fix: preserve the crash error in diagnostic logs on unexpected exit (#1757)

The TUI crash path (uncaughtException / unhandledRejection) logged the
error into an asynchronously-drained sink and then called process.exit()
on the same tick, so the one line explaining the crash was never written
to disk. Flush the diagnostic logs synchronously before exiting.

Author: RealKai42

.changeset/fix-crash-exit-log-flush.md

@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Fix the diagnostic log missing the actual error when the CLI exits unexpectedly.

apps/kimi-code/src/cli/run-shell.ts

@@ -4,6 +4,7 @@ import { join } from 'node:path';
 
 import {
   createKimiHarness,
+  flushDiagnosticLogsSync,
   log,
   type KimiHarness,
   type TelemetryClient,
@@ -159,6 +160,14 @@ export async function runShell(
   // raw mode with a hidden cursor and XON/XOFF flow control disabled. Restore
   // both before exiting so the user's shell is usable afterwards.
   const emergencyExit = (exitCode: number): void => {
+    // The crash log above is only enqueued into the async sink; flush it
+    // synchronously or the `process.exit()` below would drop the one line that
+    // explains why we crashed. Best-effort: an exit path must never throw.
+    try {
+      flushDiagnosticLogsSync();
+    } catch {
+      /* ignore */
+    }
     restoreTerminalModes();
     restoreStty();
     process.exit(exitCode);

apps/kimi-code/test/cli/run-shell.test.ts

@@ -58,6 +58,7 @@ const mocks = vi.hoisted(() => {
       track: lifecycleTrack,
     })),
     resolveKimiHome: vi.fn((homeDir?: string) => homeDir ?? '/tmp/kimi-code-test-home'),
+    flushDiagnosticLogsSync: vi.fn(),
     harnessCreatesDeviceIdOnConstruction: false,
     execSync: vi.fn(),
     TuiConfigParseError,
@@ -69,6 +70,7 @@ vi.mock('@moonshot-ai/kimi-code-sdk', async (importOriginal) => {
   return {
     ...actual,
     resolveKimiHome: mocks.resolveKimiHome,
+    flushDiagnosticLogsSync: mocks.flushDiagnosticLogsSync,
     createKimiHarness: (...args: unknown[]) => {
       const options = args[0] as { readonly homeDir?: string } | undefined;
       const homeDir = options?.homeDir ?? '/tmp/kimi-code-test-home';
@@ -508,6 +510,101 @@ describe('runShell', () => {
     });
   });
 
+  it('flushes diagnostic logs synchronously before exiting on a runtime crash', async () => {
+    mocks.loadTuiConfig.mockResolvedValue({
+      theme: 'dark',
+      editorCommand: null,
+      notifications: { enabled: true, condition: 'unfocused' },
+    });
+    mocks.tuiStart.mockResolvedValue(undefined);
+
+    const processOnSpy = vi.spyOn(process, 'on');
+    const stdout = captureProcessWrite('stdout');
+    const exitSpy = mockProcessExit();
+
+    try {
+      await runShell(
+        {
+          session: undefined,
+          continue: false,
+          yolo: false,
+          auto: false,
+          plan: false,
+          model: undefined,
+          outputFormat: undefined,
+          prompt: undefined,
+          skillsDirs: [],
+        },
+        '1.2.3-test',
+      );
+
+      const handler = processOnSpy.mock.calls.find(
+        ([event]) => event === 'uncaughtException',
+      )?.[1] as ((error: unknown) => void) | undefined;
+      expect(handler).toBeDefined();
+
+      // The async log sink cannot flush before process.exit() runs, so the
+      // crash handler must force a synchronous flush or the crash reason is
+      // lost (regression: uncaughtException logs never reached disk).
+      expect(() => handler?.(new Error('boom'))).toThrow(ExitCalled);
+      expect(mocks.flushDiagnosticLogsSync).toHaveBeenCalledOnce();
+      expect(exitSpy).toHaveBeenCalledWith(1);
+      expect(mocks.flushDiagnosticLogsSync.mock.invocationCallOrder[0]!).toBeLessThan(
+        exitSpy.mock.invocationCallOrder[0]!,
+      );
+    } finally {
+      processOnSpy.mockRestore();
+      exitSpy.mockRestore();
+      stdout.restore();
+    }
+  });
+
+  it('flushes diagnostic logs synchronously before exiting on an unhandled rejection', async () => {
+    mocks.loadTuiConfig.mockResolvedValue({
+      theme: 'dark',
+      editorCommand: null,
+      notifications: { enabled: true, condition: 'unfocused' },
+    });
+    mocks.tuiStart.mockResolvedValue(undefined);
+
+    const processOnSpy = vi.spyOn(process, 'on');
+    const stdout = captureProcessWrite('stdout');
+    const exitSpy = mockProcessExit();
+
+    try {
+      await runShell(
+        {
+          session: undefined,
+          continue: false,
+          yolo: false,
+          auto: false,
+          plan: false,
+          model: undefined,
+          outputFormat: undefined,
+          prompt: undefined,
+          skillsDirs: [],
+        },
+        '1.2.3-test',
+      );
+
+      const handler = processOnSpy.mock.calls.find(
+        ([event]) => event === 'unhandledRejection',
+      )?.[1] as ((reason: unknown) => void) | undefined;
+      expect(handler).toBeDefined();
+
+      expect(() => handler?.(new Error('boom'))).toThrow(ExitCalled);
+      expect(mocks.flushDiagnosticLogsSync).toHaveBeenCalledOnce();
+      expect(exitSpy).toHaveBeenCalledWith(1);
+      expect(mocks.flushDiagnosticLogsSync.mock.invocationCallOrder[0]!).toBeLessThan(
+        exitSpy.mock.invocationCallOrder[0]!,
+      );
+    } finally {
+      processOnSpy.mockRestore();
+      exitSpy.mockRestore();
+      stdout.restore();
+    }
+  });
+
   it('closes the harness when TUI startup fails', async () => {
     mocks.loadTuiConfig.mockResolvedValue({
       theme: 'dark',

apps/kimi-code/test/helpers/process.ts

@@ -6,7 +6,7 @@ export class ExitCalled extends Error {
   }
 }
 
-export function mockProcessExit(): { mockRestore(): void } {
+export function mockProcessExit() {
   return vi.spyOn(process, 'exit').mockImplementation(((code?: string | number | null) => {
     throw new ExitCalled(Number(code ?? 0));
   }) as never);

packages/agent-core/src/index.ts

@@ -10,6 +10,7 @@ export * from './plugin';
 export { buildReplay } from './agent/replay/build';
 export {
   flushDiagnosticLogs,
+  flushDiagnosticLogsSync,
   getRootLogger,
   log,
   redact,

packages/agent-core/src/logging/index.ts

@@ -14,6 +14,7 @@ export { LOG_LEVEL_RANK, levelEnabled } from './types';
 export {
   __resetRootLoggerForTest,
   flushDiagnosticLogs,
+  flushDiagnosticLogsSync,
   getRootLogger,
   log,
   redact,

packages/agent-core/src/logging/logger.ts

@@ -276,6 +276,16 @@ export function flushDiagnosticLogs(): Promise<boolean> {
   return getRootInternal().flush();
 }
 
+/**
+ * Synchronous variant for crash / emergency-exit paths that call
+ * `process.exit()` on the same tick: pending entries are appended with
+ * `appendFileSync`, so they survive the immediate exit that would otherwise
+ * drop everything still sitting in the async queue.
+ */
+export function flushDiagnosticLogsSync(): void {
+  getRootInternal().flushSync();
+}
+
 class LoggerImpl implements Logger {
   constructor(private readonly boundCtx: LogContext) {}
 

packages/agent-core/test/logging/logger.test.ts

@@ -1,11 +1,13 @@
 import { mkdtemp, readFile, rm } from 'node:fs/promises';
+import { readFileSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'pathe';
 
 import { afterEach, beforeEach, describe, expect, it } from 'vitest';
 
 import {
   __resetRootLoggerForTest,
+  flushDiagnosticLogsSync,
   getRootLogger,
   log,
   redact,
@@ -429,6 +431,29 @@ describe('session routing', () => {
   });
 });
 
+describe('flushDiagnosticLogsSync', () => {
+  it('persists enqueued entries synchronously, before the async drain could run', async () => {
+    await getRootLogger().configure(defaultConfig());
+    log.error('crash marker', { error: new Error('boom') });
+
+    // No `await` between enqueue and flush: the async drain (microtask + async
+    // fs) cannot have written yet, so only the synchronous append can produce
+    // the file content below. This mirrors crash paths that call
+    // process.exit() on the same tick.
+    flushDiagnosticLogsSync();
+
+    const content = readFileSync(resolveGlobalLogPath(homeDir), 'utf-8');
+    expect(content).toContain('crash marker');
+    expect(content).toContain('boom');
+  });
+
+  it('is a silent no-op before configure', () => {
+    expect(() => {
+      flushDiagnosticLogsSync();
+    }).not.toThrow();
+  });
+});
+
 describe('redact helper', () => {
   it('returns same shape with sensitive fields replaced', () => {
     const out = redact({ user: 'x', token: 'abc', nested: { apiKey: '1' } });

packages/node-sdk/src/index.ts

@@ -52,6 +52,7 @@ export {
 // RootLogger / getRootLogger / LoggingConfig stay inside agent-core.
 export {
   flushDiagnosticLogs,
+  flushDiagnosticLogsSync,
   log,
   redact,
   resolveGlobalLogPath,