diff --git a/ui/tests/unit/vite-proxy-origin.test.ts b/ui/tests/unit/vite-proxy-origin.test.ts
new file mode 100644
index 00000000..2db01199
--- /dev/null
+++ b/ui/tests/unit/vite-proxy-origin.test.ts
@@ -0,0 +1,90 @@
+import { describe, expect, it, vi } from 'vitest';
+import type { UserConfig } from 'vite';
+
+vi.mock('vite', () => ({ defineConfig: (config: UserConfig) => config }));
+vi.mock('@vitejs/plugin-react', () => ({ default: () => ({ name: 'react' }) }));
+vi.mock('@tailwindcss/vite', () => ({ default: () => ({ name: 'tailwindcss' }) }));
+
+const { default: viteConfig } = await import('../../vite.config');
+
+type ProxyRequest = {
+  getHeader(name: string): string | undefined;
+  setHeader(name: string, value: string): void;
+};
+
+type IncomingRequest = {
+  headers: { origin?: string };
+};
+
+type ProxyListener = (proxyRequest: ProxyRequest, request: IncomingRequest) => void;
+
+function getProxyListener(route: '/api' | '/ws', event: 'proxyReq' | 'proxyReqWs'): ProxyListener {
+  const proxyOptions = (viteConfig as UserConfig).server?.proxy?.[route];
+  if (!proxyOptions || typeof proxyOptions === 'string' || !proxyOptions.configure) {
+    throw new Error(`Missing configure hook for ${route}`);
+  }
+
+  const listeners = new Map<string, ProxyListener>();
+  const proxy = {
+    on(registeredEvent: string, listener: ProxyListener) {
+      listeners.set(registeredEvent, listener);
+    },
+  };
+
+  proxyOptions.configure(proxy as never, proxyOptions);
+
+  const listener = listeners.get(event);
+  if (!listener) {
+    throw new Error(`Missing ${event} listener for ${route}`);
+  }
+
+  return listener;
+}
+
+function applyOriginRewrite(listener: ProxyListener, origin?: string): string | undefined {
+  const headers = new Map<string, string>();
+  if (origin !== undefined) {
+    headers.set('origin', origin);
+  }
+
+  listener(
+    {
+      getHeader: (name) => headers.get(name),
+      setHeader: (name, value) => headers.set(name, value),
+    },
+    { headers: { origin } }
+  );
+
+  return headers.get('origin');
+}
+
+it('requires the trusted Vite development port', () => {
+  expect((viteConfig as UserConfig).server?.strictPort).toBe(true);
+});
+
+describe.each([
+  ['/api', 'proxyReq'],
+  ['/ws', 'proxyReqWs'],
+] as const)('%s Vite dev proxy origin handling', (route, event) => {
+  const listener = getProxyListener(route, event);
+
+  it.each(['http://localhost:5173', 'http://127.0.0.1:5173', 'http://[::1]:5173'])(
+    'rewrites trusted Vite origin %s to the backend origin',
+    (origin) => {
+      expect(applyOriginRewrite(listener, origin)).toBe('http://localhost:3000');
+    }
+  );
+
+  it.each([
+    'http://attacker.example.test',
+    'http://localhost.evil:5173',
+    'http://localhost:5174',
+    'https://localhost:5173',
+  ])('preserves untrusted origin %s', (origin) => {
+    expect(applyOriginRewrite(listener, origin)).toBe(origin);
+  });
+
+  it('preserves a missing origin', () => {
+    expect(applyOriginRewrite(listener)).toBeUndefined();
+  });
+});
diff --git a/ui/vite.config.ts b/ui/vite.config.ts
index 3ba84058..96e5459a 100644
--- a/ui/vite.config.ts
+++ b/ui/vite.config.ts
@@ -1,10 +1,33 @@
+import type { ClientRequest, IncomingMessage } from 'node:http';
 import { defineConfig } from 'vite';
+import type { ProxyOptions } from 'vite';
 import react from '@vitejs/plugin-react';
 import tailwindcss from '@tailwindcss/vite';
 import path from 'path';
 
 const UI_ROOT = __dirname;
 const REPO_ROOT = path.resolve(__dirname, '..');
+const BACKEND_ORIGIN = 'http://localhost:3000';
+const TRUSTED_VITE_DEV_ORIGINS = new Set([
+  'http://localhost:5173',
+  'http://127.0.0.1:5173',
+  'http://[::1]:5173',
+]);
+
+function rewriteTrustedViteDevOrigin(proxyRequest: ClientRequest, request: IncomingMessage): void {
+  const origin = request.headers.origin;
+  if (origin && TRUSTED_VITE_DEV_ORIGINS.has(origin)) {
+    proxyRequest.setHeader('origin', BACKEND_ORIGIN);
+  }
+}
+
+const configureHttpProxy: NonNullable<ProxyOptions['configure']> = (proxy) => {
+  proxy.on('proxyReq', rewriteTrustedViteDevOrigin);
+};
+
+const configureWebSocketProxy: NonNullable<ProxyOptions['configure']> = (proxy) => {
+  proxy.on('proxyReqWs', rewriteTrustedViteDevOrigin);
+};
 
 // https://vite.dev/config/
 export default defineConfig({
@@ -58,22 +81,23 @@ export default defineConfig({
   },
   server: {
     port: 5173,
+    strictPort: true,
     fs: {
       allow: [UI_ROOT, REPO_ROOT],
     },
     proxy: {
-      // The dashboard's CSRF guard rejects loopback origins on a different
-      // port, so the dev proxy must present the API server's own origin.
+      // Translate only trusted local Vite origins for the dashboard's CSRF guard.
+      // Preserve every other Origin so the backend can reject untrusted callers.
       '/api': {
-        target: 'http://localhost:3000',
+        target: BACKEND_ORIGIN,
         changeOrigin: true,
-        headers: { origin: 'http://localhost:3000' },
+        configure: configureHttpProxy,
       },
       '/ws': {
         target: 'ws://localhost:3000',
         ws: true,
         changeOrigin: true,
-        headers: { origin: 'http://localhost:3000' },
+        configure: configureWebSocketProxy,
       },
     },
   },