diff --git a/apps/sim/lib/core/utils/urls.test.ts b/apps/sim/lib/core/utils/urls.test.ts
index b476f76d073..bcbe797df6e 100644
--- a/apps/sim/lib/core/utils/urls.test.ts
+++ b/apps/sim/lib/core/utils/urls.test.ts
@@ -56,15 +56,20 @@ describe('getBaseUrl', () => {
     expect(getBaseUrl()).toBe('https://app.example.com')
   })
 
-  it('falls back to the page origin instead of throwing when the injected env is missing', () => {
+  /**
+   * Never guesses from `window.location.origin`: an opaque origin (a sandboxed
+   * iframe) serializes to the truthy string `'null'`, which would silently
+   * produce `null/api/...` rather than surfacing the misconfiguration.
+   */
+  it('throws in the browser rather than guessing from the page origin', () => {
     setLocation('https://www.sim.ai/workspace/ws-1/w/wf-1')
-    expect(getBaseUrl()).toBe('https://www.sim.ai')
+    expect(() => getBaseUrl()).toThrow('NEXT_PUBLIC_APP_URL must be configured')
   })
 
   it('treats a whitespace-only NEXT_PUBLIC_APP_URL as unset', () => {
     mockGetEnv.mockImplementation((key) => (key === 'NEXT_PUBLIC_APP_URL' ? '   ' : undefined))
     setLocation('https://www.sim.ai/')
-    expect(getBaseUrl()).toBe('https://www.sim.ai')
+    expect(() => getBaseUrl()).toThrow('NEXT_PUBLIC_APP_URL must be configured')
   })
 })
 
diff --git a/apps/sim/lib/core/utils/urls.ts b/apps/sim/lib/core/utils/urls.ts
index 59abca57512..5f13016e80b 100644
--- a/apps/sim/lib/core/utils/urls.ts
+++ b/apps/sim/lib/core/utils/urls.ts
@@ -25,31 +25,26 @@ function normalizeBaseUrl(url: string): string {
  * Returns the base URL of the application from NEXT_PUBLIC_APP_URL
  * This ensures webhooks, callbacks, and other integrations always use the correct public URL
  *
- * In the browser, falls back to the page's own origin when the injected env is
- * unavailable. Client-side callers only ever want a URL back to the app they are
- * already served from, so the origin is a correct answer — and a throw here
- * during render tears down the whole page through the error boundary. Server-side
- * callers (webhooks, callbacks, emails) have no origin to fall back to and must
- * still fail loudly on a misconfigured deployment.
+ * Deliberately has no browser fallback to `window.location.origin`. The value is
+ * injected before hydration by `<PublicEnvScript>`, so an empty read means the
+ * deployment is misconfigured — and a same-origin guess would hide that. It also
+ * would not be safe to guess: an opaque origin (a sandboxed iframe, and `/chat/*`
+ * is embeddable) serializes to the string `'null'`, which is truthy and would
+ * silently produce `null/api/...` at every call site.
  *
  * @returns The base URL string (e.g., 'http://localhost:3000' or 'https://example.com')
- * @throws Error if NEXT_PUBLIC_APP_URL is not configured and no browser origin exists
+ * @throws Error if NEXT_PUBLIC_APP_URL is not configured
  */
 export function getBaseUrl(): string {
   const baseUrl = getEnv('NEXT_PUBLIC_APP_URL')?.trim()
 
-  if (baseUrl) {
-    return normalizeBaseUrl(baseUrl)
-  }
-
-  const browserOrigin = getBrowserOrigin()
-  if (browserOrigin) {
-    return browserOrigin
+  if (!baseUrl) {
+    throw new Error(
+      'NEXT_PUBLIC_APP_URL must be configured for webhooks and callbacks to work correctly'
+    )
   }
 
-  throw new Error(
-    'NEXT_PUBLIC_APP_URL must be configured for webhooks and callbacks to work correctly'
-  )
+  return normalizeBaseUrl(baseUrl)
 }
 
 /**
diff --git a/packages/testing/src/mocks/urls.mock.ts b/packages/testing/src/mocks/urls.mock.ts
index ab013442723..2dab1b1560e 100644
--- a/packages/testing/src/mocks/urls.mock.ts
+++ b/packages/testing/src/mocks/urls.mock.ts
@@ -26,18 +26,14 @@ function hasHttpProtocol(url: string): boolean {
 
 function getBaseUrlImpl(): string {
   const baseUrl = readEnv('NEXT_PUBLIC_APP_URL')?.trim()
-  if (baseUrl) {
-    // Mirrors the real module: protocol-less values get https:// under isProd.
-    const protocol = envFlagsMock.isProd ? 'https://' : 'http://'
-    return hasHttpProtocol(baseUrl) ? baseUrl : `${protocol}${baseUrl}`
+  if (!baseUrl) {
+    throw new Error(
+      'NEXT_PUBLIC_APP_URL must be configured for webhooks and callbacks to work correctly'
+    )
   }
-  // Mirrors the real module: the browser falls back to its own origin, only
-  // server-side (no `window`) callers throw.
-  const browserOrigin = getBrowserOriginImpl()
-  if (browserOrigin) return browserOrigin
-  throw new Error(
-    'NEXT_PUBLIC_APP_URL must be configured for webhooks and callbacks to work correctly'
-  )
+  // Mirrors the real module: protocol-less values get https:// under isProd.
+  const protocol = envFlagsMock.isProd ? 'https://' : 'http://'
+  return hasHttpProtocol(baseUrl) ? baseUrl : `${protocol}${baseUrl}`
 }
 
 function getInternalApiBaseUrlImpl(): string {