diff --git a/packages/web/server/lib/tts/base-url.js b/packages/web/server/lib/tts/base-url.js
index 493d18e4..855a5365 100644
--- a/packages/web/server/lib/tts/base-url.js
+++ b/packages/web/server/lib/tts/base-url.js
@@ -47,7 +47,10 @@ export const normalizeCustomOpenAIBaseURL = (value) => {
     return { error: 'Custom server URL must not include credentials' };
   }
 
-  const allowRemote = isEnvFlagEnabled(process.env.OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS);
+  const isDesktop = (process.env.OPENCHAMBER_RUNTIME || '').trim().toLowerCase() === 'desktop';
+  const envFlagRaw = process.env.OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS;
+  const hasExplicitFlag = typeof envFlagRaw === 'string' && envFlagRaw.trim().length > 0;
+  const allowRemote = hasExplicitFlag ? isEnvFlagEnabled(envFlagRaw) : isDesktop;
   if (!allowRemote && !isAllowedLocalHost(parsed.hostname)) {
     return {
       error: 'Remote custom server URLs are disabled. Set OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS=true to allow this host.',
diff --git a/packages/web/server/lib/tts/routes.test.js b/packages/web/server/lib/tts/routes.test.js
index c2d61396..ec30ea55 100644
--- a/packages/web/server/lib/tts/routes.test.js
+++ b/packages/web/server/lib/tts/routes.test.js
@@ -1,8 +1,9 @@
-import { describe, expect, it } from 'vitest';
+import { describe, expect, it, afterEach } from 'vitest';
 import express from 'express';
 import request from 'supertest';
 
 import { registerTtsRoutes } from './routes.js';
+import { normalizeCustomOpenAIBaseURL } from './base-url.js';
 
 const createApp = () => {
   const app = express();
@@ -51,3 +52,74 @@ describe('tts routes', () => {
     });
   });
 });
+
+describe('normalizeCustomOpenAIBaseURL', () => {
+  const originalRuntime = process.env.OPENCHAMBER_RUNTIME;
+  const originalAllowRemote = process.env.OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS;
+
+  afterEach(() => {
+    // Restore env vars after each test
+    if (originalRuntime === undefined) {
+      delete process.env.OPENCHAMBER_RUNTIME;
+    } else {
+      process.env.OPENCHAMBER_RUNTIME = originalRuntime;
+    }
+    if (originalAllowRemote === undefined) {
+      delete process.env.OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS;
+    } else {
+      process.env.OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS = originalAllowRemote;
+    }
+  });
+
+  it('rejects remote URLs when OPENCHAMBER_RUNTIME is not set (web)', () => {
+    delete process.env.OPENCHAMBER_RUNTIME;
+    delete process.env.OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS;
+
+    const result = normalizeCustomOpenAIBaseURL('https://my-tts-server.example.com/v1');
+    expect(result.error).toMatch(/Remote custom server URLs are disabled/);
+    expect(result.value).toBeUndefined();
+  });
+
+  it('allows remote URLs when OPENCHAMBER_RUNTIME is desktop', () => {
+    process.env.OPENCHAMBER_RUNTIME = 'desktop';
+    delete process.env.OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS;
+
+    const result = normalizeCustomOpenAIBaseURL('https://my-tts-server.example.com/v1');
+    expect(result.error).toBeUndefined();
+    expect(result.value).toBe('https://my-tts-server.example.com/v1');
+  });
+
+  it('allows remote URLs when OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS is true', () => {
+    delete process.env.OPENCHAMBER_RUNTIME;
+    process.env.OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS = 'true';
+
+    const result = normalizeCustomOpenAIBaseURL('https://my-tts-server.example.com/v1');
+    expect(result.error).toBeUndefined();
+    expect(result.value).toBe('https://my-tts-server.example.com/v1');
+  });
+
+  it('allows localhost URLs regardless of runtime', () => {
+    delete process.env.OPENCHAMBER_RUNTIME;
+    delete process.env.OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS;
+
+    const result = normalizeCustomOpenAIBaseURL('http://localhost:8880/v1');
+    expect(result.error).toBeUndefined();
+    expect(result.value).toBe('http://localhost:8880/v1');
+  });
+
+  it('strips query strings and trailing slashes', () => {
+    process.env.OPENCHAMBER_RUNTIME = 'desktop';
+
+    const result = normalizeCustomOpenAIBaseURL('https://my-server.com/v1/?key=123');
+    expect(result.value).toBe('https://my-server.com/v1');
+  });
+
+  it('denies remote URLs on desktop when env var is explicitly false', () => {
+    process.env.OPENCHAMBER_RUNTIME = 'desktop';
+    process.env.OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS = 'false';
+
+    const result = normalizeCustomOpenAIBaseURL('https://my-tts-server.example.com/v1');
+    expect(result.error).toMatch(/Remote custom server URLs are disabled/);
+    expect(result.value).toBeUndefined();
+  });
+});