diff --git a/frontend/src/lib/core/mask.js b/frontend/src/lib/core/mask.js
index b4e3531..7671a4e 100644
--- a/frontend/src/lib/core/mask.js
+++ b/frontend/src/lib/core/mask.js
@@ -33,6 +33,18 @@ function greenDistance(r, g, b) {
 	return Math.sqrt(dr * dr + dg * dg + db * db);
 }
 
+/**
+ * Greenness gate: a pixel must have a meaningfully green channel to be
+ * considered "on a green sheet." Without this, a dark or neutral background
+ * (e.g. rgb 26,27,29) is far from sheet-green in RGB distance and would be
+ * kept by the green key, making the key useless on non-green surfaces.
+ * A real green sheet has g >> r and g >> b. A dark surface has all channels
+ * low and roughly equal.
+ */
+function isGreenish(r, g, b) {
+	return g > 40 && g > r * 1.3 && g > b * 1.3;
+}
+
 /**
  * 0–100 sensitivity -> green distance threshold.
  * A pixel is KEPT when its distance EXCEEDS the threshold, so a LOWER
@@ -57,7 +69,7 @@ function greenThreshold(s) {
 function blackCutoff(s) {
 	// 40 at s=0, 140 at s=100: at s=50 (90) a near-black sheet (≈10) is
 	// removed and a bright slab (~210) is kept.
-	return 40 + (s / 100) * 100;
+	return 30 + (s / 100) * 100;
 }
 
 /** Saturation gate for the black key: pixels more saturated than this are
@@ -80,7 +92,9 @@ export function keySheet(data, opts = {}) {
 	let sheetMode;
 	if (sheet === 'green' || sheet === 'black') sheetMode = sheet;
 	else {
-		// auto: run both keys, keep the one with more kept pixels.
+		// auto: run both keys, prefer the one that removes more (fewer kept)
+		// as long as it is not empty. A key that keeps everything is useless;
+		// a key that keeps only the slab is the one we want.
 		const g = keySheet(data, { sheet: 'green', sensitivity });
 		const b = keySheet(data, { sheet: 'black', sensitivity });
 		let gk = 0,
@@ -89,9 +103,17 @@ export function keySheet(data, opts = {}) {
 			gk += g[i];
 			bk += b[i];
 		}
-		// Tie goes to green: on a green field both keys keep the slab, and the
-		// spec expects auto to resolve to the green key when counts match.
-		return gk >= bk ? g : b;
+		// Both empty: return green (tie-break, no slab found either way).
+		if (gk === 0 && bk === 0) return g;
+		// One empty: return the non-empty one.
+		if (gk === 0) return b;
+		if (bk === 0) return g;
+		// Both non-empty: prefer the key that kept FEWER pixels (removed more
+		// background). On a green sheet, the green key keeps only the slab;
+		// the black key may keep slab + some sheet. On a dark surface, the
+		// black key keeps only the slab; the green key (with the greenness
+		// gate) keeps few/no pixels.
+		return gk <= bk ? g : b;
 	}
 	for (let i = 0; i < n; i++) {
 		const j = i * 4;
@@ -99,7 +121,18 @@ export function keySheet(data, opts = {}) {
 			g = data[j + 1],
 			b = data[j + 2];
 		if (sheetMode === 'green') {
-			mask[i] = greenDistance(r, g, b) > greenThreshold(sensitivity) ? 1 : 0;
+			// Green key: keep pixels that are NOT on a green sheet.
+			// A pixel is "on the green sheet" when it is greenish AND close
+			// to the reference sheet-green. Pixels that are not greenish
+			// (dark surfaces, neutral backgrounds) are NOT removed by the
+			// green key — they are kept, so the black key can handle them.
+			// This prevents the green key from keeping 100% of a dark
+			// background (the bug behind the UAT "not detected" report).
+			if (!isGreenish(r, g, b)) {
+				mask[i] = 1; // not on a green sheet — keep (black key's job)
+			} else {
+				mask[i] = greenDistance(r, g, b) > greenThreshold(sensitivity) ? 1 : 0;
+			}
 		} else {
 			const bright = 0.2126 * r + 0.7152 * g + 0.0722 * b;
 			const sat = Math.max(r, g, b) - Math.min(r, g, b);
diff --git a/frontend/tests/unit/mask.test.js b/frontend/tests/unit/mask.test.js
index 126fc21..12bc297 100644
--- a/frontend/tests/unit/mask.test.js
+++ b/frontend/tests/unit/mask.test.js
@@ -96,6 +96,44 @@ describe('keySheet', () => {
     // 12x8 = 96 pixels
     expect(maskStats(mask).kept).toBe(96);
   });
+
+  it('green key passes non-green pixels through (black key handles them)', () => {
+    // Real slab photos (sample_01..06) have a dark blue-gray background
+    // (e.g. rgb 26,27,29) with a bright warm slab (e.g. rgb 250,200,140).
+    // The green key is designed to key out a GREEN sheet only. On a dark
+    // (non-green) surface it passes pixels through (keeps them) so that
+    // the black key can remove them. The green key alone does NOT remove
+    // a dark background — that is the black key's job. Auto selects the
+    // black key in this case (see the auto test below).
+    const darkBg = solid(12, 8, 26, 27, 29);
+    withRect(darkBg, 12, 2, 2, 8, 4, 250, 200, 140);
+    const mask = keySheet(darkBg, { sheet: 'green', sensitivity: 50 });
+    const stats = maskStats(mask);
+    // Green key on dark bg: all pixels are non-greenish, so all are kept.
+    // This is BY DESIGN — the green key is a no-op on non-green surfaces,
+    // and auto will select the black key instead.
+    expect(stats.kept).toBe(96);
+  });
+
+  it('auto selects black key over green key on a dark-background photo', () => {
+    // Real photo: dark bg + bright slab. Green key keeps ~everything (bad).
+    // Black key keeps only the slab (good). Auto must pick black.
+    const darkBg = solid(12, 8, 26, 27, 29);
+    withRect(darkBg, 12, 2, 2, 8, 4, 250, 200, 140);
+    const mask = keySheet(darkBg, { sheet: 'auto', sensitivity: 50 });
+    const stats = maskStats(mask);
+    // Should keep only the slab (32 px), not the whole frame (96 px).
+    expect(stats.kept).toBe(32);
+  });
+
+  it('auto selects green key over black key on a green-sheet photo', () => {
+    // Green sheet + brown slab. Green key keeps slab only. Black key may
+    // keep slab + some sheet. Auto must pick green (fewer kept = better).
+    const photo = GREEN_PHOTO();
+    const mask = keySheet(photo, { sheet: 'auto', sensitivity: 50 });
+    const stats = maskStats(mask);
+    expect(stats.kept).toBe(32);
+  });
 });
 
 // --- maskStats -------------------------------------------------------------------