mcpd Project Overview

mcpd is an elastic control plane and runtime for Model Context Protocol (MCP) servers. It provides on-demand startup, scale-to-zero hibernation, unified routing, and intelligent tool filtering for managing multiple MCP servers locally.

Key Capabilities

• Elastic runtime with automatic instance lifecycle management
• Unified gateway (mcpvmcp) providing single entry point for all MCP servers
• Smart SubAgent powered by CloudWeGo/Eino for context-aware tool filtering
• GUI support (mcpvui) built with Wails 3 for configuration and monitoring
• Profile-based configuration with tag-based visibility
• Native observability with Prometheus metrics and Grafana dashboards

Architecture

Three-layer architecture for maximum decoupling:

1. Core (mcpv): Central control plane managing instance lifecycles, scheduling, and aggregation
2. Gateway (mcpvmcp): Protocol bridge acting as standard MCP server for AI clients
3. App (mcpvui): Wails-driven GUI for configuration, monitoring, and core lifecycle hosting

Internal Structure

• internal/domain/: Domain models and interfaces (ServerSpec, Instance, Transport, Scheduler, Router, etc.)
• internal/app/: Application glue layer with explicit subpackages:
  • controlplane/: Control plane facade, registry/discovery/observability/automation, reload application
  • bootstrap/: Bootstrap and server initialization management
  • catalog/: Catalog providers and state loading
  • runtime/: Runtime indexes and aggregation state
• internal/infra/: Infrastructure adapters organized by concern:
  • catalog/: Configuration loading and validation
  • scheduler/: Instance lifecycle and capacity constraints coordination
  • lifecycle/: Instance startup, handshake, and shutdown management
  • router/: Request routing to managed MCP server instances
  • transport/: stdio and streamable HTTP transport implementations
  • aggregator/: Tool/resource/prompt aggregation and indexing
  • gateway/: MCP gateway client manager and registry
  • subagent/: Eino-based intelligent tool filtering
  • telemetry/: Prometheus metrics and health checks
  • rpc/: gRPC server and client for core-gateway communication
• internal/ui/: Wails bridge layer exposing core functionality to GUI

Configuration

Configuration is YAML-based with a single file per profile. See docs/catalog.example.yaml or runtime.yaml for reference.

Key Configuration Sections:

• servers[]: MCP server definitions with transport, command, tags, lifecycle settings
• subAgent: Eino-based tool filtering configuration (model, provider, API key, enabled tags)
• observability: Prometheus metrics endpoint configuration
• rpc: gRPC server configuration for core-gateway communication
• Runtime settings: timeouts, concurrency limits, retry policies, bootstrap behavior

Server Configuration Example:

yaml
1servers:
2  - name: my-server
3    transport: stdio
4    cmd: [node, ./path/to/server.js]
5    tags: [chat]
6    idleSeconds: 60
7    maxConcurrent: 1
8    strategy: stateless
9    activationMode: on-demand
10    protocolVersion: "2025-11-25"

Profile System:

• Multiple profiles supported with caller-to-profile mapping
• Default profile used as fallback
• Tag-based visibility: clients see only servers matching their tags

Transport Types and Activation Modes

Transport Types

stdio (most common):

• Launches subprocess and communicates via stdin/stdout
• Suitable for Node.js, Python, and other scripting languages
• Process lifecycle managed by mcpv

streamable_http:

• Connects to HTTP endpoint with Server-Sent Events (SSE) streaming
• Suitable for long-running HTTP services
• No process management needed

composite:

• Advanced use case combining multiple transports
• Rarely needed

Activation Modes

on-demand (default):

• Instances start when first request arrives
• Ideal for resource efficiency
• Cold start latency on first request

always-on:

• Keeps minReady instances running at all times
• Use for latency-sensitive servers
• Higher resource usage

disabled:

• Server defined but not activated
• Useful for testing or staged rollouts

Server Strategies

stateless:

• Requests can be routed to any available instance
• Instances can be freely started/stopped
• Most common and efficient

stateful:

• Router maintains sticky sessions
• Requests from same client go to same instance
• Session expires after client inactivity timeout

Gateway Behavior

The mcpvmcp gateway:

• Registers with core on startup via gRPC
• Maintains persistent connection to core
• Translates MCP JSON-RPC to core's internal protocol
• Handles tool/resource/prompt aggregation
• Supports both single-server and tag-based modes

Flags

• --config: Specify config directory (default: current directory)
• --rpc: Override RPC address for gateway
• --tag: Filter servers by tag in gateway mode
• --server: Single-server mode for gateway

Key Technical Details

Hot Reload

The core watches the config directory for changes and automatically reloads:

• Server spec additions/removals
• Configuration updates
• Profile changes
• Active instances are gracefully drained before applying changes

Concurrency Control

• maxConcurrent: Maximum parallel requests per instance
• minReady: Minimum instances to keep warm
• Scheduler spawns new instances when capacity is reached
• Instances are reused across requests (connection pooling)

Tool Namespace Strategy

To avoid tool name collisions across servers:

• prefix: Tools named as server__toolname (default)
• suffix: Tools named as toolname__server

Bootstrap Modes

• metadata: Only fetch server metadata (tools/resources/prompts) on startup
• full: Perform full initialization including handshake
• none: Skip bootstrap entirely

Scale-to-Zero Runtime

Instances are launched on-demand and shut down after idle timeout:

• Scheduler manages instance pools with concurrency limits
• Router maintains session affinity for stateful servers
• Lifecycle manager handles startup, handshake, and graceful shutdown
• Probe system monitors instance health

Tool Aggregation

The aggregator maintains a unified index across all active instances:

• Automatic refresh on list_changed notifications
• Concurrent refresh workers with rate limiting
• Namespace strategy (prefix or suffix) to avoid tool name collisions
• Supports tools, resources, and prompts

Version Information

• Go: 1.25+
• Wails: 3.0.0-alpha.53
• MCP SDK: github.com/modelcontextprotocol/go-sdk v1.2.0
• Eino: github.com/cloudwego/eino v0.7.15
• Prometheus: github.com/prometheus/client_golang v1.23.2

Additional Resources

• MCP Specification: https://modelcontextprotocol.io/
• Wails Documentation: https://wails.io/
• Wire Documentation: https://github.com/google/wire
• CloudWeGo Eino: https://github.com/cloudwego/eino
• Frontend Guide: See frontend/CLAUDE.md for React/TypeScript frontend development