node-inspect-debugger – Coding skill for AI agents

Node.js Inspect Debugger

Overview

When console.log isn't enough, drive Node's built-in V8 inspector programmatically from the terminal. You get real breakpoints, step in/over/out, call-stack walking, local/closure scope dumps, and arbitrary expression evaluation in the paused frame.

Two tools, pick one:

• node inspect — built-in, zero install, CLI REPL. Best for quick poking.
• ndb / CDP via chrome-remote-interface — scriptable from Node/Python; best when you want to automate many breakpoints, collect state across runs, or debug non-interactively from an agent loop.

Prefer node inspect first. It's always available and the REPL is fast.

When to Use

• A Node test fails and you need to see intermediate state
• ui-tui crashes or behaves wrong and you want to inspect React/Ink state pre-render
• tui_gateway child processes (_SlashWorker, PTY bridge workers) misbehave
• You need to inspect a value in a closure that console.log can't reach without patching
• Perf: attach to a running process to capture a CPU profile or heap snapshot

Don't use for: things console.log solves in under a minute. Breakpoint-driven debugging is heavier; use it when the payoff is real.

Quick Reference: node inspect REPL

Launch paused on first line:

node inspect path/to/script.js
# or with tsx
node --inspect-brk $(which tsx) path/to/script.ts

The debug> prompt accepts:

| Command | Action |

|---|---|

| c or cont | continue |

| n or next | step over |

| s or step | step into |

| o or out | step out |

| pause | pause running code |

| sb('file.js', 42) | set breakpoint at file.js line 42 |

| sb(42) | set breakpoint at line 42 of current file |

| sb('functionName') | break when function is called |

| cb('file.js', 42) | clear breakpoint |

| breakpoints | list all breakpoints |

| bt | backtrace (call stack) |

| list(5) | show 5 lines of source around current position |

| watch('expr') | evaluate expr on every pause |

| watchers | show watched expressions |

| repl | drop into REPL in current scope (Ctrl+C to exit REPL) |

| exec expr | evaluate expression once |

| restart | restart script |

| kill | kill the script |

| .exit | quit debugger |

In the repl sub-mode: type any JS expression, including access to locals/closure variables. Ctrl+C exits back to debug>.

Attaching to a Running Process

When the process is already running (e.g. a long-lived dev server or the TUI gateway):

# 1. Send SIGUSR1 to enable the inspector on an existing process
kill -SIGUSR1 <pid>
# Node prints: Debugger listening on ws://127.0.0.1:9229/<uuid>

# 2. Attach the debugger CLI
node inspect -p <pid>
# or by URL
node inspect ws://127.0.0.1:9229/<uuid>

To start a process with the inspector from the beginning:

node --inspect script.js           # listen on 127.0.0.1:9229, keep running
node --inspect-brk script.js       # listen AND pause on first line
node --inspect=0.0.0.0:9230 script.js   # custom host:port

For TypeScript via tsx:

node --inspect-brk --import tsx script.ts
# or older tsx
node --inspect-brk -r tsx/cjs script.ts

Programmatic CDP (scripting from terminal)

When you want to automate — set many breakpoints, capture scope state, script a repro — use chrome-remote-interface:

npm i -g chrome-remote-interface        # or project-local
# Start your target:
node --inspect-brk=9229 target.js &

Driver script (save as ~/.hermes/cache/scratch/cdp-debug.js):

const CDP = require('chrome-remote-interface');

(async () => {
  const client = await CDP({ port: 9229 });
  const { Debugger, Runtime } = client;

  Debugger.paused(async ({ callFrames, reason }) => {
    const top = callFrames[0];
    console.log(`PAUSED: ${reason} @ ${top.url}:${top.location.lineNumber + 1}`);

    // Walk scopes for locals
    for (const scope of top.scopeChain) {
      if (scope.type === 'local' || scope.type === 'closure') {
        const { result } = await Runtime.getProperties({
          objectId: scope.object.objectId,
          ownProperties: true,
        });
        for (const p of result) {
          console.log(`  ${scope.type}.${p.name} =`, p.value?.value ?? p.value?.description);
        }
      }
    }

    // Evaluate an expression in the paused frame
    const { result } = await Debugger.evaluateOnCallFrame({
      callFrameId: top.callFrameId,
      expression: 'typeof state !== "undefined" ? JSON.stringify(state) : "n/a"',
    });
    console.log('state =', result.value ?? result.description);

    await Debugger.resume();
  });

  await Runtime.enable();
  await Debugger.enable();

  // Set a breakpoint by URL regex + line
  await Debugger.setBreakpointByUrl({
    urlRegex: '.*app\\.tsx$',
    lineNumber: 119,       // 0-indexed
    columnNumber: 0,
  });

  await Runtime.runIfWaitingForDebugger();
})();

Run it:

node ~/.hermes/cache/scratch/cdp-debug.js

Hermes-specific note: chrome-remote-interface is NOT in ui-tui/package.json. Install it to a throwaway location if you don't want to dirty the project:

mkdir -p ~/.hermes/cache/scratch/cdp-tools && cd ~/.hermes/cache/scratch/cdp-tools && npm i chrome-remote-interface
NODE_PATH=~/.hermes/cache/scratch/cdp-tools/node_modules node ~/.hermes/cache/scratch/cdp-debug.js

Debugging Hermes ui-tui

The TUI is built Ink + tsx. Two common scenarios:

Debugging a single Ink component under dev

ui-tui/package.json has npm run dev (tsx --watch). Add --inspect-brk by running tsx directly:

cd <hermes-agent-repo>/ui-tui
npm run build    # produce dist/ once so transpile isn't needed on first load
node --inspect-brk dist/entry.js
# In another terminal:
node inspect -p <node pid>

Then inside debug>:

sb('dist/app.js', 220)     # or wherever the suspect render is
cont

When it pauses, repl → inspect props, state refs, useInput handler values, etc.

Debugging a running hermes --tui

The TUI spawns Node from the Python CLI. Easiest path:

# 1. Launch TUI
hermes --tui &
TUI_PID=$(pgrep -f 'ui-tui/dist/entry' | head -1)

# 2. Enable inspector on that Node PID
kill -SIGUSR1 "$TUI_PID"

# 3. Find the WS URL
curl -s http://127.0.0.1:9229/json/list | jq -r '.[0].webSocketDebuggerUrl'

# 4. Attach
node inspect ws://127.0.0.1:9229/<uuid>

Interacting with the TUI (typing in its window) continues to advance execution; your debugger can pause it on a breakpoint at any sb(...).

Debugging _SlashWorker / PTY child processes

Those are Python, not Node — use the python-debugpy skill for them. Only Node portions (Ink UI, tui_gateway client, tsx-run tests under ui-tui/) use this skill.

Running Vitest Tests Under the Debugger

cd <hermes-agent-repo>/ui-tui
# Run a single test file paused on entry
node --inspect-brk ./node_modules/vitest/vitest.mjs run --no-file-parallelism src/app/foo.test.tsx

In another terminal: node inspect -p <pid>, then sb('src/app/foo.tsx', 42), cont.

Use --no-file-parallelism (vitest) or --runInBand (jest) so only one worker exists — debugging a pool is painful.

Heap Snapshots & CPU Profiles (Non-interactive)

From the CDP driver above, swap Debugger for HeapProfiler / Profiler:

// CPU profile for 5 seconds
await client.Profiler.enable();
await client.Profiler.start();
await new Promise(r => setTimeout(r, 5000));
const { profile } = await client.Profiler.stop();
require('fs').writeFileSync('~/.hermes/cache/scratch/cpu.cpuprofile', JSON.stringify(profile));
// Open ~/.hermes/cache/scratch/cpu.cpuprofile in Chrome DevTools → Performance tab

// Heap snapshot
await client.HeapProfiler.enable();
const chunks = [];
client.HeapProfiler.addHeapSnapshotChunk(({ chunk }) => chunks.push(chunk));
await client.HeapProfiler.takeHeapSnapshot({ reportProgress: false });
require('fs').writeFileSync('~/.hermes/cache/scratch/heap.heapsnapshot', chunks.join(''));

Common Pitfalls

1. Wrong line numbers in TS source. Breakpoints hit the emitted JS, not the .ts. Either (a) break in the built dist/*.js, or (b) enable sourcemaps (node --enable-source-maps) and use sb('src/app.tsx', N) — but only with CDP clients that follow sourcemaps. node inspect CLI does not.

2. --inspect vs --inspect-brk. --inspect starts the inspector but doesn't pause; your script races past your first breakpoint if you attach too late. Use --inspect-brk when you need to set breakpoints before any code runs.

3. Port collisions. Default is 9229. If multiple Node processes are inspecting, pass --inspect=0 (random port) and read the actual URL from /json/list:

   curl -s http://127.0.0.1:9229/json/list   # lists all inspectable targets on the host
   

4. Child processes. --inspect on a parent does NOT inspect its children. Use `NODE_OPTIONS='--inspect-brk' node parent.j

… (full content is in the download package)