diff --git a/frontend/src/api/runtime-config.test.ts b/frontend/src/api/runtime-config.test.ts
index 5589dddb..b646d6ef 100644
--- a/frontend/src/api/runtime-config.test.ts
+++ b/frontend/src/api/runtime-config.test.ts
@@ -15,6 +15,10 @@ describe("resolveBackendUrl", () => {
     expect(backendBaseUrl).toBe("/api");
   });
 
+  it("accepts a same-origin prefixed API base for embedded deployments", () => {
+    expect(resolveBackendUrl({ backendBaseUrl: "/embedded/api" })).toBe("/embedded/api");
+  });
+
   it.each(["/backend", "api", "//evil.test", "ftp://example.test", "https://user:pass@example.test"])(
     "rejects any browser-facing backend URL %j",
     (backendBaseUrl) => {
diff --git a/frontend/src/api/runtime-config.ts b/frontend/src/api/runtime-config.ts
index 4533bad3..f4f95639 100644
--- a/frontend/src/api/runtime-config.ts
+++ b/frontend/src/api/runtime-config.ts
@@ -1,6 +1,18 @@
-export function resolveBackendUrl(value?: string): string {
-  if (value === undefined || value === "/api") return "/api";
-  throw new Error("Invalid backend URL: the browser must use the same-origin /api route");
+export interface RuntimeConfig {
+  backendBaseUrl?: string;
+}
+
+declare global {
+  interface Window {
+    __THOTHII_CONFIG__?: RuntimeConfig;
+  }
+}
+
+export function resolveBackendUrl(value?: string | RuntimeConfig): string {
+  const candidate = typeof value === "string" ? value : value?.backendBaseUrl;
+  if (candidate === undefined || candidate === "/api") return "/api";
+  if (/^\/(?:[A-Za-z0-9._~-]+\/)+api$/.test(candidate)) return candidate;
+  throw new Error("Invalid backend URL: the browser must use a same-origin API route");
 }
 
 export function joinBackendPath(base: string, path: string): string {
@@ -9,4 +21,6 @@ export function joinBackendPath(base: string, path: string): string {
   return `${normalizedBase}/${normalizedPath}`;
 }
 
-export const backendBaseUrl = resolveBackendUrl();
+export const backendBaseUrl = resolveBackendUrl(
+  typeof window === "undefined" ? undefined : window.__THOTHII_CONFIG__,
+);