seriallm 0.2.0

Serial terminal emulator with MCP server for LLM-driven serial port interaction

SeriaLLM

Serial terminal emulator with MCP (Model Context Protocol) integration. Combines miniterm/tio-like interactive terminal access with programmatic control via MCP, so an LLM agent can interact with serial devices while the user sees everything live.

Features

• Interactive terminal on stdin/stdout (raw mode, Ctrl+] to quit)
• MCP server over stdio for LLM integration
• Supports local serial ports, RFC 2217, and TCP sockets (anything pyserial supports via serial_for_url)
• Auto-reconnect when the port disappears (USB serial going to bootloader, etc.)
• Ring buffer with absolute monotonic byte offsets — each MCP client tracks its own read position, no data is lost between calls
• Server/client architecture: multiple terminals and MCP clients share one server, server auto-spawns on first use and exits when idle
• Config file with port aliases and serial profiles

Installation

pip install seriallm

Quick start

Open a terminal to a serial port:

seriallm /dev/ttyUSB0 115200

A background server is automatically started. Ctrl+] to quit.

Configuration

Config file: ~/.config/seriallm/config.yaml

server:
  # Unix domain socket (default)
  socket: ~/.config/seriallm/server.sock

  # Or TCP socket (for remote access)
  # address: "127.0.0.1:8808"

  # Seconds before idle server exits after last client disconnects
  grace_period: 5

# Port aliases — shortcuts for serial port URLs
alias:
  target:
    url: /dev/ttyUSB0
    profile: embedded
  debug:
    url: rfc2217://192.168.1.10:2217
    profile: fast
  nucleo:
    url: /dev/ttyACM0

# Serial profiles — reusable baud rate / settings
profile:
  default:
    baudrate: 115200
  embedded:
    baudrate: 115200
  fast:
    baudrate: 921600

With this config:

seriallm target          # opens /dev/ttyUSB0 at 115200
seriallm debug            # opens rfc2217://192.168.1.10:2217 at 921600
seriallm nucleo           # opens /dev/ttyACM0 at 115200 (default profile)
seriallm /dev/ttyS0 9600  # raw URL with explicit baud rate

Commands

seriallm [attach] <target> [baudrate]

Open a terminal to a serial port. <target> is an alias name or a serial port URL. The baud rate is optional (defaults to the profile's value, or 115200).

The server is auto-spawned if not already running.

seriallm /dev/ttyUSB0 115200
seriallm target
seriallm attach target --name my-port --raw
seriallm attach /dev/ttyACM0 9600 --server http://remote-host:8808

Option	Description
--name NAME	Port name visible in MCP tools (default: alias name or URL)
--raw	Raw terminal mode (no output filtering)
--server URL	Connect to a specific server instead of config/auto-spawn
--config PATH	Use a custom config file

Terminal

When stdin is a TTY, the tool enters raw terminal mode:

• Everything you type is sent to the serial port
• Everything received is printed to stdout
• Ctrl+] quits

In default mode, output is filtered: CR is mapped to CRLF, bare LF is mapped to CRLF, non-printable control codes (except tab and ESC) are stripped, NUL bytes are removed. Use --raw to disable filtering.

When stdin is not a TTY (piped), the terminal runs in headless mode (serial output only, no keyboard input).

seriallm serve

Start the server explicitly. Normally not needed — the server auto-spawns when a client connects.

seriallm serve
seriallm serve --background
seriallm serve --buffer-size 10000000

Option	Description
--background	Suppress output (used by auto-spawn)
--buffer-size N	Max ring buffer per port in bytes (default: 1MB)
--config PATH	Use a custom config file

The server listens on a Unix domain socket (default: ~/.config/seriallm/server.sock) or a TCP address if configured. It starts with no serial ports — ports are created when terminal clients attach and removed when they disconnect.

The server exits automatically after a grace period (default: 5s) when all clients disconnect.

seriallm mcp

Run as an MCP server over stdio. This is what you configure Claude Code to launch.

seriallm mcp
seriallm mcp --config /path/to/config.yaml

The MCP client connects to the shared server (auto-spawning it if needed) and exposes all serial port tools over stdio. It stays alive until the parent process (Claude Code) terminates.

MCP tools

All tools accept a port_id parameter to select which serial port to operate on. Use list_ports to see available ports.

read_serial

Read data from the ring buffer.

Parameter	Type	Default	Description
since	int	0	Absolute byte offset to read from
up_to	int | null	null	Absolute byte offset to stop at (exclusive)
port_id	string	"default"	Port to read from

Returns { data, start, end }. The end value is the offset to pass as since on the next call to get only new data. If start > since, older data was evicted from the buffer.

send

Send a UTF-8 string to the serial port.

Parameter	Type	Description
data	string	The string to send
port_id	string	Port to send to

send_bytes

Send raw bytes (hex-encoded) to the serial port.

Parameter	Type	Description
hex_data	string	Hex string, e.g. "0d0a" for CR LF
port_id	string	Port to send to

wait_for

Wait for a regex pattern to appear in serial output.

Parameter	Type	Default	Description
pattern	string		Regex pattern to search for
since	int	0	Search from this absolute byte offset
timeout	float	10.0	Timeout in seconds
port_id	string	"default"	Port to watch

Returns { offset, end, match } — the byte offset range and matched text. Use read_serial(since=..., up_to=...) to get surrounding context.

set_control_lines

Set DTR and/or RTS control lines.

Parameter	Type	Default	Description
dtr	bool | null	null	Set DTR (null = don't change)
rts	bool | null	null	Set RTS (null = don't change)
port_id	string	"default"	Port to control

send_break

Send a serial break signal.

Parameter	Type	Default	Description
duration	float	0.25	Break duration in seconds
port_id	string	"default"	Port to send break on

get_port_info

Returns baud rate, connection status, buffer offsets, and control line states (CTS, DSR, RI, CD, DTR, RTS). The buffer_end value is the current absolute byte offset — use it as since in read_serial to start reading from "now".

get_port_events

Returns connection/disconnection events with their buffer offsets. Use to detect reconnection boundaries in the data stream.

Parameter	Type	Default	Description
since	int	0	Only return events at or after this offset
port_id	string	"default"	Port to query

set_baudrate

Change the baud rate at runtime.

Parameter	Type	Description
baudrate	int	New baud rate
port_id	string	Port to change

list_ports

List all currently attached serial ports with their connection status and baud rate.

Typical MCP usage patterns

Following serial output

The ring buffer uses absolute byte offsets that only increase. Track your read position to avoid re-reading data:

cursor = 0

# Send a command
send(data="version\r\n")

# Wait for the response
result = wait_for(pattern="v\\d+\\.\\d+", since=cursor, timeout=5.0)

# Read everything from cursor to just after the match
response = read_serial(since=cursor, up_to=result["end"])
cursor = response["end"]

Device reset flow

For devices like ESP32 that use DTR/RTS for bootloader entry:

set_control_lines(dtr=False, rts=True)   # assert RESET
set_control_lines(dtr=True, rts=False)   # assert BOOT, release RESET
set_control_lines(dtr=False, rts=False)  # release both
wait_for(pattern="waiting for download", timeout=5.0)

Detecting reconnections

When a USB serial device disappears and reappears (e.g. bootloader reset), use get_port_events to find the boundary:

events = get_port_events(since=cursor)
# [{"offset": 1523, "event": "disconnected"}, {"offset": 1523, "event": "connected"}]

Configuring with Claude Code

Add seriallm as an MCP server:

claude mcp add seriallm seriallm mcp

Or add to .mcp.json:

{
  "mcpServers": {
    "seriallm": {
      "command": "seriallm",
      "args": ["mcp"]
    }
  }
}

Then open a terminal to your device in a separate shell:

seriallm /dev/ttyUSB0 115200

Claude can now use the serial port tools. You see the serial I/O live in your terminal, and Claude interacts with the same port programmatically.

If you only need MCP access (no terminal), just configure Claude Code and start using the tools — the server and MCP client handle everything automatically. Attach a terminal later with seriallm <port> to see live output.

Architecture

seriallm serve          (background server, auto-spawned)
    ├── /ws               WebSocket for terminal clients
    └── /ws/mcp           WebSocket for MCP tool clients (JSON-RPC)

seriallm <target>       (terminal client, connects via /ws)
seriallm mcp            (MCP stdio client, connects via /ws/mcp)

The server manages serial ports and ring buffers. Terminal clients attach via WebSocket for real-time I/O. The MCP stdio client proxies tool calls to the server via JSON-RPC over WebSocket.

Port lifecycle is tied to terminal clients: when a terminal client connects, the server opens the serial port; when it disconnects, the port is closed. MCP clients can access any port that has an active terminal client.

The server auto-spawns on first client connection and exits after a configurable grace period when all clients disconnect.

License

MIT

Attribution

Claude Code has been used to create this project. Human did the whole specification, and offloaded all the boring stuff to LLM.