/create-implementation-plan

name: create-implementation-plan
description: Turn a refined requirements document into a structured implementation PLAN.md a fresh session can execute. Planning only — decides the "how", not the "what". Invoke manually only.
license: MIT
metadata:
  version: "1.11"

Create Implementation Plan

**Planning only** — no code changes, no execution. You produce one document: an implementation plan that is the contract for a later execution session.

This is the **"how", not the "what"**. The "what" was settled in an earlier refinement step and lives in the requirements document you're given; do not redefine scope. But you **must flag** any gap, ambiguity, or inconsistency you find in the requirements — surface it, never paper over it.

Golden rule

**Never guess — ask.** With one limit: anything resolvable by reading the codebase, resolve by reading the codebase; only questions the code cannot settle go to the user.

Steps

Your task is to **produce the implementation plan**. The steps below build toward it; grilling the user — interviewing relentlessly to resolve anything the code can't settle — is woven through the design, not a separate phase that runs before planning starts.

1. **Read the requirements document** the user references (e.g. a `*.REQUIREMENTS.md`). If the path is ambiguous, ask.

2. **Verify against the actual codebase.** Open the files the requirements cite and confirm the prior-art references still hold; note anything that has shifted since the requirements were written. The requirements may also carry verified codebase facts — use any that are there to save re-discovery work, aware the code may have moved meanwhile (a pinned commit makes the check cheap: `git diff <commit>..HEAD -- <cited paths>`). If the requirements gate implementation on missing data or upstream work, verify that gate independently — stale gating claims are a common failure mode and easily inflate into a plan's first step when the data is in fact already addressable.

3. **Work out the approach.** This is the core of the task: design the "how" — what existing code to reuse, what to introduce, where each change goes, and the order of operations that avoids broken intermediate states (data model before its consumers, code before its tests). Track dependencies between steps. Working this out surfaces the **decision tree**: the forks where more than one reasonable approach exists. Typical forks to design through (and grill on when the code can't settle them): whether to refactor existing code to reuse it or build anew; which API or interface to call; which unit tests to add; code style, file names, and folder structure.

**Docs generally outrank prior art.** A mirrored analogue doesn't override a documented rule, and a rule's rationale doesn't license exceptions to it — existing code may predate the rule. When docs and prior art conflict and you're unsure how to resolve it, ask the user; a deviation the user approves is recorded in the plan's Overrides section, so the execution session knows it's deliberate.

**Prior art models shape, not content.** Its settings fit its own runtime, tooling and purpose and may predate the current ones, so before a step reuses a file, check each setting against the new component's target and prescribe only what fits, citing the file for shape; "copy X verbatim" is a decision settled line by line, never by the file's existence. A technical claim a step rests on ("X needs exactly Y") is verified against a doc, a compile or a test, never read off what the prior art happens to do.

**Grill the user along the way.** Whenever the approach hits a fork you can't settle from the code, stop and resolve it with the user before continuing — don't guess, and don't defer the decision into the plan. Interview relentlessly until you reach shared understanding, walking each branch of the decision tree and resolving dependencies between decisions:

• One question at a time.
• Every question carries your recommended answer, worded via `explain-in-simple-language`

when available.

• If a question can be answered by exploring the codebase, explore instead of asking.
• Cover every gap, ambiguity, or inconsistency surfaced while reading, verifying, and designing.

4. **Write the plan** (below) — once the approach is settled and no open question remains.

Plan file

The output must be self-contained and ready for a **fresh session** that reads **only the plan file** and starts implementing — it should not need to read the requirements document or the ticket. This is the single most important constraint. Anything the execution session needs (acceptance criteria, prior-art citations, file paths, naming/string conventions, type signatures, override notes) must appear in the plan itself. Repetition from the requirements is intentional: the plan is the contract, not a diff against the requirements.

Create the file in the **same directory as the requirements document**, named by replacing `.REQUIREMENTS` with `.PLAN` (e.g. `FOO.REQUIREMENTS.md` → `FOO.PLAN.md`). If the input doesn't follow that convention, append `.PLAN` before `.md`.

Structure — five parts:

1. **Summary** — 1–3 sentences: what this plan implements and the overall shape of the change (which areas are touched). 2. **Conventions and overrides** — two distinct kinds of cross-cutting context:

• **Conventions**: rules that apply across all steps — naming patterns, user-facing string rules,

the testing approach (what to test, following existing test conventions in the codebase). Copy from the requirements verbatim where applicable.

• **Overrides**: one-time corrections where the requirements deliberately diverge from a default

or prior assumption. Each entry: what was assumed, what the actual requirement is, and why. The execution session applies each only where it fits. 3. **Steps** — ordere