TAP-MCP Bridge

Rust library and MCP server for Visa's Trusted Agent Protocol (TAP), enabling AI agents to securely authenticate with merchants and execute payment transactions.

Workspace Structure

Crate	Type	Description
tap-mcp-bridge	Library	RFC 9421 signatures, JWE encryption, TAP protocol
tap-mcp-server	Binary	MCP server exposing TAP tools for Claude and other AI agents

Installation

As a Library

[dependencies]
tap-mcp-bridge = "0.2"

As MCP Server

cargo install --path tap-mcp-server

Configure your MCP client (Claude Desktop, etc.):

{
  "mcpServers": {
    "tap": {
      "command": "tap-mcp-server",
      "env": {
        "TAP_AGENT_ID": "your-agent-id",
        "TAP_AGENT_DIRECTORY": "https://your-agent-directory.com",
        "TAP_SIGNING_KEY": "64-hex-characters-ed25519-key"
      }
    }
  }
}

Important

Requires Rust 1.85+ (Edition 2024).

Quick Example

use ed25519_dalek::SigningKey;
use tap_mcp_bridge::tap::{InteractionType, TapSigner};

let signing_key = SigningKey::from_bytes(&[0u8; 32]);
let signer = TapSigner::new(signing_key, "agent-123", "https://agent.example.com");

let signature = signer.sign_request(
    "POST",
    "merchant.example.com",
    "/checkout",
    b"request body",
    InteractionType::Checkout,
)?;

println!("Signature: {}", signature.signature);
println!("Signature-Input: {}", signature.signature_input);

MCP Tools

The server exposes tools for AI agents:

Tool	Description
checkout_with_tap	Execute payment with TAP authentication
browse_merchant	Browse merchant catalog with verified identity
verify_agent_identity	Health check and agent verification
get_products	Browse product catalog with filters
get_product	Get single product details
add_to_cart	Add item to shopping cart
get_cart	Get current cart state
update_cart_item	Update item quantity
remove_from_cart	Remove item from cart
create_order	Create order from cart
get_order	Get order status
process_payment	Complete payment with APC encryption

Features

TAP Protocol

• RFC 9421 HTTP Message Signatures with Ed25519
• RFC 7516 JWE encryption for payment data (A256GCM + RSA-OAEP-256)
• RFC 7638 JWK Thumbprints for key identification
• ID Tokens (JWT) for consumer authentication
• ACRO — Agentic Consumer Recognition Object
• APC — Agentic Payment Container with JWE encryption

Merchant Abstraction

Flexible merchant API integration with trait-based abstraction:

use tap_mcp_bridge::{DefaultMerchant, MerchantApi};

// Standard TAP merchant
let merchant = DefaultMerchant::new();

// Custom merchant from TOML configuration
let merchant = DefaultMerchant::from_toml(r#"
    name = "ACME Store"
    base_url = "https://api.acme.com"
    api_prefix = "/api/v2"

    [endpoints]
    products = "/catalog/items"
    cart = "/basket"

    [field_mappings.request]
    consumer_id = "customerId"
    product_id = "sku"
"#)?;

Tip

See examples/merchants/ for TOML configuration examples.

Transport Abstraction

Pluggable transport layer supporting multiple protocols:

use tap_mcp_bridge::transport::{HttpTransport, HttpConfig, HttpVersion};

// Default HTTP transport with connection pooling
let transport = HttpTransport::new();

// HTTP/2 with custom configuration
let config = HttpConfig {
    http_version: HttpVersion::Http2,
    timeout_secs: 60,
    pool_max_idle_per_host: 50,
    ..Default::default()
};
let transport = HttpTransport::with_config(&config)?;

Supported protocols:

• HTTP/1.1 (default)
• HTTP/2 with multiplexing
• HTTP/3 (QUIC) — planned
• gRPC — planned
• JSON-RPC — planned

Production Features

• Retry with backoff — Exponential backoff with jitter for transient failures
• Circuit breaker — Protection against cascading failures
• Rate limiting — Token bucket algorithm for request throttling
• Audit logging — Structured security events with sensitive data redaction
• Prometheus metrics — Request counters, error rates, latency tracking
• Replay protection — UUID v4 nonce with LRU cache validation

Security

• HTTPS enforcement (HTTP URLs rejected)
• Localhost/loopback blocking
• Path traversal prevention
• CRLF header injection prevention
• Field mapping injection protection
• Timeout bounds validation

Examples

# Basic checkout flow
cargo run --example basic_checkout

# Full e-commerce flow (products → cart → order → payment)
cargo run --example full_checkout_flow

# Browse merchant catalog
cargo run --example browse_catalog

# Error handling patterns
cargo run --example error_handling

# TAP signature generation
cargo run --example signature_generation

# JWKS for agent directory
cargo run --example jwks_generation

# ID Token (JWT) generation
cargo run --example id_token_generation

# ACRO generation
cargo run --example acro_generation

# APC encryption/decryption
cargo run --example apc_generation

Tip

Set AGENT_SIGNING_KEY environment variable before running examples:

export AGENT_SIGNING_KEY=$(openssl rand -hex 32)

Configuration

Merchant Configuration (TOML)

name = "My Merchant"
base_url = "https://api.merchant.com"
api_prefix = "/api/v1"

[endpoints]
products = "/products"
cart = "/cart"
checkout = "/checkout"

[field_mappings.request]
consumer_id = "customer_id"
product_id = "item_id"

[field_mappings.response]
customer_id = "consumer_id"

[auth]
type = "api_key"
header = "X-API-Key"
env_var = "MERCHANT_API_KEY"

pagination = "page_based"  # or "offset_based", "cursor_based"

Transport Configuration (TOML)

[transport]
protocol = "http2"

[transport.http]
timeout_secs = 30
connect_timeout_secs = 10
pool_max_idle_per_host = 100
http_version = "http2"  # or "http1", "auto"

Documentation

Resource	Description
API Reference	Complete API documentation
Examples	Runnable code examples
Merchant Configs	TOML configuration examples

Development

# Install tools
cargo install cargo-nextest cargo-make cargo-deny

# Quick verification
cargo make pre-commit

# Full test suite (490+ tests)
cargo nextest run --all-features

# Security audit
cargo deny check

# Documentation
cargo doc --no-deps --open

License

Licensed under MIT OR Apache-2.0 at your option.

Resources

• TAP Protocol — Official Visa documentation
• MCP Protocol — Anthropic's Model Context Protocol
• RFC 9421 — HTTP Message Signatures
• RFC 7516 — JSON Web Encryption (JWE)