diff --git a/src/client/Api.ts b/src/client/Api.ts
index a5c3ab00d..5f6177b96 100644
--- a/src/client/Api.ts
+++ b/src/client/Api.ts
@@ -679,7 +679,7 @@ export function getApiBase() {
   const domainname = getAudience();
 
   if (domainname === "localhost") {
-    const apiDomain = process?.env?.API_DOMAIN;
+    const apiDomain = process.env.API_DOMAIN;
     if (apiDomain) {
       return `https://${apiDomain}`;
     }
diff --git a/tests/Api.test.ts b/tests/Api.test.ts
new file mode 100644
index 000000000..b5bf87ebf
--- /dev/null
+++ b/tests/Api.test.ts
@@ -0,0 +1,15 @@
+import { beforeEach, describe, expect, it } from "vitest";
+import { getApiBase } from "../src/client/Api";
+
+describe("getApiBase", () => {
+  beforeEach(() => {
+    localStorage.clear();
+  });
+
+  // API_DOMAIN is forced empty under vitest via the vite.config `define`, so this
+  // regression test exercises the fallback branch deterministically regardless of
+  // any API_DOMAIN in the host shell / CI.
+  it("falls back to http://localhost:8787 on localhost when apiHost is not set and API_DOMAIN is empty", () => {
+    expect(getApiBase()).toBe("http://localhost:8787");
+  });
+});
diff --git a/vite.config.ts b/vite.config.ts
index 393dc2ca7..95067afe4 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -242,7 +242,12 @@ export default defineConfig(({ mode }) => {
       "process.env.STRIPE_PUBLISHABLE_KEY": JSON.stringify(
         env.STRIPE_PUBLISHABLE_KEY,
       ),
-      "process.env.API_DOMAIN": JSON.stringify(env.API_DOMAIN),
+      // Force empty under vitest (mode "test") so the getApiBase localhost-
+      // fallback test is deterministic regardless of any API_DOMAIN in the
+      // host shell / CI environment.
+      "process.env.API_DOMAIN": JSON.stringify(
+        mode === "test" ? "" : (env.API_DOMAIN ?? ""),
+      ),
       // Add other process.env variables if needed, OR migrate code to import.meta.env
     },