diff --git a/backend/src/auth/routes.ts b/backend/src/auth/routes.ts
index 90cabfe4..c58e17e5 100644
--- a/backend/src/auth/routes.ts
+++ b/backend/src/auth/routes.ts
@@ -175,7 +175,12 @@ export function registerAuthRoutes(app: FastifyInstance, deps: AuthRouteDependen
 
   app.get("/auth/config", async (request, reply) => {
     const snapshot = captureAuthConfigSnapshot(request, deps.authentication);
-    if (!snapshot) return unavailable(reply);
+    if (!snapshot) {
+      if (deps.authMode === "upstream") {
+        return reply.send({ mode: "upstream", localLogin: false, oidcLogin: false });
+      }
+      return unavailable(reply);
+    }
     const mode = snapshot.value.mode;
     return reply.send({ mode, localLogin: mode === "local", oidcLogin: mode === "oidc" });
   });
diff --git a/backend/test/app-auth-mode.test.ts b/backend/test/app-auth-mode.test.ts
index 7b4515dd..95933669 100644
--- a/backend/test/app-auth-mode.test.ts
+++ b/backend/test/app-auth-mode.test.ts
@@ -88,3 +88,14 @@ test("configured OIDC initializes login from the literal secret bundle without a
     rmSync(directory, { recursive: true, force: true });
   }
 });
+
+test("upstream mode advertises its public authentication contract without a local auth provider", async () => {
+  const app = buildApp(loadConfig({ NODE_ENV: "production", AUTH_MODE: "upstream", THOTH_PUBLIC_EXPOSURE: "false" }));
+  try {
+    const response = await app.inject({ method: "GET", url: "/auth/config" });
+    expect(response.statusCode).toBe(200);
+    expect(response.json()).toEqual({ mode: "upstream", localLogin: false, oidcLogin: false });
+  } finally {
+    await app.close();
+  }
+});