diff --git a/web/src/lib/backend-config.test.ts b/web/src/lib/backend-config.test.ts
index 4775d3d1..1ed1abce 100644
--- a/web/src/lib/backend-config.test.ts
+++ b/web/src/lib/backend-config.test.ts
@@ -109,3 +109,39 @@ describe('buildTransportConfig', () => {
     expect(buildTransportConfig()?.wsURL).toBe('ws://localhost:9880/ws');
   });
 });
+
+describe('VITE_BACKEND_URL build-time fallback normalization', () => {
+  it('normalizes a VITE_BACKEND_URL that contains query and hash', async () => {
+    // Re-import the module with a stubbed env so the module-level constant is
+    // re-evaluated. vi.resetModules() clears the module registry, and
+    // vi.stubEnv sets import.meta.env before the fresh import runs.
+    vi.stubEnv('VITE_BACKEND_URL', 'https://api.example.com?x=1#frag');
+    vi.resetModules();
+    const mod = await import('./backend-config');
+
+    // No localStorage override → falls back to BUILD_TIME_BACKEND_URL.
+    localStorage.clear();
+    const backend = mod.getBackendUrl();
+    expect(backend).toBe('https://api.example.com');
+
+    const cfg = mod.buildTransportConfig();
+    expect(cfg).not.toBeUndefined();
+    expect(cfg!.baseURL).toBe('https://api.example.com/api');
+    expect(cfg!.wsURL).toBe('wss://api.example.com/ws');
+
+    vi.unstubAllEnvs();
+    vi.resetModules();
+  });
+
+  it('normalizes a VITE_BACKEND_URL with trailing slash', async () => {
+    vi.stubEnv('VITE_BACKEND_URL', 'https://api.example.com/');
+    vi.resetModules();
+    const mod = await import('./backend-config');
+
+    localStorage.clear();
+    expect(mod.getBackendUrl()).toBe('https://api.example.com');
+
+    vi.unstubAllEnvs();
+    vi.resetModules();
+  });
+});
diff --git a/web/src/lib/backend-config.ts b/web/src/lib/backend-config.ts
index 36999c99..f24380af 100644
--- a/web/src/lib/backend-config.ts
+++ b/web/src/lib/backend-config.ts
@@ -34,13 +34,31 @@ export class BackendStorageError extends Error {
   }
 }
 
-/** Build-time fallback (empty string when unset). */
-const BUILD_TIME_BACKEND_URL: string =
-  (import.meta.env.VITE_BACKEND_URL as string | undefined)?.trim() ?? '';
+/**
+ * Normalizes a raw backend URL string to `origin + pathname` with no trailing
+ * slash and no query/hash. Returns an empty string for empty/whitespace input
+ * or unparseable values. This is the same contract as setBackendUrl so that
+ * VITE_BACKEND_URL and runtime overrides share identical semantics.
+ */
+function normalizeBackendOrigin(raw: string): string {
+  const trimmed = raw.trim();
+  if (!trimmed) return '';
+  try {
+    const parsed = new URL(trimmed);
+    return (parsed.origin + parsed.pathname).replace(/\/+$/, '');
+  } catch {
+    return '';
+  }
+}
+
+/** Build-time fallback, fully normalized (empty string when unset or unparseable). */
+const BUILD_TIME_BACKEND_URL: string = normalizeBackendOrigin(
+  (import.meta.env.VITE_BACKEND_URL as string | undefined) ?? '',
+);
 
 /**
  * Returns the configured backend base origin (no trailing slash), or an empty
- * string when the UI should use its own origin (same-origin default).
+
  */
 export function getBackendUrl(): string {
   let stored = '';