Profile the daemon

Use the daemon’s Bun inspector when a daemon is alive but its event loop is blocked, CPU-bound, or otherwise needs a JavaScript CPU profile. Start from a stopped daemon and run this single profile-enabled headless launch:

profile_dir=$(mktemp -d)
BUN_OPTIONS="--cpu-prof --cpu-prof-dir=$profile_dir" \
  SIGNET_INSPECTOR_PUBLIC=127.0.0.1:9229/json \
  signet daemon restart --no-sync

SIGNET_INSPECTOR_PUBLIC keeps 127.0.0.1:9229 as the public inspector endpoint, then gives the daemon a private Bun inspector endpoint and runs a local discovery proxy. The proxy is what makes this work across the daemon’s systemd-run boundary and across the daemon’s detached launch. It also supplies the discovery routes that are missing from the Bun inspector in the current runtime. restart --no-sync makes the profile-enabled invocation replace any stale daemon without running an unrelated workspace sync.

Keep the /json suffix. It is the endpoint form expected by Bun’s inspector protocol and by the discovery proxy.

Verify the launch

The daemon and public inspector become ready independently. Poll the daemon’s health endpoint first, then poll all public inspector discovery endpoints. Do not continue to the attach step until both checks succeed:

ready=0
for attempt in $(seq 1 180); do
  if curl -fsS --max-time 2 http://127.0.0.1:3850/health/live >/dev/null; then
    if curl -fsS --max-time 2 http://127.0.0.1:9229/json/version >/dev/null && \
       curl -fsS --max-time 2 http://127.0.0.1:9229/json >/dev/null && \
       curl -fsS --max-time 2 http://127.0.0.1:9229/json/list >/dev/null && \
       curl -fsS --max-time 2 http://127.0.0.1:9229/json/protocol >/dev/null; then
      ready=1
      break
    fi
  fi
  sleep 1
done
if [ "$ready" -ne 1 ]; then
  echo "daemon and inspector did not become ready within 180 seconds" >&2
  exit 1
fi

The daemon and inspector are independent endpoints:

curl -fsS http://127.0.0.1:3850/health/live
curl -fsS http://127.0.0.1:9229/json/version
curl -fsS http://127.0.0.1:9229/json
curl -fsS http://127.0.0.1:9229/json/list
curl -fsS http://127.0.0.1:9229/json/protocol

The expected inspector responses are HTTP 200. /json returns one target with a webSocketDebuggerUrl on the public inspector port. Use that URL for the WebSocket connection. The proxy returns /json, /json/list, and /json/protocol; the target Bun inspector itself may return 404 for those routes on this runtime and only expose /json/version.

Capture a CPU profile

The current Bun runtime exposes the inspector WebSocket for runtime attachment, but it does not expose the CDP Profiler domain. The profile-enabled launch above uses Bun’s built-in sampling profiler. BUN_OPTIONS is forwarded through the Linux transient systemd-run --user service boundary to the daemon child and writes a Chrome-compatible .cpuprofile when the daemon exits.

After the readiness and discovery checks succeed, exercise the workload, then stop the daemon cleanly so Bun can flush the profile. The parent handoff and discovery proxy may also write small profiles; the daemon profile is the largest file because it contains the workload:

signet daemon stop
profile=$(find "$profile_dir" -maxdepth 1 -type f -name '*.cpuprofile' -printf '%s %p\n' \
  | sort -nr | head -1 | cut -d' ' -f2-)
test -n "$profile"
printf 'CPU profile: %s\n' "$profile"

If the daemon is wedged and the CLI cannot stop it, send a graceful SIGTERM to the daemon PID from .daemon/pid. Do not use SIGKILL if you need Bun to write the profile. The resulting file can be opened in Chromium-based DevTools under the Performance panel, or imported into another CDP-compatible CPU profile viewer. Verify that the daemon exited after the profile was flushed:

if curl -fsS --max-time 2 http://127.0.0.1:3850/health/live; then
  echo "daemon is still running"
  exit 1
fi
echo "daemon stopped and profile flushed"

Why the environment and service manager matter

Bun binds BUN_INSPECT before application code runs. A Bun CLI process cannot also bind a public discovery proxy on that same port. The compiled CLI’s outer process can therefore exit during the inspector handoff before daemon start has a chance to report a useful error. Set the public endpoint with SIGNET_INSPECTOR_PUBLIC instead. The daemon runtime then reserves a private inspector port and starts the public discovery proxy without relying on that outer handoff.

On Linux, the daemon is started in a transient systemd-run --user unit. The CLI forwards the private inspector endpoint as BUN_INSPECT through that unit and keeps the public proxy outside the unit. This is why setting systemctl --user set-environment BUN_INSPECT=... is not the canonical launch: that manager environment is not the source environment of the daemon runtime, and service-manager delegation can drop or replace it. Put SIGNET_INSPECTOR_PUBLIC on the signet daemon restart --no-sync command itself.

The same public/private split avoids the parent-child EADDRINUSE failure. The daemon owns the private Bun inspector, while the proxy owns the public endpoint and exposes the stable discovery handshake.

#1607: reproducible daemon profiling.
#1534: Bun inspector handshake and discovery failures.
#1513: the startup integrity wedge that motivated the current profiling work.

Profile the daemon

Verify the launch

Capture a CPU profile

Why the environment and service manager matter

Related investigations