feat: MPP machine payments — agent-native pay-per-tool-call

[stackbilt-admin]

Context

Stripe launched Machine Payments Protocol (MPP) on 2026-03-18. mppx SDK is installed. Foundation code stashed on main.

What's Ready (git stash)

• src/mpp.ts — pricing map (18 tools priced), server factory
• src/rest-mpp.ts — REST endpoint handler skeleton
• src/types.ts — MPP_SECRET_KEY + STRIPE_SECRET_KEY in GatewayEnv
• src/index.ts — /api/pay/:toolName + /api/pay catalog routes
• mppx installed in package.json

What Needs Building

1. Wire mppx server with Stripe SPT method (stripe.charge() from mppx/server)
2. MCP-native path using Transport.Mcp (JSON-RPC error -32042 for payment required)
3. Test with a real agent client
4. Deploy with MPP_SECRET_KEY + STRIPE_SECRET_KEY as Worker secrets

Key Discovery

mppx has native MCP support:

• `Transport.Mcp` type for JSON-RPC payment flow
• Error code `-32042` (payment required)
• `_meta["org.paymentauth/credential"]` for credentials
• `_meta["org.paymentauth/receipt"]` for receipts

This means we can do MPP inside MCP JSON-RPC — agents using our MCP server get native payment flow without a separate REST surface.

References

• mppx npm
• Stripe MPP docs
• Working example: Hono + Workers + mppx
• Stripe machine-payments samples

Estimated Effort

2-3 focused hours with mppx docs loaded.