IOS Safari Debug

iOS Safari Debug

Drive a real iPhone's Safari from a Linux (or Windows) workstation — no Mac required. Evaluate JS, stream console logs, and probe runtime state (crossOriginIsolated, SharedArrayBuffer, service-worker registration, response headers) — all from the terminal.

This is the equivalent of Safari Web Inspector that ships with macOS, but built on top of google/ios-webkit-debug-proxy so you can use it from any non-Apple host.

When to Use

• A page works in desktop browsers but fails on mobile Safari
• Need to inspect crossOriginIsolated, SharedArrayBuffer, COOP/COEP, or service-worker behavior on a real iPhone
• Want to reproduce a stale-cache / bfcache issue that only surfaces on iOS
• Want to script repeatable iPhone checks (CI smoke tests, manual reload loops, header verification) without poking at the device by hand

Preflight (always run this first)

Agent instructions: before suggesting any debug step, run check.sh and read its output. It diagnoses every prerequisite layer in one pass and prints a precise fix for whatever's missing. It is read-only — safe to run any time.

.github/skills/ios-safari-debug/scripts/check.sh

What the exit code means:

Friendly-reminder rules for the agent

When prerequisites are missing, never fail silently or dump a stack trace. Instead:

1. Run check.sh and quote the relevant [MISSING] / [WARN] lines back to the user verbatim.
2. If it's something you can fix: offer to run the fix command and wait for confirmation. Examples: install.sh, start-proxy.sh, killing a stale proxy with pkill -x ios_webkit_debug_proxy.
3. If it requires the user (physical access, iPhone settings): give a short, numbered checklist of what they need to tap, and offer to re-run check.sh once they say they're done.
4. Web Inspector is invisible from the host — always mention it explicitly when no tabs show up: "Open Settings → Safari → Advanced and confirm Web Inspector is ON."

Prerequisites (what check.sh verifies)

On the host:

• Linux (Ubuntu 22.04 tested; WSL2 + usbipd-win also works)
• libimobiledevice tools: idevice_id, ideviceinfo, ideviceinstaller
• ios_webkit_debug_proxy binary on PATH
• Python 3 with the websockets module
• usbmuxd daemon (usually auto-starts)

On the iPhone:

• Plugged in via USB, unlocked
• "Trust This Computer" granted (one-time pairing prompt)
• Settings → Safari → Advanced → Web Inspector = ON
• A Safari tab open on the page you want to inspect

Setup (one time)

Install libimobiledevice and build ios-webkit-debug-proxy:

.github/skills/ios-safari-debug/scripts/install.sh

What it does:

1. apt install libimobiledevice tools, ideviceinstaller, usbmuxd, build deps
2. Clones google/ios-webkit-debug-proxy, configures, builds, sudo make install
3. Verifies idevice_id -l lists your phone
4. Installs the Python websockets module (used by iosdbg.py)

Usage

1. Start the proxy

.github/skills/ios-safari-debug/scripts/start-proxy.sh

Backgrounds ios_webkit_debug_proxy -F. Devices show up on http://localhost:9221/json and per-device DevTools endpoints on http://localhost:9222/, :9223/, …

2. Open Safari on the iPhone

Navigate to the page you want to inspect. Until a tab is open with Web Inspector enabled, iosdbg.py tabs returns [].

3. Drive it from the terminal

PY=.github/skills/ios-safari-debug/scripts/iosdbg.py

$PY tabs                                       # list inspectable Safari tabs
$PY eval --match learnc "self.crossOriginIsolated"
$PY eval --match learnc "$(cat probe.js)"      # multi-line JS via file
$PY logs --match learnc --for 30               # stream console for 30s

--match <substr> filters by URL or title. Without it, the first tab is used.

Multi-line JS pattern

iOS WebKit's Runtime.evaluate returns whatever the expression evaluates to. For async work, set a global from inside an IIFE then read it back:

// probe.js
window.__out = null;
(async () => {
  const r = await fetch(location.href, { cache: 'no-store' });
  window.__out = {
    coi: self.crossOriginIsolated,
    sab: typeof SharedArrayBuffer !== 'undefined',
    coep: r.headers.get('cross-origin-embedder-policy'),
  };
})();
"started"

$PY eval --match learnc "$(cat probe.js)"
sleep 1
$PY eval --match learnc "JSON.stringify(window.__out)"

Why two calls: iOS WebKit honors Runtime.evaluate synchronously but is inconsistent about awaitPromise: true. Setting a global + polling is the reliable pattern.

Protocol Gotchas (iOS 13+)

ios-webkit-debug-proxy exposes a Chrome DevTools-style WebSocket, but every CDP message must be wrapped in Target.sendMessageToTarget and responses arrive inside Target.dispatchMessageFromTarget. Sending bare Runtime.evaluate gets you 'Runtime' domain was not found.

iosdbg.py handles the wrapping for you — see its CDP class. If you need to extend it (DOM inspection, network capture, etc.), just add new await cdp.call("Domain.method", params) calls; everything goes through the same envelope.

Common Diagnostic Snippets

Cross-origin isolation health

JSON.stringify({
  crossOriginIsolated: self.crossOriginIsolated,
  hasSAB: typeof SharedArrayBuffer !== 'undefined',
  swController: !!navigator.serviceWorker.controller,
  swState: navigator.serviceWorker.controller && navigator.serviceWorker.controller.state,
})

Did the SW actually serve this navigation?

JSON.stringify((() => {
  const n = performance.getEntriesByType('navigation')[0] || {};
  return { workerStart: n.workerStart, transferSize: n.transferSize, type: n.type };
})())

workerStart: 0 and transferSize: 0 mean Safari served the page from bfcache and the SW was bypassed. Force a hard reload (or bust the SW URL with a query string) to recover.

Response headers from a real fetch

window.__h = null;
fetch(location.href, { cache: 'no-store' }).then(async r => {
  window.__h = {
    coop: r.headers.get('cross-origin-opener-policy'),
    coep: r.headers.get('cross-origin-embedder-policy'),
    corp: r.headers.get('cross-origin-resource-policy'),
  };
});
"started"

Cleanup

pkill ios_webkit_debug_proxy

The libimobiledevice/usbmuxd packages stay installed (they're cheap).

Files

• scripts/check.sh — read-only preflight; always run first
• scripts/install.sh — installs apt deps, builds iwdp, pip-installs websockets
• scripts/start-proxy.sh — runs ios_webkit_debug_proxy -F in the background
• scripts/iosdbg.py — CDP driver: tabs, eval, logs