fix(gemini-vertex): address CodeRabbit review on #1614

- onboard-github: clear CLAUDE_CODE_USE_GEMINI_VERTEX in the fast-path
  (applyGithubOnboardingProcessEnv + buildGithubOnboardingSettingsEnv), so
  GitHub activation resets Vertex routing on both onboarding paths, not just
  the device-flow finalize path. Covered by an assertion in the existing test.
- messages.normalizeMessages: guard against falsy/non-object slots before the
  switch reads message.type, matching the default arm's intent.
- toolUseIds.sanitizeToolUseIdForWire: keep at least one prefix char if a
  caller passes maxLength < tail length.
- providerFlag.test: add the Gemini Vertex env vars to the cleanup lists.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/commands/onboard-github/onboard-github.test.ts

@@ -97,6 +97,7 @@ describe('onboarding auth precedence cleanup', () => {
       GITHUB_ENTERPRISE_URL: 'https://github.old.example.com',
       CLAUDE_CODE_PROVIDER_PROFILE_ENV_APPLIED: '1',
       CLAUDE_CODE_PROVIDER_PROFILE_ENV_APPLIED_ID: 'profile_old',
+      CLAUDE_CODE_USE_GEMINI_VERTEX: '1',
     }
 
     applyGithubOnboardingProcessEnv('github:copilot', undefined, env)
@@ -115,6 +116,9 @@ describe('onboarding auth precedence cleanup', () => {
     expect(env.GITHUB_ENTERPRISE_URL).toBeUndefined()
 
     expect(env.CLAUDE_CODE_USE_OPENAI).toBeUndefined()
+    // The fast path must clear Gemini Vertex routing too, like the
+    // device-flow finalize path (PROVIDER_SPECIFIC_KEYS).
+    expect(env.CLAUDE_CODE_USE_GEMINI_VERTEX).toBeUndefined()
     expect(env.CLAUDE_CODE_PROVIDER_PROFILE_ENV_APPLIED).toBeUndefined()
     expect(env.CLAUDE_CODE_PROVIDER_PROFILE_ENV_APPLIED_ID).toBeUndefined()
 
@@ -127,6 +131,7 @@ describe('onboarding auth precedence cleanup', () => {
     expect(settingsEnv.OPENAI_PROJECT).toBeUndefined()
     expect(settingsEnv.OPENAI_ORGANIZATION).toBeUndefined()
     expect(settingsEnv.GITHUB_ENTERPRISE_URL).toBeUndefined()
+    expect(settingsEnv.CLAUDE_CODE_USE_GEMINI_VERTEX).toBeUndefined()
   })
 
   test('persists Enterprise URL when switching to GitHub Enterprise', () => {

src/commands/onboard-github/onboard-github.tsx

@@ -114,6 +114,7 @@ export function buildGithubOnboardingSettingsEnv(
     OPENAI_API_BASE: undefined,
     CLAUDE_CODE_USE_OPENAI: undefined,
     CLAUDE_CODE_USE_GEMINI: undefined,
+    CLAUDE_CODE_USE_GEMINI_VERTEX: undefined,
     CLAUDE_CODE_USE_BEDROCK: undefined,
     CLAUDE_CODE_USE_VERTEX: undefined,
     CLAUDE_CODE_USE_FOUNDRY: undefined,
@@ -144,6 +145,7 @@ export function applyGithubOnboardingProcessEnv(
 
   delete env.CLAUDE_CODE_USE_OPENAI
   delete env.CLAUDE_CODE_USE_GEMINI
+  delete env.CLAUDE_CODE_USE_GEMINI_VERTEX
   delete env.CLAUDE_CODE_USE_BEDROCK
   delete env.CLAUDE_CODE_USE_VERTEX
   delete env.CLAUDE_CODE_USE_FOUNDRY

src/utils/messages.ts

@@ -770,6 +770,13 @@ export function normalizeMessages(messages: Message[]): NormalizedMessage[] {
   // and remains true for all subsequent messages in the normalization process.
   let isNewChain = false
   return messages.flatMap(message => {
+    // Defensive: a falsy slot (sparse array) or a non-object would throw on
+    // the message.type read below. Drop it — this mirrors the default arm
+    // and keeps the render pipeline alive instead of crashing the UI.
+    if (!message || typeof message !== 'object' || !('type' in message)) {
+      return []
+    }
+
     switch (message.type) {
       case 'assistant': {
         isNewChain = isNewChain || message.message.content.length > 1

src/utils/providerFlag.test.ts

@@ -17,6 +17,7 @@ import {
 const ENV_KEYS = [
   'CLAUDE_CODE_USE_OPENAI',
   'CLAUDE_CODE_USE_GEMINI',
+  'CLAUDE_CODE_USE_GEMINI_VERTEX',
   'CLAUDE_CODE_USE_GITHUB',
   'CLAUDE_CODE_USE_MISTRAL',
   'CLAUDE_CODE_USE_BEDROCK',
@@ -26,6 +27,7 @@ const ENV_KEYS = [
   'OPENAI_API_KEY',
   'OPENAI_MODEL',
   'GEMINI_MODEL',
+  'GEMINI_VERTEX_MODEL',
   'NVIDIA_API_KEY',
   'NVIDIA_NIM',
   'BNKR_API_KEY',
@@ -53,6 +55,7 @@ beforeEach(async () => {
 const RESET_KEYS = [
   'CLAUDE_CODE_USE_OPENAI',
   'CLAUDE_CODE_USE_GEMINI',
+  'CLAUDE_CODE_USE_GEMINI_VERTEX',
   'CLAUDE_CODE_USE_GITHUB',
   'CLAUDE_CODE_USE_MISTRAL',
   'CLAUDE_CODE_USE_BEDROCK',
@@ -62,6 +65,7 @@ const RESET_KEYS = [
   'OPENAI_API_KEY',
   'OPENAI_MODEL',
   'GEMINI_MODEL',
+  'GEMINI_VERTEX_MODEL',
   'NVIDIA_API_KEY',
   'NVIDIA_NIM',
   'BNKR_API_KEY',

src/utils/toolUseIds.ts

@@ -34,5 +34,8 @@ export function sanitizeToolUseIdForWire(
   if (clean.length === 0) {
     return `tool${tail}`
   }
-  return clean.slice(0, maxLength - tail.length) + tail
+  // Defensive: keep at least one prefix char if a caller ever passes a
+  // maxLength smaller than the hash tail (all current call sites use >= 40).
+  const prefixLength = Math.max(1, maxLength - tail.length)
+  return clean.slice(0, prefixLength) + tail
 }