adr

ADR — Architecture Decision Records

An ADR records that a decision was made and why, so a future reader (human or agent) doesn't undo it by accident or burn a week rediscovering the reasoning. It is not a spec, a design doc, or a task list. The value is in the reasoning, not in filling out sections.

This skill covers the full lifecycle: deciding whether a decision deserves an ADR, writing it, reading existing ones before you touch code, and maintaining them as decisions get revisited.

Where ADRs live

• Default: docs/adr/ at the repo root.
• Multi-context repos (e.g. a monorepo with separate bounded contexts) may also keep context-scoped decisions in src/<context>/docs/adr/. Root docs/adr/ holds system-wide decisions; per-context dirs hold local ones.
• Create the directory lazily — only when the first ADR is actually needed. Don't scaffold empty folders.

Before you change code: read first

When you're about to work in an area, read the ADRs that touch it before writing anything. They tell you which decisions are settled. Two rules follow:

1. Don't re-litigate. If an ADR already settled something, build on it — don't re-propose the alternative it rejected.

2. Surface conflicts, don't silently override. If your plan contradicts an existing ADR, say so explicitly and let the human decide:

   Contradicts ADR-0007 (event-sourced orders) — but worth reopening because the projection lag now breaks the 200ms SLA.

   Only raise it when the friction is real enough to justify reopening the decision. Don't list every theoretical change an ADR forbids.

Whether to write one: all three must be true

Most decisions don't need an ADR. Write one only when all three hold:

1. Hard to reverse — changing your mind later carries real cost. (Easy to reverse? Skip it — you'll just reverse it.)
2. Surprising without context — a future reader looks at the code and asks "why on earth did they do it this way?" (Not surprising? Nobody will wonder.)
3. The result of a real trade-off — there were genuine alternatives and you picked one for specific reasons. (No real alternative? There's nothing to record beyond "we did the obvious thing.")

What qualifies

• Architectural shape — "monorepo", "write model is event-sourced, read model projected into Postgres".
• Integration patterns between contexts — "Ordering and Billing talk via domain events, not synchronous HTTP".
• Technology choices that carry lock-in — database, message bus, auth provider, deployment target. Not every library — only the ones that'd take a quarter to swap.
• Boundary and scope decisions — "Customer data is owned by the Customer context; others reference it by ID only". The explicit nos are as valuable as the yeses.
• Deliberate deviations from the obvious path — "manual SQL instead of an ORM because X". Anything where a reasonable reader would assume the opposite — this stops the next engineer from "fixing" something that was intentional.
• Constraints not visible in the code — "can't use AWS due to compliance", "responses must be under 200ms per the partner contract".
• Rejected alternatives where the rejection is non-obvious — picked REST over GraphQL for subtle reasons? Record it, or someone re-suggests GraphQL in six months.

How to write one

Keep it to the minimum that carries the reasoning. The template, numbering rules, optional sections, and worked examples live in references/format.md — read it when you're about to create or revise an ADR.

The short version: a title line plus 1–3 sentences (context, decision, why) is a complete ADR. Add optional sections only when they pull their weight.

Maintaining ADRs

Decisions get revisited. Rather than editing history into something that never happened, record the change:

• Don't delete or rewrite a superseded ADR. Leave it; it's the record of what was once true and why.
• Supersede by writing a new ADR and marking the old one superseded by ADR-NNNN (a status frontmatter field — see format reference). The new one should say what changed and why the original reasoning no longer holds.
• A status of proposed | accepted | deprecated | superseded is only worth adding once decisions in the repo actually start getting revisited. Don't add ceremony before it earns its keep.