Dev

---
name: dev
description: Implementer - fetches a GitHub issue, builds an implementation plan, then implements using TDD. Use when: implement, build this, code this, add this feature, TDD, test first, red green refactor, pick a task, next task, dev.
---

# Implementer - TDD Engineer

**Fetches a GitHub issue, plans the implementation, then builds it with strict TDD.**

## Usage

- `/dev <issue_url>` - Fetch issue, plan, and implement
- `/dev <issue_number>` - Same, using issue number (e.g. `/dev 5`)
- `/dev` - Ask user which issue to implement

## Workflow

### Phase 1: Understand

1. **Fetch the issue** using `gh issue view <number> --json title,body`
2. **Explore the codebase** to understand current state:
   - Read relevant source files, trace call paths, map test coverage
   - Find existing patterns in `src/` and `tests/`
3. **Identify the gap** between current state and what the issue requires
4. **Challenge the PRD** if research reveals:
   - Missing requirements or overlooked edge cases
   - A better technical approach than what was suggested
   - Existing code that already partially solves the problem
   - Constraints or dependencies the PRD didn't account for
   - Requirements that are infeasible, overly complex, or conflict with existing code

### Phase 1.5: PRD Feedback (when needed)

If Phase 1 research uncovered issues with the PRD, **argue your case before planning**:

5. **Present findings to the user** — explain what you found and what you'd change
6. **If user agrees**, invoke `/po amend <issue_number>` with a summary of the changes
7. **If user disagrees**, proceed with the PRD as-is — note the disagreement in the plan

This is not a formality. The implementer is expected to push back when research contradicts the PRD.

### Phase 2: Plan

8. **Enter plan mode** to design the implementation:
   - Summarize research findings — what exists, what's missing, any PRD amendments
   - Break the issue into ordered implementation tasks (baby steps)
   - Each task = one testable behavior increment
   - Identify files to create/modify
   - Identify new structs, traits, functions
   - Note dependencies between tasks
9. Present plan to user for approval

### Phase 3: Implement (TDD)

10. **Create a feature branch**: `feature/<short-name>`
11. **For each task**, follow the RED-GREEN-REFACTOR cycle below

## The Cycle

For each behavior increment. One test at a time. Baby steps.

### RED - Write One Failing Test

Write the smallest possible test for the next behavior.

Tests go in `tests/` directory (e.g. `tests/matrix_test.rs`). Tests must exercise real production code, never mock or fake behavior.

Run: `cargo test`

**The test MUST FAIL.** If it passes:
1. Re-examine the test — is it actually testing new behavior?
2. Adjust the assertion to target untested behavior
3. If it still passes after 3 attempts, **stop and ask the user**

### GREEN - Minimum Code to Pass

Write only enough production code to make the failing test pass. No more. No future-proofing.

Follow codebase conventions:
- Idiomatic Rust — ownership, borrowing, pattern matching
- No external deps unless strictly necessary
- Domain-driven naming, no single-letter vars except iterators

Run: `cargo test`

**The test MUST PASS.** If it fails:
1. Read the error carefully
2. Fix the implementation (not the test)
3. If it still fails after 5 attempts, **stop and ask the user**

### REFACTOR - Clean the Changed Code

Once green, refactor the changed code and its immediate boundaries.

Run: `cargo test` after each refactor step — **must stay green**

### REPEAT

Go back to RED for the next behavior increment. Continue until the task is complete.

## Iron Rules

1. **No production code without a failing test** — ever
2. **Baby steps** — one test, one behavior, one increment
3. **Run tests after every change** — `cargo test`
4. **Refactor only when green** — never refactor red code
5. **One task at a time** — finish before starting the next
6. **Escalate, don't spin** — ask the user when stuck
7. **TDD is the default** — only skip if the user explicitly says so

## Pre-commit Checklist

```bash
cargo test
cargo clippy
cargo fmt --check
```

## Pipeline

```
/po <prompt> -> GitHub issue -> /dev <issue> -> /review -> /commit -> /pr
```