diff --git a/extensions/google-gemini-cli-auth/oauth.test.ts b/extensions/google-gemini-cli-auth/oauth.test.ts
index afc0971b16e..cec301a13e7 100644
--- a/extensions/google-gemini-cli-auth/oauth.test.ts
+++ b/extensions/google-gemini-cli-auth/oauth.test.ts
@@ -239,14 +239,15 @@ describe("loginGeminiCliOAuth", () => {
     "GOOGLE_CLOUD_PROJECT_ID",
   ] as const;
 
-  function getExpectedPlatform(): "WINDOWS" | "MACOS" | "LINUX" {
+  function getExpectedPlatform(): "WINDOWS" | "MACOS" | "LINUX" | "PLATFORM_UNSPECIFIED" {
     if (process.platform === "win32") {
       return "WINDOWS";
     }
-    if (process.platform === "linux") {
-      return "LINUX";
+    if (process.platform === "darwin") {
+      return "MACOS";
     }
-    return "MACOS";
+    // Matches updated resolvePlatform() which uses PLATFORM_UNSPECIFIED for Linux
+    return "PLATFORM_UNSPECIFIED";
   }
 
   function getRequestUrl(input: string | URL | Request): string {
diff --git a/extensions/google-gemini-cli-auth/oauth.ts b/extensions/google-gemini-cli-auth/oauth.ts
index 7e2280b9c9f..4f8f2938e47 100644
--- a/extensions/google-gemini-cli-auth/oauth.ts
+++ b/extensions/google-gemini-cli-auth/oauth.ts
@@ -224,14 +224,16 @@ function generatePkce(): { verifier: string; challenge: string } {
   return { verifier, challenge };
 }
 
-function resolvePlatform(): "WINDOWS" | "MACOS" | "LINUX" {
+function resolvePlatform(): "WINDOWS" | "MACOS" | "LINUX" | "PLATFORM_UNSPECIFIED" {
   if (process.platform === "win32") {
     return "WINDOWS";
   }
-  if (process.platform === "linux") {
-    return "LINUX";
+  if (process.platform === "darwin") {
+    return "MACOS";
   }
-  return "MACOS";
+  // Google's loadCodeAssist API rejects "LINUX" as an invalid Platform enum value.
+  // Use "PLATFORM_UNSPECIFIED" for Linux and other platforms to match the pi-ai runtime.
+  return "PLATFORM_UNSPECIFIED";
 }
 
 async function fetchWithTimeout(