocable() -> None:
         thread.join(timeout=5)
 
 
+
+def test_admin_session_requests_partition_cache_without_bearer() -> None:
+    """A cookie-authenticated admin POST must not 401 in the cache partitioner.
+
+    Regression: #772's cache partitioner required a bearer header, breaking
+    every state-changing admin route for opaque-session operators after #788.
+    """
+    server = build_server(build(), port=0, security=SecurityConfig(auth_token="secret_token"))
+    thread = threading.Thread(target=server.serve_forever, daemon=True)
+    thread.start()
+    base = f"http://127.0.0.1:{server.server_address[1]}"
+    try:
+        status, body, headers = request_json(
+            f"{base}/admin/session",
+            "POST",
+            body={"token": "secret_token"},
+        )
+        assert status == 200
+        set_cookie = headers.get("set-cookie") or headers.get("Set-Cookie") or ""
+        cookie_pair = set_cookie.split(";", 1)[0]
+        status, evaluation, _ = request_json(
+            f"{base}/api/v1/evaluation_runs",
+            "POST",
+            body={"prompts": ["evaluate this"]},
+            headers={"cookie": cookie_pair, "origin": base},
+        )
+        assert status == 201 and evaluation["prompt_count"] == 1
+
+        status, other, other_headers = request_json(
+            f"{base}/admin/session",
+            "POST",
+            body={"token": "secret_token"},
+        )
+        other_cookie = (other_headers.get("set-cookie") or "").split(";", 1)[0]
+        status2, evaluation2, _ = request_json(
+            f"{base}/api/v1/evaluation_runs",
+            "POST",
+            body={"prompts": ["evaluate this"]},
+            headers={"cookie": other_cookie, "origin": base},
+        )
+        assert status2 == 201
+    finally:
+        server.shutdown()
+        thread.join(timeout=5)
+
 def test_http_api_requires_bearer_token_and_hides_trace_by_default() -> None:
     server = build_server(build(), port=0, security=SecurityConfig(auth_token="secret_token"))
     thread = threading.Thread(target=server.serve_forever, daemon=True)