harald

install  /  how it works  /  devices  /  bluetooth classic  /  add your device

fig. 1 · claude + harald find a sony wh-1000xm5's control channel over bluetooth classic, wait for you to arm it, then turn noise cancelling on. real hardware, sped up.

一 how it works

harald is an mcp server, a cli and a playbook for coding agents. it gives your agent hands and ears for bluetooth le and classic, so it can work out the protocol of a gadget you own. then you control that gadget from your own code instead of the vendor's app.

listen	decode	speak
harald reads what the vendor app says to the gadget, from your phone's bluetooth log.	your agent works out the language, keys and checksums included, by diffing payloads, testing keys and searching the vendor's android app.	you get the protocol written up and a standalone driver script with its own small cli. no vendor app.

二 install

you need uv. then:

uv tool install harald

that puts harald on your path for the commands below. if your shell can't find it, run uv tool update-shell. to try harald without installing it, run uvx harald --help: uvx fetches harald and runs it once. tested on mac.

the first time harald scans, macos asks whether your terminal app may use bluetooth. allow it. if you missed the prompt, turn it on in system settings › privacy & security › bluetooth.

two optional tools, for the steps that need them:

• brew install jadx lets harald read a vendor's android app.
• brew install libimobiledevice records your iphone's bluetooth log from the terminal.

harald also works without an agent:

harald scan                          list bluetooth le devices nearby
harald connect <address>             list a device's services and characteristics
harald read <address> <char>         read a characteristic once
harald write <address> <char> <hex>  write bytes, once you have armed the device
harald capture open <file>           show a phone capture as a timeline
harald apk <file>                    search a vendor android app for uuids and keys
harald devices                       list the device library
harald drive <slug> <command>        run a library device's driver

harald --help lists the rest, and bluetooth classic has the commands for classic gadgets.

三 devices

device	what harald found	status
sony wh-1000xm5 headphones	noise cancelling, ambient and battery over bluetooth classic	decoded
your device	open a pull request with its device.toml, protocol.md and driver.py	wanted

each decoded device lives in devices/ with its protocol written up and a standalone driver. harald devices lists them, and harald drive <slug> <command> runs one, for example harald drive <slug> on.

四 agent setup

add the mcp server to your agent, then install the playbook: the step-by-step method your agent follows to crack a gadget.

claude code, in your project folder:

claude mcp add harald -- uvx harald mcp
harald playbook install claude-code --dest .claude/skills/harald

use --dest ~/.claude/skills/harald to have the skill in every project.

codex

add this to ~/.codex/config.toml, or run codex mcp add harald -- uvx harald mcp:

[mcp_servers.harald]
command = "uvx"
args = ["harald", "mcp"]

then, in your project folder:

harald playbook install codex

this writes AGENTS.md. if your project already has one, harald leaves it alone; use --dest ~/.codex to install it for every project instead.

cursor

add this to .cursor/mcp.json in your project, or to ~/.cursor/mcp.json for every project:

{
  "mcpServers": {
    "harald": {
      "command": "uvx",
      "args": ["harald", "mcp"]
    }
  }
}

cursor has no playbook; it uses harald's tools directly.

then ask your agent to work out your gadget with harald. it will ask you for two things only you can do.

record your phone talking to the gadget. a computer cannot overhear phone-to-gadget traffic without extra hardware, so the log comes from your own iphone. a bluetooth classic gadget's log comes from an android phone instead; see bluetooth classic.

1. on the iphone, install apple's bluetooth logging profile: "bluetooth for ios/ipados" on apple's profiles and logs page. it is free.
2. connect the iphone to your mac by cable. open packetlogger, from apple's additional tools for xcode, and choose file › new ios trace. or, from the terminal: idevicebtlogger capture.pklg, which records until you press ctrl-c.
3. in the vendor app, do one thing at a time, a few seconds apart, in the order your agent asks for.
4. stop the trace, save the .pklg file and give your agent its path. to look at it yourself: harald capture open capture.pklg.

if packetlogger records nothing, remove the logging profile and install it again.

get the vendor's android app. if the commands look encrypted, your agent asks for the apk of the vendor app you already use. you don't need an android phone, and harald never downloads apps: download it yourself from apkmirror, which verifies each app's signature, or with apkeep (apkeep -a <package name> .). the package name is the id= part of the app's play store address.

五 bluetooth classic

some gadgets, often headphones and speakers, are controlled over bluetooth classic instead of bluetooth le: the vendor app talks to an rfcomm channel, a plain byte stream. harald reaches those too, on a mac only for now; bluetooth le works as before.

pair the gadget in system settings > bluetooth first and keep it connected there. harald doesn't search for or pair classic gadgets, and harald scan only sees bluetooth le. classic addresses look like aa:bb:cc:11:22:33.

harald classic devices                         list the gadgets paired with this mac
harald classic services <address>              list a gadget's rfcomm services
harald classic send <address> <service> <hex>  send messages, once you have armed it
harald classic listen <address> <service>      show what a channel says, sending nothing
harald capture open <file> --channel n         show one rfcomm channel of a capture

<service> is a service uuid or channel number from harald classic services; add --refresh there when the list looks out of date. harald classic send takes one or more messages and sends them in order, listening for replies after each one (--listen s, 2 seconds by default). if the grant runs out or the link fails partway, it prints what it sent and received, says why on stderr and exits 1. harald classic listen sends nothing and listens for --seconds s (5 by default). either listens for at most 30 seconds at a time.

like harald write, harald classic send needs harald arm <address> first, and every message spends one write from the grant, a query too. harald never opens the channels of protected profiles such as hands-free or file transfer; see safety. your agent gets the same four as mcp tools: classic_devices, classic_services, classic_send and classic_listen. none of them arms.

classic captures come from android. an iphone capture of a classic gadget shows only apple's iap2 wrapper, which harald can't unwrap yet, so record the vendor app from an android phone's bluetooth log:

1. turn on developer options (tap the build number seven times in the phone's about screen), then turn on the bluetooth hci snoop log in them, choosing enabled rather than filtered if it asks.
2. with the vendor app closed, turn the phone's bluetooth off and on, so the log is running before the app opens its channel.
3. in the vendor app, do one thing at a time, a few seconds apart, as your agent asks.
4. with usb debugging on, run adb bugreport. its zip holds btsnoop_hci.log; give your agent that file's path, or look at it yourself with harald capture open btsnoop_hci.log.

harald apk flags the rfcomm sockets and uuids in a vendor's android app as well as its bluetooth le calls.

六 safety

harald is for gadgets you own: interoperability and research. don't use it on anyone else's devices.

• writes need you. harald refuses every write until you run, in your own terminal:

  harald arm <address> --writes n --minutes m
  

  that grants a budget of n writes for m minutes (50 and 30 if you leave the flags out), and each write spends one. it shows the grant and waits for you to type arm, and it refuses to run without a terminal. harald status shows what is armed and harald disarm <address> revokes it. there is no mcp tool that arms.

• the gate has limits. harald arm stops casual or accidental arming by an agent, but it is not a sandbox. grants live in a file, ~/.harald/arm.json, and any agent that can run arbitrary commands or write files on your machine could get around the gate. what keeps a cooperative agent out is the playbook's rule: it never runs harald arm, harald write, harald drive, harald classic send or a driver from its own shell; you do. it sends classic messages only through its classic_send tool.

• firmware updates are blocked. writes to firmware-update (dfu) services stay refused even when a device is armed, so a stray write cannot start a firmware update. only --allow-dfu on harald arm lifts that, and working out a protocol never needs it. harald also blocks the classic firmware-update channels it knows, so far airoha's tooling channel, the same way unless you arm with --allow-dfu. but many classic gadgets take firmware over their normal control channel, where harald can't tell an update from any other message, so your agent must never send a message whose purpose it doesn't know.

• classic channels are fenced. harald never opens a channel whose service record lists hands-free, headset, dial-up networking, obex object push and file transfer, phonebook, messages, sim access or apple iap2. the serial port profile is allowed, since many gadgets' control channels use it. harald also refuses channels missing from the device's service list. with harald classic send, every message spends one write, a query too.

• library drivers need no arming. harald drive runs a devices/ entry's standalone driver, which you start yourself, so it sits outside the gate. read a driver before you run it, as you would any script.

• only app-wide keys are shared. a devices/ entry may carry a key only if it is the same on every unit and ships inside the vendor app. per-unit keys and pairing keys never go in the library; they stay in your own harald-work/ folder.

七 contributing a device

cracked a gadget? your agent's work is in harald-work/<slug>/. turn it into devices/<slug>/ with a device.toml, protocol.md and driver.py, then open a pull request. the contributing guide has the details, including the keys rule.

_{mit license. harald is named after king harald "bluetooth" of denmark, after whom bluetooth is named; ᚼᛅᚱᛅᛚᛏᚱ is his name as it is carved on the jelling stone.}