Valksor Assern

A Go-based MCP (Model Context Protocol) aggregator with project-level configuration support. Assern combines multiple MCP servers into a unified interface while allowing different configurations per project.

Why Assern?

Assern solves the problem of managing multiple MCP servers across different projects and contexts.

Key benefits:

• Unified Interface – Single MCP client connection to all your servers
• Full MCP Protocol – Aggregates tools, resources, and prompts from all backends
• Tool Prefixing – All tools are namespaced by server name (github_search, jira_get_ticket) preventing conflicts
• Resource Prefixing – Resources use custom URI scheme (assern://server/original-uri)
• Prompt Prefixing – Prompts are namespaced like tools (assistant_code_review)
• Project Contexts – Different configurations per project (tokens, env vars, enabled servers)
• TOON Format – Token-optimized output format for LLM consumption (40-60% token reduction)
• Directory Matching - Auto-detect projects based on directory patterns
• Environment Merging - Configurable overlay or replace modes for env variables
• Tool Filtering – Expose only allowed tools per server for security and simplicity
• Instance Sharing – Prevents cascade spawning when LLMs call nested LLMs via shared Unix socket

Client Compatibility

Assern speaks standard stdio MCP protocol, so it works with any MCP-compatible client:

Note: If your client speaks stdio MCP, it works with Assern – no special integration needed.

How It Works

┌─────────────────────────────────────────────────────────────┐
│                                                             │
│  MCP Client (Claude, etc.)                                  │
│       │                                                     │
│       ▼                                                     │
│  ┌─────────────────────────────────────────┐                │
│  │           Assern Aggregator             │                │
│  │  • Tool/Resource/Prompt prefixing       │                │
│  │  • Project detection                    │                │
│  │  • Config merging                       │                │
│  └─────────────────────────────────────────┘                │
│       │                                                     │
│       ├─────────────────┬─────────────┬───────────────┐     │
│       ▼                 ▼             ▼               ▼     │
│  ┌─────────┐      ┌─────────┐   ┌─────────┐    ┌─────────┐  │
│  │ GitHub  │      │ Jira    │   │  Slack  │    │    ...  │  │
│  │ Server  │      │ Server  │   │ Server  │    │ Server  │  │
│  └─────────┘      └─────────┘   └─────────┘    └─────────┘  │
│                                                             │
└─────────────────────────────────────────────────────────────┘

1. Configure – Define MCP servers and projects in ~/.valksor/assern/config.yaml
2. Detect – Assern auto-detects your project from the current directory
3. Aggregate – All enabled servers are spawned; tools, resources, and prompts are prefixed
4. Route – Requests are routed to the appropriate backend server

Requirements

• Go 1.25+ (only if building from source – not needed for pre-built binaries)

Note: Individual MCP servers have their own requirements (npx/Node.js, API tokens, etc.). These are your responsibility to configure – Assern simply spawns the commands you define.

Features

• MCP Aggregation: Combine multiple MCP servers into one unified interface
• Full MCP Protocol: Aggregates tools, resources, and prompts from backend servers
• Multi-Transport: Support for stdio (local), HTTP, SSE, and OAuth-authenticated (remote) MCP servers
• Authentication: HTTP headers (API keys, Bearer tokens) and OAuth 2.0 with PKCE support
• Tool Prefixing: All tools are prefixed with server name (github_search, jira_get_ticket)
• Resource Prefixing: Resources use custom URI scheme (assern://github/file:///repo/README.md)
• Prompt Prefixing: Prompts are prefixed like tools (assistant_code_review)
• Project Contexts: Different configurations per project (tokens, env vars, servers)
• Directory Matching: Auto-detect projects based on directory patterns
• Environment Merging: Configurable overlay or replace modes for env variables
• Tool Filtering: Expose only allowed tools per server
• Instance Sharing: Prevents cascade spawning when nested LLMs launch assern
• Hot-Reload: Update configuration without restarting (assern reload or SIGHUP). Note: Connected clients (e.g., Claude Code) need to reconnect to see updated tools.

Instance Sharing

When LLMs use MCP servers that themselves call LLMs (creating nested chains), each nested invocation would normally spawn a new assern instance with all its MCP servers. This creates a cascade of redundant instances.

Assern solves this with instance sharing:

Primary Instance (first invocation):
┌─────────────┐
│ LLM (Claude)│
└──────┬──────┘
       │ stdio
┌──────▼──────┐
│assern       │
│ - ServeStdio│
│ - SocketSrv │◄───┐
└──────┬──────┘    │
       │           │ Unix Socket
┌──────▼──────┐    │
│ Aggregator  │    │
│ (MCP Servers)    │
└─────────────┘    │
                   │
Secondary Instance:│
┌─────────────┐    │
│ Nested LLM  │    │
└──────┬──────┘    │
       │ stdio     │
┌──────▼──────┐    │
│assern       │────┘
│ (proxy mode)│
└─────────────┘

• First invocation → becomes primary instance (starts aggregator + socket server)
• Subsequent invocations → detect socket, run as proxy to primary instance
• Socket location: ~/.valksor/assern/assern.sock
• Disable: Set ASSERN_NO_INSTANCE_SHARING=1 environment variable

Installation

Quick Install

curl -fsSL https://raw.githubusercontent.com/valksor/go-assern/master/install.sh | bash

The install script automatically:

• Detects your OS and architecture (Linux/macOS, AMD64/ARM64)
• Finds the best install location (~/.local/bin, ~/bin, or /usr/local/bin)
• Verifies checksums (and Cosign signatures if available)
• Provides shell-specific PATH instructions

Install options:

# Install specific version
curl -fsSL ... | bash -s -- -v v1.0.0

# Install nightly build
curl -fsSL ... | bash -s -- --nightly

# Show help
bash install.sh --help

Go Install

go install github.com/valksor/go-assern/cmd/assern@latest

Docker

Build and run with Docker Compose:

# Create config directory
mkdir -p ~/.valksor/assern

# Add your mcp.json configuration
cat > ~/.valksor/assern/mcp.json << 'EOF'
{
  "mcpServers": {
    "github": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-github"],
      "env": {
        "GITHUB_TOKEN": "${GITHUB_TOKEN}"
      }
    }
  }
}
EOF

# Start with docker-compose
docker-compose up -d

Or build and run manually:

docker build -t assern .
docker run -d --name assern-mcp \
  -v ~/.valksor/assern:/home/assern/.valksor/assern:ro \
  assern

See docs/docker.md for complete Docker documentation.

Manual Download

Download the latest release for your platform:

Download from GitHub Releases:

# Download and install (example for macOS ARM64)
curl -L https://github.com/valksor/go-assern/releases/latest/download/assern-darwin-arm64 -o assern
chmod +x assern
sudo mv assern /usr/local/bin/

# Verify
assern version

Quick Start

1. Initialize configuration:

assern config init

1. Add MCP servers interactively OR edit config files manually:

Option A: Interactive CLI (Recommended)

# Add a server with guided prompts
assern mcp add

# List all configured servers
assern mcp list

# Edit an existing server
assern mcp edit github

# Delete servers
assern mcp delete

Option B: Manual Configuration

Add servers to ~/.valksor/assern/mcp.json (local or remote):

{
  "mcpServers": {
    "github": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-github"],
      "env": {
        "GITHUB_TOKEN": "${GITHUB_TOKEN}"
      }
    },
    "filesystem": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-filesystem"]
    },
    "context7": {
      "url": "https://mcp.context7.com/mcp"
    },
    "api-with-key": {
      "url": "https://api.example.com/mcp",
      "headers": {
        "Authorization": "Bearer ${API_TOKEN}"
      }
    }
  }
}

1. Configure projects in ~/.valksor/assern/config.yaml:

projects:
  work:
    directories:
      - ~/work/*
    env:
      GITHUB_TOKEN: "${WORK_GITHUB_TOKEN}"
    servers:
      filesystem:
        allowed:
          - read_file
          - list_directory

settings:
  log_level: info
  timeout: 60s

1. Start the aggregator:

cd ~/work/myproject
assern serve

Documentation

Full documentation available at valksor.com/docs/assern

• Quick Start
• Docker Setup - Run Assern in containers
• Integration - Claude Desktop, Claude Code, IDEs
• Configuration
• Projects
• Servers
• Concepts
• Troubleshooting

CLI Commands

Note: All commands support colon notation for faster typing (e.g., mcp:add, config:init, list:servers).

Global Flags

Configuration

Global MCP Servers (~/.valksor/assern/mcp.json)

Standard MCP format - copy-paste from Claude Desktop or any MCP example.

Global Config (~/.valksor/assern/config.yaml)

Defines project registry with directory patterns, settings, and server overrides.

Local MCP Servers (.assern/mcp.json)

Optional project-specific MCP servers.

Local Config (.assern/config.yaml)

Optional project-level overrides in any directory.

Environment Variables

• Global: ~/.valksor/assern/.env
• Project: .assern/.env

Variables can use ${VAR} syntax for expansion.

Project Detection

1. Check for .assern/config.yaml in current or parent directories
2. Match against projects[*].directories patterns in global config
3. Auto-detect from directory name (e.g., my-repo from /path/to/my-repo)
4. Use explicit --project flag to override

Works in any directory without configuration - the directory name becomes the project name.

Development

make build        # Build binary to ./build/assern
make install      # Install to $GOPATH/bin
make test         # Run tests with coverage
make coverage     # Generate coverage report
make quality      # Run golangci-lint + govulncheck
make fmt          # Format code (gofmt, goimports, gofumpt)
make tidy         # Tidy dependencies
make hooks        # Enable versioned git hooks
make lefthook     # Install pre-commit hooks (auto-format + lint)

CI/CD: PRs trigger lint/test/build via GitHub Actions. Releases use GoReleaser.

See CONTRIBUTING.md for development guidelines.

License

MIT License – see LICENSE for details.