feat(api): add native MPP (Machine Payments Protocol) support

[decofe]

Summary

Add native HTTP 402 payment support via the Machine Payments Protocol so AI agents can pay for E2B sandbox usage with crypto (Tempo USDC) instead of API keys.

Motivation

MPP enables no-signup, pay-per-use access to E2B sandboxes. An AI agent with a Tempo wallet can create and manage sandboxes by paying per request — no API key, no account, no billing setup. This is especially useful for autonomous agents that need compute on demand.

Changes

OpenAPI spec (spec/openapi.yml)

• Add PaymentAuth security scheme (http/payment)
• Add PaymentAuth as an alternative auth option on sandbox endpoints: create, kill, connect, timeout, refreshes

Config (packages/api/internal/cfg/model.go)

• Add MPP_ENABLED, MPP_SECRET_KEY, MPP_REALM, MPP_CURRENCY, MPP_RECIPIENT, MPP_TEAM_ID env vars

Auth (packages/api/internal/handlers/payment_auth.go)

• PaymentAuthenticator implements auth.Authenticator for the PaymentAuth security scheme
• Maps MPP-authenticated requests to a pre-configured team via MPP_TEAM_ID

Middleware (packages/api/internal/middleware/mpp.go)

• Gin middleware handling the full 402 challenge/credential flow
• Issues WWW-Authenticate: Payment challenges when no auth is present
• Verifies payment credentials and sets Payment-Receipt header
• Skips requests that already have API key or bearer auth

Payment method (packages/api/internal/handlers/mpp_method.go)

• TempoMethod / TempoChargeIntent implementing mpp-go/server.Method interface
• Placeholder verification (HMAC challenge binding verified by mpp-go; full on-chain verification can be added later)

Wiring (packages/api/main.go)

• Add PaymentAuthenticator to authenticator chain when MPP_ENABLED=true
• Add MPP middleware after LaunchDarkly context, before rate limiting
• Add Accept-Payment to CORS allowed headers

Store (packages/api/internal/handlers/store.go)

• Add GetTeamByID using existing GetTeamWithTierByTeamID query

Dependency

• Add github.com/tempoxyz/mpp-go to go.mod

Protocol flow

Agent                                    E2B API
  │                                        │
  │  POST /sandboxes (no auth)             │
  ├───────────────────────────────────────>│
  │                                        │
  │  402 + WWW-Authenticate: Payment ...   │
  │<───────────────────────────────────────┤
  │                                        │
  │  POST /sandboxes                       │
  │  Authorization: Payment <credential>   │
  ├───────────────────────────────────────>│
  │                                        │
  │  201 Created                           │
  │  Payment-Receipt: <receipt>            │
  │<───────────────────────────────────────┤

Testing

This is gated behind MPP_ENABLED=false (default), so existing behavior is unchanged. When enabled, requires MPP_SECRET_KEY, MPP_REALM, MPP_CURRENCY, MPP_RECIPIENT, and MPP_TEAM_ID.

Prompted by: georgios

[claude]

Claude Code Review

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Register MPP middleware before OpenAPI auth validation

In packages/api/main.go, the OAPI validator is installed before MPPMiddleware, so unauthenticated POST /sandboxes requests are rejected during security validation and never reach the middleware that generates WWW-Authenticate: Payment challenges. This breaks the first step of the documented 402 handshake (no-auth request -> challenge), because clients cannot obtain a challenge to construct a payment credential.

Useful? React with 👍 / 👎.

[chatgpt-codex-connector]

Exempt Supabase-authenticated calls from MPP interception

The skip condition in packages/api/internal/middleware/mpp.go only recognizes X-API-Key and Authorization: Bearer, so requests authenticated via X-Supabase-Token/X-Supabase-Team are treated as unauthenticated and get challenged/charged by MPP when it is enabled. That regresses existing Supabase auth flows on billable sandbox endpoints despite valid credentials.

Useful? React with 👍 / 👎.

[gakonst]

Closing per offline convo - will bring proxy repo instead.

[tvi]

Thanks for the prototype. We are going to be implemeting this feature differently.