Blast Radius Control

• Introduction
• Tag-Based Filtering
• The tags() Method
• Runtime Tag Filtering
• Use Cases

Introduction

In a large MCP server with 100+ tools, you don't always want the agent to see everything. A billing agent shouldn't see user management tools. A read-only dashboard agent shouldn't see mutations. Blast Radius Control uses tags to filter which tools are visible to each agent session.

Tag-Based Filtering

Tag your tools during definition:

const f = initFusion<AppContext>();

export const listInvoices = f.query('billing.list')
  .describe('List invoices')
  .tags('billing', 'read')
  .returns(InvoicePresenter)
  .handle(async (input, ctx) => {
    return ctx.db.invoices.findMany();
  });

export const chargeInvoice = f.mutation('billing.charge')
  .describe('Charge an invoice')
  .tags('billing', 'write', 'destructive')
  .withString('id', 'Invoice ID')
  .handle(async (input, ctx) => {
    await ctx.db.invoices.charge(input.id);
    return { charged: true };
  });

export const listUsers = f.query('users.list')
  .describe('List users')
  .tags('users', 'read', 'admin')
  .returns(UserPresenter)
  .handle(async (input, ctx) => {
    return ctx.db.users.findMany();
  });

The tags() Method

.tags() accepts any number of string arguments. Tags are metadata — they don't affect tool behavior, only visibility:

f.query('projects.list')
  .tags('projects', 'read', 'v1')    // Multiple tags

Runtime Tag Filtering

Filter tools at server attachment time based on session context:

registry.attachToServer(server, {
  contextFactory: createContext,
  toolFilter: (tool, session) => {
    // Billing agent: only billing tools
    if (session.agentType === 'billing') {
      return tool.tags.includes('billing');
    }

    // Read-only agent: no write tools
    if (session.agentType === 'readonly') {
      return tool.tags.includes('read');
    }

    // Admin: everything
    return true;
  },
});

The toolFilter runs during tools/list — filtered tools are invisible to the agent. They don't appear in the tool list and can't be called.

Use Cases

Agent Type	Tag Filter	Effect
Billing agent	billing	Sees only invoice/payment tools
Dashboard agent	read	Sees only query tools, no mutations
Admin agent	No filter	Sees everything
External agent	public	Sees only public-facing tools
Debug agent	internal	Sees monitoring and debug tools

IMPORTANT

Tag filtering is a presentation-layer control. It hides tools from the agent's view but doesn't prevent direct tools/call invocations if the agent guesses the tool name. For true authorization, combine tag filtering with Authentication Middleware.