YEWLINE PROTOCOL / ROBINHOOD CHAIN

Secure settlement
for agent payments.

YEWLINE is an x402-compatible security layer that binds authorization, settlement, and service delivery into one verifiable flow.

OPEN SPECx402 COMPATIBLERH TESTNET

The verification gap

In a facilitator-mediated payment, /verify and /settle are separate operations. A valid response from the first does not guarantee that funds will move in the second.

verify(payload)  ==  valid
execute(service) ==  delivered
settle(payload)  ==  ???

YEWLINE makes the unknown state explicit. A resource is released only against a bound settlement ticket and a confirmed receipt.

Protocol flow

STEPOPERATIONGUARANTEE

01AUTHORIZEIntent is bound to request, amount, asset, recipient, expiry.

02VERIFYConditions checked; nonce and context become single-use.

03SETTLEExact transaction is simulated, bounded, and confirmed on-chain.

04DELIVERA signed receipt binds payment to service delivery.

Security rules

SR-01 Bind payment requirements to signed payload fields.

SR-02 Enforce freshness, nonce uniqueness, and idempotency.

SR-03 Never report success before settlement is confirmed.

SR-04 Reject zero-value and economically meaningless payments.

SR-05 Bound sponsor-paid gas and execution complexity.

SR-06 Revalidate time- and state-dependent conditions.

SR-07 Execute only allow-listed programs and instruction templates.

SR-08 Bind delivery to a signed settlement receipt.

Research basis

Motivated by When HTTP 402 Meets the Blockchain. The study tested 15 facilitators, found 49 rule violations and 31 previously unknown vulnerabilities.

arXiv:2607.19545 ↗

Implementation status

● SPECIFICATION　 PUBLIC DRAFT
● FACILITATOR　 LOCAL SIMULATION
● ROBINHOOD PROFILE　 TESTNET TARGET

network   Robinhood Chain Testnet
chainId   46630
gas       ETH

Why this project exists

x402 makes machine-to-machine commerce possible with an HTTP-native payment flow. But the facilitator sits between an HTTP request and an irreversible blockchain transaction. That boundary is where “valid” can diverge from “paid”.

Our goal is not to replace x402. YEWLINE adds a verifiable safety layer that can be adopted incrementally by existing clients, resource servers, and facilitators.

What we reproduced

We built an isolated x402 test environment with our own merchant, client, facilitator, wallets, and testnet funds. We reproduced the failure modes described in the paper without touching third-party systems.

CASEOBSERVATIONIMPACT

A-01Replay raceOne authorization accepted by concurrent requests.

A-02Verify / settle gapService released before settlement finality.

A-03Gas abuseFailed or zero-value submissions burn sponsor fees.

A-04Payload mutationRecipient or instruction semantics diverge from intent.

Experiments remain isolated and testnet-only. Production claims require public traces, test vectors, and independent review.

System architecture

agent ── payment intent ──▶ resource server
  │                              │
  │                        YEWLINE policy
  │                              │
  └──────── settlement ticket ──▶ facilitator
                                 │
                          bounded EVM call
                                 │
                         signed delivery receipt

YEWLINE is a protocol layer, not a new chain. Its first network profile targets Robinhood Chain, an EVM-compatible Layer 2. x402 handles negotiation; YEWLINE handles proof, safety, and delivery.

Threat model

We model malicious clients, forged or mutated payment payloads, replay attempts, timing races, malformed EVM transactions, and abusive use of sponsor-paid execution.

We do not claim to solve compromised wallets, malicious merchants, colluding facilitators, or vulnerabilities in the underlying blockchain. Those remain separate trust boundaries.

Open by design

The specification, reference facilitator, test vectors, and attack harness are intended for public review. Security claims should be challenged before the protocol is used for valuable services.

● SPEC / PUBLIC DRAFT
● REPRODUCTION LAB / ISOLATED
● IMPLEMENTATION / IN PROGRESS

Explore Robinhood Chain docs ↗

YEWLINE is an independent project. It is not affiliated with, endorsed by, or operated by Robinhood Markets, Inc. or its affiliates.

Secure settlementfor agent payments.