writing-adrs — Claude Code skill: ADRs in docs/adr/

---
name: writing-adrs
description: >
  Format, numbering, and when-to-write criteria for Architecture Decision
  Records in docs/adr/. Use when recording an architectural decision, when
  another skill flags a decision as ADR-worthy, or when the user asks to
  write or review an ADR.
metadata.derived-from: lifted from mattpocock/skills' `domain-modeling` reference file ADR-FORMAT.md (https://github.com/mattpocock/skills/blob/391a2701dd948f94f56a39f7533f8eea9a859c87/skills/engineering/domain-modeling/ADR-FORMAT.md)
---

# Writing ADRs

ADRs live in `docs/adr/` and use sequential numbering: `0001-slug.md`, `0002-slug.md`, etc.

Create the `docs/adr/` directory lazily — only when the first ADR is needed.

## Template

```md
# {Short title of the decision}

{1-3 sentences: what's the context, what did we decide, and why.}
```

That's it. An ADR can be a single paragraph. The value is in recording *that* a decision was made and *why* — not in filling out sections.

Written and checked with the `writing-prose` skill if available, surface `markdown`. Hard limit: precision and understandability must not suffer.

## Optional sections

Only include these when they add genuine value. Most ADRs won't need them.

- **Status** frontmatter (`proposed | accepted | deprecated | superseded by ADR-NNNN`) — useful when decisions are revisited
- **Considered Options** — only when the rejected alternatives are worth remembering
- **Consequences** — only when non-obvious downstream effects need to be called out

## Numbering

Scan `docs/adr/` for the highest existing number and increment by one.

## Grilling gate

ADRs record load-bearing decisions. Decision made in-conversation, never grilled (`docs/glossary/grilling-session.md`) → ask user: "Decision wasn't grilled — record as-is?" Proceed only on explicit confirmation. Transcribing decision user already stated explicitly → gate doesn't apply.

## When to write an ADR

All three of these must be true:

1. **Hard to reverse** — the cost of changing your mind later is meaningful
2. **Surprising without context** — a future reader will look at the code and wonder "why on earth did they do it this way?"
3. **The result of a real trade-off** — there were genuine alternatives and you picked one for specific reasons

If a decision is easy to reverse, skip it — you'll just reverse it. If it's not surprising, nobody will wonder why. If there was no real alternative, there's nothing to record beyond "we did the obvious thing."

### What qualifies

- **Architectural shape.** "We're using a monorepo." "The write model is event-sourced, the read model is projected into Postgres."
- **Integration patterns between contexts.** "Ordering and Billing communicate via domain events, not synchronous HTTP."
- **Technology choices that carry lock-in.** Database, message bus, auth provider, deployment target. Not every library — just the ones that would take a quarter to swap out.
- **Boundary and scope decisions.** "Customer data is owned by the Customer context; other contexts reference it by ID only." The explicit no-s are as valuable as the yes-s.
- **Deliberate deviations from the obvious path.** "We're using manual SQL instead of an ORM because X." Anything where a reasonable reader would assume the opposite. These stop the next engineer from "fixing" something that was deliberate.
- **Constraints not visible in the code.** "We can't use AWS because of compliance requirements." "Response times must be under 200ms because of the partner API contract."
- **Rejected alternatives when the rejection is non-obvious.** If you considered GraphQL and picked REST for subtle reasons, record it — otherwise someone will suggest GraphQL again in six months.