rn-debugging

rn-debugging — CDP Usage, Error Identification, and Connection Troubleshooting

How to diagnose problems in React Native apps using the CDP MCP server, native device logs, and bash tools.

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CDP vs Bash Decision Table

What you need to know	Tool	Command / MCP call
Is Metro running?	MCP	cdp_status
Is app showing a crash (JS)?	MCP	cdp_error_log
Is app showing a crash (native)?	MCP	collect_logs(sources=["native_ios"]) or collect_logs(sources=["native_android"])
Unified JS + native logs?	MCP	collect_logs(sources=["js_console","native_ios"], durationMs=3000)
What screen is the user on?	MCP	cdp_navigation_state
What does the component render?	MCP	cdp_component_tree(filter="MyComponent")
What is in the store?	MCP	cdp_store_state(path="cart.items")
What API calls were made?	MCP	cdp_network_log(limit=10)
What did console.log output?	MCP	cdp_console_log(level="all")
Did the JS engine pause?	MCP	cdp_status (reports isPaused)
Is there a RedBox overlay?	MCP	cdp_component_tree (auto-detects and warns)
Dismiss RedBox / toggle inspector	MCP	cdp_dev_settings(action="dismissRedBox")
Is Metro bundler alive?	bash	curl localhost:8081/status
Is a specific element on screen?	MCP	device_find(text="element") or Maestro assertVisible
Tap an element by text or ref	MCP	device_find(text="Login", action="click") or device_press(ref="@e3")
Fill a text input	MCP	device_fill(ref="@e5", text="hello")
What are all UI elements?	MCP	device_snapshot (cross-platform accessibility tree with @refs)
What are all UI elements' positions?	bash	adb shell uiautomator dump (Android only, raw XML)
Arbitrary runtime value	MCP	cdp_evaluate(expression="...")

Key rule: If cdp_error_log is empty but the app is visibly broken, the problem is native. CDP only sees JavaScript — check native logs as fallback.

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Error Types Matrix

Error Type	Where to Find It	Tool
JS runtime error (throw, TypeError)	cdp_error_log	MCP
Unhandled promise rejection	cdp_error_log	MCP
Uncaught error overlay (RedBox)	cdp_component_tree (APP_HAS_REDBOX warning)	MCP
console.error() call	cdp_console_log(level="error")	MCP
Metro bundle syntax error	curl localhost:8081/status	bash
Native crash (iOS)	collect_logs(sources=["native_ios"], logLevel="error")	MCP
Native crash (Android)	collect_logs(sources=["native_android"], logLevel="error")	MCP
Cross-layer crash diagnosis	collect_logs(sources=["js_console","native_ios"], durationMs=3000, logLevel="error")	MCP
Network failure	cdp_network_log (look for status=0 or missing status)	MCP

Note: Replace "/YourApp" with the actual binary name and com.example.app with the real bundle ID. Find the binary name: ls $(xcrun simctl get_app_container booted <bundle-id>)

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Environment Status Check (Always First)

Before any testing or debugging, call cdp_status. It returns:

{
  "metro": { "running": true, "port": 8081 },
  "cdp": { "connected": true, "device": "iPhone 16 Pro", "pageId": 3 },
  "app": {
    "platform": "ios", "dev": true, "hermes": true,
    "rnVersion": "0.83.1",
    "dimensions": { "width": 393, "height": 852 },
    "hasRedBox": false, "isPaused": false, "errorCount": 0
  },
  "capabilities": {
    "networkDomain": true, "fiberTree": true, "networkFallback": false
  }
}

Decision tree:

• metro.running = false → start Metro: npx expo start or npx react-native start
• app.hasRedBox = true → read cdp_error_log, fix the error, then cdp_reload
• app.isPaused = true → cdp_reload (auto-reconnects after reload)
• app.errorCount > 0 → check cdp_error_log before continuing
• capabilities.networkDomain = false → network logging uses injected hooks (RN < 0.83)
• capabilities.fiberTree = false → release build or non-Hermes engine

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Connection Troubleshooting Guide

Symptom	Cause	Fix
Metro not found	Dev server not running	npx expo start or npx react-native start
No Hermes target	App not loaded	Open simulator, wait for app bundle, retry
Error code 1006	Another debugger connected	Close React Native DevTools, Flipper, or Chrome DevTools
Evaluate timeout (5s)	JS thread blocked or paused	Search for debugger; statements; check for long sync ops
"hook not available"	Release build or JSC engine	Only works in __DEV__ mode with Hermes
APP_HAS_REDBOX	Error overlay showing	Read cdp_error_log, fix code, cdp_reload
"No store found"	Zustand not exposed	Add if (__DEV__) global.__ZUSTAND_STORES__ = { ... }
All CDP calls fail	Zombie Hermes target	Reload app and reconnect with cdp_status
dev: false in status	Connected to wrong JS context	CDP auto-selects targets by probing __DEV__; if all targets report false, restart Metro or the app
fiberTree/navRef missing	Wrong Bridgeless context	RN 0.76+ Bridgeless exposes multiple Hermes targets; cdp_status warns if dev: false

Code 1006: Hermes allows only one CDP client. Close all debugger UIs first. Code 1001: Normal close from a reload — handled automatically, no action needed.

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Post-Reload Readiness

After cdp_reload, the server auto-reconnects and waits up to 30 seconds for React DevTools hook. If cdp_component_tree returns "No fiber roots" immediately after reload, wait 2 seconds and retry.

Manual readiness check:

cdp_evaluate(expression="typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ !== 'undefined' && __REACT_DEVTOOLS_GLOBAL_HOOK__.renderers?.size > 0")

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CDP Technical Constraints

5-Second Timeout on All Calls

Every CDP call has a hard 5-second timeout. Common causes of timeout:

• debugger; statement in code
• Long synchronous computation
• Unresolved promise with awaitPromise=true
• Metro busy bundling

Single CDP Session

Hermes allows exactly one CDP client at a time. Things that consume a session:

• React Native DevTools (built into RN 0.73+)
• Flipper (older projects)
• Chrome DevTools connected via Metro
• The MCP server itself

Fiber Tree Limitations

• Only works in __DEV__ builds with Hermes engine
• Component in fiber tree does NOT mean visible on screen
• Full tree dumps are expensive — always use filter parameter

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Common Error Patterns

For diagnostic recipes covering "Cannot read property X of undefined", "Invariant Violation", native crashes with no CDP error, truncated results, and missing network requests, consult references/common-error-patterns.md.

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Metro Health Check

curl localhost:8081/status          # Expected: "packager-status:running"
curl localhost:8081/json/list       # List Hermes debug targets
curl localhost:19000/status         # Expo alternative port (Expo Dev Client)

The MCP server auto-discovers Metro on ports 8081, 8082, 19000, 19006. Pass metroPort to cdp_status when using non-standard ports.

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Native Log Commands

For full native log command reference (iOS log stream, Android logcat, binary name discovery), see the rn-device-control skill.

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Common Rationalizations

Agents routinely skip diagnostic steps because "the problem looks obvious." Don't.

Excuse	Reality
"The error message is clear — I'll just fix what it says"	Error messages are symptoms. A "Cannot read property 'x' of undefined" may mean a store slice isn't hydrated, a race condition in a selector, or a stale cache. Read cdp_store_state and cdp_error_log together before editing code.
"Native crashes look rare — skip collect_logs(sources=['native_ios'])"	Blank screens, unresponsive apps, and reload loops are often native crashes that show nothing in JS. If the screen is empty and cdp_error_log is empty, native is where the truth lives.
"CDP is flaky, I'll work around it with xcrun simctl"	xcrun simctl bypasses the app state — you fix the wrong layer. If CDP is unstable, debug CDP (cdp_status, check Metro, check target). Don't route around it.
"I know what's wrong, skip the component tree"	cdp_component_tree(filter="X") is ~300ms. Confirming the rendered fiber state takes less time than one wrong guess. Always verify your mental model before editing.
"The fix is small, I don't need to reproduce first"	Without a before-state, you can't prove the fix actually works. Reproduce → fix → reproduce-again is the only trustworthy loop.
"Reloading will make the problem go away"	Reload is a lazy workaround, not a fix. If the bug re-appears after navigation or state changes, you haven't fixed it. Find the root cause.

Red Flags — Stop and Reconsider

If you notice yourself doing any of these, stop and reassess:

• Editing code without having read cdp_error_log + cdp_component_tree first
• About to run xcrun simctl or adb logcat directly instead of collect_logs
• Claiming "fixed" without running the reproduction steps again
• Declaring "looks fine now" based on a screenshot alone — check store state too
• Suggesting a reload without having identified the actual bug
• Adding try/catch to silence the error instead of understanding it

Verification — Before Declaring a Fix Complete

• cdp_status returns ok:true with no errors
• Reproduction steps executed AGAIN after the fix → bug no longer reproduces
• cdp_error_log(clear: true) → cdp_error_log() shows zero new errors
• If the bug showed symptoms on screen, device_screenshot now shows expected state
• If the bug was state-related, cdp_store_state(path="<slice>") returns expected shape
• Cross-platform: check the same flow on the OTHER platform (cross_platform_verify)

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Additional Resources

• references/common-error-patterns.md — Diagnostic recipes for common RN errors
• references/navigation-patterns.md — Navigation debugging: modal→tab navigation (B75), nested navigator patterns, CDP navigate usage
• references/capability-matrix.md — CDP feature support by React Native version