Test Writer Skill

Generate comprehensive tests for code changes, with emphasis on error paths and edge cases.

When to trigger

• •User asks: "write tests", "add tests for this", "test this"
• •After implementing new functionality
• •When reviewing code with missing test coverage

Instructions

1. •

   Analyze the code:

   • •Read the file/function to test
   • •Identify: inputs, outputs, side effects, dependencies, error conditions
   • •Check existing tests to match style/framework
   • •List all possible error returns/throws
2. •

   Determine test type:

   • •Unit: Pure functions, isolated logic
   • •Integration: Component + hooks, API + DB interactions
   • •E2E: Full user flows
3. •

   Generate test cases (in priority order):

   a. Error paths (CRITICAL):

   • •Test EVERY error return/throw variant
   • •Verify error types/messages remain stable
   • •Why: Internal implementation changes break error handling
   • •Error tests act as regression tests for refactors
   • •Examples:
     • •Rust: Test all Err(...) variants in Result
     • •TypeScript: Test all thrown error types
     • •Go: Test all error return values
     • •Python: Test all raised exceptions

   b. Happy path:

   • •Expected behavior with valid inputs
   • •Primary use cases

   c. Edge cases:

   • •Empty, null, undefined, zero
   • •Boundary values (min, max, off-by-one)
   • •Large inputs, deeply nested data
   • •Special characters, unicode

   d. State changes:

   • •Before/after assertions
   • •Side effects verification

   e. Concurrency/timing:

   • •Race conditions (if applicable)
   • •Timeout scenarios
4. •

   Test structure (AAA pattern):

   typescript

   describe('functionName', () => {
     it('should [expected behavior] when [condition]', () => {
       // Arrange: Setup
       const input = ...
   
       // Act: Execute
       const result = functionName(input)
   
       // Assert: Verify
       expect(result).toBe(expected)
     })
   })
   

5. •

   Test naming:

   • •Describe behavior, not implementation
   • •should return user when valid ID provided
   • •should return NotFoundError when user does not exist
   • •should throw ValidationError when email is invalid
   • •should handle empty array without errors
6. •

   Error testing examples:

   Rust:

   rust

   #[test]
   fn should_return_not_found_error_when_user_missing() {
       let result = get_user(999);
       assert!(matches!(result, Err(UserError::NotFound(_))));
   }
   
   #[test]
   fn should_return_validation_error_for_negative_id() {
       let result = get_user(-1);
       assert!(matches!(result, Err(UserError::InvalidId(_))));
   }
   

   TypeScript:

   typescript

   it('should throw NotFoundError when user does not exist', () => {
     expect(() => getUser(999)).toThrow(NotFoundError);
     expect(() => getUser(999)).toThrow('User not found');
   });
   

7. •

   Coverage checklist:

   • • ALL error paths tested (every Err/throw variant)
   • • Error messages/types verified (API contract stability)
   • • All code branches executed
   • • Edge cases covered
   • • Mocks for external dependencies
   • • State changes verified
   • • No flaky tests (consistent results)

Framework detection

• •Detect from imports: Jest, Vitest, pytest, RSpec, cargo test, etc.
• •Match existing test patterns in codebase
• •Use same mocking/assertion libraries
• •Follow project conventions (file naming, directory structure)

Tool access

allowed-tools:

• •Read
• •Write
• •Grep
• •Glob
• •Bash

Output

• •Write complete test file or add to existing tests
• •Include necessary imports and setup/teardown
• •Group related tests in describe/context blocks
• •Add comments only for non-obvious test logic
• •Prioritize error path tests first
• •Ensure tests fail when implementation breaks

Principles

• •Error tests are regression tests - treat them as API contracts
• •If a function can return 5 different errors, write 5 error tests
• •Test what users observe, not internal implementation
• •Make tests readable - they're documentation
• •Fast, isolated, deterministic tests only