mcp-discovery

MCP Discovery Policy Examples

This example demonstrates different discovery policies for controlling how MCP tools and resources are exposed through discovery endpoints.

Discovery Policies

1. DiscoveryCount - Only shows counts, no tool names
2. DiscoveryAuthenticated - Shows full list only with Authorization header
3. DiscoveryPublic - Shows all discoverable tools (default)
4. DiscoveryNone - Hides all tool information
5. Custom Filter - Context-aware filtering based on request

Running the Example

go run main.go

This starts three servers demonstrating different policies:

• Port 8081: Count-only policy
• Port 8082: Authenticated policy
• Port 8083: Custom IP-based filter

Testing

Server 1: Count-only (port 8081)

curl http://localhost:8081/.well-known/mcp.json | jq '.capabilities.tools'
# Output: {"supported": true, "count": 2}

Server 2: Authenticated (port 8082)

# Without auth - only counts
curl http://localhost:8082/.well-known/mcp.json | jq '.capabilities.tools'
# Output: {"supported": true, "count": 2}

# With auth - full list
curl -H "Authorization: Bearer token" http://localhost:8082/.well-known/mcp.json | jq '.capabilities.tools'
# Output: {"supported": true, "count": 2, "available": ["public_info"]}
# Note: secret_operation is hidden because IsDiscoverable() returns false

Server 3: Custom filter (port 8083)

# From localhost - see non-secret tools
curl http://localhost:8083/.well-known/mcp.json | jq '.capabilities.tools'
# Output: {"supported": true, "count": 3, "available": ["public_info", "admin_tool"]}

Key Concepts

1. IsDiscoverable() - Tools can opt out of discovery by implementing this method
2. Discovery Filter - Custom logic for context-aware filtering
3. RBAC Compatible - Filters can decode JWT tokens from Authorization headers
4. Default Behavior - Tools without IsDiscoverable() default to being discoverable

Security Notes

• Dev tools like server_control are automatically hidden in production
• Tools prefixed with internal_ or _ are always hidden
• Custom filters can implement complex RBAC logic
• Discovery policies work independently of actual tool access control