# Adr Writing

> Architecture Decision Record format and rules for the developer agent. Load when the implementation introduces a structural decision worth remembering 6 months later — new pattern, new dependency, deliberate departure from convention, non-obvious schema choice. Skip for naming and file-layout micro-choices. No ADR is the default.

- Skill: `marmelab/adr-writing` (Agent Skill)
- Install (CLI): `npx skillmds@latest add marmelab/adr-writing`
- Raw SKILL.md: https://api.skillmd.com/api/skills/marmelab/adr-writing/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: marmelab (https://skillmd.com/u/marmelab)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/marmelab/adr-writing

---


# ADR — Architecture Decision Record

Load only when you have a structural decision to record. Default behaviour for any developer is **no ADR**.

## When to write one

Yes:

- New pattern introduced into the codebase (not just used once — recurring).
- New runtime dependency (npm package, new Supabase extension, external service).
- Deliberate departure from an existing convention.
- Non-obvious schema choice (denormalisation, computed column, soft-delete strategy).

No:

- File naming, folder placement.
- A bug fix or refactor with no structural impact.
- Style decisions already captured by lint/prettier.

## File naming — namespaced by session + ticket

Path: `<WORKTREE_PATH>/adr/ADR-<SESSION_SHORT_ID>-<TASK-XXX>-<slug>.md`

- `<SESSION_SHORT_ID>` is your session id — the prefix of your `BRANCH_NAME` before the `/` (e.g. `BRANCH_NAME=58e4ec51/TASK-004` → `58e4ec51`). It scopes the ADR to this session.
- `<TASK-XXX>` is the literal ticket id. Together with the session id it forms the namespace — there is no separate sequence number (e.g. `ADR-58e4ec51-TASK-004-supabase-rls-strategy.md`).
- `<slug>` is kebab-case, ASCII, ≤ 40 chars.
- One ADR per ticket is the common case. If the same ticket genuinely needs two ADRs, the distinct slugs keep them apart.

**Why the session+ticket namespace**: in a wave, N developers run in parallel on worktrees branched from the same base. A monotonically-incremented `ADR-NNN-…` scheme makes every worker independently pick the same next number (e.g. all land on `ADR-003`); different slugs hide the collision from `git merge`, so duplicate numbers land in `$CLAUDE_PROJECT_DIR/adr/` silently. `<TASK-XXX>` removes the intra-wave collision; the `<SESSION_SHORT_ID>` prefix removes the cross-session one — ticket numbering restarts at TASK-001 each session, so two sessions both producing a `TASK-004` would otherwise overwrite each other's ADR in the shared `$CLAUDE_PROJECT_DIR/adr/`.

## Source-code reference

One comment at the most representative line of the change, pointing at the ADR you just wrote:

- TS/JS/CSS: `// See adr/ADR-<SESSION_SHORT_ID>-<TASK-XXX>-<slug>.md`
- Python/SQL/shell: `# See adr/ADR-<SESSION_SHORT_ID>-<TASK-XXX>-<slug>.md`

No need for more than one reference; reviewers follow the link.

## Commit

The ADR + its reference comment go in the same commit as the code they describe (WORKFLOW step 3). Subject:

```
docs(TASK-XXX): ADR-<SESSION_SHORT_ID>-<TASK-XXX> <title>
```

Reviewers see the ADR alongside the implementation.

## Template (≤ 25 lines)

```markdown
# ADR-<SESSION_SHORT_ID>-<TASK-XXX> — <decision title>

- **Date**: YYYY-MM-DD
- **Ticket**: TASK-XXX
- **Session**: <SESSION_SHORT_ID>

## Context

2–4 lines on what made this decision necessary.

## Decision

1–3 lines on what was chosen.

## Consequences

- Up to 4 bullets: what this enables, costs, locks in.

## Alternatives considered

- Up to 3, one line each, with reason for rejection. If none were captured, write `- _Not captured at decision time._` — never invent.
```

Keep it short. An ADR longer than 25 lines is a sign that the decision either belongs in code comments or is actually several decisions that should be split.

## Red Flags

- An ADR written for a naming, file-placement, or pure-style choice (no ADR is the default).
- A monotonic `ADR-NNN-…` filename instead of the `ADR-<SESSION_SHORT_ID>-<TASK-XXX>-<slug>` namespace parallel workers collide on the same number.
- An ADR file written outside the worktree, or committed separately from the code it describes.
- `Alternatives considered` invented after the fact instead of `- _Not captured at decision time._`.
- An ADR past 25 lines likely belongs in code comments or is several decisions.

## Verification

- [ ] The change is a genuine structural decision (new pattern, dependency, convention departure, non-obvious schema) otherwise no ADR.
- [ ] Filename is `<WORKTREE_PATH>/adr/ADR-<SESSION_SHORT_ID>-<TASK-XXX>-<slug>.md`.
- [ ] Exactly one source-code reference comment points at the ADR.
- [ ] ADR + reference comment are in the same commit as the code, with the `docs(TASK-XXX):` subject.
- [ ] Body follows the template and is ≤ 25 lines.