diff --git a/frontend/brand-guidelines.md b/frontend/brand-guidelines.md
index 38eaa588c5..8264f075a9 100644
--- a/frontend/brand-guidelines.md
+++ b/frontend/brand-guidelines.md
@@ -658,6 +658,16 @@ When implementing any component:
 
 We use the **[Lucide icon library](https://lucide.dev/)** to ensure a consistent, modern, and lightweight visual language. Icons are line-only, aligning with our clean and technical aesthetic.
 
+## Brand marks and the safe area
+
+Third-party brand marks live in `frontend/src/lib/components/icons/`. They sit next to Lucide glyphs at the same requested size — in resource rows, pickers and trigger strips — so they have to occupy the same box, or the coloured mark reads as larger than the grey glyph beside it.
+
+Lucide draws on a 24×24 grid and its artwork, stroke included, spans 22 of those 24 units. Every brand mark matches that: **a square `viewBox` centred on the artwork, sized so the painted artwork spans 22/24 of it**, leaving roughly 4% of the box as margin on the tightest side.
+
+When adding an icon, do not paste the brand's own `viewBox` from its press kit — those are cropped to the artwork, so the mark lands edge to edge and draws about 10% larger than everything around it. Instead, measure the painted bounding box, then set the `viewBox` to a square of `max(width, height) × 24/22` centred on it. `/kitchen_sink/icons` renders the whole library with an **Icon box** toggle that outlines each icon's box, which is the quickest way to see whether a new mark sits with its neighbours.
+
+`iconViewBox.test.ts` fails on a non-square `viewBox`, the usual symptom of a pasted one. The 22/24 ratio itself needs a renderer to check, so it is not enforced automatically.
+
 ## Do's and Don'ts
 
 ### ✅ Do
diff --git a/frontend/src/lib/components/icons/iconViewBox.test.ts b/frontend/src/lib/components/icons/iconViewBox.test.ts
new file mode 100644
index 0000000000..e1adc5f0a6
--- /dev/null
+++ b/frontend/src/lib/components/icons/iconViewBox.test.ts
@@ -0,0 +1,46 @@
+import { describe, expect, it } from 'vitest'
+import { readdirSync, readFileSync } from 'node:fs'
+import { dirname, join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+
+// Brand marks share the box lucide draws in: a square viewBox centred on the artwork, sized so
+// the painted artwork spans 22/24 of it. Squareness is the half that can be checked without a
+// renderer — the 22/24 ratio needs rasterising, so it is enforced by review, not here. A brand's
+// own viewBox pasted from a press kit is the way this drifts, and it is almost always non-square.
+const iconsDir = dirname(fileURLToPath(import.meta.url))
+
+function svelteFiles(dir: string): string[] {
+	return readdirSync(dir, { withFileTypes: true }).flatMap((entry) => {
+		const full = join(dir, entry.name)
+		if (entry.isDirectory()) return svelteFiles(full)
+		return entry.name.endsWith('.svelte') ? [full] : []
+	})
+}
+
+describe('icon viewBox', () => {
+	const boxes = svelteFiles(iconsDir).flatMap((file) => {
+		const source = readFileSync(file, 'utf8')
+		return [...source.matchAll(/viewBox="([^"]*)"/g)].map((m) => ({
+			icon: file.slice(iconsDir.length + 1),
+			viewBox: m[1]
+		}))
+	})
+
+	it('is square on every icon', () => {
+		const offenders = boxes.filter(({ viewBox }) => {
+			const parts = viewBox
+				.trim()
+				.split(/[\s,]+/)
+				.map(Number)
+			if (parts.length !== 4 || parts.some((n) => !Number.isFinite(n))) return true
+			const [, , width, height] = parts
+			return !(width > 0) || width !== height
+		})
+		expect(offenders.map((o) => `${o.icon}: ${o.viewBox}`)).toEqual([])
+	})
+
+	// Guards the walk itself: a broken path would make the assertion above pass over nothing.
+	it('finds the icon library', () => {
+		expect(boxes.length).toBeGreaterThan(300)
+	})
+})