feat: colorize snapshot diff output

README.md

@@ -39,11 +39,10 @@ Use `press` as the canonical tap command; `click` is an equivalent alias.
 ```bash
 agent-device open Contacts --platform ios # creates session on iOS Simulator
 agent-device snapshot
-agent-device diff snapshot # first run initializes baseline
 agent-device press @e5
+agent-device diff snapshot # subsequent runs compare against previous baseline
 agent-device fill @e6 "John"
 agent-device fill @e7 "Doe"
-agent-device diff snapshot # subsequent runs compare against previous baseline
 agent-device press @e3
 agent-device close
 ```

skills/agent-device/SKILL.md

@@ -15,6 +15,7 @@ agent-device snapshot -i
 agent-device press @e3
 agent-device wait text "Camera"
 agent-device alert wait 10000
+agent-device diff snapshot -i
 agent-device fill @e5 "test"
 agent-device close
 ```

skills/agent-device/references/snapshot-refs.md

@@ -56,15 +56,15 @@ agent-device snapshot -i -s @e3
 Use `diff snapshot` when you need compact state-change visibility between nearby UI states:
 
 ```bash
-agent-device diff snapshot           # First run initializes baseline
+agent-device snapshot -i           # First snapshot initializes baseline
 agent-device press @e5
-agent-device diff snapshot           # Shows +/− structural lines vs prior baseline
+agent-device diff snapshot -i           # Shows +/− structural lines vs prior snapshot
 ```
 
 Efficient pattern:
 - Initialize once at a stable point.
 - Mutate UI (`press`, `fill`, `swipe`).
-- Run `diff snapshot` to confirm expected change shape.
+- Run `diff snapshot` to confirm expected change shape. Prefer `diff snapshot` before interactions for smaller token usage.
 - Re-run full/scoped `snapshot` only when you need fresh refs for next step selection.
 
 ## Troubleshooting

src/utils/__tests__/output.test.ts

@@ -2,38 +2,52 @@ import test from 'node:test';
 import assert from 'node:assert/strict';
 import { formatSnapshotDiffText } from '../output.ts';
 
-test('formatSnapshotDiffText renders unified diff lines with summary', () => {
-  const text = formatSnapshotDiffText({
-    baselineInitialized: false,
-    summary: { additions: 2, removals: 2, unchanged: 4 },
-    lines: [
-      { kind: 'unchanged', text: '@e0 [application]' },
-      { kind: 'unchanged', text: '@e2 [window]' },
-      { kind: 'removed', text: '  @e3 [other] "67"' },
-      { kind: 'removed', text: '  @e4 [text] "67"' },
-      { kind: 'added', text: '  @e3 [other] "134"' },
-      { kind: 'added', text: '  @e4 [text] "134"' },
-      { kind: 'unchanged', text: '  @e5 [button] "Increment"' },
-      { kind: 'unchanged', text: '  @e6 [text] "Footer"' },
-    ],
-  });
+const DIFF_DATA = {
+  mode: 'snapshot',
+  baselineInitialized: false,
+  summary: { additions: 1, removals: 1, unchanged: 1 },
+  lines: [
+    { kind: 'unchanged', text: '@e2 [window]' },
+    { kind: 'removed', text: '  @e3 [text] "67"' },
+    { kind: 'added', text: '  @e3 [text] "134"' },
+  ],
+} as const;
 
-  assert.doesNotMatch(text, /^@e0 \[application\]$/m);
-  assert.match(text, /^@e2 \[window\]/m);
-  assert.match(text, /^-  @e3 \[other\] "67"$/m);
-  assert.match(text, /^\+  @e3 \[other\] "134"$/m);
-  assert.match(text, /^  @e5 \[button\] "Increment"$/m);
-  assert.doesNotMatch(text, /^  @e6 \[text\] "Footer"$/m);
-  assert.match(text, /2 additions, 2 removals, 4 unchanged/);
+test('formatSnapshotDiffText renders plain text when color is disabled', () => {
+  const originalForceColor = process.env.FORCE_COLOR;
+  const originalNoColor = process.env.NO_COLOR;
+  process.env.FORCE_COLOR = '0';
+  delete process.env.NO_COLOR;
+  try {
+    const text = formatSnapshotDiffText({ ...DIFF_DATA });
+    assert.match(text, /^@e2 \[window\]/m);
+    assert.match(text, /^-  @e3 \[text\] "67"$/m);
+    assert.match(text, /^\+  @e3 \[text\] "134"$/m);
+    assert.match(text, /1 additions, 1 removals, 1 unchanged/);
+    assert.equal(text.includes('\x1b['), false);
+  } finally {
+    if (typeof originalForceColor === 'string') process.env.FORCE_COLOR = originalForceColor;
+    else delete process.env.FORCE_COLOR;
+    if (typeof originalNoColor === 'string') process.env.NO_COLOR = originalNoColor;
+    else delete process.env.NO_COLOR;
+  }
 });
 
-test('formatSnapshotDiffText renders baseline initialization text', () => {
-  const text = formatSnapshotDiffText({
-    baselineInitialized: true,
-    summary: { additions: 0, removals: 0, unchanged: 5 },
-    lines: [],
-  });
-
-  assert.match(text, /Baseline initialized \(5 lines\)\./);
-  assert.doesNotMatch(text, /additions|removals|unchanged/);
+test('formatSnapshotDiffText renders ANSI colors when forced', () => {
+  const originalForceColor = process.env.FORCE_COLOR;
+  const originalNoColor = process.env.NO_COLOR;
+  process.env.FORCE_COLOR = '1';
+  delete process.env.NO_COLOR;
+  try {
+    const text = formatSnapshotDiffText({ ...DIFF_DATA });
+    assert.equal(text.includes('\x1b[31m'), true);
+    assert.equal(text.includes('\x1b[32m'), true);
+    assert.equal(text.includes('\x1b[2m'), true);
+    assert.match(text, /\x1b\[[0-9;]+m/);
+  } finally {
+    if (typeof originalForceColor === 'string') process.env.FORCE_COLOR = originalForceColor;
+    else delete process.env.FORCE_COLOR;
+    if (typeof originalNoColor === 'string') process.env.NO_COLOR = originalNoColor;
+    else delete process.env.NO_COLOR;
+  }
 });

src/utils/output.ts

@@ -1,6 +1,7 @@
 import { AppError, normalizeError, type NormalizedError } from './errors.ts';
 import { buildSnapshotDisplayLines, formatSnapshotLine } from './snapshot-lines.ts';
 import type { SnapshotNode } from './snapshot.ts';
+import { styleText } from 'node:util';
 
 type JsonResult =
   | { success: true; data?: Record<string, unknown> }
@@ -80,19 +81,34 @@ export function formatSnapshotDiffText(data: Record<string, unknown>): string {
   const additions = toNumber(summaryRaw.additions);
   const removals = toNumber(summaryRaw.removals);
   const unchanged = toNumber(summaryRaw.unchanged);
+  const useColor = supportsColor();
   if (baselineInitialized) {
     return `Baseline initialized (${unchanged} lines).\n`;
   }
   const rawLines = Array.isArray(data.lines) ? (data.lines as SnapshotDiffLine[]) : [];
   const contextLines = applyContextWindow(rawLines, 1);
   const lines = contextLines.map((line) => {
     const text = typeof line.text === 'string' ? line.text : '';
-    if (line.kind === 'added') return text.startsWith(' ') ? `+${text}` : `+ ${text}`;
-    if (line.kind === 'removed') return text.startsWith(' ') ? `-${text}` : `- ${text}`;
-    return text;
+    if (line.kind === 'added') {
+      const prefix = text.startsWith(' ') ? `+${text}` : `+ ${text}`;
+      return useColor ? colorize(prefix, 'green') : prefix;
+    }
+    if (line.kind === 'removed') {
+      const prefix = text.startsWith(' ') ? `-${text}` : `- ${text}`;
+      return useColor ? colorize(prefix, 'red') : prefix;
+    }
+    return useColor ? colorize(text, 'dim') : text;
   });
   const body = lines.length > 0 ? `${lines.join('\n')}\n` : '';
-  return `${body}${additions} additions, ${removals} removals, ${unchanged} unchanged\n`;
+  if (!useColor) {
+    return `${body}${additions} additions, ${removals} removals, ${unchanged} unchanged\n`;
+  }
+  const summary = [
+    `${colorize(String(additions), 'green')} additions`,
+    `${colorize(String(removals), 'red')} removals`,
+    `${colorize(String(unchanged), 'dim')} unchanged`,
+  ].join(', ');
+  return `${body}${summary}\n`;
 }
 
 function toNumber(value: unknown): number {
@@ -117,3 +133,18 @@ function applyContextWindow(lines: SnapshotDiffLine[], contextWindow: number): S
   }
   return lines.filter((_, index) => keep[index]);
 }
+
+function supportsColor(): boolean {
+  const forceColor = process.env.FORCE_COLOR;
+  if (typeof forceColor === 'string') {
+    return forceColor !== '0';
+  }
+  if (typeof process.env.NO_COLOR === 'string') {
+    return false;
+  }
+  return Boolean(process.stdout.isTTY);
+}
+
+function colorize(text: string, format: Parameters<typeof styleText>[0]): string {
+  return styleText(format, text);
+}