diff --git a/src/routes/__tests__/gateway.test.ts b/src/routes/__tests__/gateway.test.ts
index d4111dd..37f137a 100644
--- a/src/routes/__tests__/gateway.test.ts
+++ b/src/routes/__tests__/gateway.test.ts
@@ -6,10 +6,12 @@ import request from 'supertest';
 import { createTestApp, signedCookie } from '../../__tests__/test-helpers.js';
 
 let previousUsers: string | undefined;
+let previousWebPort: string | undefined;
 let stateDir: string;
 
 beforeEach(() => {
     previousUsers = process.env.USERS;
+    previousWebPort = process.env.WEB_PORT;
     stateDir = mkdtempSync(join(tmpdir(), 'gateway-route-settings-'));
     process.env.USERS = JSON.stringify([{ id: 'u1', name: 'User', workDir: stateDir, stateDir }]);
 });
@@ -17,6 +19,8 @@ beforeEach(() => {
 afterEach(() => {
     if (previousUsers === undefined) delete process.env.USERS;
     else process.env.USERS = previousUsers;
+    if (previousWebPort === undefined) delete process.env.WEB_PORT;
+    else process.env.WEB_PORT = previousWebPort;
     rmSync(stateDir, { recursive: true, force: true });
 });
 
@@ -58,4 +62,15 @@ describe('/api/gateway', () => {
         const disabled = await request(server).post('/api/gateway').set('Cookie', cookie).send({ enabled: false });
         expect(disabled.body.gateway).toMatchObject({ enabled: false, configured: false, source: 'state' });
     });
-});
\ No newline at end of file
+
+    it('returns backend WEB_PORT base URL when host is vite dev server', async () => {
+        process.env.WEB_PORT = '3000';
+        const cookie = signedCookie('u1');
+        const res = await request(await app())
+            .get('/api/gateway')
+            .set('Cookie', cookie)
+            .set('Host', 'localhost:5173');
+        expect(res.status).toBe(200);
+        expect(res.body.gateway.baseUrl).toBe('http://localhost:3000/v1');
+    });
+});
diff --git a/src/routes/gateway.ts b/src/routes/gateway.ts
index 4ab78bc..3788d1e 100644
--- a/src/routes/gateway.ts
+++ b/src/routes/gateway.ts
@@ -11,6 +11,19 @@ function userConfig(userId: string) {
     return getUsersConfig().find((user) => user.id === userId);
 }
 
+function gatewayBaseUrl(origin: string | null | undefined): string {
+    const rawOrigin = typeof origin === 'string' ? origin : '';
+    try {
+        const url = new URL(rawOrigin);
+        if (url.port === '5173') {
+            url.port = process.env.WEB_PORT ?? '3000';
+        }
+        return `${url.origin}/v1`;
+    } catch {
+        return rawOrigin ? `${rawOrigin.replace(/\/$/, '')}/v1` : '/v1';
+    }
+}
+
 export function gateway(router: Router): void {
     router.get('/api/gateway', async (ctx) => {
         const userId = ctx.state.userId as string | undefined;
@@ -26,7 +39,8 @@ export function gateway(router: Router): void {
             return;
         }
         const userCtx = await calcUser(userId);
-        ctx.body = { gateway: await getGatewayStatus(cfg, userCtx.stateDir, `${ctx.origin}/v1`) };
+        const requestOrigin = ctx.origin ?? `${ctx.protocol}://${ctx.host}`;
+        ctx.body = { gateway: await getGatewayStatus(cfg, userCtx.stateDir, gatewayBaseUrl(requestOrigin)) };
     });
 
     router.post('/api/gateway', async (ctx) => {
@@ -44,11 +58,12 @@ export function gateway(router: Router): void {
         }
         const body = (ctx.request.body ?? {}) as Record<string, unknown>;
         const userCtx = await calcUser(userId);
+        const requestOrigin = ctx.origin ?? `${ctx.protocol}://${ctx.host}`;
         ctx.body = {
             gateway: await updateGatewayStatus(cfg, userCtx.stateDir, {
                 enabled: typeof body.enabled === 'boolean' ? body.enabled : undefined,
                 rotate: body.rotate === true,
-            }, `${ctx.origin}/v1`),
+            }, gatewayBaseUrl(requestOrigin)),
         };
     });
-}
\ No newline at end of file
+}