Settlens

npx add-mcp 'https://settlens.com/mcp'

Settlens

Security and conformance evidence for agentic payment endpoints.

Settlens inspects the x402 V2 HTTP handshake, validates advertised payment requirements, detects unexpected changes in price or recipient, enforces a local policy, and can safely check that an intentionally invalid authorization does not unlock the resource.

Settlens uses the official @x402/core parser as a pinned runtime dependency. It never holds funds, wallets or payment keys and never calls /settle. The only key it uses is the optional ed25519 key that signs evidence attestations.

Product overview and full documentation: settlens.com (docs, changelog).

Source availability

This repository publishes a reviewed snapshot of the Settlens implementation. The original development history, commercial prospecting records and production operations history remain in a separate private repository. Deployment hostnames and IP examples in this snapshot are illustrative; configure your own host.

Source visibility does not establish an open-source license. No npm package or general open-source license is published here. The hosted service and accompanied technical pilots remain available at settlens.com.

What it checks

• the unauthenticated resource returns HTTP 402;
• PAYMENT-REQUIRED exists and contains Base64-encoded JSON;
• the decoded object uses x402 V2 and the required core fields;
• the same object is accepted by the pinned official x402 V2 SDK;
• resource.url binds payment to the URL under test;
• scheme, CAIP-2 network, atomic amount, asset, recipient and timeout are valid;
• schemes, networks, recipients, assets and maximum price match an optional local policy;
• identical requests do not unexpectedly change their payment terms;
• remote endpoints use HTTPS;
• a facilitator's GET /supported schema, signer metadata and coverage agree with the advertised requirements;
• with explicit authorization, an invalid payment signature fails closed, both at the resource and at the facilitator's /verify endpoint;
• a captured payment authorization can be cryptographically verified offline.

The implementation follows the official x402 V2 core specification and HTTP transport specification.

Requirements

• Node.js 20.19 or newer.

Try the local demonstration

Install the exact lockfile and run the quality gate:

npm ci --ignore-scripts
npm run quality

Start the deliberately mixed-quality local target:

npm run demo:server

In another terminal, scan the compliant endpoint:

npm run settlens -- scan http://127.0.0.1:4402/secure

Run the safe active probe:

npm run settlens -- scan http://127.0.0.1:4402/secure \
  --active \
  --authorized \
  --authorization-ref DEMO-APPROVAL-001

Detect the demonstration payment bypass:

npm run settlens -- scan http://127.0.0.1:4402/bypass \
  --active \
  --authorized \
  --authorization-ref DEMO-APPROVAL-002

The CLI writes self-contained JSON, HTML and SARIF evidence to reports/. JSON reports contain a deterministic SHA-256 integrity block. It exits with code 1 when a failing check meets the configured severity threshold, and code 2 for CLI or configuration errors.

Verify that a JSON report has not changed:

npm run settlens -- verify-report reports/settlens-REPORT_ID.json

Scan a real authorized endpoint

Passive mode is the default:

npm run settlens -- scan https://merchant.example/api/data \
  --expect-scheme exact \
  --expect-network eip155:8453 \
  --expect-pay-to 0xYourReviewedRecipient \
  --max-amount 10000

Settlens does not follow redirects and does not include response bodies in its reports. URL credentials and common secret query parameters are redacted. Request header values are also omitted from evidence. A user-supplied PAYMENT-SIGNATURE is rejected outright so passive scans cannot accidentally send a real payment authorization.

Only use --active --authorized after the endpoint owner has explicitly approved the assessment, and provide its ticket or scope identifier with --authorization-ref. The active probe submits one intentionally invalid PAYMENT-SIGNATURE; it never calls a facilitator's /settle endpoint and cannot move funds.

When custom headers are used, their destination must be independently allowlisted:

npm run settlens -- scan https://merchant.example/paid \
  --header "Authorization: Bearer ${ASSESSMENT_TOKEN}" \
  --allow-host merchant.example

This prevents a modified configuration from forwarding CI secrets to an attacker-controlled host. CI may also set SETTLENS_ALLOWED_HOSTS as a comma-separated list outside the assessment configuration.

Independent signature verification

Settlens can cryptographically verify a captured x402 payment authorization offline — no wallet, private key or RPC required:

npm run settlens -- verify-payment payment.json

The file contains { "requirement": { … }, "payload": { … } }. For the EVM exact scheme, Settlens recovers the EIP-3009 / EIP-712 signer and checks that it matches the authorized from, that the transfer is bound to the advertised recipient and amount, and that the check time is strictly inside the authorization window (validAfter < now < validBefore, as EIP-3009 requires). The check time is the current clock, or --now <unix-seconds|ISO-8601> to check a capture at the moment it was used. The EIP-712 domain is rebuilt from the payment requirement itself, so no on-chain call is needed.

For Solana it checks every signer slot of the transaction against its account key with ed25519. An empty slot is reported as pending: an x402 client payload is partially signed, and the facilitator adds the fee payer's signature at settlement. The transaction verifies when at least one signature is present, every present signature is valid and the slot count matches the message header. If no payment terms (asset, amount, payTo, maxTimeoutSeconds, extra) are supplied, this is explicitly a signatures-only result: payment terms are not checked. Providing asset, amount, payTo and extra.feePayer also requires the supported offline SPL TransferChecked profile to bind the mint, atomic amount and recipient. Partial payment terms are a usage error. This static profile does not establish runtime execution or settlement; unsupported instructions, lookup tables and token extensions remain unverified.

The command exits 0 when the requested verification succeeds, 1 when signatures or the requested payment profile fail, and 2 for invalid input or usage.

Continuous monitoring

Once an endpoint is live, settlens monitor turns point-in-time scans into continuous assurance. Each run compares the current passive scan with the previous run and raises an alert for each change for the worse: a check that drops from pass to warn or fail, a new failing or warning check, or a check that disappeared (removed: coverage lost). Drift is caught on the first scheduled run after an endpoint starts behaving differently:

settlens monitor --config settlens.config.json --state settlens.state.json

Run it on a schedule (cron or CI). An alert blocks when its check's severity is at or above --fail-on (default high); below that it is printed as a note. The command exits 1 on a blocking alert and can POST the alerts with --webhook <url>. The state file is keyed by method and URL. Monitoring is fully passive and never sends a payment.

Shareable evidence attestations

Every report carries a deterministic SHA-256 digest. An evidence attestation binds that digest to an ed25519 signing identity and a timestamp, so a customer, partner or auditor can verify the result offline:

settlens evidence keygen --out settlens-evidence.key
settlens evidence attest reports/settlens-REPORT_ID.json \
  --key settlens-evidence.key --out attestation.json
settlens evidence verify attestation.json --report reports/settlens-REPORT_ID.json

verify checks the ed25519 signature and that the report is intact and matches the attestation. Add --expect-key <file> (an ed25519 PEM public key, or the 0x hex key printed by keygen) to also reject an attestation signed by any other key: the public key inside an attestation only proves that some key signed it. keygen --out refuses to overwrite an existing file and creates the key readable by its owner only (mode 0600). Signing uses node:crypto only; no HSM or network is required.

AI agents (MCP)

settlens mcp runs a Model Context Protocol server over stdio so Claude Desktop, Claude Code or any MCP client can check an x402 payment request before paying:

claude mcp add settlens -- node /absolute/path/to/settlens/src/cli.js mcp

On every HTTP 402 the agent calls evaluate_payment_requirements with the PAYMENT-REQUIRED header, the requested URL and its spending policy, and gets pay, review or do_not_pay with the reasons. Offline tools also verify payment signatures, Solana transactions, reports and attestations; the network tools (scan_endpoint, scan_facilitator) are passive only. No tool pays, signs or needs a key.

The decision is pay only when every protocol check passes, at least one advertised payment option satisfies the policy, and the policy sets all four of pay_to, assets, networks and max_amount. A policy that leaves any of them open gets review. The agent then pays only with an option listed in acceptable.

The same tools are served over MCP Streamable HTTP at https://settlens.com/mcp: the offline tools are public, the network tools need an API token. See docs/MCP.md for the remote endpoint, resources, the x402_preflight prompt, every tool's inputs and outputs, and the security model.

Configuration and CI

Copy the example:

cp settlens.config.example.json settlens.config.json
npm run settlens -- check --config settlens.config.json

Configuration values can reference environment variables:

{
  "headers": {
    "Authorization": "Bearer ${ASSESSMENT_TOKEN}"
  }
}

The value is expanded at runtime and never written into the report. See action.yml and examples/github-workflow.yml for the local composite GitHub Action. SARIF results are anchored at line 1 of the configuration file, so GitHub code scanning can display them (code scanning on private repositories requires GitHub Code Security).

The repository CI tests Node.js 20, 22 and 24, enforces coverage thresholds, audits dependencies, produces a CycloneDX SBOM, uploads SARIF, proves bypass detection and builds the pinned non-root container:

docker build -t settlens:local .
docker run --rm settlens:local --version

For an isolated Plesk host deployment, compose.plesk.yml provides a default network-disabled service and a separate network-enabled scan profile. Neither service publishes an inbound port.

Assurance documents

• docs/METHODOLOGY.md defines what each result proves;
• docs/THREAT_MODEL.md documents trust boundaries, controls and residual risks;
• docs/PILOT_ACCEPTANCE.md is the objective go/no-go checklist for a customer pilot;
• docs/READINESS.md is the historical 0.2.0 readiness snapshot (verified and open gates at that release, without self-certification);
• docs/EVIDENCE.md provides reproducible proof commands;
• SECURITY.md defines the vulnerability reporting policy.

Current boundary

Settlens now independently and cryptographically verifies payment authorizations (see Independent signature verification above), and its authorized active probe calls a facilitator's /verify endpoint to confirm that an invalid signature fails closed. It still does not:

• call facilitator /settle;
• submit, replay or broadcast real transactions;
• hold wallets or payment keys, sign payments or take custody of funds;
• prove complete application security or regulatory compliance.

Those exclusions are intentional: Settlens produces useful, reproducible evidence without ever holding wallets or payment keys, moving funds or taking custody.

Settlens reports observed protocol behavior. It is not a proof of complete application security, a certification, or legal advice.

Persistent monitoring and private workspace

Monitoring now compares payment-term fingerprints between runs even when check statuses remain unchanged. Use monitor --history .settlens/monitor.json for a bounded, atomic journal and history --history .settlens/monitor.json --format html for an export. The optional private /dashboard supports target configuration, scheduling and history; it uses a dedicated credential, not the scan API token. SMTP notifications can send pilot requests ([Settlens][PILOTE]) and blocking changes ([Settlens][ALERTE]) to contact@settlens.com. See monitoring setup.

For agent integration, src/client.js provides a dependency-free HTTP MCP client. examples/preflight-client.mjs demonstrates a request followed by a policy check. An offline CLI check is also available:

node src/cli.js preflight payment-required.txt --policy examples/agent-policy.json --resource-url https://merchant.example/paid

The example policy is illustrative Base Sepolia data; review and replace its recipient, asset, network and amount. Exit 0 means pay; both review and do_not_pay exit 1. These tools never pay or sign, and do not verify on-chain state.

Approved references and incident lifecycle

Journal mode now separates observations from explicitly approved payment terms, tracks open/acknowledged/resolved incidents, and queues owner email/webhook alerts atomically with observations. Persistent incidents gate CLI/CI until recovery; acknowledgement pauses reminders. The private dashboard supports endpoint policy editing, reviewed approval, comments, severity filters and delivery retries. See monitoring behavior and limits, operational checks and encrypted recovery and the first pilot acceptance protocol.

Run npm run conformance for the deterministic synthetic corpus and pinned SDK compatibility checks. Its measured outcomes apply only to that corpus; see benchmark methodology. Solana requirements can additionally be checked against the supported static SPL TransferChecked profile offline; this cannot establish on-chain balances, CPI outcomes, Token-2022 net transfers or settlement.