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) 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 with Exponential Backoff: Automatically detects VM disconnects and restarts the bridge with intelligent exponential backoff (1s initial, 60s max) to prevent rapid reconnection storms.
• 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.
• Resource Subscriptions: Control live VM output notifications using standard MCP resources/subscribe and resources/unsubscribe methods.
• Process Reaping: Built-in SIGCHLD handler prevents zombie processes from forks.
• Non-Blocking Write System: Truly non-blocking writes with optional buffer queue for reliable data delivery.
• Configurable Log Levels: Debug, info, and error log levels with priority-based filtering.
• Progress Notifications: Supports MCP progress notifications for long-running operations.
• Robust Cleanup: END block ensures proper cleanup on abnormal exit (crash, _exit, die).

Prerequisites

• Operating System: Strictly requires a *nix-like system (Linux, macOS, BSD, etc.). Windows is NOT supported (unless via WSL).
• Perl Modules: The following non-core modules are required:
  • IO::Pty - Pseudo-terminal creation
• 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.
• Command-Line Options:
  • --socket <path>: Run in Unix socket client mode (connect to existing bridge) (don't care about it unless in non-MCP environement)
  • --terminal <name>: Specify preferred terminal emulator to use (bypasses auto-detection)

Supported Terminal Emulators

The script automatically detects and supports the following terminal emulators (tested for availability at runtime):

macOS (Priority):

• Ghostty.app (GPU-accelerated)
• WezTerm.app
• iTerm.app / iTerm2.app
• Terminal.app (built-in macOS terminal)

Modern Linux/Unix:

• wezterm (cross-platform)
• kitty (GPU-accelerated, tested)
• alacritty (GPU-accelerated)
• ghostty (modern GPU-accelerated)
• foot (Wayland terminal)

Mid-tier:

• konsole (KDE)
• gnome-terminal (GNOME)
• tilix (GTK3 tiling terminal)
• terminator (advanced tiling terminal)
• xfce4-terminal (XFCE desktop)

Legacy:

• xterm (classic X11 terminal)
• urxvt (Unicode rxvt)

The script uses a priority-based detection system:

1. First honors user's explicit --terminal option
2. Then checks TERM_PROGRAM environment variable (macOS/VSCode/Warp/Hyper)
3. Then checks TERMINAL environment variable
4. Finally tries terminals in priority order until one works

The detection now performs a test launch to verify the terminal can actually spawn before selecting it, ensuring more reliable terminal spawning. If no terminal is detected, it provides fallback mechanisms and error notifications. It always prioritize best graphical terminal emulators.

Configuration

Default Constants (Version 1.1)

• 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)
• Restart Backoff: Initial 1s, Maximum 60s (exponential backoff)
• Write Buffer: 1MB max per destination
• Protocol Version: 2025-06-18

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
    }
}

You can also specify a terminal explicitly:

"mcp": {
    "serencp": {
        "type": "local",
        "command": ["perl", "/path/to/serencp.pl","--terminal","wezterm"],
        "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 [email protected]
systemctl start [email protected]

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 supports real-time VM output streaming through MCP protocol notifications and resource subscriptions. This provides immediate feedback without requiring polling.

The MCP client should use the standard MCP resources/subscribe method to start receiving VM output notifications, and resources/unsubscribe to stop receiving them.

VM Output Notifications

VM output is automatically streamed as JSON-RPC 2.0 notifications using the MCP resources pattern:

{
    "jsonrpc": "2.0",
    "method": "notifications/resources/updated",
    "params": {
        "uri": "vm://<vm_name>/output",
        "content": "output data here",
        "stream": "stdout"
    }
}

Notification Parameters

• uri: Resource URI in the format vm://<vm_name>/output
• content: The actual output data (UTF-8 safe)
• stream: "stdout" or "stderr"

Benefits

• Real-time Feedback: VM output appears immediately without polling
• Efficient: Push-based model reduces overhead compared to polling
• UTF-8 Safe: Binary data is converted to UTF-8 with escaped representations for non-printable characters
• Backward Compatible: Existing read tool continues to work for pull-based access
• MCP Resources Pattern: Uses standardized notifications/resources/updated for VM output
• URI-based Access: VM output accessible via vm://<vm_name>/output resource URI

Log Level Control

The server supports configurable log levels: debug, info, and error. By default, debug logging is enabled.

Available Tools

All tools include enhanced MCP annotations for better UI integration (title, readOnlyHint, destructiveHint, idempotentHint, openWorldHint).

The server provides 5 tools for VM serial console management. Live VM output notifications are controlled via standard MCP resources/subscribe and resources/unsubscribe methods (not separate tools).

1. start

Starts the bridge for a specific VM. If a bridge already exists, it is restarted to ensure a clean slate with fresh exponential backoff state. 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}}
• Annotations: Non-destructive, idempotent, open world

2. status

Checks the status of the bridge.

• Arguments: {"vm_name": "string"}
• Returns: {"running": true/false, "vm_name": "...", "port": ..., "buffer_size": ...}
• Annotations: Read-only, closed world

3. read

Reads all available output from the VM serial console's dedicated output Unix socket with a 2-second timeout. Live output is also streamed via notifications.

• Arguments: {"vm_name": "string"}
• Returns: {"success": true, "output": "..."}
• Annotations: Read-only, open world

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 /"}}
• Annotations: Non-destructive, non-idempotent, open world

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": "..."}
• Annotations: Destructive (stops bridge), idempotent, closed world

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.

Exponential Backoff Restart

When a VM disconnects, the bridge now uses exponential backoff to prevent reconnection storms:

• Initial backoff: 1 second
• Maximum backoff: 60 seconds
• Per-VM state tracking: Each VM maintains its own backoff timer
• Backoff resets on successful connection

Non-Blocking Write System

Version 1.1 introduces a truly non-blocking write system with three modes:

• Mode 0 (Pure Non-Blocking): Returns immediately if would block
• Mode 1 (Buffered): Queues data if can't write immediately
• Mode 2 (Legacy): Retry with timeout (default for backward compatibility)
• Maximum write buffer: 1MB per destination
• Automatic buffer flushing in the event loop

Enhanced UTF-8 Handling

• Binary data from VM is converted to UTF-8 safely
• Non-printable characters are escaped for JSON transport
• Preserves data integrity while ensuring JSON compatibility

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.

Explicit Terminal Selection

You can explicitly specify which terminal to use:

./serencp.pl --terminal wezterm

This bypasses automatic detection and uses the specified terminal.

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

• No terminal window is opened: If the automatic terminal spawning fails, first check if graphical apps can be launched from a root terminal (e.g : pluma) Then check the following environment variables are properly set:

  • XAUTHORITY: Required for X11 authentication (e.g., /root/.Xauthority)
  • XDG_RUNTIME_DIR: Should be set to the user's runtime directory (e.g., /run/user/0)
  • DBUS_SESSION_BUS_ADDRESS: Required for D-Bus session communication

  To fix these issues, run the script from a proper X11 session where these variables are automatically set, or export them manually:

  export XAUTHORITY=/root/.Xauthority
  export XDG_RUNTIME_DIR=/run/user/0
  

• Terminal detection fails: Use the --terminal option to explicitly specify your terminal emulator:

  ./serencp.pl --terminal wezterm
  

• 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. Use standard MCP resources/subscribe method to subscribe to vm://<vm_name>/output resources.

• Socket Permission: Ensure /tmp is writable by the user running the MCP server.

• Syntax Check: Run perl -c serencp.pl to verify script integrity.

• The MCP server failed to start: Check that all required Perl modules are loaded. Run perl -c serencp.pl to verify syntax and module loading. If you see "Can't locate ... in @INC", install the missing module (e.g., cpan IO::Pty for non-core modules).

• Write buffer full: If you see "Write buffer full" warnings, the destination is not keeping up with data. This is normal during high-throughput scenarios and data will be dropped.

• Exponential backoff active: If the bridge keeps restarting, you'll see increasing delays between reconnection attempts (1s, 2s, 4s... up to 60s). This is intentional to prevent connection storms.

About

The name serencp is a play on words:

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

Feel free to contribute

Since it's a complex script, your help / pull requests are much appreciated !