Writing Plans

---
name: writing-plans
description: Writes a step-by-step implementation plan of bite-sized TDD tasks with exact file paths, interfaces, test code, and commits, from an approved spec. Use when a design or spec has been approved for a multi-step task and before touching any code.
---

# Writing Plans

## Overview

Write comprehensive implementation plans assuming the engineer has zero context for our codebase and questionable taste. Document everything they need to know: which files to touch for each task, code, testing, docs they might need to check, how to test it. Give them the whole plan as bite-sized tasks. DRY. YAGNI. TDD. Frequent commits.

Assume they are a skilled developer, but know almost nothing about our toolset or problem domain. Assume they do not know good test design very well. In `opm:executing-plans`, each task's implementer sees ONLY its own task text plus the plan's interface and constraint blocks, so every task must stand on its own.

**Announce at start:** "Using opm:writing-plans to create the implementation plan."

**Save plans to:** `docs/plans/YYYY-MM-DD-<topic>.md` (user preferences for plan location override this default).

**Scope note:** for scope larger than roughly a week, or work with several distinct phases, use `opm:milestone-planning` instead and write one plan per milestone phase.

## Scope Check

If the spec covers multiple independent subsystems, it should have been broken into sub-project specs during `opm:brainstorming`. If it was not, suggest breaking this into separate plans, one per subsystem. Each plan should produce working, testable software on its own.

## 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.
- You reason best about code you can hold in context at once, and edits are more reliable when files are focused. Prefer smaller, focused files over large ones that do too much.
- Files that change together should live together. Split by responsibility, not by technical layer.
- In existing codebases, follow established patterns. If the codebase uses large files, do not unilaterally restructure, but if a file you are modifying has grown unwieldy, including a split in the plan is reasonable.

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

## Task Right-Sizing

A task is the smallest unit that carries its own test cycle and is worth a fresh reviewer's gate. When drawing task boundaries: fold setup, configuration, scaffolding, and documentation steps into the task whose deliverable needs them; split only where a reviewer could meaningfully reject one task while approving its neighbor. Each task ends with an independently testable deliverable.

## Bite-Sized Step Granularity

**Each step is one action (2-5 minutes):**
- "Write the failing test" - step
- "Run it to make sure it fails" - step
- "Implement the minimal code to make the test pass" - step
- "Run the tests and make sure they pass" - step
- "Commit" - step

## Plan Document Header

**Every plan MUST start with this header:**

```markdown
# [Feature Name] Implementation Plan

> **For agentic workers:** REQUIRED SUB-SKILL: Use opm:executing-plans to implement this plan task-by-task. Each task follows opm:tdd-workflow. Steps use checkbox (`- [ ]`) syntax for tracking.

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

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

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

**Spec:** [path to the spec this plan implements, e.g. docs/specs/YYYY-MM-DD-<topic>.md. The plan argues from the spec, so the spec travels with it; executors read both.]

## Global Constraints

[The spec's project-wide requirements: version floors, dependency limits, naming and copy rules, platform requirements. One line each, with exact values copied verbatim from the spec. Every task's requirements implicitly include this section.]

---
```

## Task Structure

````markdown
### 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`

**Interfaces:**
- Consumes: [what this task uses from earlier tasks: exact signatures]
- Produces: [what later tasks rely on: exact function names, parameter and return types. A task's implementer sees only their own task; this block is how they learn the names and types neighboring tasks use.]

- [ ] **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: `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: `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"
```
````

## 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 sees only their own task)
- 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

## 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 the patterns in 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.

**4. Interface completeness:** Does every task's Consumes block name something an earlier task's Produces block defined? Does every Produces entry get consumed, or is it dead weight?

If you find issues, fix them inline. No need to re-review; fix and move on. If you find a spec requirement with no task, add the task.

## Execution Handoff

After saving the plan, hand off:

**"Plan complete and saved to `docs/plans/<filename>.md`. Ready to execute with `opm:executing-plans`: a fresh subagent per task with a reviewer gate between tasks, or inline execution in this session for plans of 3 or fewer tasks. Shall I start?"**

On yes, invoke `opm:executing-plans`. Do not begin implementing tasks inside this skill.

<!-- Adapted from obra/superpowers (MIT) -->