BDD Test File Generator

Generate behavior-driven test files that focus on public API and observable behavior.

Quick Start Workflow

1. Read the source file to understand what needs to be tested
2. Identify the public API - all exported functions, constants, hooks, or component props
3. Check for existing test setup - look for setup.ts, vitest.config.ts, or existing test files
4. Ask the user: unit, integration, or both? (see below)
5. Delegate context + edge-case analysis to an agent (see below) — do not try to produce the full case list from the source file alone
6. Review the agent's list with the user, prune/add cases, then generate the test file using patterns from references/patterns.md

Delegate Analysis to a Subagent

A single file rarely tells the full story: callers pass specific shapes, sibling files encode invariants, existing tests hint at conventions, and the bug that prompted the test may live in the git log. Before writing cases, spawn a subagent to gather that context and draft the case list.

Use the Explore agent (or general-purpose if deeper reasoning is needed) via the Agent tool. Give it a self-contained brief that includes:

• The exact source path(s) being tested
• The testing level chosen in the previous step (unit / integration / both)
• The framework stack (e.g. vitest + testing-library, jest + supertest)
• The edge-case dimensions listed under Edge Cases: Analyze from Context — ask the agent to walk those dimensions against the actual code, not as a generic checklist
• An instruction to read callers, sibling modules, existing tests, and any obvious schema/constant files before proposing cases

Ask the agent to report back using a BDD-native shape — the same describe / context / it nesting with GIVEN/WHEN/THEN that the generated test file will use. This keeps review cheap (the scenarios map 1:1 to the code you're about to write) and keeps the skill internally consistent.

Prefer spec style (matches vitest / jest / RSpec output):

describe <unit under test>
  context <condition, state, or collaborator setup>
    it <expected observable behavior>
      GIVEN ...
      WHEN ...
      THEN ...
      [src/foo.ts:42 — `if (!user)` branch]
    it ...
  context ...

Open questions
  - <ambiguity the agent couldn't resolve from the code>

Use Gherkin-style Feature / Scenario instead only if the project already uses Cucumber or a Gherkin runner.

Notes for the agent's report:

• Group scenarios by the code branch or collaborator state they exercise (the context), not by a "happy / edge / error" bucket — BDD treats them as a single flat list of scenarios.
• Annotate each it with the file:line or named branch that motivates it, so pruning is reviewable.
• Cap report length (e.g. "under 400 lines") so it stays reviewable.

When the report comes back:

1. Resolve the Open Questions with the user before generating code
2. Drop any case the code doesn't actually distinguish
3. Add anything the agent missed that you can justify from the source

Only then move on to generating the test file.

Ask: Unit, Integration, or Both?

Before generating anything, ask the user which level(s) of tests they want — don't assume. Use AskUserQuestion (or a plain question if that tool isn't available) with these choices, explained in terms of this file:

• Unit tests — exercise the module in isolation; collaborators (network, DB, other modules, timers, the DOM beyond what a single hook/component needs) are mocked or stubbed. Fast, many cases, focused on one unit's contract.
• Integration tests — let the real collaborators run and test how this module behaves inside its actual neighborhood (real DB/driver, real HTTP client hitting a test server, real router, multiple hooks/components composed together). Slower, fewer cases, focused on wiring and boundaries.
• Both — produce separate files (e.g. foo.test.ts and foo.integration.test.ts) so they can run under different configs.

Recommend a default based on the file:

File kind	Default recommendation
Pure function / utility / constants	Unit
Hook / component with mockable deps	Unit
Repository / DB query / HTTP client	Integration
Router, workflow, or multi-module orchestrator	Both

State the recommendation and why, but defer to the user's answer.

Edge Cases: Analyze from Context, Don't Use a Generic Checklist

"Empty string / null / zero" is a starting list, not the finish line. Before writing cases, read the source carefully and derive edge cases from what the code actually does. Check each of these against the file in front of you:

• Inputs and types — what does each parameter accept? For every type, what are its degenerate values (empty, zero-length, undefined, NaN, -0, very large, very small, unicode, trailing whitespace)?
• Branches and guards — every if, switch, ?., ??, try/catch, early return. Each branch is an edge case worth naming.
• Boundaries — off-by-one around lengths/indices, inclusive vs. exclusive ranges, min/max of numeric domains, first/last element behavior.
• State and time — initial render vs. after update, before vs. after async resolution, stale closures, cleanup on unmount, race between two in-flight requests, timers firing after teardown.
• Collaborator failures — what happens when the thing this code calls throws, times out, returns null, or returns an unexpected shape? Cover the ones the code handles, and at least one it doesn't (to document the contract).
• Concurrency & ordering — duplicate events, rapid re-renders, out-of-order responses, double-submits.
• Authorization & identity — missing user, wrong role, expired token — wherever the code branches on identity.
• Environment — feature-flag on/off, locale/timezone, SSR vs. client, dev vs. prod env checks in the code.

Only include cases that the code's behavior actually distinguishes. Don't pad the file with cases the implementation treats identically — one test per observable behavior.

Core Principle

Test public API and observable behavior only, never internal implementation:

• Hooks/Components: Test user interactions, props, rendered output
• Functions/Utilities: Test inputs → outputs, not internal algorithm steps
• Constants: Test exported values are correct

Test Description Style

Use BDD-style descriptions with flexible GIVEN/WHEN/THEN/AND comments:

// Full form
// GIVEN an unauthenticated user
// WHEN the protected route is accessed
// THEN the user should be redirected

// With AND for multiple assertions
// GIVEN a user is authenticated
// WHEN the profile page loads
// THEN the username should be displayed
// AND the avatar should be visible

// Simple form (no WHEN needed)
// GIVEN an empty array
// THEN length should be zero

// Multiple conditions
// GIVEN a valid token
// AND the user has admin role
// WHEN accessing admin panel
// THEN access should be granted

Coverage Requirements

• Primary success paths (happy path)
• Edge cases derived from the code (see Edge Cases: Analyze from Context above — not a generic "empty / null / zero" checklist)
• Error states (graceful error handling for every catch, rejection, or fallback branch in the source)
• All exported items
• For integration tests: the real wiring between this module and each collaborator it owns in production (don't re-test the collaborator itself)

References

• references/patterns.md - Detailed test patterns and mock handling
• references/examples.md - Complete example test files