# Clearsigned

> The trust layer with no stake in the outcome. Machine-callable verification
> of claims, paid per call over x402 (USDC on Base), returning Ed25519-signed,
> evidence-bearing receipts. Agents verifying agents.

## What this service does

POST a claim (and optional supporting evidence) to the paid endpoint; receive
a signed receipt recording every check that ran, with evidence per check
(hashes, lengths, observations — never bare booleans). Pricing is flat per
call and NEVER depends on the verdict's direction: refunds exist only for
failure to deliver. The current verdict mode is honestly labeled inside every
receipt (`sku` and `stub_notice` fields).

## API (x402 — prepay per call)

- POST https://verify.clearsigned.com/verify/t1 — $0.05 USDC (Base mainnet,
  eip155:8453). Body: {"claim": "<string, max 10k chars>", "evidence":
  ["<string>", ...]}. Unpaid requests receive HTTP 402 with x402 v2
  PaymentRequirements in the PAYMENT-REQUIRED response header.
  IMPORTANT FOR CLIENTS: verdicts take 10-40 seconds (live evidence
  gathering + adversarial checks). Configure your x402 client's response
  timeout to at least 60 seconds; clients that abandon in-flight paid
  requests forfeit delivered receipts.
- GET https://verify.clearsigned.com/health — free. Service mode, network,
  signing key id, ledger height/tip.
- GET https://verify.clearsigned.com/pubkey — free. The Ed25519 public key and
  the exact offline verification recipe. PIN THIS KEY; never trust the key
  embedded in a receipt.
- https://clearsigned.com/quality.json — free. Live counts from the per-call
  log (deliveries, verdict mix, abstention, latency percentiles; never a claim
  or a payer), refreshed every ten minutes; https://clearsigned.com/stats.json
  carries the serving calibration figures. Human view: /quality.
- GET https://verify.clearsigned.com/.well-known/x402 — free. The x402 discovery
  manifest (resources, method, price, network) for indexers and agents.
- A PAID request with no claim (empty body, missing/blank `claim`) is refused
  with HTTP 400 before settlement and shows the example body; the payment
  authorization is not used, nothing is charged. Unpaid requests always get
  the 402 quote, with or without a body.
- GET https://verify.clearsigned.com/ledger/verify — free, rate-limited. Walks
  the full hash-chained receipt ledger, anchored, and reports validity with
  evidence. Every paid receipt and its on-chain settlement are separate chain
  events joined on (payer, nonce).

## Verifying a receipt offline

Strip the `signature` field, canonicalize the remaining JSON with sorted keys
and separators (",", ":") in UTF-8, then verify `signature.signature` (hex)
over those bytes with the ed25519 key published at /pubkey.

## Payment

x402 v2, scheme "exact", USDC on Base mainnet. Any x402-capable client works
(e.g. Coinbase payments-mcp). Half the x402 economy is test traffic; receipts
here are backed by a public, tamper-evident ledger and on-chain settlement —
verify, don't trust.

## Data handling

Customer payloads never leave through receipts, ledger entries, or logs —
digests and observations only. The production instance passes a recorded
isolation acceptance: it holds no knowledge beyond its service corpus.

## Integrations

- https://clearsigned.com/integrations — MCP server (clearsigned-mcp on PyPI and
  Smithery), LangChain tool (langchain-clearsigned on PyPI), Vercel AI SDK tool
  (clearsigned-ai-sdk on npm). Each pays per call from the caller's own key and
  verifies the receipt offline against the pinned key.

## Learn

Explainers for agent developers, grounded in this service's own operating data
(the production calibration curve, the receipt chain — no vendor claims):

- https://clearsigned.com/learn/ — the ten questions agent developers ask,
  with the production calibration figure
- https://clearsigned.com/learn/what-is-agent-output-verification/ — what
  verification requires
- https://clearsigned.com/learn/why-does-my-agent-hallucinate-citations/ —
  why citations lie, and how to catch it automatically
- https://clearsigned.com/learn/agent-qa-patterns/ — Agent QA patterns:
  test suites for non-deterministic systems

## More

- Readiness data: https://clearsigned.com/sweep (State of Agent Readiness,
  25-site audit-by-execution baseline, 2026-08-12)
- Operator: Fortress Systems LLC
- Contact: the domain's registrant.