Add tool filtering support with --ignore-tool flag

[JonDotsoy]

Overview

This PR implements a comprehensive tool filtering mechanism that allows users to selectively ignore specific tools from remote MCP servers using the --ignore-tool command line flag.

Changes Made

🚀 New Features

1. Tool Filtering with --ignore-tool flag

   • Added support for multiple --ignore-tool flags to specify tools that should be filtered out
   • Supports wildcard patterns using * for flexible matching:
     • delete* - ignores all tools starting with "delete"
     • *account - ignores all tools ending with "account"
     • exactTool - ignores only the tool named exactly "exactTool"

2. Bidirectional Filtering

   • Response Filtering: Automatically filters ignored tools from tools/list responses
   • Request Blocking: Prevents tools/call requests for ignored tools and returns appropriate error responses

3. Pattern Matching Engine

   • Implemented patternToRegex() function to convert glob patterns to regular expressions
   • Added shouldIncludeTool() function for efficient pattern matching
   • Case-insensitive matching for better usability

🔧 Implementation Details

Command Line Parsing (src/lib/utils.ts)

• Extended parseCommandLineArgs() to parse multiple --ignore-tool flags
• Added validation and logging for ignored tool patterns
• Returns ignoredTools array in the configuration object

Message Transformation (src/lib/utils.ts)

• Created createMessageTransformer() utility for intercepting and transforming MCP messages
• Implemented request interception to block calls to ignored tools
• Implemented response transformation to filter tools from tools/list responses
• Added proper error handling with JSON-RPC compliant error responses

Proxy Integration (src/proxy.ts)

• Updated runProxy() function to accept and pass through ignoredTools parameter
• Integrated tool filtering into the MCP proxy workflow

MCP Proxy Enhancement (src/lib/utils.ts)

• Enhanced mcpProxy() function to support tool filtering
• Added message blocking mechanism using symbols for type safety
• Proper error response generation for blocked tool calls

📚 Documentation Updates

Updated README.md with comprehensive usage examples:

• Basic usage with single and multiple --ignore-tool flags
• Wildcard pattern examples with clear explanations
• JSON configuration examples for MCP client setups

🧪 Comprehensive Test Suite

Added extensive test coverage in src/lib/utils.test.ts:

Command Line Parsing Tests (18 scenarios)

• Basic server URL parsing
• Port and transport strategy parsing
• Custom headers and host configuration
• Single and multiple ignored tool parsing
• Mixed argument parsing validation

Tool Filtering Tests (6 scenarios)

• Wildcard pattern matching (create*, *account)
• Multiple pattern support
• Exact match scenarios
• Empty pattern handling
• Non-matching pattern behavior

MCP Proxy Tests (8 scenarios)

• Message proxying between client and server
• Transport lifecycle management
• Tool filtering in tools/list responses
• Request blocking for ignored tools
• Error response generation

Usage Examples

Basic Usage

npx mcp-remote https://remote.mcp.server/sse --ignore-tool deleteTask

Multiple Patterns

npx mcp-remote https://remote.mcp.server/sse --ignore-tool "delete*" --ignore-tool "remove*"

MCP Client Configuration

{
  "args": [
    "mcp-remote",
    "https://remote.mcp.server/sse",
    "--ignore-tool",
    "delete*",
    "--ignore-tool", 
    "remove*"
  ]
}

Manual Testing Checklist

✅ VS Code Integration

• Test MCP client configuration with ignored tools
• Verify filtered tools don't appear in tool lists
• Confirm blocked tools return appropriate errors
• Validate wildcard patterns work correctly

✅ Command Line Interface

• Test single --ignore-tool flag parsing
• Test multiple --ignore-tool flags parsing
• Test wildcard patterns (delete*, *account, exact)
• Test mixed arguments with other flags
• Verify logging output shows ignored tools

Test Execution

Run the unit tests to validate all functionality:

pnpm run test:unit

Expected results:

• ✅ All command line parsing tests pass (18/18)
• ✅ All tool filtering tests pass (6/6)
• ✅ All MCP proxy tests pass (8/8)
• ✅ Total: 32 test scenarios passing

Breaking Changes

None. This is a backward-compatible addition that doesn't affect existing functionality when no --ignore-tool flags are provided.

Security Considerations

• Tool filtering happens at the proxy level, providing an additional security layer
• Blocked tools return proper JSON-RPC error responses instead of potentially leaking information
• Pattern matching uses escaped regex to prevent injection attacks

[pkg-pr-new]

Open in StackBlitz

npx https://pkg.pr.new/mcp-remote@137

commit: 94059ce

[geelen]

Nice idea! I'm on parental leave at the moment so I don't have much capacity to test this out, but given that it's additive, I'm going to merge & release it as-is

[geelen]

Released as v0.1.22