CtrlK
BlogDocsLog inGet started
Tessl Logo

pantheon-ai/test-driven-development

Master Test-Driven Development with deterministic red-green-refactor workflows, test-first feature delivery, bug reproduction through failing tests, behavior-focused assertions, and refactoring safety; use when implementing new functions, changing APIs, fixing regressions, or restructuring code under test.

Review — 100%

Does it follow best practices?

Evaluation — 86%

↑ 1.05x

Agent success when using this tile

Validation — 11 / 11 Passed

Validation for skill structure

Overview
Skills
Evals
Files

assert-error-messages.mdreferences/

title:
Assert on Error Messages and Types
impact:
MEDIUM
impactDescription:
prevents false positives from wrong errors
tags:
assert, errors, exceptions, specificity

Assert on Error Messages and Types

When testing error conditions, verify both the error type and message. Catching any error isn't enough - the right error must be thrown.

Incorrect (any error passes):

test('throws on invalid email', () => {
  // Passes if ANY error is thrown, even unrelated ones
  expect(() => createUser({ email: 'invalid' })).toThrow()
})

test('throws on missing required field', async () => {
  // Catches network errors, type errors, anything
  await expect(saveUser({})).rejects.toBeDefined()
})

Correct (specific error assertions):

test('throws ValidationError for invalid email', () => {
  expect(() => createUser({ email: 'invalid' }))
    .toThrow(ValidationError)
})

test('error message indicates invalid email format', () => {
  expect(() => createUser({ email: 'invalid' }))
    .toThrow('Invalid email format')
})

test('throws with specific error details', () => {
  expect(() => createUser({ email: 'invalid' }))
    .toThrow(expect.objectContaining({
      code: 'VALIDATION_ERROR',
      field: 'email'
    }))
})

test('async operation throws NotFoundError', async () => {
  await expect(getUser('nonexistent'))
    .rejects.toThrow(NotFoundError)
})

test('error includes resource identifier', async () => {
  await expect(getUser('user-999'))
    .rejects.toThrow(/user-999/)
})

What to assert:

  • Error class/type when using custom errors
  • Error message content (exact or partial match)
  • Error code or status when applicable
  • Associated data (field name, invalid value)

Reference: Unit Testing Best Practices - Microsoft Learn

Install with Tessl CLI

npx tessl i pantheon-ai/test-driven-development@0.2.4

references

assert-custom-matchers.md

assert-error-messages.md

assert-no-assertions-antipattern.md

assert-snapshot-testing.md

assert-specific-assertions.md

cycle-maintain-test-list.md

cycle-minimal-code-to-pass.md

cycle-refactor-after-green.md

cycle-small-increments.md

cycle-verify-test-fails-first.md

cycle-write-test-first.md

data-avoid-mystery-guests.md

data-builder-pattern.md

data-minimal-setup.md

data-unique-identifiers.md

data-use-factories.md

design-aaa-pattern.md

design-avoid-logic-in-tests.md

design-descriptive-test-names.md

design-one-assertion-per-test.md

design-test-behavior-not-implementation.md

design-test-edge-cases.md

isolate-deterministic-tests.md

isolate-mock-external-dependencies.md

isolate-no-shared-state.md

isolate-prefer-stubs-over-mocks.md

isolate-use-dependency-injection.md

org-file-structure.md

org-group-by-behavior.md

org-parameterized-tests.md

org-setup-teardown.md

org-test-utilities.md

perf-avoid-network-calls.md

perf-avoid-sleep.md

perf-fast-unit-tests.md

perf-fix-flaky-tests.md

perf-parallelize-tests.md

strat-coverage-targets.md

strat-e2e-critical-paths.md

strat-integration-boundaries.md

strat-mutation-testing.md

strat-test-pyramid.md

SKILL.md

tile.json