Codex Plan Writer

TL;DR

Create {task-slug}.md plan file for complex requests. Map file structure first. Required sections: Overview, Success Criteria, Tech Stack, File Structure, Task Breakdown (TDD steps, dependencies, verify), Phase X Checklist. No placeholders. Self-review before presenting. Offer execution handoff. Wait for user approval.

Plan Writer

Activation

1. Activate during workflow planning phases.
2. Activate on explicit $codex-plan-writer or $plan.
3. Mandatory for complex requests with scope medium or large.

Announce at start: "I'm using codex-plan-writer to create the implementation plan."

Output File Naming

• Derive a short kebab-case slug from the task intent (2-3 keywords).
• Maximum 30 characters.
• Examples: ecommerce-cart.md, login-fix.md, dark-mode.md.

Output Location

Write plan file to project root: ./{task-slug}.md

Scope Check

If the spec covers multiple independent subsystems, it should have been broken into sub-project specs during the design phase. If it wasn't, suggest breaking this into separate plans — one per subsystem. Each plan should produce working, testable software on its own.

Plan Document Header

Every plan MUST start with this header:

# [Feature Name] Implementation Plan

> **For agentic workers:** Use `$sdd` (codex-subagent-execution) or execute inline.
> Follow `$tdd` (RED-GREEN-REFACTOR) for each implementation step.
> to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.

**Goal:** [One sentence describing what this builds]

**Architecture:** [2-3 sentences about approach]

**Tech Stack:** [Key technologies/libraries]

---

Required Plan Structure

1. Overview

• what is being built or changed
• why it matters

2. Success Criteria

• measurable "done" conditions

3. Tech Stack (if relevant)

• selected technologies and rationale

4. File Structure

Before defining tasks, map out which files will be created or modified and what each one is responsible for. This is where decomposition decisions get locked in.

• Design units with clear boundaries and well-defined interfaces
• Each file should have one clear responsibility
• Prefer smaller, focused files over large ones
• Files that change together should live together
• Split by responsibility, not by technical layer
• In existing codebases, follow established patterns

This structure informs the task decomposition. Each task should produce self-contained changes that make sense independently.

5. Task Breakdown

For each task include:

• task name and id
• domain (frontend | backend | mobile | debug)
• priority (P0 | P1 | P2 | P3)
• dependencies
• input
• output
• verify method
• rollback strategy

5.5 Evidence & Monitoring

For medium or high-risk tasks, also include:

• what evidence is required before claiming success
• what signal should be monitored after the change
• what drift or failure would look like
• what fallback path exists if the signal goes red

6. Phase X Verification Checklist

• lint passes
• tests pass
• security scan reviewed
• all tasks completed
• $gate passes

Plan Granularity: Bite-Sized Tasks

Each task in an implementation plan should be completable in 2-5 minutes. Break down as follows:

1. "Write the failing test" - one step
2. "Run it to verify it fails" - one step
3. "Implement minimal code to pass" - one step
4. "Run tests to verify they pass" - one step
5. "Commit" - one step

Task Structure

### Task N: [Component Name]

**Files:**
- Create: `exact/path/to/file.py`
- Modify: `exact/path/to/existing.py:123-145`
- Test: `tests/exact/path/to/test.py`

- [ ] **Step 1: Write the failing test**

```python
def test_specific_behavior():
    result = function(input)
    assert result == expected
```

- [ ] **Step 2: Run test to verify it fails**

Run: `python -m pytest tests/path/test.py::test_name -v`
Expected: FAIL with "function not defined"

- [ ] **Step 3: Write minimal implementation**

```python
def function(input):
    return expected
```

- [ ] **Step 4: Run test to verify it passes**

Run: `python -m pytest tests/path/test.py::test_name -v`
Expected: PASS

- [ ] **Step 5: Commit**

```bash
git add tests/path/test.py src/path/file.py
git commit -m "feat: add specific feature"
```

Each task MUST include:

• Files: Exact paths to create/modify/test
• Code: Complete code (not "add validation here")
• Commands: Exact commands with expected output
• Verification: How to confirm this task is done
• Evidence: What makes the step specific and believable

No Placeholders

Every step must contain the actual content an engineer needs. These are plan failures — never write them:

• "TBD", "TODO", "implement later", "fill in details"
• "Add appropriate error handling" / "add validation" / "handle edge cases"
• "Write tests for the above" (without actual test code)
• "Similar to Task N" (repeat the code — the engineer may be reading tasks out of order)
• Steps that describe what to do without showing how (code blocks required for code steps)
• References to types, functions, or methods not defined in any task

Rules

• Do not use vague placeholders in implementation tasks.
• Keep tasks small and verifiable.
• Explain why each task exists, not only what to do.
• Exact file paths always.
• Complete code in every step — if a step changes code, show the code.
• Exact commands with expected output.
• DRY, YAGNI, TDD, frequent commits.

Self-Review

After writing the complete plan, look at the spec with fresh eyes and check the plan against it. This is a checklist you run yourself — not a subagent dispatch.

1. Spec coverage: Skim each section/requirement in the spec. Can you point to a task that implements it? List any gaps.

2. Placeholder scan: Search your plan for red flags — any of the patterns from the "No Placeholders" section above. Fix them.

3. Type consistency: Do the types, method signatures, and property names you used in later tasks match what you defined in earlier tasks? A function called clearLayers() in Task 3 but clearFullLayers() in Task 7 is a bug.

If you find issues, fix them inline. If you find a spec requirement with no task, add the task.

Execution Handoff

After saving the plan, offer execution choice:

"Plan complete and saved to ./{task-slug}.md. Two execution options:

1. Subagent-Driven ($sdd) — I dispatch a fresh subagent per task, review between tasks, fast iteration

2. Inline Execution — Execute tasks in this session, batch execution with checkpoints

Which approach?"

If Subagent-Driven chosen:

• Use codex-subagent-execution skill
• Fresh subagent per task + two-stage review

If Inline Execution chosen:

• Execute tasks sequentially in current session
• Run $gate after each batch of 3 tasks

Script Invocation Discipline

1. When plan execution calls scripts from other skills, run --help first.
2. Treat scripts as black-box helpers and execute by contract.
3. Read script source only when customization or bug fixing is required.

Exit Gate

Plan writing is complete only when:

1. plan file exists at ./{task-slug}.md
2. all required sections exist (including File Structure and No Placeholders)
3. self-review completed (spec coverage, placeholder scan, type consistency)
4. the plan can survive strict deliverable validation via run_gate.py --output-file <plan.md> without falling back to generic filler
5. user has reviewed and approved the plan

Reference Files

• references/plan-document-reviewer-prompt.md: subagent dispatch template for automated plan quality verification.

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