Generating Test Doubles

What it does

When your agent uses it

• Test isolation fixtures
• With phrases like generate mocks
• Create test doubles

Example prompts

• “generate mocks”
• “create test doubles”
• “setup stubs”
• “/generating-test-doubles”

Requirements

• Python 3
• Compatibility (from SKILL.md): Designed for Claude Code
• Pre-approved tools (allowed-tools): Read, Write, Edit, Grep, Glob, Bash(test:doubles-*)

Workflow steps

7 steps, taken from the first numbered list in SKILL.md.

1. 1Scan the codebase with Glob and Grep to identify modules with external dependencies (database clients, HTTP clients, file system access…
2. 2Read each module under test and catalog its dependency interfaces -- list every method signature, return type, and side effect.
3. 3Determine the appropriate test double type for each dependency
4. 4Generate test double files following the project's existing test directory structure (e.g., mocks/, test/doubles/, testutil/).
5. 5For each test double, implement
6. 6Wire test doubles into existing test files using the framework's dependency injection pattern (jest.mock(), @patch, constructor injection…
7. 7Validate all test doubles compile and pass type checks by running tsc --noEmit or equivalent.

What it can do on your machine

Read from SKILL.md and the folder at commit 80f86df. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves these tools, so the agent can use them without asking each time:

  • Read
  • Write
  • Edit
  • Grep
  • Glob
  • Bash(test:doubles-*)

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  Ships 3 files in scripts/ (Python), which the agent can run.

  Shell commands in SKILL.md call:

  • tsc

  From the folder's file list and the shell code blocks in SKILL.md.

• Network

  Links to these hosts (documentation or services it may open):

  • jestjs.io
  • sinonjs.org
  • docs.python.org
  • github.com
  • martinfowler.com

  From URLs in SKILL.md, links to its own repository left out.

• Credentials

  Names no API keys, tokens, secrets or passwords.

  From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

• Compatibility

  Designed for Claude Code

  From compatibility in the SKILL.md frontmatter.

Context cost

Generating Test Doubles loads about 1.3k tokens when it runs, and up to ~1.3k if it reads all its reference files. Until then it costs about 59 tokens; SKILL.md has 488 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it

~59

When it runs · the whole SKILL.md, loaded when a task matches

~1.3k

With references · SKILL.md plus every file in references/, read only if the agent opens them

~1.3k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); the scripts in this folder are not scanned.

SKILL.md

The full file from jeremylongshore/tons-of-skills-marketplace at commit 80f86df, republished under its MIT licence (© jeremylongshore). 488 words, ~1,309 tokens.

name

generating-test-doubles

description

Generate mocks, stubs, spies, and fakes for dependency isolation. Use when creating mocks, stubs, or test isolation fixtures. Trigger with phrases like "generate mocks", "create test doubles", or "setup stubs".

allowed-tools

Read, Write, Edit, Grep, Glob, Bash(test:doubles-*)

compatibility

Designed for Claude Code

version

1.22.0

author

Jeremy Longshore <jeremy@intentsolutions.io>

license

MIT

tags

testing, test-doubles

// __mocks__/userRepository.ts
export const createMockUserRepo = () => ({
  findById: jest.fn().mockResolvedValue({ id: '1', name: 'Test User' }),
  save: jest.fn().mockResolvedValue(undefined),
  delete: jest.fn().mockResolvedValue(true),
});

from unittest.mock import patch, MagicMock

@patch('myapp.client.requests.get')
def test_fetch_data(mock_get):
    mock_get.return_value = MagicMock(status_code=200, json=lambda: {"key": "value"})  # HTTP 200 OK
    result = fetch_data("https://api.example.com/data")
    assert result == {"key": "value"}
    mock_get.assert_called_once()

type FakeStore struct {
    data map[string]string
}
func (f *FakeStore) Get(key string) (string, error) {
    v, ok := f.data[key]
    if !ok { return "", ErrNotFound }
    return v, nil
}

© jeremylongshore, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 5 other files (scripts, references, assets) in skills/.curated/generating-test-doubles of jeremylongshore/tons-of-skills-marketplace.

• SKILL.md
• assets/README.md
• references/README.md
• scripts/README.md
• scripts/dependency_analyzer.py
• scripts/generate_doubles.py

Open the folder on GitHubat commit 80f86df