diff --git a/server/opensandbox_server/api/lifecycle.py b/server/opensandbox_server/api/lifecycle.py
index 39bbc62b8..391ec1889 100644
--- a/server/opensandbox_server/api/lifecycle.py
+++ b/server/opensandbox_server/api/lifecycle.py
@@ -21,7 +21,7 @@
 
 from typing import List, Optional
 
-from fastapi import APIRouter, Body, Header, Query, Request, status
+from fastapi import APIRouter, Body, Header, HTTPException, Query, Request, status
 from fastapi.responses import Response
 
 from opensandbox_server.extensions import validate_extensions
@@ -45,6 +45,7 @@
     Snapshot,
     SnapshotFilter,
 )
+from opensandbox_server.services.constants import SandboxErrorCodes
 from opensandbox_server.services.factory import create_sandbox_service
 from opensandbox_server.services.snapshot_service import create_snapshot_service
 
@@ -504,6 +505,7 @@ def delete_snapshot(
     response_model_exclude_none=True,
     responses={
         200: {"description": "Endpoint retrieved successfully"},
+        400: {"model": ErrorResponse, "description": "The request was invalid or malformed"},
         401: {"model": ErrorResponse, "description": "Authentication credentials are missing or invalid"},
         403: {"model": ErrorResponse, "description": "The authenticated user lacks permission for this operation"},
         404: {"model": ErrorResponse, "description": "The requested resource does not exist"},
@@ -546,6 +548,18 @@ def get_sandbox_endpoint(
         HTTPException: If sandbox not found, endpoint not available, or signed
             routes are not supported by the runtime/configuration (400).
     """
+    if use_server_proxy and expires is not None:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail={
+                "code": SandboxErrorCodes.INVALID_PARAMETER,
+                "message": (
+                    "use_server_proxy cannot be combined with expires; "
+                    "signed endpoints require the gateway route."
+                ),
+            },
+        )
+
     # Delegate to the service layer for endpoint resolution
     endpoint = sandbox_service.get_endpoint(sandbox_id, port, expires=expires)
 
diff --git a/server/tests/test_routes_endpoint_behavior.py b/server/tests/test_routes_endpoint_behavior.py
index 6d71c6853..40afedb6c 100644
--- a/server/tests/test_routes_endpoint_behavior.py
+++ b/server/tests/test_routes_endpoint_behavior.py
@@ -91,7 +91,34 @@ def get_endpoint(sandbox_id: str, port: int, **kwargs) -> Endpoint:
     )
 
     assert response.status_code == 200
-    assert response.json()["endpoint"] == "sandbox.example.com/opensandbox/sandboxes/sbx-001/proxy/44772"
+    assert (
+        response.json()["endpoint"]
+        == "sandbox.example.com/opensandbox/sandboxes/sbx-001/proxy/44772"
+    )
+
+
+def test_get_endpoint_rejects_server_proxy_with_expires(
+    client: TestClient,
+    auth_headers: dict,
+    monkeypatch,
+) -> None:
+    class StubService:
+        @staticmethod
+        def get_endpoint(sandbox_id: str, port: int, **kwargs) -> Endpoint:
+            raise AssertionError("signed endpoint resolution should not run")
+
+    monkeypatch.setattr(lifecycle, "sandbox_service", StubService())
+
+    response = client.get(
+        "/v1/sandboxes/sbx-001/endpoints/44772",
+        params={"use_server_proxy": "true", "expires": "2000000000"},
+        headers=auth_headers,
+    )
+
+    assert response.status_code == 400
+    payload = response.json()
+    assert payload["code"] == "SANDBOX::INVALID_PARAMETER"
+    assert "use_server_proxy cannot be combined with expires" in payload["message"]
 
 
 def test_get_endpoint_rejects_non_numeric_port(
diff --git a/specs/sandbox-lifecycle.yml b/specs/sandbox-lifecycle.yml
index 9809d7d99..61eeda3fd 100644
--- a/specs/sandbox-lifecycle.yml
+++ b/specs/sandbox-lifecycle.yml
@@ -640,7 +640,7 @@ paths:
             maximum: 65535
         - name: use_server_proxy
           in: query
-          description: Whether to return a server-proxied URL
+          description: Whether to return a server-proxied URL. Cannot be combined with `expires`.
           schema:
             type: boolean
             default: false
@@ -652,7 +652,7 @@ paths:
             is **Linux / Unix epoch seconds** — a decimal `uint64` count of **whole seconds** since
             the Unix epoch (`1970-01-01 00:00:00` UTC, same as POSIX / `time(2)`), not
             milliseconds. Normalized to `expires_b36` for the four-segment route token. Omit to
-            get the unsigned/legacy response shape.
+            get the unsigned/legacy response shape. Cannot be combined with `use_server_proxy=true`.
           schema:
             type: string
             pattern: '^(0|[1-9][0-9]*)$'