Create implementation plan

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: never ask what the code can answer. 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.

   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.
   • 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 — ordered implementation steps. Each is concrete enough to act on but doesn't dictate every line. Each step has:

   • What — a short imperative.
   • Where — concrete file paths and identifiers.
   • How — the approach in prose. Reuse decisions go here, with path:line-range citations for prior art.
   • Snippet — only when prose is genuinely more ambiguous than code (a non-obvious type signature, a tricky nested structure, an unfamiliar call shape). Default to omitting; keep under ~10 lines.
   • Depends on — earlier step numbers this one requires, if any.

   Each step is self-contained — readable without scrolling back. No "as in step 2" without restating what step 2 did. Step 1 must never be discovery work ("find the field names", "explore where this lives") — discovery already happened in the steps above. A small deterministic sanity check of a known fact is fine; open-ended investigation is not.

4. Acceptance criteria — flat checklist the implementation must satisfy, copied from the requirements' AC section. If unnumbered, number them (AC-1, …). Each references the step number(s) that satisfy it (e.g. "AC-3 → steps 4, 7"). Note any AC that needs manual verification only. The execution session uses this as its done-check.

5. Decisions log — a standing instruction copied into the plan for the execution session: "When a settled decision deviates from or extends this plan, append one line — the decision and its why — to <slug>.DECISIONS.md beside this plan, the moment it's settled." Without it, the why behind mid-implementation pivots is lost to later handover and review.

The plan must capture all the thinking — every decision needed to implement the feature, so the execution session re-derives nothing. It may omit only mechanical, locally-reversible detail with no cross-cutting consequence (a variable name, the exact wording of a log line). Anything that spans files, depends on knowing the codebase, or is costly to get wrong (which utility to reuse, what order to change things in, what new types to introduce) belongs in the plan. When unsure whether something is a decision or a mechanical detail, put it in.

Boundaries

• Do not modify any source files. The only file you write is the plan.
• Do not start implementing.
• When preparing the plan from a *.REQUIREMENTS.md file, you normally shouldn't need to go back to the *.TICKET.md — close any gaps with the user instead. Never assume a requirement; ask.
• Write the plan to disk only after every open question is resolved.

State clearly when done that the plan file is ready, referring to it by its project-relative path (relative to the current working directory — never absolute). Then tell the user to start a fresh execution session with a clean context that reads only the plan file, implements it, then runs the project's validation (lint, tests, build).

End with a single copy-pasteable launch command — session name and prompt combined, so one paste starts the execution session. Use the launch syntax of the agent tool in use (vendor-agnostic — claude below is only the example). Name the session execute-plan-<slug>, where <slug> is the plan filename's slug (without id prefix or extension), so the phase is recognizable in the session list, e.g.:

claude --name execute-plan-report-approval "Execute the plan .agents/plans/123-report-approval/123-report-approval.PLAN.md"

Then offer the alternative — clearing the current session instead (vendor-agnostic — /clear below is only the example; use the clear command of the agent tool in use):

OR /clear and run:

Execute the plan .agents/plans/123-report-approval/123-report-approval.PLAN.md