Bambu Lab MQTT, in plain words

The connection

The printer is the server: apps connect to it directly over your network, without Bambu Lab's cloud.

Reports: what the printer sends

After connecting, an app asks once for a full report (a pushall request), then receives changes as they happen, roughly every second while printing. Reports are JSON and include the job and its stage, progress and layers, time left, nozzle, bed and chamber temperatures, fans, the AMS trays with their filament, and printer errors (HMS codes).

Bambu Bar merges those changes into one picture of the printer, which is what the menu bar, the window and the widgets show. Different models report different fields, which is why an A1 shows no chamber temperature and an H2D shows two nozzles.

Requests: what apps send

Commands go to the request topic: pause, resume, stop, print speed, the chamber light, or a request for a full report. The printer decides what to accept: on current firmware, third-party apps can send print commands only when the printer is in LAN Only mode with Developer Mode on. Bambu Bar shows a lock on those buttons otherwise. See why controls are locked.

Certificates: making sure it's your printer

The connection is encrypted with TLS, and each printer presents a certificate. Bambu Bar accepts the printer only if that certificate chains to Bambu Lab's own certificate authorities, which ship with the app, and is issued to the printer's serial number. That way no other device on your network can pretend to be the printer and collect the access code.

Discovery and the camera

• Discovery: printers announce themselves on your network over UDP (port 2021). That's how apps list printers without an IP address, and how Bambu Bar notices when a printer gets a new one.
• The camera isn't on MQTT: X1, H2, H3C, P2S and X2D printers stream it over RTSPS (port 322, with LAN Only Liveview on), and P1, A1 and A2L printers over an encrypted JPEG stream (port 6000). See the camera on your Mac.

Look at the reports yourself

For diagnostics, any MQTT client can read the reports. With Mosquitto's command-line tools, and your printer's values in place of the placeholders:

mosquitto_sub -h <printer-ip> -p 8883 -u bblp -P <access-code> --insecure -t "device/<serial>/report"

--insecure skips the certificate check, so use it only on a network you trust, and only to read. Never paste your access code or serial number into forums or bug reports.

Not a public API

This is the protocol Bambu Lab's own apps use on the local network, not a documented, stable API: fields differ between models and firmware versions, and Bambu Lab can change what the printer accepts. Bambu Bar tracks those changes so you don't have to. See LAN mode explained for the printer settings involved and where to find the access code.