feat(telemetry): Update API response in telemetry

Adds the text content of the API response to the  telemetry event. This provides more context for debugging and analysis without logging the entire, potentially large, response object.

- Adds an optional  field to the  type.
- Updates  to include the  field in the logged attributes.
- Modifies the  to extract the response text using  and pass it to the logger.
- Adds a new test file for the telemetry loggers, including tests for the  function to verify the new functionality.

4 files changed, 124 insertions, 0 deletions

diff --git a/packages/core/src/core/client.ts b/packages/core/src/core/client.ts
index a2deca4e..596ddcd7 100644
--- a/packages/core/src/core/client.ts
+++ b/packages/core/src/core/client.ts
@@ -250,6 +250,7 @@ export class GeminiClient {
         response.usageMetadata?.cachedContentTokenCount ?? 0,
       thoughts_token_count: response.usageMetadata?.thoughtsTokenCount ?? 0,
       tool_token_count: response.usageMetadata?.toolUsePromptTokenCount ?? 0,
+      response_text: getResponseText(response),
     });
   }
 
diff --git a/packages/core/src/telemetry/loggers.test.ts b/packages/core/src/telemetry/loggers.test.ts
new file mode 100644
index 00000000..3493dc49
--- /dev/null
+++ b/packages/core/src/telemetry/loggers.test.ts
@@ -0,0 +1,119 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { logs } from '@opentelemetry/api-logs';
+import { SemanticAttributes } from '@opentelemetry/semantic-conventions';
+import { Config } from '../config/config.js';
+import { EVENT_API_RESPONSE } from './constants.js';
+import { logApiResponse } from './loggers.js';
+import * as metrics from './metrics.js';
+import * as sdk from './sdk.js';
+import { vi, describe, beforeEach, it, expect } from 'vitest';
+
+describe('logApiResponse', () => {
+  const mockConfig = {
+    getSessionId: () => 'test-session-id',
+  } as Config;
+
+  const mockLogger = {
+    emit: vi.fn(),
+  };
+
+  const mockMetrics = {
+    recordApiResponseMetrics: vi.fn(),
+    recordTokenUsageMetrics: vi.fn(),
+  };
+
+  beforeEach(() => {
+    vi.spyOn(sdk, 'isTelemetrySdkInitialized').mockReturnValue(true);
+    vi.spyOn(logs, 'getLogger').mockReturnValue(mockLogger);
+    vi.spyOn(metrics, 'recordApiResponseMetrics').mockImplementation(
+      mockMetrics.recordApiResponseMetrics,
+    );
+    vi.spyOn(metrics, 'recordTokenUsageMetrics').mockImplementation(
+      mockMetrics.recordTokenUsageMetrics,
+    );
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date('2025-01-01T00:00:00.000Z'));
+  });
+
+  it('should log an API response with all fields', () => {
+    const event = {
+      model: 'test-model',
+      status_code: 200,
+      duration_ms: 100,
+      attempt: 1,
+      output_token_count: 50,
+      cached_content_token_count: 10,
+      thoughts_token_count: 5,
+      tool_token_count: 2,
+      response_text: 'test-response',
+    };
+
+    logApiResponse(mockConfig, event);
+
+    expect(mockLogger.emit).toHaveBeenCalledWith({
+      body: 'API response from test-model. Status: 200. Duration: 100ms.',
+      attributes: {
+        'session.id': 'test-session-id',
+        'event.name': EVENT_API_RESPONSE,
+        'event.timestamp': '2025-01-01T00:00:00.000Z',
+        [SemanticAttributes.HTTP_STATUS_CODE]: 200,
+        model: 'test-model',
+        status_code: 200,
+        duration_ms: 100,
+        attempt: 1,
+        output_token_count: 50,
+        cached_content_token_count: 10,
+        thoughts_token_count: 5,
+        tool_token_count: 2,
+        response_text: 'test-response',
+      },
+    });
+
+    expect(mockMetrics.recordApiResponseMetrics).toHaveBeenCalledWith(
+      mockConfig,
+      'test-model',
+      100,
+      200,
+      undefined,
+    );
+
+    expect(mockMetrics.recordTokenUsageMetrics).toHaveBeenCalledWith(
+      mockConfig,
+      'test-model',
+      50,
+      'output',
+    );
+  });
+
+  it('should log an API response with an error', () => {
+    const event = {
+      model: 'test-model',
+      duration_ms: 100,
+      attempt: 1,
+      error: 'test-error',
+      output_token_count: 50,
+      cached_content_token_count: 10,
+      thoughts_token_count: 5,
+      tool_token_count: 2,
+      response_text: 'test-response',
+    };
+
+    logApiResponse(mockConfig, event);
+
+    expect(mockLogger.emit).toHaveBeenCalledWith({
+      body: 'API response from test-model. Status: N/A. Duration: 100ms.',
+      attributes: {
+        'session.id': 'test-session-id',
+        ...event,
+        'event.name': EVENT_API_RESPONSE,
+        'event.timestamp': '2025-01-01T00:00:00.000Z',
+        'error.message': 'test-error',
+      },
+    });
+  });
+});
diff --git a/packages/core/src/telemetry/loggers.ts b/packages/core/src/telemetry/loggers.ts
index d2b01f65..48275829 100644
--- a/packages/core/src/telemetry/loggers.ts
+++ b/packages/core/src/telemetry/loggers.ts
@@ -195,6 +195,9 @@ export function logApiResponse(
     'event.name': EVENT_API_RESPONSE,
     'event.timestamp': new Date().toISOString(),
   };
+  if (event.response_text) {
+    attributes.response_text = event.response_text;
+  }
   if (event.error) {
     attributes['error.message'] = event.error;
   } else if (event.status_code) {
diff --git a/packages/core/src/telemetry/types.ts b/packages/core/src/telemetry/types.ts
index 4e2933a0..f62bd23e 100644
--- a/packages/core/src/telemetry/types.ts
+++ b/packages/core/src/telemetry/types.ts
@@ -53,6 +53,7 @@ export interface ApiResponseEvent {
   cached_content_token_count: number;
   thoughts_token_count: number;
   tool_token_count: number;
+  response_text?: string;
 }
 
 export interface CliConfigEvent {