fix(diff): include maximum RGB distance at threshold one

src/__tests__/cli-diff.test.ts

@@ -269,6 +269,46 @@ describe('cli diff commands', () => {
     }
   });
 
+  test.each([false, true])(
+    'diff screenshot honors threshold 1 for saved images (json=%s)',
+    async (json) => {
+      const dir = mkdtempForTestSync('cli-diff-threshold-');
+      const baseline = path.join(dir, 'baseline.png');
+      const current = path.join(dir, 'current.png');
+      const diffOut = path.join(dir, 'diff.png');
+      fs.writeFileSync(baseline, solidPngBuffer(2, 2, { r: 0, g: 0, b: 0 }));
+      fs.writeFileSync(current, solidPngBuffer(2, 2, { r: 255, g: 255, b: 255 }));
+      fs.writeFileSync(diffOut, 'stale diff');
+
+      const result = await runCliCapture([
+        'diff',
+        'screenshot',
+        '--baseline',
+        baseline,
+        current,
+        '--threshold',
+        '1',
+        '--out',
+        diffOut,
+        ...(json ? ['--json'] : []),
+      ]);
+      assert.equal(result.code, null);
+      assert.equal(result.calls.length, 0);
+      assert.equal(result.stderr, '');
+      if (json) {
+        const payload = JSON.parse(result.stdout);
+        assert.equal(payload.success, true);
+        assert.equal(payload.data.match, true);
+        assert.equal(payload.data.differentPixels, 0);
+        assert.equal(payload.data.diffPath, undefined);
+      } else {
+        assert.match(result.stdout, /Screenshots match\./);
+        assert.doesNotMatch(result.stdout, /Diff image:/);
+      }
+      assert.equal(fs.existsSync(diffOut), false);
+    },
+  );
+
   test('diff screenshot rejects overlay refs with supplied current image', async () => {
     const dir = mkdtempForTestSync('cli-diff-test-');
     const baseline = path.join(dir, 'baseline.png');

src/commands/capture/diff.ts

@@ -59,7 +59,7 @@ export const diffCommandFacet = defineCommandFacet({
   text: {
     summary: 'Diff snapshot or screenshot',
     cliDetail:
-      'Live iOS simulator screenshot diffs normalize status-bar chrome by default; use screenshot --normalize-status-bar when capturing reusable baselines.',
+      'Screenshot --threshold is a per-pixel RGB tolerance: 0 requires exact colors and 1 ignores color differences; image dimensions must still match. Live iOS simulator screenshot diffs normalize status-bar chrome by default; use screenshot --normalize-status-bar when capturing reusable baselines.',
   },
   metadata: diffCommandMetadata,
   run: (client, input) => client.capture.diff(input),

src/screenshot-diff/__tests__/screenshot-diff.test.ts

@@ -343,6 +343,49 @@ test('threshold controls sensitivity: small differences ignored at default thres
   assert.equal(strict.differentPixels, 25);
 });
 
+test.each([
+  [
+    { r: 0, g: 0, b: 0 },
+    { r: 255, g: 255, b: 255 },
+  ],
+  [
+    { r: 255, g: 0, b: 0 },
+    { r: 0, g: 255, b: 255 },
+  ],
+  [
+    { r: 0, g: 255, b: 0 },
+    { r: 255, g: 0, b: 255 },
+  ],
+  [
+    { r: 0, g: 0, b: 255 },
+    { r: 255, g: 255, b: 0 },
+  ],
+])('threshold 1 includes the maximum RGB distance from %j to %j', async (before, after) => {
+  const dir = tmpDir();
+  const baseline = path.join(dir, 'baseline.png');
+  const current = path.join(dir, 'current.png');
+  const outputPath = path.join(dir, 'diff.png');
+  writeSolidPng(baseline, 2, 2, before);
+  writeSolidPng(current, 2, 2, after);
+
+  const belowMaximum = await compareScreenshots(baseline, current, {
+    threshold: 1 - Number.EPSILON / 2,
+    outputPath,
+  });
+  assert.equal(belowMaximum.match, false);
+  assert.equal(belowMaximum.differentPixels, 4);
+  assert.equal(fs.existsSync(outputPath), true);
+
+  const maximum = await compareScreenshots(baseline, current, { threshold: 1, outputPath });
+  assert.equal(maximum.match, true);
+  assert.equal(maximum.differentPixels, 0);
+  assert.equal(maximum.mismatchPercentage, 0);
+  assert.equal(maximum.totalPixels, 4);
+  assert.equal(maximum.regions, undefined);
+  assert.equal(maximum.diffPath, undefined);
+  assert.equal(fs.existsSync(outputPath), false);
+});
+
 test('throws INVALID_ARGS when baseline file does not exist', async () => {
   const dir = tmpDir();
   const current = path.join(dir, 'current.png');

src/screenshot-diff/screenshot-diff.ts

@@ -72,7 +72,8 @@ export type ScreenshotDiffOptions = {
 // white (255,255,255): √(255² + 255² + 255²) = 255√3 ≈ 441.67.
 // We use this as the denominator so threshold 0–1 maps linearly to the full
 // color distance range: 0 = exact match only, 1 = everything matches.
-const COLOR_DISTANCE_SCALE = 255 * Math.sqrt(3);
+// Match the per-pixel square-root rounding so the maximum stays inclusive.
+const COLOR_DISTANCE_SCALE = Math.sqrt(3 * 255 ** 2);
 
 export async function compareScreenshots(
   baselinePath: string,

website/docs/docs/commands.md

@@ -952,6 +952,7 @@ agent-device record stop                # Stop active recording
 - `screenshot --scale <factor> --overlay-refs` writes a smaller image and draws refs for that final image size; avoid very small scales when text, icons, or labels need to remain readable.
 - `diff screenshot` compares the current live screenshot to `--baseline`, or compares `--baseline` to an optional saved `current.png` path without requiring an active session. Its text output reports ranked changed regions with screen-space rectangles, changed-pixel counts, and each region's share of the diff; JSON also includes normalized rectangles. The earlier best-effort `ocr` and `nonTextDeltas` analyzers are retired; their optional result fields remain for source compatibility but are no longer emitted, so use the baseline/current images and diff artifact with vision for qualitative interpretation. It writes a diff PNG with a light grayscale current-screen context, red-tinted changed pixels, and outlined changed regions when `--out` is provided. Live iOS simulator diffs normalize status-bar chrome by default; use `screenshot --normalize-status-bar` when capturing reusable baselines.
 - `diff screenshot --overlay-refs` additionally writes a separate current-screen overlay guide for live captures without using that annotated image for the pixel comparison. If current-screen refs intersect changed regions, the output lists the best ref matches under those regions. Saved-image comparisons do not have live accessibility refs, so `--overlay-refs` is unavailable when a `current.png` path is provided.
+- `diff screenshot --threshold <0-1>` sets the per-pixel RGB tolerance (default `0.1`): `0` requires exact colors and `1` ignores all color differences. Image dimensions must still match at every threshold.
 - In `--json` mode, each overlay ref also includes a screenshot-space `center` point for coordinate fallback like `press <x> <y>`.
 - Burned-in touch overlays are exported only on macOS hosts, because the overlay pipeline depends on Swift + AVFoundation helpers.
 - On Linux or other non-macOS hosts, `record stop` still succeeds and returns the raw video plus telemetry sidecar, and includes `overlayWarning` when burn-in overlays were skipped.