Lists multiple specific concrete actions: 'mock external dependencies — HTTP, databases, filesystems, clocks — isolating the unit under test while still exercising realistic interactions.' This clearly describes what the skill does with concrete examples.

Clearly answers both what ('Generates tests that mock external dependencies...isolating the unit under test') AND when ('Use when the code has side effects you can't run in a test, when external services are slow or unavailable, or when testing error paths that are hard to trigger'). Explicit 'Use when' clause with multiple trigger scenarios.

Includes some relevant technical terms like 'mock', 'HTTP', 'databases', 'filesystems', but misses common user phrases like 'unit tests', 'test doubles', 'stub', 'fake', or 'mocking library'. Users might say 'mock API calls' or 'stub database' which aren't explicitly covered.

Clear niche focused specifically on mocking external dependencies in tests. The specific mention of HTTP, databases, filesystems, clocks, and the focus on isolation distinguishes it from general testing skills or integration testing skills.

The content is lean and efficient, using tables for quick reference and avoiding explanations of concepts Claude already knows. Every section adds value without padding or unnecessary context.

Provides fully executable Python code examples with pytest-mock, concrete scenarios (happy path, retry, error), and copy-paste ready test implementations. The worked example is complete and runnable.

Clear decision tables guide when to mock and what type to use. The worked example demonstrates a logical progression through test scenarios, and the 'Do not' section provides explicit validation checkpoints for avoiding common mistakes.

Content is well-organized with clear sections and tables, but it's a moderately long single file. The output format template at the end could potentially be a separate reference, and there are no links to external resources for deeper topics like contract testing.