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Testing Conventions

This document covers every testing pattern and convention used in the Claude Code codebase. Tests use Vitest and follow specific patterns for resettability, determinism, and isolation.

Test Framework

  • Vitest — Test runner (Jest-compatible API)
  • Configuration in vitest.config.*
  • Run with: npm test or npx vitest

Reset Helpers for Module State

Since the codebase uses module-level state extensively, every stateful module provides a _reset*ForTesting() function:

Pattern: _resetForTesting()

/**
 * Reset error log state for testing purposes only.
 * @internal
 */
export function _resetErrorLogForTesting(): void {
  errorLogSink = null
  errorQueue.length = 0
  inMemoryErrorLog = []
}

Naming Convention

  • Prefix with _ (indicates internal/private)
  • Include ForTesting suffix
  • Tag with @internal in JSDoc
  • Reset ALL module-level mutable state

Usage in Tests

import { _resetErrorLogForTesting } from '../src/utils/log.js'

beforeEach(() => {
  _resetErrorLogForTesting()
})

afterEach(() => {
  _resetErrorLogForTesting()
})

NODE_ENV Guards

Production code uses process.env.NODE_ENV === 'test' to alter behavior in tests:

Suppressing Side Effects

function shouldLogDebugMessage(message: string): boolean {
  // Don't write debug logs during tests (noisy, filesystem side effects)
  if (process.env.NODE_ENV === 'test' && !isDebugToStdErr()) {
    return false
  }
  // ... normal logic
}

Deterministic Sorting

const sortedMatches = results
  .map((_, i) => [_, stats[i]!] as const)
  .sort((a, b) => {
    if (process.env.NODE_ENV === 'test') {
      // In tests, always sort by filename for deterministic results
      return a[0].localeCompare(b[0])
    }
    // In production, sort by modification time
    return b[1] - a[1]
  })

Test-Only Behavior

// Disable error reporting in tests
if (process.env.NODE_ENV === 'test') {
  return
}

Test-Only Exports

Some modules export values exclusively for testing:

Test-Only Tool Implementations

// src/tools/testing/
// Contains tool implementations only used in tests
export const TestingPermissionTool = buildTool({
  name: 'TestingPermission',
  // ... simplified for testing
})

Exported for Testing Comments

// Exported for testing purposes
export const getDebugFilter = memoize((): DebugFilter | null => { ... })

Deterministic Output

Tests must produce deterministic results. Key patterns:

Sort by Name in Tests

if (process.env.NODE_ENV === 'test') {
  return a[0].localeCompare(b[0])  // Alphabetical, not by mtime
}

Fixed Timestamps

Use fixed dates/timestamps in test fixtures instead of Date.now().

Seeded Random Values

When randomness is needed, use deterministic seeds in tests.

Test File Organization

Tests typically mirror the source structure:

src/
├── utils/
│   ├── errors.ts
│   └── __tests__/
│       └── errors.test.ts
├── tools/
│   ├── GrepTool/
│   │   ├── GrepTool.ts
│   │   └── __tests__/
│   │       └── GrepTool.test.ts

Or in a top-level test/ directory:

test/
├── utils/
│   └── errors.test.ts
├── tools/
│   └── GrepTool.test.ts

Mocking Patterns

Mocking Module-Level State

Use the _resetForTesting() helpers rather than mocking internals:

beforeEach(() => {
  _resetErrorLogForTesting()
})

Mocking Environment Variables

beforeEach(() => {
  process.env.NODE_ENV = 'test'
  process.env.USER_TYPE = 'ant'
})

afterEach(() => {
  delete process.env.USER_TYPE
})

Clearing Memoize Caches

beforeEach(() => {
  isDebugMode.cache.clear?.()
  getDebugFilter.cache.clear?.()
})

Test Naming

Follow descriptive test names that explain the behavior:

describe('toError', () => {
  it('returns the same Error if given an Error', () => { ... })
  it('wraps a string in a new Error', () => { ... })
  it('converts undefined to Error("undefined")', () => { ... })
})

describe('isENOENT', () => {
  it('returns true for ENOENT errors', () => { ... })
  it('returns false for other errno codes', () => { ... })
  it('returns false for non-Error values', () => { ... })
})

Promise Testing

Testing Async Functions

it('handles abort signal', async () => {
  const controller = new AbortController()
  const promise = sleep(10000, controller.signal)
  controller.abort()
  await promise  // Should resolve immediately
})

Testing Timeouts

it('rejects after timeout', async () => {
  const slowPromise = new Promise(() => {})  // Never resolves
  await expect(
    withTimeout(slowPromise, 100, 'Timed out')
  ).rejects.toThrow('Timed out')
})

Integration Testing Patterns

Testing Tools

it('searches for pattern in files', async () => {
  const result = await GrepTool.call(
    { pattern: 'function', path: testDir },
    mockContext,
  )
  expect(result.data.numFiles).toBeGreaterThan(0)
})

Testing Commands

it('compacts conversation', async () => {
  const result = await call('', mockContext)
  expect(result.type).toBe('compact')
})

Key Testing Guidelines

  1. Always reset module state in beforeEach/afterEach
  2. Use NODE_ENV === 'test' guards for non-deterministic behavior
  3. Sort deterministically in tests (by name, not by time)
  4. Don't test internal implementation — test public API behavior
  5. Mock at boundaries (filesystem, network) not at function level
  6. Clear memoize caches between tests to prevent state leakage
  7. Use descriptive test names that explain expected behavior
  8. Keep test files close to the code they test