SERENCP - VM Console MCP Server & live viewer Usage Guide

(tested with QEMU/KVM/virt-manager and OpenCode)

Overview

The serencp.pl script provides a standard MCP (Model Context Protocol) 1.0 server for bidirectional communication with VM serial consoles via an internal Perl-based socket bridge.

It uses IO::Pty to create a pseudo-terminal (PTY) for the VM serial console. It manages multiple VMs by assigning unique TCP ports for communication and provides a high-performance multiplexed event loop.

Core Features:

• Persistent PTY: Maintains a stable connection to the VM serial console.
• Auto-Restart: Automatically detects VM disconnects and restarts the bridge for persistent interaction.
• Ring Buffer: Maintains a ring buffer of the last 1000 lines of output (10MB max per VM).
• Multi-Client Access: Supports multiple simultaneous clients via dedicated Unix sockets for input (/tmp/serial_${VM_NAME}.in) and output (/tmp/serial_${VM_NAME}.out).
• Standard MCP: Supports standard tools/list and tools/call methods for tool discovery and execution.
• Zombie Management: Built-in process reaper prevents zombie processes from forks.

Prerequisites

• Operating System: Strictly requires a *nix-like system (Linux, macOS, BSD, etc.). Windows is NOT supported (unless via WSL).
• Perl with IO::Pty and JSON::PP modules installed.
• VM running with serial console on a TCP port (default starts at 4555).
• It does not require root permissions.
• Automatic Terminal Feature: Requires a supported terminal emulator to automatically spawn a window. It uses an internal client mode and does not require socat.

Supported Terminal Emulators

The script automatically detects and supports the following terminal emulators:

Linux/Unix:

• konsole (KDE)
• gnome-terminal (GNOME)
• xterm (classic X11 terminal)
• terminator (advanced tiling terminal)
• guake (drop-down terminal)
• tilix (GTK3 tiling terminal)
• alacritty (GPU-accelerated terminal)
• kitty (GPU-accelerated terminal)
• urxvt (Unicode rxvt)
• xfce4-terminal (XFCE desktop)
• lxterminal (LXDE desktop)
• deepin-terminal (Deepin desktop)
• mate-terminal (MATE desktop)
• qterminal (LXQt desktop)
• wezterm (cross-platform terminal)
• ghostty (modern GPU-accelerated terminal)

macOS:

• Terminal.app (built-in macOS terminal)
• iTerm.app (iTerm2)

The script will automatically detect which terminal is available on your system and use the appropriate command-line arguments to spawn a new terminal window connected to the VM serial console session. If no terminal is detected, it provides fallback mechanisms and error notifications.

Configuration

Default Constants

• Default VM Port: 4555
• Ring Buffer Size: 1000 lines
• Max Buffer Bytes: 10MB per VM
• Console History Lines: 60 lines (sent to new clients)
• Read Timeout: 2 seconds (internal, for legacy read tool)

MCP Server Configuration

Make sure the MCP server is configured in opencode.jsonc:

"mcp": {
    "serencp": {
        "type": "local",
        "command": ["perl", "/path/to/serencp.pl"],
        "enabled": true
    }
}

Make sure your guest OS is configured to use the serial console.

GRUB example:

GRUB_CMDLINE_LINUX_DEFAULT="console=ttyS0,115200n8"

/etc/inittab example:

T0:23:respawn:/sbin/getty -L ttyS0 115200 vt100

systemd example:

systemctl enable serial-getty@ttyS0.service
systemctl start serial-getty@ttyS0.service

QEMU/KVM example XML config:

<serial type="tcp">
  <source mode="bind" host="127.0.0.1" service="4555" tls="no"/>
  <protocol type="raw"/>
  <target type="isa-serial" port="0">
    <model name="isa-serial"/>
  </target>
  <alias name="serial0"/>
</serial>

Standard MCP Methods

tools/list

Lists all available tools.

• Request: {"jsonrpc": "2.0", "id": 1, "method": "tools/list"}
• Response: List of tools with their input schemas.

tools/call

Executes a specific tool.

• Request Format:

  {
    "jsonrpc": "2.0",
    "id": 1,
    "method": "tools/call",
    "params": {
      "name": "tool_name",
      "arguments": { ... }
    }
  }
  

Live Output Notifications

The server now supports real-time VM output streaming through MCP protocol notifications. This provides immediate feedback without requiring polling.

Notification Format

VM output is automatically streamed as JSON-RPC 2.0 notifications:

{
    "jsonrpc": "2.0",
    "method": "notifications/tool_stream",
    "params": {
        "toolName": "serencp/vm1",
        "content": [
            {
                "type": "text",
                "text": "output data here"
            }
        ],
        "isError": false
    }
}

Notification Parameters

• toolName: The tool identifier in format "serencp/{vm_name}"
• content: Array containing the output data with type and text fields
• isError: Boolean flag indicating if this is an error message

Client-Side Handling

MCP clients can listen for notifications:

client.on('notification', (notification) => {
    if (notification.method === 'notifications/tool_stream' && notification.params.toolName.startsWith('serencp/')) {
        const vmName = notification.params.toolName.replace('serencp/', '');
        const content = notification.params.content[0];
        console.log(`[${vmName}]: ${content.text}`);
        // Render live output to UI
    }
});

Benefits

• Real-time Feedback: VM output appears immediately without polling
• Efficient: Push-based model reduces overhead compared to polling
• Timestamped: Each chunk includes precise timing information
• Backward Compatible: Existing read tool continues to work for pull-based access

notifications/message

The server sends all log messages for errors, warnings, debug info (if enabled), and VM output using the standardized MCP logging notification. This is the preferred way for clients to receive live updates.

{
    "jsonrpc": "2.0",
    "method": "notifications/message",
    "params": {
        "level": "info", // or "error", "debug"
        "logger": "serencp", // or "vm/vm1" for VM output
        "message": "Description of the event or raw VM output",
        "data": "Description of the event or raw VM output"
    }
}

Available Tools

1. start

Starts the bridge for a specific VM. If a bridge already exists, it is restarted to ensure a clean slate. New behavior: Automatically spawns a graphical terminal window linked to the session using the internal client of serencp.pl. The PID of this terminal is stored to avoid duplicate windows.

• Arguments: {"vm_name": "string", "port": "number"} (port is optional, default: 4555)
• Returns: {"success": true, "message": "...", "port": 4555, "socket_in": "/tmp/serial_VM_NAME.in", "socket_out": "/tmp/serial_VM_NAME.out", "session_id": "session_...", "terminal_pid": 1234}
• Example: tools/call {"name": "start", "arguments": {"vm_name": "MYVM", "port": 4555}}

2. status

Checks the status of the bridge.

• Arguments: {"vm_name": "string"}
• Returns: {"running": true/false, "vm_name": "...", "port": ..., "buffer_size": ...}

3. read

Reads all available output from the VM serial console's dedicated output Unix socket with a 2-second timeout. This is for pull-based legacy access.

• Arguments: {"vm_name": "string"}
• Returns: {"success": true, "output": "..."}

4. write

Sends a command to the VM serial console via its dedicated input Unix socket.

• Arguments: {"vm_name": "string", "text": "command"}
• Returns: {"success": true/false, "message": "..."}
• Example: tools/call {"name": "write", "arguments": {"vm_name": "MYVM", "text": "ls -l /"}}

5. stop

Stops the bridge for a specific VM, cleaning up all PTYs, child processes, and temporary Unix sockets.

• Arguments: {"vm_name": "string"}
• Returns: {"success": true/false, "message": "..."}

Architecture

The script connects to the VM serial console as a client and provides two Unix socket servers: one for input at /tmp/serial_${VM_NAME}.in and one for output at /tmp/serial_${VM_NAME}.out. It supports both an internal Unix socket client mode and automatic terminal spawning. The MCP server handles JSON-RPC commands and replies via MCP-compliant notifications.

Internal Unix Socket Client Mode

The script can be run in client mode to connect to an existing bridge:

./serencp.pl --socket=/tmp/serial_${VM_NAME}.out

This mode provides direct terminal access to the VM serial console through the Unix socket interface.

graph TD
    VM["VM Serial Console (TCP:127.0.0.1:4555+)"]
    Bridge["Perl Bridge Child (Forked)"]
    PTY["Pseudo-Terminal (PTY Master)"]
    MCP["serencp MCP Server (Main Event Loop)"]
    Client["MCP Client (LLM / Opencode)"]
    UnixIn["Unix Input Socket (/tmp/serial_VM_NAME.in)"]
    UnixOut["Unix Output Socket (/tmp/serial_VM_NAME.out)"]
    ScriptClient["serencp.pl --socket=/tmp/serial_VM_NAME.out"]
    ExtClients["External Clients (optional)"]
    LiveTerminal["Live Terminal View (Auto-Spawned)"]

    VM <--> Bridge
    Bridge <--> PTY
    PTY <--> MCP
    MCP <--> Client
    MCP -.-> UnixIn
    MCP -.-> UnixOut
    UnixOut <--> ScriptClient
    UnixOut <--> LiveTerminal
    UnixOut -.-> ExtClients
    ScriptClient --> UnixIn
    LiveTerminal --> UnixIn
    ExtClients --> UnixIn

The parent MCP server uses IO::Select to multiplex:

1. STDIN: JSON-RPC commands from the LLM or Opencode.
2. PTY Master: Real-time data from/to the VM via the child bridge.
3. Unix Input/Output Sockets: Listeners for external terminal connections.
4. Unix Clients: Active terminal sessions connected to the Unix sockets.

When the VM disconnects, the parent detects the PTY closure and automatically restarts the bridge child to maintain persistence.

Sequence Diagram

sequenceDiagram
autonumber
participant VM as VM (serial console)
participant TCP as IO::Socket::INET (TCP 127.0.0.1:port)
participant Bridge as Bridge process (child)
participant PTY as IO::Pty (master/slave)
participant MCP as MCP server (parent)
participant USockIn as IO::Socket::UNIX (/tmp/serial_<vm>.in)
participant USockOut as IO::Socket::UNIX (/tmp/serial_<vm>.out)
participant Term as Terminal client

%% Initial connection
MCP->>Bridge: fork() + PTY creation
Bridge->>TCP: TCP connection to the VM's serial port
TCP-->>Bridge: OK (socket connected)
Bridge-->>MCP: READY via pipe

%% VM -> user flow
VM-->>TCP: Serial output (bytes)
TCP-->>Bridge: Raw data
Bridge-->>PTY: Write into PTY slave
PTY-->>MCP: Data read from PTY master
MCP->>MCP: Buffer ring + JSON stdout notification
MCP-->>USockOut: Make data available to output clients

Term-->>USockOut: Unix output socket connection (read-only)
USockOut-->>MCP: New connection accepted()
MCP-->>Term: History + live stream

%% User -> VM flow
Term->>USockIn: Unix input socket connection (write-only)
USockIn-->>MCP: New connection accepted()
Term->>USockIn: Keyboard input (command)
USockIn->>MCP: Client data
MCP-->>PTY: Write into PTY master
PTY-->>Bridge: Data read from PTY slave
Bridge-->>TCP: Write to TCP socket
TCP-->>VM: Command received on the serial console

Terminal Access

For direct interaction outside of the MCP environment, you can use the script itself as a client by specifying the output socket:

./serencp.pl --socket=/tmp/serial_${VM_NAME}.out

New output connections automatically receive the last 60 lines of history. Live output notifications are sent automatically when VM data is received, providing real-time streaming without polling. To send input, the client automatically opens and writes to the corresponding input socket (/tmp/serial_${VM_NAME}.in).

Troubleshooting

• Failed to get tools: Ensure the script is run in an environment where standard input/output is captured. Use tools/list to verify connectivity.
• Bridge not running: Call start before attempting to read or write.
• No live notifications: Ensure your MCP client supports notification handling. Notifications are sent automatically when VM output is received.
• Socket Permission: Ensure /tmp is writable by the user running the MCP server.
• Syntax Check: Run perl -c serencp.pl to verify script integrity.

About

The name serencp is a play on words:

• seren - Serenity / Serial
• cp - MCP (Model Context Protocol)

Feel free to contribute

i still can't see output notifications on Opencode, the LLM still have to poll using the read tool to get output. Maybe Opencode does not support them correctly or maybe i'm doing something wrong. Since it's a really complex script, your help / pull requests are much appreciated !