diff --git a/packages/core/src/telemetry/loggers.test.ts b/packages/core/src/telemetry/loggers.test.ts
index 904e410940..0988a84218 100644
--- a/packages/core/src/telemetry/loggers.test.ts
+++ b/packages/core/src/telemetry/loggers.test.ts
@@ -231,6 +231,7 @@ describe('loggers', () => {
           output_format: 'json',
           extension_ids: '',
           extensions_count: 0,
+          auth_type: 'vertex-ai',
         },
       });
     });
diff --git a/packages/core/src/telemetry/types.ts b/packages/core/src/telemetry/types.ts
index a6778fc851..a3aa5426b8 100644
--- a/packages/core/src/telemetry/types.ts
+++ b/packages/core/src/telemetry/types.ts
@@ -68,6 +68,7 @@ export class StartSessionEvent implements BaseTelemetryEvent {
   output_format: OutputFormat;
   extensions_count: number;
   extension_ids: string;
+  auth_type?: string;
 
   constructor(config: Config, toolRegistry?: ToolRegistry) {
     const generatorConfig = config.getContentGeneratorConfig();
@@ -102,6 +103,7 @@ export class StartSessionEvent implements BaseTelemetryEvent {
     const extensions = config.getExtensions();
     this.extensions_count = extensions.length;
     this.extension_ids = extensions.map((e) => e.id).join(',');
+    this.auth_type = generatorConfig?.authType;
     if (toolRegistry) {
       const mcpTools = toolRegistry
         .getAllTools()
@@ -135,6 +137,7 @@ export class StartSessionEvent implements BaseTelemetryEvent {
       output_format: this.output_format,
       extensions_count: this.extensions_count,
       extension_ids: this.extension_ids,
+      auth_type: this.auth_type,
     };
   }