wsjtx-mcp

An MCP server that controls WSJT-X (FT8/FT4/JT65/MSK144/Q65/WSPR…) from MCP clients such as Claude Desktop and the MCP Inspector.

It is the weak-signal leg of an "operate → log" trio for amateur radio:

• fldigi-mcp — operate broad digital modes via fldigi (XML-RPC).
• contest-mcp — log QSOs to N3FJP (TCP API).
• wsjtx-mcp (this one) — operate the FT8/FT4 weak-signal world via WSJT-X (UDP message protocol).

⚠️ Experimental (v0.1). Transmit is gated behind your callsign and you keep the operator in command. Read the Transmit safety section.

How it is different

WSJT-X does not offer a request/response API. It broadcasts state over UDP (Status, Decode, QSOLogged, Heartbeat, …) and honours a small set of inbound control messages. So this server runs a background UDP listener that continuously parses datagrams and keeps the latest status, a buffer of decodes, and completed QSOs — you read those, and nudge WSJT-X with control messages.

Consequences worth knowing up front:

• No dial-frequency control over UDP. You can read the dial frequency from Status, and set mode/sub-mode/Rx DF/T-R period via configure, but QSY is a rig-control concern (Hamlib/CAT or the UI), not this server.
• You can start and halt Tx, but cannot toggle "Enable Tx". Transmission is started by answering a CQ (reply) or by free_text with send=true, and stopped by transmit halt. There is no UDP command for the "Enable Tx", "Auto Seq", "Call 1st", or "Hold Tx Freq" checkboxes — those stay UI settings.
• reply gives hands-free QSOs only when WSJT-X's "Auto Seq" is on. A reply is equivalent to double-clicking a CQ; with Auto Seq enabled (the usual FT8/FT4 default) WSJT-X then sequences the whole exchange to QSOLogged with no further calls. With Auto Seq off, reply starts only the first transmission.
• Best for search-and-pounce, weak for RUN. The API is built to answer CQs (and only CQ/QRZ decodes), so S&P is fully automatable. It has no clean way to drive a repeating call-CQ (RUN) cycle — that needs WSJT-X's own "Enable Tx", or a free_text CQ re-sent each period.

Requirements

• WSJT-X 2.1 through 3.x — the UDP message protocol is schema 3 / Qt_5_4 and has been stable across those releases (verified live against WSJT-X 3.0.2).
• In WSJT-X: Settings → Reporting → UDP Server
  • UDP Server = the host running this server (default 127.0.0.1), port 2237.
  • Accept UDP requests = ON to allow control (it is OFF by default). Observing decodes/status works without it; commanding does not.

Install (Claude Desktop)

Download the wsjtx-mcp.mcpb from the latest release and double-click it, or drag it onto Claude Desktop → Settings → Extensions. Fill in the settings form (callsign, host, port). See docs/INSTALL.md.

Configuration

Variable	Default	Purpose
WSJTX_HOST	127.0.0.1	UDP address to bind/listen on.
WSJTX_PORT	2237	WSJT-X UDP Server port.
WSJTX_CALLSIGN	(empty)	Operator callsign — the single transmit gate. Blank = receive-only.
WSJTX_MULTICAST	(off)	Optional multicast group to join (coexist with other UDP consumers).
WSJTX_INSTANCE	(auto)	Target a specific WSJT-X Id when several instances broadcast.

Host/port are where this server listens; control replies are sent back to the address each datagram arrived from.

Tools

Tool	Kind	What it does
status	observe	Latest Status snapshot + listener/instance health.
diagnostics	observe	Host/network + bind status + datagram counts + gate state.
decodes	observe/nudge	read / drain (poll new) / clear_local / replay. The RX plane.
log	observe	Buffered completed QSOs (QSOLogged + LoggedADIF) → feed N3FJP.
reply	transmit	Answer a buffered CQ/QRZ decode (auto-sequences the QSO when WSJT-X "Auto Seq" is on).
free_text	transmit if send	Set the Tx5 free-text message; send=true keys the radio.
transmit	control	halt / halt_auto — stop transmitting (UDP can't enable Tx).
configure	control	Mode/sub-mode/Rx DF/T-R period/freq-tol/DX call+grid. No dial freq.
clear	control	Clear the Band Activity / Rx Frequency windows.
highlight	control	Colour or clear a callsign in Band Activity.
location	control	Override the session Maidenhead grid.
switch_config	control	Switch to a named WSJT-X configuration.
annotate	control	Set a Fox/Hound sort-order annotation for a DX call (niche, DXpedition).
wsjtx_call	escape hatch	Build & send any message type by name (gate still applies).

Transmit safety

The callsign is the single transmit gate, exactly as in fldigi-mcp. With WSJTX_CALLSIGN blank the server is receive-only: it refuses every transmit-initiating message — reply, free_text with send=true, and any keying message via wsjtx_call. transmit halt, clear, configure, highlight, location, replay, and all reads are always available (they don't put you on the air; halt takes you off).

Beyond that gate:

• Per-transmit approval comes from the Claude Desktop tool-permission prompt — lean on it for human-in-the-loop control.
• WSJT-X's own Tx Watchdog and the Tx Enabled / Transmitting flags (surfaced in status) are extra safety signals.
• Operating under Part 97 automatic/remote control is the operator's responsibility: ensure station identification and a control operator who can intervene.

Running alongside other UDP tools

Only one process can normally own UDP 2237 on a host. If JTAlert, GridTracker, or N1MM already consume it, either point WSJT-X's secondary UDP server here, use a multicast group (WSJTX_MULTICAST) so several listeners coexist, or run this server on a different host. To reach a WSJT-X on another machine, install the mcp-host-bridge tool (mcp-host-bridge install wsjtx --to <rig-host>) and set WSJTX_PORT=2238 — sandboxed MCP clients reach only loopback, and the bridge does the LAN hop. See docs/REMOTE-HOST.md.

Development

uv sync
uv run ruff check .
uv run pytest

The protocol codec is pure standard library and unit-tested against byte fixtures, so the tests need no running WSJT-X. A smoke_test.py proves a live WSJT-X is reachable receive-only. The field-tested message reference lives in docs/WSJTX-API.md.

License

MIT © 2026 Stefan Brunner (AE5VG)