diff --git a/CLAUDE.md b/CLAUDE.md
index 61ab99c..9c53700 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -35,6 +35,12 @@ Definitions and STATUS: `docs/architecture.md`.
 Name the primitive before writing code. If it reduces to none it is a **trunk** (wants a permanent
 service or a core-model change): stop, say so in a paragraph, propose the leaf, wait.
 
+**Name the layer too.** Content on disk · engine (data only the engine can produce) · theme (markup) ·
+browser (CSS, and only then JS). Build it at the outermost layer that can do the job: a line-breaking or
+spacing problem CSS solves is not an engine feature, and code that edits an author's text to fix how it
+*looks* is at the wrong layer by definition (ADR-0045 — a whole feature was deleted for this). Layers and
+the test: `docs/architecture.md`.
+
 ## 3. Hard rules
 
 1. **No abstraction before its second concrete use** — pipeline, resolver, interface, generic, config
diff --git a/HARNESS.md b/HARNESS.md
index a36a69d..926f131 100644
--- a/HARNESS.md
+++ b/HARNESS.md
@@ -52,6 +52,13 @@ must do — not the theme. `verify.sh` fails if the embedded reference theme cha
 changing, because in practice those two drift together — and it fails on a `<script>` tag in that theme,
 because a reference theme that grows taste stops being a reference (ADR-0026).
 
+**The agent names the layer as well as the primitive.** Content on disk, engine, theme, browser — and it builds
+at the outermost layer that can do the job (`CLAUDE.md` §2, `docs/architecture.md`). This exists because a whole
+feature was built at the wrong one: widow prevention as a Markdown transform that inserted a non-breaking space
+into an author's prose. It worked and it had tests; `text-wrap: pretty` does it better with no bytes in the
+content, so the feature was deleted (ADR-0045). If a feature only rearranges how something looks, expect the
+agent to push back toward CSS or a template.
+
 **The core ceiling moved once, on purpose.** `CORE_LOC_MAX` went 2000 → 2800 (ADR-0041) because the original
 figure was costed before any code existed and never budgeted what `cmd/` would hold — `check`, `new`, `-dev`,
 change detection. Treat a *second* raise as evidence that something belongs in `internal/ext/` instead: the
diff --git a/docs/architecture.md b/docs/architecture.md
index ba7db66..b5d0456 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -70,6 +70,27 @@ URL shape so the mux has one entry.
 
 ---
 
+## Layers — build at the outermost one that can do the job
+
+A primitive says *what* a feature is. This says *where* it belongs, and it is the question that catches a
+feature nothing else does (ADR-0045).
+
+| Layer | Owns | Test |
+|---|---|---|
+| Content | bytes an author wrote | Would a reader's own words change? Then only the author may touch it |
+| Engine | facts only the engine can produce — which bundles exist, an image's size, a derivative's name, a permalink | Could anything downstream work this out for itself? If yes, it does not belong here |
+| Theme | markup and which fields go where | Is the answer "how it looks"? Then it is a template, not Go |
+| Browser | line breaking, spacing, colour, motion — CSS, and only then JS | Does the browser already know something the engine cannot, like the line box? Then leave it there |
+
+Two rules fall out. **The engine never edits authored text to change how it looks**: it may transform text
+(`--` into an en dash is a transformation no stylesheet can express) but never lay it out. And **a feature
+that needs no engine fact is not an engine feature** — it is a stylesheet, a template, or nothing.
+
+The failure this exists to prevent: widow prevention was built as a Markdown transform that inserted a
+non-breaking space into an author's prose. It worked, it had tests, and it was wrong at the layer —
+`text-wrap: pretty` does it better with no bytes in the content, and the engine could never see the line box
+it was guessing about.
+
 ## Invariants — violating one is a stop condition
 
 1. **Open page object.** Known fields as struct members plus a `Meta`/`Extra` bag. Absence equals