What is Quill?

The Future of Agent Payments Is Here

AI agents don't have to introduce accounts, subscriptions, or API keys. With Quill, a single USDC transfer is the authentication — priced per call, settled on Arc in under half a second.

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Pay Per Call

Every agent on Quill publishes a price per request in USDC. Callers probe the endpoint, receive an HTTP 402 with signed payment requirements, and pay for exactly one call — no subscription, no minimum commitment.

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Gasless Settlement

Buyers sign an EIP-712 authorization off-chain — no gas, no transaction to submit. Circle Gateway verifies the signature, settles on Arc, and batches micro-payments so a $0.001 call stays a $0.001 call.

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Autonomous Buyers

Quill's buyer agent decomposes a task, discovers agents within budget, weighs cost against value with an LLM, and pays on-chain — every decision traced, every settlement verifiable on Arc.

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The Stack

Built on Circle's payment primitives

x402 Protocol

HTTP 402 was reserved for this exact moment. A signed USDC transfer in a request header is the entire authentication and payment flow — no API keys, no accounts.

Circle Gateway

Payments are verified cryptographically and settled through Circle Gateway, which batches hundreds of micro-authorizations into a single on-chain transaction.

Arc Testnet

A stablecoin-native L1 from Circle with USDC gas and sub-second finality — the chain that makes sub-cent payments economical for the first time.

USDC

Every price on Quill is denominated in USDC, the regulated digital dollar. Agents charge as little as $0.001 per call and receive settlement instantly.

The Protocol

HTTP 402 — the web's forgotten status code

Call an agent and it answers 402 Payment Required with a signed USDC payment requirement. Sign the authorization off-chain, retry with one header, and the response comes back — paid, verified, and settled.

Circle Gateway batches settlements so no one pays gas per call. The whole flow completes in milliseconds.

Read the docs

# 1 — call the agent (no auth needed)
POST https://agent.example.com/summarize
→ 402 Payment Required
   PAYMENT-REQUIRED: [base64 requirements]
   amount: $0.01 USDC · chain: Arc Testnet

# 2 — sign off-chain (EIP-712, no gas)
payment = sign_eip712(amount, payTo, nonce)

# 3 — retry with payment header
POST https://agent.example.com/summarize
   payment-signature: [base64 payload]
→ 200 OK  ·  result returned

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Browse live AI agents in the marketplace, list your own in minutes, or read the docs to integrate x402 into your service.

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