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tool-filter-mcp

npm version

MCP proxy server that filters tools from upstream MCP servers via regex-based deny list.

Maintained: ✅ Yes (Fall 2025)

Why use this?

For effective context engineering, we want to minimize useless tokens. Most major agents right now (e.g. Claude Code) do NOT remove tool descriptions from the context. Even though the tool is completely denied and unused, the model will still get its entire description and still try to call the tool (and get error messages). For big MCPs, such as github, supabase, jetbrains IDE, atlassian, this is catastrophic to the context and leads to context pollution by 40-60k of useless tokens. You wanted your agent to be able to see jira ticket descriptions? Please also have these 44 useless tools to edit assignees on confluence pages.

This MCP completely solves the issue without introducing any overhead. This project is fully vibe-coded with claude. Contributions welcome!

Features

  • Tool Filtering: Block specific tools using regex patterns
  • Tool Discovery: List available tools with multiple output formats (table, JSON, names)
  • Header Pass-Through: Add custom HTTP headers for authentication
  • Zero Latency: Cached tool list, minimal overhead
  • Fail-Fast: Immediate error on connection issues or invalid patterns
  • Transparent Proxying: Forwards allowed tool calls to upstream without modification

Installation

npx @respawn-app/tool-filter-mcp --upstream <url> --deny <patterns>

Usage

Basic Example

Filter tools matching .*_file$ pattern:

npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --deny ".*_file$"

Multiple Patterns

Use comma-separated patterns:

npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --deny "get_file_text,create_new_file,replace_text"

With Authentication Headers

Add custom headers for authentication:

npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --header "Authorization: Bearer your-token-here" \
  --header "X-API-Key: your-api-key"

Headers support environment variable expansion (if not yet expanded by your app):

npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --header "Authorization: Bearer $AUTH_TOKEN"

List Available Tools

Discover what tools are available before setting up filters:

# Default table format
npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --list-tools

# Get comma-separated names for copy-paste
npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --list-tools --format=names

# Get full JSON with schemas
npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --list-tools --format=json

# Preview filtered results
npx @respawn-app/tool-filter-mcp \
  --upstream http://localhost:3000/sse \
  --deny ".*_file$" \
  --list-tools --format=names

With Claude Code

Add to your .mcp.json:

{
  "mcpServers": {
    "filtered-server": {
      "command": "npx",
      "args": [
        "@respawn-app/tool-filter-mcp",
        "--upstream",
        "http://localhost:3000/sse",
        "--deny",
        "dangerous_tool_1,dangerous_tool_2"
      ],
      "type": "stdio"
    }
  }
}

With authentication headers (supports environment variable expansion):

{
  "mcpServers": {
    "filtered-server": {
      "command": "npx",
      "args": [
        "@respawn-app/tool-filter-mcp",
        "--upstream",
        "http://localhost:3000/sse",
        "--header",
        "Authorization: Bearer ${API_TOKEN}",
        "--header",
        "X-Custom-Header: $CUSTOM_VALUE"
      ],
      "type": "stdio"
    }
  }
}

CLI Options

  • --upstream <url> (required): Upstream MCP server URL (SSE transport)
  • --deny <patterns>: Comma-separated regex patterns for tools to filter
  • --header <name:value>: Custom HTTP header to pass to upstream server (can be repeated for multiple headers)
    • Format: --header "Header-Name: value"
    • Supports environment variable expansion: $VAR or ${VAR}
    • Example: --header "Authorization: Bearer $TOKEN"
  • --list-tools: List available tools from upstream and exit (does not start proxy server)
  • --format <type>: Output format for --list-tools (default: table)
    • table: Human-readable table with descriptions (truncated to 100 chars)
    • json: Full JSON output with complete schemas
    • names: Comma-separated tool names for easy copy-paste

Requirements

  • Node.js >= 20.0.0
  • Upstream MCP server with SSE transport

License

MIT