Agentic Payments with MPP Router

Pay-per-call for AI agents. Call 402-gated HTTP APIs and settle automatically in Stellar USDC via MPP Router.

Agentic payments let AI agents spend Stellar USDC to call paid HTTP APIs — no API keys, no subscriptions, no credit card on file. The agent makes a request, the server returns 402 Payment Required with a Stellar payment challenge, the agent signs with its wallet, retries the request, and receives the response.

ROZO's agentic payment stack runs on Stellar mainnet via MPP Router — a Stellar-native 402 proxy that accepts Stellar USDC and forwards the request to upstream merchants (Parallel.ai, Exa, Firecrawl, OpenRouter, and more).

Why Stellar for agentic payments

• Sub-cent fees. Each 402-gated call settles for fractions of a cent, making microtransactions practical.

• Fast finality. Stellar closes ledgers every ~5 seconds, so the 402 → pay → retry loop completes in one round trip.

• Sponsored transactions. The agent's wallet does not need XLM for fees — MPP Router sponsors the fee payer.

• One stablecoin balance, many APIs. You hold USDC on Stellar once and spend it across dozens of upstream services — no need to fund Base, Tron, or Solana.

The flow

Agent                           Server (402-gated)
  │                                      │
  │ ───── 1. POST /endpoint ────────────▶ │
  │                                      │
  │ ◀──── 402 Payment Required ───────── │
  │       WWW-Authenticate: Payment      │
  │       request="<challenge>"          │
  │                                      │
  │  sign challenge with                 │
  │  Stellar wallet (sponsored mode)     │
  │                                      │
  │ ───── 2. POST /endpoint ────────────▶ │
  │       Authorization: Payment <cred>  │
  │                                      │
  │ ◀──── 200 OK + Payment-Receipt ──── │

1. First request: plain HTTP POST to the service URL.

2. Server responds 402 Payment Required with a Stellar charge challenge (either MPP's WWW-Authenticate: Payment header or the x402 Payment-Required header).

3. Agent signs the challenge with its Stellar key. The inner transaction is a sponsored SAC transfer — the agent's account is ALL_ZEROS and only the auth entries are signed, so the server can fee-bump and broadcast.

4. Agent retries the request with the signed credential in the Authorization header.

5. Server validates the credential, broadcasts the signed transaction, and returns the upstream response plus a Payment-Receipt header.

Protocols supported

MPP Router emits a single 402 response that carries both dialects. The inner signed XDR is identical — a single signer produces both envelopes.

Aspect	x402	MPP
Outer envelope	X-Payment header (base64 JSON)	Authorization: Payment <credential> header
Field names	payTo, asset, amount	recipient, currency, amount
Sponsored flag	extra.areFeesSponsored	feePayer: true
Inner XDR	Same sponsored SAC transfer	Same sponsored SAC transfer

The MPP and x402 payTo addresses in a single challenge are different and HMAC-bound — pay the address that matches the dialect you chose. Never mix.

Discovering services

The service catalog is live at https://apiserver.mpprouter.dev/v1/services/catalog. Never hardcode service paths — the catalog is the source of truth.

Example services (as of publication — always fetch the live catalog):

id	price	upstream
parallel_search	$0.01 / request	Parallel.ai web search
exa_search	$0.005 / request	Exa neural search
firecrawl_scrape	$0.002 / request	Firecrawl HTML → markdown
openrouter_chat	dynamic	OpenRouter LLM proxy

Getting started

The fastest path for Claude Code or similar AI agents is the stellar-agent-wallet plugin, which ships two skills:

• discover — fetch and filter the MPP Router service catalog

• pay-per-call — execute the full 402 → sign → retry loop, handling both MPP and x402 dialects

Install the plugin, then:

# Find a service
npx tsx skills/discover/run.ts --query "web search"

# Call it — the skill pays automatically
npx tsx skills/pay-per-call/run.ts \
  "https://apiserver.mpprouter.dev/v1/services/parallel/search" \
  --body '{"query": "Summarize https://stripe.com/docs"}' \
  --method POST

For custom integrations, use the @stellar/mpp SDK on npm with @stellar/stellar-sdk ^14.6.1 and mppx ^0.4.11 as peer dependencies.

Safety notes

• Credentials are single-use. The HMAC binding to amount, currency, and recipient is the router's defense against replay. If the retry fails, start fresh with a new 402 challenge — do not re-send the same credential.

• Confirm above a threshold. For mainnet calls above ~$1.00, prompt the user before signing. The pay-per-call skill enforces this by default (--max-auto <usd> to override).

• Validate the challenge amount. If you know the advertised price from the catalog, verify the 402 challenge amount matches before signing.

• Mainnet only. MPP Router does not run on Stellar testnet.

Relationship to ROZO

MPP Router is the agentic payments rail that lets ROZO's Stellar-native wallets pay for off-chain services with the same USDC balance used for cross-chain intents. An agent holding USDC on Stellar can:

1. Bridge from another chain to Stellar USDC via a ROZO intent.

2. Call any MPP Router service and pay per request.

3. Send the result to an intent for further action (e.g. swap, settle, bridge back).

Further reading

• MPP Router: https://www.mpprouter.dev

• Live service catalog: https://apiserver.mpprouter.dev/v1/services/catalog

• Stellar Soroban smart-account payments: Stellar Smart Account Payments

• stellar-agent-wallet plugin skills: discover, pay-per-call, bridge, send-payment, check-balance