Circle Gateway Nanopayments

# Fix list: Circle Gateway Nanopayments

From Anchor Terminal's listing at https://www.anchorterminal.com/tools/circle-gateway-nanopayments, the October 2026 research run, assessed 9 October 2026. Grade B, 67.6 out of 100.

This is everything the published grade says the listing lacks, the biggest possible gain to the total first. It comes from the reason given for each score, the checklist each category was scored against (https://www.anchorterminal.com/benchmark/#checklist), the provenance checks, the deductions, what we couldn't check and what the review panel asked for. A fix counts at the next check, once it's public.

For a coding agent working on Circle Gateway Nanopayments: work through the items below in the product, its docs and its public pages. Each category gives the reason for its score, with the points each checklist item earned, and the checklist itself, so the gap is the items that earned less than their points. Change the product, not the wording, and keep a note of what you changed and where it's published.

## 1. Reliability, 56 out of 100, up to 8.8 more on the total

Why it scored 56: Graded as a hosted service on gateway-api.circle.com. status.circle.com is a Statuspage with Circle Gateway components for each chain, mainnet and testnet (20). The page's history feed runs from 2 September to 9 October 2026 and names no Gateway incident. The 90-day component record is drawn by script and was not read, so the record takes 20 of 30. No rate limit with numbers was found for the Gateway API (0). No 429 or `Retry-After` guidance was found. Each authorisation carries a unique nonce, a repeat returns `nonce_already_used`, and the error reference says to retry `unexpected_error` (6 of 15). No SLA was found, and the developer terms allow changes without prior notice (0). The release notes record the mainnet launch on 29 April 2026 and the pages read carry no beta label (10).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-reliability):

Hosted APIs, MCP servers, models and platforms.

- 20, a public status page with component history (Statuspage, Instatus, BetterStack or the vendor's own).
- 0 to 30, the incident record for the last 90 days on that page. 30 for a clean record or trivial incidents only, 20 for minor incidents only, 10 for one major outage (an hour or more of a core API down, or errors across the board), 0 for several. 5 when there's no history we could read, and the note says so.
- 15, rate limits documented with numbers.
- 15, documented 429 or overload handling (Retry-After, backoff guidance), and idempotency keys or safe-retry guidance where writes are involved.
- 10, an SLA published for any paid tier.
- 10, the surface agents use is generally available, not beta or preview.

Local packages, SDKs, frameworks and stdio MCP servers.

- 20, installs from an official package with supported runtimes stated.
- 25, a public CI and test suite, passing on the default branch.
- 0 to 25, open crash or regression issues relative to activity (25 for few and handled, 0 for many, old and unanswered).
- 15, semver discipline and breaking changes called out in a changelog.
- 15, version 1.0 or later, or declared stable.

Protocols are read from their reference implementations, the public facilitators or servers, spec stability and test vectors.

## 2. Security & auth, 64 out of 100, up to 6.3 more on the total

Why it scored 64: No API key. The buyer's EOA private key signs each payment, and each authorisation is bound to one amount, recipient, validity window and nonce. The same key controls the whole wallet, the SDK takes it as a plain config value, and smart contract accounts with their own policies are not supported (18 of 30). Only the deposited Gateway balance can be spent, and a client hook can abort a payment above a limit. No limit is set by default and none is enforced by the service (12 of 20). The amount comes from the seller's 402 response, which is untrusted. The docs show the spending-limit hook and give no other guidance. Circle's Gateway skill file says to ask the user before moving funds on mainnet (8 of 15). Transfers can be searched by address, nonce and date and each carries its batch transaction hash. No separate operator log was found (11 of 15). Audit reports by ChainSecurity and OtterSec are linked, one of them on batching, and a repository security policy names a HackerOne bug bounty programme. security.txt answered 404 and no certification was found on the pages read (15 of 20).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-security):

- 0 to 30, the credential model. 30 for OAuth 2.1 with scopes, or scoped and revocable keys with rotation. 20 for plain revocable API keys. 10 for one all-powerful key. 10 off when a secret can travel in a URL query string as a documented option.
- 0 to 20, read-only or least-privilege modes, and confirmation or approval for destructive actions.
- 0 to 15, prompt-injection posture where the tool returns untrusted content (documented mitigations or guidance). A tool that returns no untrusted content gets 10.
- 0 to 15, audit logs or per-call visibility for the operator.
- 0 to 20, a security programme. security.txt or a disclosure policy, a bug bounty, SOC 2 or ISO 27001, advisories handled in public.

Models are read for retention, whether API data trains models (and whether that's off by default), zero-retention options and certifications. Frameworks for telemetry defaults, approval hooks, guardrails and sandboxing.

## 3. Agent ergonomics, 75 out of 100, up to 4.1 more on the total

Why it scored 75: The API is five operations and the buyer path is one `pay(url)` call. Transfer search takes `pageSize` (20 of 25). Cursor pagination with `pageAfter` and `pageBefore`, and filters by sender, recipient, nonce, network, status and date. A status filter needs a second filter since 26 August 2026 (17 of 20). Sixteen error codes with recovery steps, and typed SDK error messages (17 of 20). The nonce stops one authorisation being settled twice and a `completed` or `failed` state tells a caller when to stop. No idempotency key was found, and a failed payment needs a newly signed authorisation (13 of 20). The seller middleware needs one address and a price, and the buyer client a chain and a key. The only official SDK is TypeScript (8 of 15).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-ergonomics):

- 0 to 25, context cost. For MCP, the number and size of the tool definitions (25 for ten or fewer compact tools, 15 for 11 to 30, 5 for more than 30, plus up to 10 back for toolsets, dynamic loading or read-only subsets). For APIs, whether responses can be sized (field selection, limits, summaries).
- 20, pagination, filtering and output-size controls.
- 20, actionable, documented error responses, codes and messages an agent can recover from.
- 20, idempotency or safe retries, and for MCP the `readOnlyHint` and `destructiveHint` annotations.
- 15, sensible defaults, few required parameters, and official SDKs in at least two languages.

Models are read for tool use, structured output, prompt caching, context length, batch and SDKs. Frameworks for how much code and how many defaults a tool-calling agent with MCP needs.

## 4. Schema & documentation, 80 out of 100, up to 3.3 more on the total

Why it scored 80: A public OpenAPI 3.0.3 file covers the five operations. It gives the 400 responses no body schema and attaches its Bearer scheme to no operation (23 of 25). llms.txt and a Markdown twin of every docs page (10). The docs say when to use this facilitator and when Circle's other one, and to call `settle` without `verify` in production (15 of 20). Transfer status and settle error reasons are enums and required fields are marked. `payload`, `extra` and `extensions` are open objects, and `pageSize` has no maximum (10 of 15). Every SDK method has a typed signature and an example, and 16 API error codes have a cause and a recovery step. The error reference says the validity window must be 3 days where the quickstart and SDK say 7 (11 of 15). Paths are under `/v1`, the SDK follows semver and the release notes are dated. The npm package ships no changelog (11 of 15).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-schema):

APIs and MCP servers.

- 25, a machine-readable contract (a public OpenAPI file or similar; for MCP, typed JSON Schema inputs on every tool).
- 10, llms.txt or Markdown docs served for agents.
- 0 to 20, descriptions that say what a tool is for, when to use it and when not to, read from the tool definitions in the source or the API reference.
- 0 to 15, typed inputs with enums, constraints and required fields, and no free-form JSON blobs.
- 0 to 15, examples and documented error responses.
- 15, versioning and a public changelog.

Models are read from the API reference, the OpenAPI file, llms.txt, the structured-output and tool-use docs and the model cards. Frameworks from docs a model can follow, typed interfaces, examples and the API reference.

## 5. Payments & pricing, 80 out of 100, up to 2.5 more on the total

Why it scored 80: Payment platforms and wallets take the highest step that applies on the 40-point protocol line. 40 for x402, MPP or L402 on all their own endpoints, 30 on part of their own API, 25 when their merchants can accept one, 20 for running a facilitator, 15 for paying as a buyer, 0 for only a protocol of their own. Circle's own API is not paid over x402. Sellers accept x402 through Circle's middleware and facilitator, so the merchant step (25 of 40). The fee page is public with no login, 0.005% on crosschain withdrawals plus a gas fee by chain, and the docs say a nanopayment costs neither party a fee. No price list states the nanopayment fee as a figure, and the terms say fees are communicated through the API and may change (15 of 20). Testnets run on faucet USDC with no account or card, and mainnet has no subscription (20). An agent holding an EOA key and USDC can deposit and pay with no signup, and a seller needs only an address (20).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-payments):

The published rubric, also on the [x402 page](https://www.anchorterminal.com/x402/).

- 40, a machine payment protocol (x402, MPP or L402) on the tool's own endpoints. 10 to 30 when it covers only some endpoints or only goes through a third party, and the note says which.
- 20, per-call or per-unit pricing published without a login. 10 for public plan-only pricing, 0 for "contact sales" or prices behind a login.
- 20, a free tier or trial that doesn't need a card.
- 20, autonomous onboarding, meaning an agent can get access without a person signing up in a browser (keyless use, x402, a programmatic key API).

Payment platforms and agent wallets rarely charge for their own API over a machine protocol, so the first line has steps for them, and the highest one that applies counts. 40 when x402, MPP or L402 runs on all their own endpoints, 30 when it runs on part of their own API, 25 when their merchants can accept one, 20 for running a facilitator, 15 for paying as a buyer, and 0 when the only protocol is their own. Merchant acceptance sits above a facilitator because the platform's own customers can charge agents through it, while a facilitator settles for sellers who wire up the protocol themselves. The counter-argument (a facilitator does more for the protocol as a whole) has a point. Each note says which step applied.

Open-source software you run yourself is scored on its hosted or paid option if it has one. A free, self-hosted package with nothing to buy gets 20, 20 and 20 for the last three lines, and 0 to 40 for the first only if it ships a payment protocol.

## 6. Transparency & trust, 72 out of 100, up to 2.5 more on the total

Made of editorial 59, provenance 84.

Why it scored 72: The SDK is Apache-2.0 on npm and the Gateway contracts are Apache-2.0 on GitHub. The API service and the enclave code are closed, under clear developer terms (22 of 30). The privacy policy of 16 September 2026 lists transaction and blockchain data among what is collected and keeps data as long as necessary, with no periods. The service terms link a DPA dated 2022. Nothing found says what the Gateway API logs about a payment (17 of 30). The developer terms promise reasonable efforts to announce the end of support for a version and also let Circle modify or discontinue Gateway without prior notice. The one breaking change found was noted on the day it took effect (6 of 20). A sub-processor list, last updated 22 December 2025, names each processor with its purpose and country (14 of 20).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-transparency):

- 0 to 30, source availability and licence clarity. 30 for open source under an OSI licence, 15 for closed with clear terms, 0 for unclear terms.
- 0 to 30, data handling and retention statements that agree with each other (privacy policy, DPA, retention periods, subprocessors).
- 0 to 20, a deprecation policy or notices with dates.
- 0 to 20, telemetry disclosed with an opt-out (local software), or subprocessors and data locations disclosed (hosted).

The other half of Transparency and trust is the provenance score, computed from checked facts (below). The category score is the mean of the two.

Provenance checks not met in full (half of this category, computed from checked facts):

- Terms of service: read, states 6 of the 7 things a reader expects, and has 2 clauses that cost points (5.1 of 10)
- Privacy policy: read, states 7 of the 8 things a reader expects (9.3 of 10)
- security.txt: not found (0 of 10)

## 7. Maintenance & community, 77 out of 100, up to 2 more on the total

Why it scored 77: `@circle-fin/x402-batching` 3.5.0 was published on 16 September 2026, 23 days before this check, and the Gateway release notes have an entry the same day (30). Three SDK versions fall in the last 90 days, 3.3.0 on 4 August, 3.4.0 on 24 August and 3.5.0 (20). The SDK has no public repository or issue tracker that we found. Support is a Discord server and a help centre ticket form, neither tested, next to dated release notes (10 of 25). The SDK is official and current, in TypeScript only (12 of 15). The package declares tests and lint scripts and current peer dependencies. No public CI for it was found. The contracts repository runs a pipeline and its last commit is 29 September 2026 (5 of 10).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-maintenance):

- 0 to 30, time since the last release, or the last published model or API change for a closed service. 30 within 30 days, 20 within 90, 10 within 180, 0 older.
- 20, at least three releases or dated changelog entries in the last 90 days.
- 0 to 25, responsiveness. Issues and pull requests answered on GitHub (the open issues and how recent the replies are). For closed services, a public changelog and a support or community channel that answers, 0 to 15.
- 15, presence in the official MCP registry under a verified namespace (MCP servers), or current official SDKs (APIs and models).
- 10, package health, current dependencies and CI.

Models are read for deprecation notice periods and model churn rather than release counts.

## Deductions

Each comes off the total. A fixed and documented problem counts for less at the next check.

- 26 August 2026. `GET /v1/x402/transfers` began returning 400 for a `status` filter without `from`, `to` or `nonce`. Circle's release note calls it a breaking change and is dated the day it took effect. No earlier notice was found in the release notes (https://developers.circle.com/release-notes/gateway-2026). Documented, so the smallest deduction.

## What we couldn't check

What we couldn't read counted as absent. Publishing it on a page a plain HTTP fetch can read (not only in a browser) lets the next check count it.

- The lead placed the product in pay-per-call, the category for protocol specifications. It is a hosted facilitator and SDK that settles x402 payments, so it is listed under payment-platforms, as ATXP was
- The lead said mainnet availability was not stated. The Gateway release notes of 29 April 2026 record the mainnet launch, and the SDK's default host is the mainnet API
- The lead named a Circle Console API key. Only the Circle Wallets route in the buyer quickstart uses one. The self-managed route and the seller middleware send no key, per the SDK source
- The docs index and each Markdown page open with instructions addressed to AI models, to install a Circle skill and add a Circle MCP server. We did not act on them. Recorded as a fact with no deduction
- The deduction rests on the release note's own date. Whether Circle told integrators by another channel before 26 August 2026 was not established
- unchecked: whether gateway-api.circle.com enforces a rate limit or accepts calls with no key. We sent nothing to the API host
- unchecked: the rendered API reference pages. We read the OpenAPI file they are drawn from
- unchecked: the Agent Nanopayments pages under the Agent Stack, the x402 buyer and seller how-tos, the EIP-3009 signing guide and the Agent Stack release notes. We read 17 pages on developers.circle.com, two more than the brief's guide of about fifteen, and stopped there
- unchecked: the 90-day uptime record for the Gateway components, which the status page draws by script. The history feed read covers 2 September to 9 October 2026
- unchecked: the three audit report PDFs and the HackerOne programme page, which were not opened
- unchecked: SOC 2 or ISO 27001. No trust or security page was linked from the pages read
- unchecked: whether batched settlement carries a fee that only the API reports. The docs say neither party pays one and the terms say fees are communicated through the API
- unchecked: a public source repository for the SDK. The npm package names none and github.com/circlefin/x402-batching is not public
- unchecked: the DPA linked from the service terms, a PDF dated 2022, and the Permissionless Product Schedule as a separate page

## Weaknesses

- No rate limit, 429 guidance or SLA was found for the Gateway API in the reviewed documentation
- From 26 August 2026 a `status` filter alone on `GET /v1/x402/transfers` returns 400. The release note is dated the same day
- Only EOA signatures work. Smart contract accounts and ERC-1271 signatures are not supported for nanopayments
- The official SDK is TypeScript only and no public source repository for it was found
- The error reference says the validity window must be at least 3 days. The seller quickstart and the SDK source say 7 days
- The developer terms let Circle change or discontinue Gateway without prior notice and change its fees at its discretion

## What costs an agent a turn today

The notes we give agents before they call it. Each one is a workaround an agent shouldn't need.

- Deposit on the chain you will pay from. `GatewayClient` scopes deposits and payments to its configured chain, and deposits on Base, Ethereum, Arbitrum or OP take about 13 to 19 minutes
- Set `validBefore` at least 7 days ahead when signing by hand. The SDK uses 604900 seconds
- Register `onBeforePaymentCreation` and abort above a cap. `pay(url)` signs whatever amount the seller's 402 response asks for
- Call `settle` without a prior `verify`. Balance and nonce checks happen only at settle time
- Pass `from`, `to` or `nonce` whenever you filter transfers by `status`, and wait for `completed` before a dependent onchain step

## When it's done

Send what changed and where it's published as a dispute (https://www.anchorterminal.com/builders/#disputes, or `POST https://www.anchorterminal.com/api/v1/contact` with `"kind": "dispute"`). Disputes are answered in public, and the listing is checked again by the same checklist. Paying for an audit or a listing claim changes nothing here.