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@hmrc-sync/agent-skill

HMRC Fraud Prevention Headers SDK and API for generating, validating, and submitting HMRC-compliant headers.

When to use this package vs engine/collector

Use @hmrc-sync/agent-skill when:

  • You want a simple one-function API (prepareHmrcRequest) that handles the entire workflow
  • You need built-in guardrails and policy enforcement
  • You're integrating with AI agents or MCP-native clients
  • You want human-readable validation explanations

Use @hmrc-sync/engine + @hmrc-sync/collector when:

  • You need fine-grained control over each step
  • You want to collect client data separately from header generation
  • You're building custom orchestration logic

1. SDK Usage

Installation

pnpm add @hmrc-sync/agent-skill

Core contract

import { prepareHmrcRequest, ConnectionMethod } from '@hmrc-sync/agent-skill'

// In production, collect client data using @hmrc-sync/collector
// For this example, we provide mock data
const clientData = {
  userAgent: 'Mozilla/5.0',
  browserJsUserAgent: 'Mozilla/5.0',
  timezone: 'UTC+00:00',
  localIPs: ['192.168.1.10'],
  publicIP: '203.0.113.10',
  macAddresses: ['00:11:22:33:44:55'],
  deviceId: '550e8400-e29b-41d4-a716-446655440000'
}

const result = prepareHmrcRequest({
  method: ConnectionMethod.WEB_APP_VIA_SERVER,
  clientData,
  serverIP: '203.0.113.6', // Your server's public IP
  serverPort: 8443, // Your server's port
  vendorConfig: {
    productName: 'MyTaxProduct',
    version: { MyTaxProduct: '1.0.0' }
  },
  requestMeta: {
    executionContext: 'backend',
    submissionTarget: 'hmrc',
    integrationType: 'web'
  },
  explainMode: true
})

console.log(result.headers)
console.log(result.validation)
console.log(result.nextActions)

Tool catalog

  • hmrc.collect_client_data(environment)
  • hmrc.generate_headers(...)
  • hmrc.validate_headers(...)
  • hmrc.build_submission(...)

Use createHmrcMcpServer() to expose tool definitions and an executeTool(...) handler.

MCP stdio server (for chat interfaces)

This package also provides a real MCP stdio transport server for MCP-native clients like Cursor, Windsurf, or Claude Desktop.

Note: The stdio server requires the package to be built first.

In a monorepo (this repo):

pnpm --filter @hmrc-sync/agent-skill build
pnpm --filter @hmrc-sync/agent-skill mcp:stdio

After npm install (standalone):

npx hmrc-agent-skill-mcp

Programmatic start:

import { startHmrcMcpStdioServer } from '@hmrc-sync/agent-skill'

await startHmrcMcpStdioServer()

2. HTTP API Usage

API endpoint

Deploy the HTTP API to your own infrastructure (Railway, Vercel, or any Node.js hosting).

Set the API_URL environment variable to your deployment URL:

API_URL=https://your-deployment-url.com

Authentication

To use the HTTP API, you need an API key. Contact the service administrator to obtain your API key, then include it in the request header:

x-api-key: YOUR_API_KEY

Or use OAuth bearer token:

Authorization: Bearer YOUR_TOKEN

Endpoints

Get available tools

curl $API_URL/v1/mcp/tools

Execute a tool

curl -X POST $API_URL/v1/mcp/execute \
  -H "Content-Type: application/json" \
  -H "x-api-key: YOUR_API_KEY" \
  -d '{
    "tool": "hmrc.validate_headers",
    "input": {
      "method": "WEB_APP_VIA_SERVER",
      "headers": {
        "Gov-Client-Connection-Method": "WEB_APP_VIA_SERVER",
        "Gov-Client-Timezone": "UTC+00:00"
      }
    }
  }'

Example integration (JavaScript)

const API_URL = process.env.API_URL || 'http://localhost:3001'
const API_KEY = process.env.API_KEY || 'your-api-key'

async function validateHeaders(headers) {
  const response = await fetch(`${API_URL}/v1/mcp/execute`, {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
      'x-api-key': API_KEY
    },
    body: JSON.stringify({
      tool: 'hmrc.validate_headers',
      input: {
        method: 'WEB_APP_VIA_SERVER',
        headers: headers
      }
    })
  })

  const result = await response.json()
  return result
}

Features

  • High-level orchestration via prepareHmrcRequest(...)
  • MCP-style tool handlers for collection, generation, validation, and submission planning
  • Built-in guardrails for risky/invalid flows
  • Explain mode for human-readable remediation guidance
  • Starter templates for common integration intents

Guardrails

The skill enforces non-negotiable checks:

  • *_VIA_SERVER methods require serverIP and serverPort
  • Browser-originated direct HMRC submissions are blocked by policy checks
  • Invalid method values are rejected before execution

Explain mode

When explainMode: true, responses include a concise human explanation of:

  • what failed
  • why it failed
  • what to change next

Starter templates

skillTemplates includes built-in intent templates:

  • Set up web via server
  • Set up desktop direct
  • Validate before submit