diff --git a/SKILLS_INDEX.md b/SKILLS_INDEX.md
index e808bc0..9df9960 100644
--- a/SKILLS_INDEX.md
+++ b/SKILLS_INDEX.md
@@ -9,6 +9,7 @@ the SKILL.md, not the table.
 <!-- BEGIN skills-table -->
 | Skill | Description | Use when | Surfaces |
 |-------|-------------|----------|----------|
+| `adr` | Architecture Decision Records — house format, and how to record a decision you cannot make yet | Recording an architectural decision; revisiting one; or deciding between options where the right answer genuinely is not knowable yet | claude-code, library |
 | `atdd` | Acceptance Test-Driven Development workflow | Feature development with acceptance criteria | claude-code, library |
 | `cad-babysit` | Focused human-in-the-loop session working CAD's stuck queue (read-only) | Picking up whatever `cad stuck` flags, with a human present | claude-code, library |
 | `clean-code` | Clean code principles with Go adaptation | Code review, refactoring, new code | claude-code, library |
@@ -21,6 +22,7 @@ the SKILL.md, not the table.
 | `feature-spec` | Implementation-level feature spec (tighter than spec-driven-dev) | Scoping one feature inside an already-specced project | claude-code, library |
 | `gitea-ci` | Gitea CI/CD pipeline with act_runner, buildah, k3s, GitHub mirror | Setting up or debugging CI on this infra | claude-code, library |
 | `grill-me` | Structured plan interrogation — Quick Poke, Full Grill, Pre-mortem | Stress-testing a plan before committing; end of Diamond 1; before promoting to pre-prod | claude-code, library, chat |
+| `lineage` | Backward decision archaeology — was this tried before, and does the reason it failed still hold | Before proposing something that feels new; when an idea keeps resurfacing; when you inherit code whose reason is gone | claude-code, library |
 | `own-the-task` | Delegation ownership — Intake → Brief → Plan → Execute | A task is handed over open-ended; \"take this off my plate\"; before a plan exists | claude-code, library, chat |
 | `planning` | Planning and task breakdown | Sprint planning, task decomposition | claude-code, library |
 | `playwright` | Playwright E2E testing — setup, HTMX patterns, SSE streams, CI wiring | Adding browser tests to any project with a web UI | claude-code, library |
diff --git a/skills/adr/SKILL.md b/skills/adr/SKILL.md
new file mode 100644
index 0000000..b9058d0
--- /dev/null
+++ b/skills/adr/SKILL.md
@@ -0,0 +1,204 @@
+---
+name: adr
+surfaces: [claude-code, library]
+summary: "Architecture Decision Records — house format, and how to record a decision you cannot make yet"
+use_when: "Recording an architectural decision; revisiting one; or deciding between options where the right answer genuinely is not knowable yet"
+description: Write an Architecture Decision Record in this estate's format — YAML frontmatter, Status/Context/Decision/Consequences/Validation — including the case where the tradeoff is genuinely context-dependent and forcing a resolution would be worse than preserving the tension. Use when recording or superseding an architectural decision, or when a decision keeps getting re-litigated because nobody wrote down why.
+---
+
+# ADR
+
+## Overview
+
+An ADR is the *record* of a decision — the artifact someone reads in eight months when
+they are about to undo it without knowing why it exists. It does not confer the decision;
+a person with authority does that, and this skill never is that person.
+
+Two things make one useful, and neither is the prose quality: a **format consistent
+enough to grep across repos**, and an honest account of **what was not decided**.
+
+## Before you write
+
+Both of these change what goes in the ADR, so they are not cross-references, they are
+steps:
+
+1. **`lineage`** — was this decided before and reversed? The reason it was dropped is
+   usually the most valuable sentence in your Context, and if an earlier ADR exists,
+   yours supersedes it rather than ignoring it.
+2. **`grill-me`** (Full Grill) — find the hole now. It is cheaper here than in the
+   Consequences section of the ADR that supersedes yours.
+
+## When to Use
+
+- Any choice where a reasonable alternative existed and was not taken
+- Superseding or amending an earlier ADR (never edit the old one — see Status)
+- A decision that keeps getting re-litigated, which means the reasoning was never recorded
+- A tradeoff that is genuinely context-dependent — see *Preserving a productive tension*
+
+**Do not use for:** implementation choices with no alternative worth naming, or anything
+a code comment answers. An ADR per function is an ADR nobody reads.
+
+## The format
+
+**Where it goes:** `docs/adr/NNNN-kebab-slug.md`, zero-padded to four digits, numbered
+sequentially within the repo. Frontmatter you cannot locate is not greppable, and half
+the value of the format is that `docs/adr/*` is a known path in every repo here.
+
+**The number is repo-local.** `cad` ADR-0002 and `infra` ADR-0002 are different
+documents. When referring across repos — including in `supersedes` — qualify it:
+`cad#0002`. Within a repo the bare number is fine.
+
+Frontmatter first. It exists so the corpus is machine-readable — "which ADRs does 0004
+supersede", "what is still `proposed`" — and a bespoke header block, however well
+written, cannot answer those.
+
+```markdown
+---
+adr:           0004
+title:         <the decision, as a sentence — not "database choice">
+status:        proposed | accepted | superseded | amended
+date:          2026-08-29
+deciders:      <who accepted it, by name — or "pending: <name>" while proposed>
+supersedes:    <adr number, or ->
+---
+
+# ADR-0004 — <same title>
+
+## Status
+
+**The frontmatter is authoritative for status and date.** This section exists for the one
+thing the frontmatter cannot hold: *why* it moved — what superseded it and on what
+evidence, or what accepting it was contingent on. If it would only restate the YAML,
+delete it.
+
+## Context
+
+The forces. What is true that makes this a decision rather than a default — constraints,
+the failure that prompted it, what was tried. Name the unknowns explicitly; a decision
+made under uncertainty is not weaker for saying so, it is auditable.
+
+## Decision
+
+One sentence of what was decided, then why *this* option rather than the named
+alternatives. Address the strongest case against, not the weakest.
+
+## Consequences
+
+**Positive:** what this buys.
+
+**Negative / watch:** what it costs, and the specific thing to watch that would tell you
+it was wrong. "Some added complexity" is not a consequence; "the event schema is now the
+coupling surface between cad and every executor — version it deliberately" is.
+
+## Validation
+
+How you will know. A link to the experiment or the metric that proves or kills it, and
+the condition that fires a revisit. An ADR with no validation section is a preference
+with a template around it.
+```
+
+## Preserving a productive tension
+
+Some tradeoffs are not unresolved because nobody thought hard enough. They are
+context-dependent, the context is not settled, and picking now means picking blind.
+Forcing resolution there produces a confident record that is wrong half the time.
+
+Record the tension instead, in the Decision section:
+
+```markdown
+## Decision
+
+Provisional: **<option A>**, on this reasoning: <what makes it safe to pick now>.
+
+<The strongest case for the option you did NOT take, and why it does not win today. The
+plain format demands this and a preserved tension does not get an exemption — the whole
+point is that the loser is still live.>
+
+### The tension, preserved
+
+- **Option A** — <what it assumes, what it costs>
+- **Option B** — <what it assumes, what it costs>
+
+**What we do not know:** <the variable that decides it>
+
+**Preservation strategy:** <what we build now so BOTH stay reachable. This is the
+technical content of the ADR, not a one-liner — enumerate it. It is usually the part both
+options need anyway, which is often most of the work: the schema, the interface seam, the
+idempotency key. Name each piece and say which option it keeps alive.>
+
+**Resolution triggers:** <plural, and one per unknown. Two independent unknowns do not
+collapse into one condition, and a trigger may exist that neither unknown implies.>
+```
+
+Two rules keep this from becoming an excuse:
+
+1. **A preserved tension still ships a decision.** "We will decide later" is not one.
+   Prefer the option whose correctness does not depend on the unknown, say so, and mark
+   it provisional.
+
+   **When no such option exists** — and often none does, because a tension is productive
+   precisely when both options are hostage to the same unknown — do not manufacture a
+   reason that one of them is. That reads as reasoned and will not be re-examined, which
+   makes it worse than an honest arbitrary pick. Fall back, in order: the option that is
+   cheaper to reverse; the one with the lower switching cost given your preservation
+   strategy; the one already built. Then **say that is why you picked it.** "Chosen
+   because it is the cheaper mistake" is a real justification and an auditable one.
+2. **The resolution trigger must be observable by someone who is not you.** "When it
+   becomes a problem" is not a trigger. "When executor count passes five and they are all
+   long-lived services in one cluster" is. A trigger nobody can evaluate is a decision
+   that quietly becomes permanent.
+
+`cad`'s ADR-0001 does this informally under *"Known refinements (recorded, deliberately
+not built yet)"* — sovereignty deferred to a 3-class model *"until a client actually opts
+into an EU provider"*. That clause is the pattern: option, cost, and the event that
+forces the choice.
+
+## Status discipline
+
+- **Never edit an accepted ADR to change its decision.** Write a new one and set
+  `supersedes`. The record of what was believed, and when, is the whole value.
+- **Amend** for corrections that do not change the decision (a broken link, a clarified
+  consequence). Note the amendment inline with its date.
+- **`proposed` is not a parking space.** An ADR that sits proposed for a month *with no
+  named decider* is a decision nobody is making; either accept it provisionally or close
+  it as declined. With a named decider it is a decision waiting on a person, which is a
+  different thing and is chased rather than closed.
+
+## Anti-patterns
+
+| Anti-pattern | Why it fails |
+|---|---|
+| Consequences that are all positive | Then it was not a decision, it was an announcement |
+| "We chose X because it is better" | Names no alternative and no force; unfalsifiable |
+| Written after the code shipped | Reverse-engineers a justification. Record the reasoning while it is still contested |
+| One ADR amended repeatedly | Destroys the history the format exists to keep |
+| No Validation section | Nothing distinguishes a good decision from a lucky one |
+| A tension "resolved" by picking arbitrarily | The next person re-litigates it, having lost the reasoning |
+
+## Escalation
+
+Writes a file, so it acts — but it decides nothing.
+
+Stop and hand back when: the deciders are not you — which, for an agent, is always. An
+ADR records a decision someone with authority made; writing one to *create* that
+authority is how a preference acquires a document number.
+
+So the deliverable is a draft marked `proposed` with **the person who must accept it named
+in `deciders` as `pending: <name>`**, and that hand-back stated in the response, not left
+implied. Naming the decider is the escalation, not a formality — an unnamed one is how a
+proposal sits for a month.
+
+That is the intended state for an agent-authored ADR, and it is not the parking-space
+failure below: a `proposed` ADR with a named decider is waiting on someone; one without is
+waiting on nobody.
+
+Also stop when the Context cannot be written without inventing a constraint. A missing
+force means the decision has not been thought through yet, and an ADR built on an
+invented one is worse than none: it will be cited.
+
+## Cross-References
+
+- `lineage` and `grill-me` — see *Before you write*; they change the content, so they are
+  steps rather than references.
+- `spec-driven-dev` / `feature-spec` — an ADR is the *why*; a spec is the *what*. If you
+  are describing behaviour, you are writing a spec.