diff --git a/integration-tests/json-output.test.ts b/integration-tests/json-output.test.ts
index 3b77b2d935..1892cf67a3 100644
--- a/integration-tests/json-output.test.ts
+++ b/integration-tests/json-output.test.ts
@@ -37,6 +37,15 @@ describe('JSON output', () => {
     expect(typeof parsed.stats).toBe('object');
   });
 
+  it('should return a valid JSON with a session ID', async () => {
+    const result = await rig.run('Hello', '--output-format', 'json');
+    const parsed = JSON.parse(result);
+
+    expect(parsed).toHaveProperty('session_id');
+    expect(typeof parsed.session_id).toBe('string');
+    expect(parsed.session_id).not.toBe('');
+  });
+
   it('should return a JSON error for sd auth mismatch before running', async () => {
     process.env['GOOGLE_GENAI_USE_GCA'] = 'true';
     await rig.setup('json-output-auth-mismatch', {
@@ -87,6 +96,9 @@ describe('JSON output', () => {
       "enforced authentication type is 'gemini-api-key'",
     );
     expect(payload.error.message).toContain("current type is 'oauth-personal'");
+    expect(payload).toHaveProperty('session_id');
+    expect(typeof payload.session_id).toBe('string');
+    expect(payload.session_id).not.toBe('');
   });
 
   it('should not exit on tool errors and allow model to self-correct in JSON mode', async () => {
@@ -129,5 +141,9 @@ describe('JSON output', () => {
 
     // Should NOT have an error field at the top level
     expect(parsed.error).toBeUndefined();
+
+    expect(parsed).toHaveProperty('session_id');
+    expect(typeof parsed.session_id).toBe('string');
+    expect(parsed.session_id).not.toBe('');
   });
 });
diff --git a/packages/cli/src/nonInteractiveCli.test.ts b/packages/cli/src/nonInteractiveCli.test.ts
index 67764f0345..f40dc68cbd 100644
--- a/packages/cli/src/nonInteractiveCli.test.ts
+++ b/packages/cli/src/nonInteractiveCli.test.ts
@@ -637,7 +637,11 @@ describe('runNonInteractive', () => {
     );
     expect(processStdoutSpy).toHaveBeenCalledWith(
       JSON.stringify(
-        { response: 'Hello World', stats: MOCK_SESSION_METRICS },
+        {
+          session_id: 'test-session-id',
+          response: 'Hello World',
+          stats: MOCK_SESSION_METRICS,
+        },
         null,
         2,
       ),
@@ -720,7 +724,15 @@ describe('runNonInteractive', () => {
 
     // This should output JSON with empty response but include stats
     expect(processStdoutSpy).toHaveBeenCalledWith(
-      JSON.stringify({ response: '', stats: MOCK_SESSION_METRICS }, null, 2),
+      JSON.stringify(
+        {
+          session_id: 'test-session-id',
+          response: '',
+          stats: MOCK_SESSION_METRICS,
+        },
+        null,
+        2,
+      ),
     );
   });
 
@@ -755,7 +767,15 @@ describe('runNonInteractive', () => {
 
     // This should output JSON with empty response but include stats
     expect(processStdoutSpy).toHaveBeenCalledWith(
-      JSON.stringify({ response: '', stats: MOCK_SESSION_METRICS }, null, 2),
+      JSON.stringify(
+        {
+          session_id: 'test-session-id',
+          response: '',
+          stats: MOCK_SESSION_METRICS,
+        },
+        null,
+        2,
+      ),
     );
   });
 
@@ -792,6 +812,7 @@ describe('runNonInteractive', () => {
     expect(consoleErrorJsonSpy).toHaveBeenCalledWith(
       JSON.stringify(
         {
+          session_id: 'test-session-id',
           error: {
             type: 'Error',
             message: 'Invalid input provided',
@@ -837,6 +858,7 @@ describe('runNonInteractive', () => {
     expect(consoleErrorJsonSpy).toHaveBeenCalledWith(
       JSON.stringify(
         {
+          session_id: 'test-session-id',
           error: {
             type: 'FatalInputError',
             message: 'Invalid command syntax provided',
diff --git a/packages/cli/src/nonInteractiveCli.ts b/packages/cli/src/nonInteractiveCli.ts
index 1f7308c6f2..07a2bf5ca9 100644
--- a/packages/cli/src/nonInteractiveCli.ts
+++ b/packages/cli/src/nonInteractiveCli.ts
@@ -428,7 +428,9 @@ export async function runNonInteractive({
           } else if (config.getOutputFormat() === OutputFormat.JSON) {
             const formatter = new JsonFormatter();
             const stats = uiTelemetryService.getMetrics();
-            textOutput.write(formatter.format(responseText, stats));
+            textOutput.write(
+              formatter.format(config.getSessionId(), responseText, stats),
+            );
           } else {
             textOutput.ensureTrailingNewline(); // Ensure a final newline
           }
diff --git a/packages/cli/src/utils/errors.test.ts b/packages/cli/src/utils/errors.test.ts
index b2267bd5a0..f56edf5768 100644
--- a/packages/cli/src/utils/errors.test.ts
+++ b/packages/cli/src/utils/errors.test.ts
@@ -29,18 +29,20 @@ vi.mock('@google/gemini-cli-core', async (importOriginal) => {
       return `API Error: ${String(error)}`;
     }),
     JsonFormatter: vi.fn().mockImplementation(() => ({
-      formatError: vi.fn((error: Error, code?: string | number) =>
-        JSON.stringify(
-          {
-            error: {
-              type: error.constructor.name,
-              message: error.message,
-              ...(code && { code }),
+      formatError: vi.fn(
+        (error: Error, code?: string | number, sessionId?: string) =>
+          JSON.stringify(
+            {
+              ...(sessionId && { session_id: sessionId }),
+              error: {
+                type: error.constructor.name,
+                message: error.message,
+                ...(code && { code }),
+              },
             },
-          },
-          null,
-          2,
-        ),
+            null,
+            2,
+          ),
       ),
     })),
     StreamJsonFormatter: vi.fn().mockImplementation(() => ({
@@ -77,6 +79,8 @@ describe('errors', () => {
   let processExitSpy: MockInstance;
   let consoleErrorSpy: MockInstance;
 
+  const TEST_SESSION_ID = 'test-session-123';
+
   beforeEach(() => {
     // Reset mocks
     vi.clearAllMocks();
@@ -93,6 +97,7 @@ describe('errors', () => {
     mockConfig = {
       getOutputFormat: vi.fn().mockReturnValue(OutputFormat.TEXT),
       getContentGeneratorConfig: vi.fn().mockReturnValue({ authType: 'test' }),
+      getSessionId: vi.fn().mockReturnValue(TEST_SESSION_ID),
     } as unknown as Config;
   });
 
@@ -166,6 +171,7 @@ describe('errors', () => {
         expect(consoleErrorSpy).toHaveBeenCalledWith(
           JSON.stringify(
             {
+              session_id: TEST_SESSION_ID,
               error: {
                 type: 'Error',
                 message: 'Test error',
@@ -188,6 +194,7 @@ describe('errors', () => {
         expect(consoleErrorSpy).toHaveBeenCalledWith(
           JSON.stringify(
             {
+              session_id: TEST_SESSION_ID,
               error: {
                 type: 'Error',
                 message: 'Test error',
@@ -210,6 +217,7 @@ describe('errors', () => {
         expect(consoleErrorSpy).toHaveBeenCalledWith(
           JSON.stringify(
             {
+              session_id: TEST_SESSION_ID,
               error: {
                 type: 'FatalInputError',
                 message: 'Fatal error',
@@ -246,6 +254,7 @@ describe('errors', () => {
         expect(consoleErrorSpy).toHaveBeenCalledWith(
           JSON.stringify(
             {
+              session_id: TEST_SESSION_ID,
               error: {
                 type: 'Error',
                 message: 'Error with status',
@@ -398,6 +407,7 @@ describe('errors', () => {
           expect(consoleErrorSpy).toHaveBeenCalledWith(
             JSON.stringify(
               {
+                session_id: TEST_SESSION_ID,
                 error: {
                   type: 'FatalToolExecutionError',
                   message: 'Error executing tool test-tool: Tool failed',
@@ -467,6 +477,7 @@ describe('errors', () => {
         expect(consoleErrorSpy).toHaveBeenCalledWith(
           JSON.stringify(
             {
+              session_id: TEST_SESSION_ID,
               error: {
                 type: 'FatalCancellationError',
                 message: 'Operation cancelled.',
@@ -529,6 +540,7 @@ describe('errors', () => {
         expect(consoleErrorSpy).toHaveBeenCalledWith(
           JSON.stringify(
             {
+              session_id: TEST_SESSION_ID,
               error: {
                 type: 'FatalTurnLimitedError',
                 message:
diff --git a/packages/cli/src/utils/errors.ts b/packages/cli/src/utils/errors.ts
index 29a527f865..e46db098f1 100644
--- a/packages/cli/src/utils/errors.ts
+++ b/packages/cli/src/utils/errors.ts
@@ -100,6 +100,7 @@ export function handleError(
     const formattedError = formatter.formatError(
       error instanceof Error ? error : new Error(getErrorMessage(error)),
       errorCode,
+      config.getSessionId(),
     );
 
     console.error(formattedError);
@@ -152,6 +153,7 @@ export function handleToolError(
       const formattedError = formatter.formatError(
         toolExecutionError,
         errorType ?? toolExecutionError.exitCode,
+        config.getSessionId(),
       );
       console.error(formattedError);
     } else {
@@ -191,6 +193,7 @@ export function handleCancellationError(config: Config): never {
     const formattedError = formatter.formatError(
       cancellationError,
       cancellationError.exitCode,
+      config.getSessionId(),
     );
 
     console.error(formattedError);
@@ -231,6 +234,7 @@ export function handleMaxTurnsExceededError(config: Config): never {
     const formattedError = formatter.formatError(
       maxTurnsError,
       maxTurnsError.exitCode,
+      config.getSessionId(),
     );
 
     console.error(formattedError);
diff --git a/packages/core/src/output/json-formatter.test.ts b/packages/core/src/output/json-formatter.test.ts
index 587030a980..7df4aa770a 100644
--- a/packages/core/src/output/json-formatter.test.ts
+++ b/packages/core/src/output/json-formatter.test.ts
@@ -13,18 +13,30 @@ describe('JsonFormatter', () => {
   it('should format the response as JSON', () => {
     const formatter = new JsonFormatter();
     const response = 'This is a test response.';
-    const formatted = formatter.format(response);
+    const formatted = formatter.format(undefined, response);
     const expected = {
       response,
     };
     expect(JSON.parse(formatted)).toEqual(expected);
   });
 
+  it('should format the response as JSON with a session ID', () => {
+    const formatter = new JsonFormatter();
+    const response = 'This is a test response.';
+    const sessionId = 'test-session-id';
+    const formatted = formatter.format(sessionId, response);
+    const expected = {
+      session_id: sessionId,
+      response,
+    };
+    expect(JSON.parse(formatted)).toEqual(expected);
+  });
+
   it('should strip ANSI escape sequences from response text', () => {
     const formatter = new JsonFormatter();
     const responseWithAnsi =
       '\x1B[31mRed text\x1B[0m and \x1B[32mGreen text\x1B[0m';
-    const formatted = formatter.format(responseWithAnsi);
+    const formatted = formatter.format(undefined, responseWithAnsi);
     const parsed = JSON.parse(formatted);
     expect(parsed.response).toBe('Red text and Green text');
   });
@@ -33,7 +45,7 @@ describe('JsonFormatter', () => {
     const formatter = new JsonFormatter();
     const responseWithControlChars =
       'Text with\x07 bell\x08 and\x0B vertical tab';
-    const formatted = formatter.format(responseWithControlChars);
+    const formatted = formatter.format(undefined, responseWithControlChars);
     const parsed = JSON.parse(formatted);
     // Only ANSI codes are stripped, other control chars are preserved
     expect(parsed.response).toBe('Text with\x07 bell\x08 and\x0B vertical tab');
@@ -42,7 +54,7 @@ describe('JsonFormatter', () => {
   it('should preserve newlines and tabs in response text', () => {
     const formatter = new JsonFormatter();
     const responseWithWhitespace = 'Line 1\nLine 2\r\nLine 3\twith tab';
-    const formatted = formatter.format(responseWithWhitespace);
+    const formatted = formatter.format(undefined, responseWithWhitespace);
     const parsed = JSON.parse(formatted);
     expect(parsed.response).toBe('Line 1\nLine 2\r\nLine 3\twith tab');
   });
@@ -114,7 +126,7 @@ describe('JsonFormatter', () => {
         totalLinesRemoved: 0,
       },
     };
-    const formatted = formatter.format(response, stats);
+    const formatted = formatter.format(undefined, response, stats);
     const expected = {
       response,
       stats,
@@ -129,7 +141,7 @@ describe('JsonFormatter', () => {
       message: 'Invalid input provided',
       code: 400,
     };
-    const formatted = formatter.format(undefined, undefined, error);
+    const formatted = formatter.format(undefined, undefined, undefined, error);
     const expected = {
       error,
     };
@@ -144,7 +156,7 @@ describe('JsonFormatter', () => {
       message: 'Request timed out',
       code: 'TIMEOUT',
     };
-    const formatted = formatter.format(response, undefined, error);
+    const formatted = formatter.format(undefined, response, undefined, error);
     const expected = {
       response,
       error,
@@ -167,6 +179,23 @@ describe('JsonFormatter', () => {
     });
   });
 
+  it('should format error using formatError method with a session ID', () => {
+    const formatter = new JsonFormatter();
+    const error = new Error('Something went wrong');
+    const sessionId = 'test-session-id';
+    const formatted = formatter.formatError(error, 500, sessionId);
+    const parsed = JSON.parse(formatted);
+
+    expect(parsed).toEqual({
+      session_id: sessionId,
+      error: {
+        type: 'Error',
+        message: 'Something went wrong',
+        code: 500,
+      },
+    });
+  });
+
   it('should format custom error using formatError method', () => {
     class CustomError extends Error {
       constructor(message: string) {
@@ -177,7 +206,7 @@ describe('JsonFormatter', () => {
 
     const formatter = new JsonFormatter();
     const error = new CustomError('Custom error occurred');
-    const formatted = formatter.formatError(error);
+    const formatted = formatter.formatError(error, undefined);
     const parsed = JSON.parse(formatted);
 
     expect(parsed).toEqual({
@@ -217,7 +246,7 @@ describe('JsonFormatter', () => {
       code: 429,
     };
 
-    const formatted = formatter.format(response, stats, error);
+    const formatted = formatter.format(undefined, response, stats, error);
     const expected = {
       response,
       stats,
diff --git a/packages/core/src/output/json-formatter.ts b/packages/core/src/output/json-formatter.ts
index 83ea3e3862..dd3e558a6f 100644
--- a/packages/core/src/output/json-formatter.ts
+++ b/packages/core/src/output/json-formatter.ts
@@ -9,9 +9,18 @@ import type { SessionMetrics } from '../telemetry/uiTelemetry.js';
 import type { JsonError, JsonOutput } from './types.js';
 
 export class JsonFormatter {
-  format(response?: string, stats?: SessionMetrics, error?: JsonError): string {
+  format(
+    sessionId?: string,
+    response?: string,
+    stats?: SessionMetrics,
+    error?: JsonError,
+  ): string {
     const output: JsonOutput = {};
 
+    if (sessionId) {
+      output.session_id = sessionId;
+    }
+
     if (response !== undefined) {
       output.response = stripAnsi(response);
     }
@@ -27,13 +36,17 @@ export class JsonFormatter {
     return JSON.stringify(output, null, 2);
   }
 
-  formatError(error: Error, code?: string | number): string {
+  formatError(
+    error: Error,
+    code?: string | number,
+    sessionId?: string,
+  ): string {
     const jsonError: JsonError = {
       type: error.constructor.name,
       message: stripAnsi(error.message),
       ...(code && { code }),
     };
 
-    return this.format(undefined, undefined, jsonError);
+    return this.format(sessionId, undefined, undefined, jsonError);
   }
 }
diff --git a/packages/core/src/output/types.ts b/packages/core/src/output/types.ts
index 0448354255..81de667b22 100644
--- a/packages/core/src/output/types.ts
+++ b/packages/core/src/output/types.ts
@@ -19,6 +19,7 @@ export interface JsonError {
 }
 
 export interface JsonOutput {
+  session_id?: string;
   response?: string;
   stats?: SessionMetrics;
   error?: JsonError;