diff --git a/src/commands/model/model.test.tsx b/src/commands/model/model.test.tsx
index d35c1f0fe..a1e0888ef 100644
--- a/src/commands/model/model.test.tsx
+++ b/src/commands/model/model.test.tsx
@@ -21,6 +21,7 @@ type SettingsModule = typeof import('../../utils/settings/settings.js')
 const originalEnv = {
   CLAUDE_CODE_USE_OPENAI: process.env.CLAUDE_CODE_USE_OPENAI,
   CLAUDE_CODE_USE_GEMINI: process.env.CLAUDE_CODE_USE_GEMINI,
+  CLAUDE_CODE_USE_GEMINI_VERTEX: process.env.CLAUDE_CODE_USE_GEMINI_VERTEX,
   CLAUDE_CODE_USE_GITHUB: process.env.CLAUDE_CODE_USE_GITHUB,
   CLAUDE_CODE_USE_MISTRAL: process.env.CLAUDE_CODE_USE_MISTRAL,
   CLAUDE_CODE_USE_BEDROCK: process.env.CLAUDE_CODE_USE_BEDROCK,
@@ -202,6 +203,10 @@ afterEach(() => {
     settingsForTest = {}
     restoreEnv('CLAUDE_CODE_USE_OPENAI', originalEnv.CLAUDE_CODE_USE_OPENAI)
     restoreEnv('CLAUDE_CODE_USE_GEMINI', originalEnv.CLAUDE_CODE_USE_GEMINI)
+    restoreEnv(
+      'CLAUDE_CODE_USE_GEMINI_VERTEX',
+      originalEnv.CLAUDE_CODE_USE_GEMINI_VERTEX,
+    )
     restoreEnv('CLAUDE_CODE_USE_GITHUB', originalEnv.CLAUDE_CODE_USE_GITHUB)
     restoreEnv('CLAUDE_CODE_USE_MISTRAL', originalEnv.CLAUDE_CODE_USE_MISTRAL)
     restoreEnv('CLAUDE_CODE_USE_BEDROCK', originalEnv.CLAUDE_CODE_USE_BEDROCK)
diff --git a/src/components/StartupScreen.ts b/src/components/StartupScreen.ts
index df8ba7cfd..ec886dc8a 100644
--- a/src/components/StartupScreen.ts
+++ b/src/components/StartupScreen.ts
@@ -85,10 +85,10 @@ const LOGO_CLAUDE = [
 
 export function detectProvider(modelOverride?: string): { name: string; model: string; baseUrl: string; isLocal: boolean } {
   const useGeminiVertex = isEnvTruthy(process.env.CLAUDE_CODE_USE_GEMINI_VERTEX)
-  const useGemini = process.env.CLAUDE_CODE_USE_GEMINI === '1' || process.env.CLAUDE_CODE_USE_GEMINI === 'true'
-  const useGithub = process.env.CLAUDE_CODE_USE_GITHUB === '1' || process.env.CLAUDE_CODE_USE_GITHUB === 'true'
-  const useOpenAI = process.env.CLAUDE_CODE_USE_OPENAI === '1' || process.env.CLAUDE_CODE_USE_OPENAI === 'true'
-  const useMistral = process.env.CLAUDE_CODE_USE_MISTRAL === '1' || process.env.CLAUDE_CODE_USE_MISTRAL === 'true'
+  const useGemini = isEnvTruthy(process.env.CLAUDE_CODE_USE_GEMINI)
+  const useGithub = isEnvTruthy(process.env.CLAUDE_CODE_USE_GITHUB)
+  const useOpenAI = isEnvTruthy(process.env.CLAUDE_CODE_USE_OPENAI)
+  const useMistral = isEnvTruthy(process.env.CLAUDE_CODE_USE_MISTRAL)
 
   if (useGeminiVertex) {
     // Use the shared resolvers so this display matches the runtime/provider
diff --git a/src/services/api/promptCacheBreakDetection.test.ts b/src/services/api/promptCacheBreakDetection.test.ts
index 5ad9423e3..fbefb41b5 100644
--- a/src/services/api/promptCacheBreakDetection.test.ts
+++ b/src/services/api/promptCacheBreakDetection.test.ts
@@ -50,6 +50,7 @@ const PROVIDER_ENV_KEYS = [
   'CLAUDE_CODE_USE_OPENAI',
   'CLAUDE_CODE_USE_GITHUB',
   'CLAUDE_CODE_USE_GEMINI',
+  'CLAUDE_CODE_USE_GEMINI_VERTEX',
   'CLAUDE_CODE_USE_MISTRAL',
   'CLAUDE_CODE_USE_BEDROCK',
   'CLAUDE_CODE_USE_VERTEX',
diff --git a/src/services/api/withRetry.test.ts b/src/services/api/withRetry.test.ts
index 63bc273d8..e747c42b6 100644
--- a/src/services/api/withRetry.test.ts
+++ b/src/services/api/withRetry.test.ts
@@ -22,6 +22,7 @@ let originalProvidersModule: ProvidersModule | undefined
 const envKeys = [
   'CLAUDE_CODE_USE_OPENAI',
   'CLAUDE_CODE_USE_GEMINI',
+  'CLAUDE_CODE_USE_GEMINI_VERTEX',
   'CLAUDE_CODE_USE_GITHUB',
   'CLAUDE_CODE_USE_BEDROCK',
   'CLAUDE_CODE_USE_VERTEX',
diff --git a/src/utils/betas.test.ts b/src/utils/betas.test.ts
index 1e38e0ea5..be9e90d88 100644
--- a/src/utils/betas.test.ts
+++ b/src/utils/betas.test.ts
@@ -22,6 +22,7 @@ import { setSdkBetas } from '../bootstrap/state.js'
 const PROVIDER_ENV_KEYS = [
   'CLAUDE_CODE_USE_OPENAI',
   'CLAUDE_CODE_USE_GEMINI',
+  'CLAUDE_CODE_USE_GEMINI_VERTEX',
   'CLAUDE_CODE_USE_MISTRAL',
   'CLAUDE_CODE_USE_GITHUB',
   'CLAUDE_CODE_USE_BEDROCK',
diff --git a/src/utils/conversationRecovery.test.ts b/src/utils/conversationRecovery.test.ts
index 5d6043e9d..478826584 100644
--- a/src/utils/conversationRecovery.test.ts
+++ b/src/utils/conversationRecovery.test.ts
@@ -17,6 +17,7 @@ const tempDirs: string[] = []
 const originalSimple = process.env.CLAUDE_CODE_SIMPLE
 const providerEnvKeys = [
   'CLAUDE_CODE_USE_GEMINI',
+  'CLAUDE_CODE_USE_GEMINI_VERTEX',
   'CLAUDE_CODE_USE_MISTRAL',
   'CLAUDE_CODE_USE_GITHUB',
   'CLAUDE_CODE_USE_BEDROCK',
diff --git a/src/utils/envFile.test.ts b/src/utils/envFile.test.ts
index 433516b06..7df9ee449 100644
--- a/src/utils/envFile.test.ts
+++ b/src/utils/envFile.test.ts
@@ -11,6 +11,7 @@ import {
   reapplyRememberedEnvFileValues,
   rememberLoadedEnvFileValues,
 } from './envFile.js'
+import { PROVIDER_SELECTION_FLAGS } from './providerSelectionFlags.js'
 
 const TEST_ENV_KEYS = [
   'NODE_OPTIONS',
@@ -21,6 +22,16 @@ const TEST_ENV_KEYS = [
   'OPENAI_API_KEY',
   'OPENAI_BASE_URL',
   'OPENAI_MODEL',
+  ...PROVIDER_SELECTION_FLAGS,
+  'GEMINI_ACCESS_TOKEN',
+  'GEMINI_VERTEX_PROJECT',
+  'GEMINI_VERTEX_LOCATION',
+  'GEMINI_VERTEX_MODEL',
+  'GEMINI_VERTEX_AUTH_MODE',
+  'GOOGLE_APPLICATION_CREDENTIALS',
+  'GOOGLE_CLOUD_PROJECT',
+  'GCLOUD_PROJECT',
+  'GOOGLE_PROJECT_ID',
   'WEB_AUTH_HEADER',
   'WEB_AUTH_SCHEME',
   'WEB_BODY_TEMPLATE',
@@ -335,6 +346,49 @@ describe('loadEnvFile', () => {
     expect(process.env.CODEX_AUTH_JSON_PATH).toBe('/tmp/codex-auth.json')
   })
 
+  it('accepts every registered provider-selection flag', () => {
+    // Guards against a new CLAUDE_CODE_USE_* flag (like Gemini Vertex) being
+    // added to PROVIDER_SELECTION_FLAGS but silently rejected by the env-file
+    // allowlist, which would break --provider-env-file end-to-end wiring.
+    const filePath = writeTempEnvFile(
+      PROVIDER_SELECTION_FLAGS.map(flag => `${flag}=1`).join('\n'),
+    )
+
+    const loaded = loadEnvFile(filePath)
+
+    for (const flag of PROVIDER_SELECTION_FLAGS) {
+      expect(loaded[flag]).toBe('1')
+      expect(process.env[flag]).toBe('1')
+    }
+  })
+
+  it('loads the documented Gemini Vertex provider setup', () => {
+    const filePath = writeTempEnvFile([
+      'CLAUDE_CODE_USE_GEMINI_VERTEX=1',
+      'GEMINI_VERTEX_PROJECT=my-project',
+      'GEMINI_VERTEX_LOCATION=us-central1',
+      'GEMINI_VERTEX_MODEL=gemini-2.0-flash',
+      'GEMINI_VERTEX_AUTH_MODE=adc',
+      'GOOGLE_APPLICATION_CREDENTIALS=/tmp/sa.json',
+      'GCLOUD_PROJECT=my-project',
+      'GOOGLE_PROJECT_ID=my-project',
+    ].join('\n'))
+
+    const loaded = loadEnvFile(filePath)
+
+    expect(loaded).toEqual({
+      CLAUDE_CODE_USE_GEMINI_VERTEX: '1',
+      GEMINI_VERTEX_PROJECT: 'my-project',
+      GEMINI_VERTEX_LOCATION: 'us-central1',
+      GEMINI_VERTEX_MODEL: 'gemini-2.0-flash',
+      GEMINI_VERTEX_AUTH_MODE: 'adc',
+      GOOGLE_APPLICATION_CREDENTIALS: '/tmp/sa.json',
+      GCLOUD_PROJECT: 'my-project',
+      GOOGLE_PROJECT_ID: 'my-project',
+    })
+    expect(process.env.CLAUDE_CODE_USE_GEMINI_VERTEX).toBe('1')
+  })
+
   it('rejects unsupported variables before mutating process environment', () => {
     const filePath = writeTempEnvFile([
       'OPENAI_MODEL=from-file',
diff --git a/src/utils/envFile.ts b/src/utils/envFile.ts
index be01239d7..e521fd6de 100644
--- a/src/utils/envFile.ts
+++ b/src/utils/envFile.ts
@@ -37,6 +37,7 @@ const ALLOWED_ENV_FILE_KEYS = new Set([
   'CLAUDE_CODE_USE_BEDROCK',
   'CLAUDE_CODE_USE_FOUNDRY',
   'CLAUDE_CODE_USE_GEMINI',
+  'CLAUDE_CODE_USE_GEMINI_VERTEX',
   'CLAUDE_CODE_USE_GITHUB',
   'CLAUDE_CODE_USE_MISTRAL',
   'CLAUDE_CODE_USE_OPENAI',
@@ -53,11 +54,16 @@ const ALLOWED_ENV_FILE_KEYS = new Set([
   'FIRECRAWL_API_KEY',
   'FIRECRAWL_API_URL',
   'FIREWORKS_API_KEY',
+  'GCLOUD_PROJECT',
   'GEMINI_ACCESS_TOKEN',
   'GEMINI_API_KEY',
   'GEMINI_AUTH_MODE',
   'GEMINI_BASE_URL',
   'GEMINI_MODEL',
+  'GEMINI_VERTEX_AUTH_MODE',
+  'GEMINI_VERTEX_LOCATION',
+  'GEMINI_VERTEX_MODEL',
+  'GEMINI_VERTEX_PROJECT',
   'GH_TOKEN',
   'GITHUB_COPILOT_ALLOW_SUBAGENTS',
   'GITHUB_COPILOT_FORCE_SYNC_SUBAGENTS',
@@ -65,8 +71,10 @@ const ALLOWED_ENV_FILE_KEYS = new Set([
   'GITHUB_COPILOT_OPTIMIZATION_DISABLED',
   'GITHUB_TOKEN',
   'GOOGLE_API_KEY',
+  'GOOGLE_APPLICATION_CREDENTIALS',
   'GOOGLE_CLOUD_PROJECT',
   'GOOGLE_CSE_ID',
+  'GOOGLE_PROJECT_ID',
   'GROQ_API_KEY',
   'HICAP_API_KEY',
   'JINA_API_KEY',
diff --git a/src/utils/providerFlag.ts b/src/utils/providerFlag.ts
index 0b834d887..87d4a22f9 100644
--- a/src/utils/providerFlag.ts
+++ b/src/utils/providerFlag.ts
@@ -225,18 +225,10 @@ export function applyModelFlagFromArgs(args: string[]): void {
   if (!model) return
 
   const useGeminiVertex = isEnvTruthy(process.env.CLAUDE_CODE_USE_GEMINI_VERTEX)
-  const useGemini =
-    process.env.CLAUDE_CODE_USE_GEMINI === '1' ||
-    process.env.CLAUDE_CODE_USE_GEMINI === 'true'
-  const useMistral =
-    process.env.CLAUDE_CODE_USE_MISTRAL === '1' ||
-    process.env.CLAUDE_CODE_USE_MISTRAL === 'true'
-  const useOpenAI =
-    process.env.CLAUDE_CODE_USE_OPENAI === '1' ||
-    process.env.CLAUDE_CODE_USE_OPENAI === 'true'
-  const useGithub =
-    process.env.CLAUDE_CODE_USE_GITHUB === '1' ||
-    process.env.CLAUDE_CODE_USE_GITHUB === 'true'
+  const useGemini = isEnvTruthy(process.env.CLAUDE_CODE_USE_GEMINI)
+  const useMistral = isEnvTruthy(process.env.CLAUDE_CODE_USE_MISTRAL)
+  const useOpenAI = isEnvTruthy(process.env.CLAUDE_CODE_USE_OPENAI)
+  const useGithub = isEnvTruthy(process.env.CLAUDE_CODE_USE_GITHUB)
 
   if (useGeminiVertex) {
     process.env.GEMINI_VERTEX_MODEL = model