write-tests - SKILL.md Agent Skill

name: write-tests type: atomic license: MIT description: > Use when writing, reviewing, or configuring RSpec tests in Ruby on Rails — must execute the spec via `bundle exec rspec` and capture the actual test output (failure message or stack trace) rather than describing expected behavior, prefer behavioral confidence over implementation coupling, pick the smallest spec type exercising the behavior (model > service > request > system), mirror the file paths of the source, use # frozen_string_literal: true, define subject(:result) for service specs, and consult `assets/tdd_proof_checklist.md` when the task involves new behavior. Use when adding test coverage, refactoring specs, or practicing TDD. Trigger words: write spec, rspec, test-driven development, testing, write tests. metadata: version: 1.0.0 user-invocable: "true"

Write Tests

Use this skill when the task is to write, review, or clean up RSpec tests.

Quick Reference

Aspect	Rule
Spec types	Model: domain logic / pure domain → start here; Request: HTTP endpoints; Job: background processing; Service/PORO: clean Ruby; System: E2E cross-layer journey (sparingly)
Assertions	Test behavior, not implementation
Factories	Minimal attributes; traits for options; prefer `build`/`build_stubbed` over `create`
Mocking	Stub external boundaries at class level (e.g. `allow(Client).to receive`); no Active Record mocking
Service specs	Required: `describe '.call'` and `subject(:result)` (see assets/output_checklist.md)
`let` vs `let!`	Default to `let`; use `let!` only when object must exist before action
Example names	Present tense; no `should`; no `and` (see assets/output_checklist.md)

TDD Workflow (RED → GREEN → REFACTOR)

HARD GATE: DO NOT write implementation code before a failing test exists.

When driving new behaviour with RSpec, follow this sequence:

Write the failing spec — pick the smallest spec type that exercises the intended behaviour (model > service > request > system), as shown in the Spec types row above.
Run it and confirm the failure message — show the concrete RED failure class/message. Do not leave this as a placeholder template; do not use illustrative e.g. failure examples in the final artifact.
Implement the minimum code to make the spec pass.
Refactor — clean up duplication and naming while keeping the suite green.
Verify — run the full relevant spec file, then the suite, before committing.

For output format and RED/GREEN proof requirements, see assets/tdd_proof_checklist.md.

Service Spec (anchor pattern)

RSpec.describe Invoices::MarkOverdue do
  describe '.call' do
    subject(:result) { described_class.call(invoice: invoice) }

    context 'when the invoice is overdue and unpaid' do
      let(:invoice) { create(:invoice, due_date: 2.days.ago, paid_at: nil) }
      it 'marks the invoice overdue' do
        expect { result }.to change { invoice.reload.overdue? }.from(false).to(true)
      end
    end
  end
end

One Behavior Per Example

Split and in it/specify descriptions every time — no exceptions.

# BAD — two assertions; if the first fails, the second never runs
it 'returns 201 and creates the record' do; end

# GOOD — one observable outcome per example
it 'returns 201' do; end
it 'creates the record' do; end

Flaky Tests & Deterministic Assertions

Cause	Fix
Time-dependent logic	`freeze_time` / `travel_to`; never set past dates as shortcut
State leakage	Each example sets up own state; avoid `before(:all)`
Async jobs	`queue_adapter = :test` + `have_enqueued_job`; never assert side-effects imperatively
External HTTP	`WebMock` / `VCR`; never allow real network in CI
DB state bleed	Transactional fixtures or `DatabaseCleaner`; never share `let!` across contexts
Race conditions	Explicit Capybara waits; avoid `sleep`
Imprecise assertions	`change.from().to()` over final state; exact values over `be_truthy`/`be_falsey`; see rule 16

Extended Resources (Progressive Disclosure)

Load these files only when their specific content is needed:

assets/complete_example.md — A complete, step-by-step example of a high-scoring answer.md showing plan, spec, realistic Observed RED/GREEN outputs, and verification tables.
assets/examples.md — For code examples of service specs, shared examples, and factory design.
assets/spec_templates.md — Standard templates for different types of specs.
assets/tdd_proof_checklist.md — Use when the task involves new behavior; defines RED/GREEN proof format, output formatting rules, and GREEN progress bar requirements.
assets/output_checklist.md — Complete 18-point checklist for RSpec output structure, conventions, and self-auditing; also defines all output style rules (spec structure, file path mirroring, frozen_string_literal, subject(:result), and TDD proof format).