MCP Tool Filtering

Overview

MCP Tool Filtering allows you to control which tools are available to AI models on a per-request basis using Virtual Keys (VKs). By configuring a VirtualKey, you can create a strict allow-list of MCP clients and tools, ensuring that only approved tools can be executed.

Make sure you have at least one MCP client set up. Read more about it here.

What it does

A Virtual Key’s MCP configuration controls exactly which tools requests using that key can execute:

• Leave the Virtual Key’s MCP configuration empty (default) - All tools from all enabled MCP clients are available. In this state, a caller can still narrow tools for a single request by passing the x-bf-mcp-include-tools header.
• Configure MCP clients on the Virtual Key - The key’s configuration becomes a strict allow-list and takes precedence over any x-bf-mcp-include-tools header sent with the request. Use this to guarantee that a key can only ever reach the tools you approved.

When you do configure MCP clients on a Virtual Key, set the allowed tools for each client:

• Select specific tools - Only the chosen tools from that client are available.
• Use the * wildcard - All available tools from that client are permitted.
• Leave the tool list empty - All tools from that client are blocked.
• Do not add a client at all - Every tool from that client is blocked (once any other client is configured).

Setting MCP Tool Restrictions

You can configure which tools a Virtual Key has access to via the Web UI.

1. Go to Virtual Keys page.
2. Create/Edit virtual key
3. In MCP Client Configurations section, add the MCP client you want to restrict the VK to
4. Add the tools you want to restrict the VK to, or leave it blank to allow all tools for this client
5. Click on the Save button

For example, to allow only the check-status tool from billing-client while allowing all tools from support-client, add both clients in the MCP Client Configurations section, set billing-client’s allowed tools to check-status, and leave support-client’s tool list empty (or use the * wildcard) to permit all of its tools. Any other MCP client is then implicitly blocked for this key.

Example Scenario

Available MCP Clients & Tools:

• billing-client: with tools [create-invoice, check-status]
• support-client: with tools [create-ticket, get-faq]

VK with Full Access

Configuration:

• billing-client -> Allowed Tools: [*] (wildcard)
• support-client -> Allowed Tools: [*] (wildcard)

Result: A request with this Virtual Key can access all four tools: create-invoice, check-status, create-ticket, and get-faq.

VK with Partial Access

Configuration:

• billing-client -> Allowed Tools: [check-status]
• support-client -> Not configured

Result: A request with this Virtual Key can only access the check-status tool. All other tools are blocked.

VK with No Tools

Configuration:

• billing-client -> Allowed Tools: [] (empty list)

Result: A request with this Virtual Key cannot access any tools. All tools from all clients are blocked.

Note

When a Virtual Key has MCP configurations, its tool rules are always enforced and override any x-bf-mcp-include-tools header you send with the request. You can still use the x-bf-mcp-include-clients header to further narrow which MCP clients a single request may use.