diff --git a/src/__tests__/bugfixes.test.ts b/src/__tests__/bugfixes.test.ts
index 9b3cf3f54..bdb5dcf4c 100644
--- a/src/__tests__/bugfixes.test.ts
+++ b/src/__tests__/bugfixes.test.ts
@@ -594,6 +594,17 @@ describe('Gemini Vertex provider drift guards', () => {
     expect(is3PSection).toContain('CLAUDE_CODE_USE_GEMINI_VERTEX')
   })
 
+  test('isUsing3PServices() includes Gemini Vertex flag', async () => {
+    const content = await file('utils/auth.ts').text()
+    const using3PSection = sectionBetween(
+      content,
+      'export function isUsing3PServices',
+      'function getConfiguredOtelHeadersHelper',
+    )
+
+    expect(using3PSection).toContain('CLAUDE_CODE_USE_GEMINI_VERTEX')
+  })
+
   test('API preconnect skips Gemini Vertex routing', async () => {
     const content = await file('utils/apiPreconnect.ts').text()
     const cloudSkipSection = sectionBetween(
diff --git a/src/utils/auth.ts b/src/utils/auth.ts
index 1e975a113..aba9d401d 100644
--- a/src/utils/auth.ts
+++ b/src/utils/auth.ts
@@ -1759,6 +1759,7 @@ export function isUsing3PServices(): boolean {
     isEnvTruthy(process.env.CLAUDE_CODE_USE_FOUNDRY) ||
     isEnvTruthy(process.env.CLAUDE_CODE_USE_OPENAI) ||
     isEnvTruthy(process.env.CLAUDE_CODE_USE_GEMINI) ||
+    isEnvTruthy(process.env.CLAUDE_CODE_USE_GEMINI_VERTEX) ||
     isEnvTruthy(process.env.CLAUDE_CODE_USE_MISTRAL) ||
     isEnvTruthy(process.env.CLAUDE_CODE_USE_GITHUB)
   )