Tersign is the record layer for the machine age: what AI agents do and pay, counter-signed by a party outside the deal, anchored where no one can move it, checkable by anyone. Machines can generate anything but the past. Sellers sign EIP-712 receipts; Tersign chains them per seller and counter-signs every entry. When the dispute comes, the transcript is already sealed.
Venues rotate. The transcript endures.
Verify a Real Entry — Right Now
No account. No API key. This is the genesis receipt, seq 1 on the production chain. It is a self-signed demo: Tersign signed it as seller and named its own key as payer, so its EIP-712 payload signature (domain {name: "x402 receipt", version: "1", chainId: 1}, per the x402 offer-and-receipt extension) recovers to the payload's own payer, 0x36f82906859E5B0bd076069f8cdfAea355358b14. /v1/genesis serves the record with the recipe for each of its signatures.
npx tersign verify 0xe5874f1ffe87f0a6dd9eb157730f67b86ee4538b125fe30fcc4e165213dd3fc4
ledger: https://tersign.ai
reports: found, counter-signed chain intact (seller tersign-first, seq 1, …) — not checked locally
VALID (ledger-reported) — https://tersign.ai reports the record and its counter-signed chain; nothing was verified locally
npx tersign verify <receipt.json | 0xdigest> [--signer 0xaddr] [--ledger url] takes a receipt file or a digest.
- A receipt file is checked locally and touches no network unless you add
--ledger. The EIP-712 signature is recovered, and--signercompares it with the issuer's address, which you take from the issuer through a channel you trust, never from the receipt. Without--signerthe signer is reportedUNAUTHENTICATEDand the verdict readsVALID (signer UNAUTHENTICATED): recovery yields an address for any payload, so an edited receipt reaches that same line with a different address. - A receipt signed with a published test key (the 20 Hardhat/Anvil default dev-mnemonic accounts, or private key 1, 2 or 3) is flagged
published test keyeither way: anyone can produce that signature. Fields in the file that the signature does not cover are listed by name, and a signed field in another JSON type ("version": "1") is refused. The signature must be the one canonical encoding (0x+ 130 lower-case hex digits, v 27/28, low-s): a re-encoding of a genuine signature (v 0/1, upper-case hex, the high-s twin) also recovers the issuer, under a different digest, so it is refused. - The file is a receipt,
{receipt, record}, or an evidence-bundle record file (records/NNNNNN.json, verdict qualifiedrecord artifact only: its chain fields are not checked here). Anything else that nests a receipt — a second receipt at the top level, or any other field beside it — is refused, because a reader would take those fields for the receipt that was checked. Duplicate keys and non-integer numbers are refused too. - A digest is looked up on the Tersign ledger unless
--ledgernames another; the answer is that ledger's own counter-signed chain check, nothing in it is re-verified locally, and the verdict says so (VALID (ledger-reported)). The ledger consulted is always printed. The local check is the receipt file with--signer.
Exit status: 0 valid (read the last line — for a file, a bare VALID means a bound, non-test-key signer on a receipt or {receipt, record}; a digest is always VALID (ledger-reported)), 1 invalid, 2 usage (an unknown or valueless flag, --signer with a digest, a missing or non-JSON file). Prefer raw HTTP? The same proof, no CLI:
curl https://tersign.ai/v1/receipts/0xe5874f1ffe87f0a6dd9eb157730f67b86ee4538b125fe30fcc4e165213dd3fc4/verify
One-Call Disclosure Evidence
Counter-signed evidence that your agent presented a disclosure — one command, no account:
npx tersign disclose "You are chatting with an AI assistant." --medium chat --agent-id my-agent
The text is digested locally (only the digest travels — data-minimization by construction). Your key signs the record; the ledger counter-signs it into a per-signer hash chain whose head is submitted for Bitcoin anchoring on a six-hourly cron. The first call self-provisions a free signer-keyed account bound set-once to your key, unless that key is already registered to an API-key ledger account (409: submit through that account) or the day's provisioning caps are reached (429). Key resolution: TERSIGN_SELLER_KEY env → macOS keychain tersign-signer → ~/.tersign/signer.key, created on first use. Free tier is quota- and rate-limited — limits. What this is: independently verifiable evidence the disclosure was attested at that time. What it is not: a compliance certification.
Chain of Custody
Every entry takes the same path: the seller signs the receipt (EIP-712, x402 offer-receipt extension) → Tersign computes the keccak256 canonical digest → the digest joins that seller's hash chain, each seq n bound to seq n−1 → the neutral ledger counter-signs (secp256k1) → anyone verifies, and any venue gets a serialized envelope.
Since 2026-08-28 each anchor stamps a chain commitment — an accumulator over every counter-signed link — so one anchored digest covers the whole prefix; rows anchored earlier bind the head record only and say so (subjectSchema).
graph LR
A["agent transaction<br/>x402"] --> B["seller-signed receipt<br/>EIP-712"]
B --> C["canonical digest<br/>keccak256"]
C --> D["per-seller hash chain<br/>seq n binds seq n−1"]
D --> E["neutral counter-signature<br/>secp256k1 ledger"]
E --> F["verifiable by anyone<br/>venue-formatted envelope"]
Diagram renders on GitHub. On npm, the paragraph above IS the diagram.
A refund record references the record it corrects by digest (refundOf). Disputes attach to the digest with objective reason codes. Party statements are structurally segregated behind an UNVERIFIED marker — the evidence stays prompt-injection-hardened.
Enter the Record
npm i tersign
withAssure() wraps your x402 fetch handler. On a settled call it signs a receipt and a compliance-fields record with your key and merges both into the PAYMENT-RESPONSE header, the base64 JSON settlement response x402 wallets already read; the response body is left alone. When it cannot issue them (a settlement network that is neither CAIP-2 nor a v1 name in its table, such as "bsc", or a ledger error), the paid response goes out exactly as your handler returned it, with no receipt, and the error goes to onError (default: console.error); with idempotency on, a retry with the same payment id replays that response. On a 402 it adds compliance-fields to the PAYMENT-REQUIRED header. When the Assure is built with a ledger, the ledger counter-signs the receipt into your chain; the compliance-fields record is not counter-signed, and is bound to its receipt only by your own signature, which covers the receipt's digest. Without a ledger, both carry your signature alone.
import { privateKeyToAccount } from 'viem/accounts';
import { Assure, withAssure } from 'tersign';
const signer = privateKeyToAccount(process.env.TERSIGN_SELLER_KEY as `0x${string}`);
const assure = new Assure({ signer, issuer: { name: 'Example API Ltd', jurisdiction: 'HK' } });
export default { fetch: withAssure(app.fetch, { assure }) }; // app: your x402-protected Hono app
A wallet finds the receipt at extensions["offer-receipt"].info.receipt and the record at extensions["compliance-fields"].info.record of the decoded PAYMENT-RESPONSE (X-PAYMENT-RESPONSE on x402 v1). Sellers whose readers still parse the JSON body can pass legacyBodyPlacement: true for one more minor release.
The full register:
Capture at the MCP Boundary — tersign intercept
An agent's tool calls are usually recorded, if at all, inside the agent's own code. Record them at the wire instead:
npx tersign intercept -- npx your-mcp-server
The proxy is a pure observer: bytes reach the server and the client exactly as sent, in
order, unmodified. It runs on your side with your key, so each record is still your own report.
Every tools/call that gets a response becomes an ActionRecordV1 signed by your key and, when a ledger is
configured, counter-signed into a hash chain; a call cancelled or never answered leaves no record. Digests only: each record carries a digest of
the call's arguments and one of its result or error (canonical JSON), never the content. Records
go to a configured ledger, and fall back to a local ~/.tersign/intercepts-<date>.jsonl, where
they carry your signature only, so evidence is never silently dropped.
Experimental, and deliberately unopinionated about where the protocol lands: it implements the observation semantics of the audit-mode validator described in MCP SEP-2624 (Draft) as a transport-level proxy today, and is structured to move onto the interceptor primitive if and when that stabilizes. It makes no conformance claim to that draft.
For Agents — the MCP Server
npx tersign starts the MCP server (stdio). Official registry entry: io.github.tersignhq/evidence (active).
{
"mcpServers": {
"tersign": {
"command": "npx",
"args": ["tersign"]
}
}
}
No configuration is needed. With TERSIGN_SELLER_KEY unset or empty, the server signs with the key in the macOS keychain (tersign-signer) or ~/.tersign/signer.key, and generates one there on first run; the key never leaves your machine. Set TERSIGN_SELLER_KEY only to bring your own.
Tools — issue_receipt · verify_receipt · verify_compliance_record · record_disclosure · record_refund · open_dispute · submit_dispute_evidence · adjudicate_dispute · get_dispute
No tool moves money: issue_receipt and record_refund record payments and refunds that already happened. adjudicate_dispute runs deterministic triage and records its result: fixed rules, stated in the tool's description, classify a duplicate charge or an unanswered claim; the result is data, not an adjudication, and decides no legal claim. The duplicate-charge rule, R1, reads receipts from one seller to one payer that share one settlement transaction as one payment.
Cold to counter-signed in one session, with no configuration: call record_disclosure, then check the digest it returns with npx tersign verify <digest> (a bare digest checks against https://tersign.ai). issue_receipt counter-signs as well once TERSIGN_LEDGER_URL, TERSIGN_LEDGER_API_KEY and TERSIGN_LEDGER_SELLER_ID are set; without them it returns a locally signed, unchained receipt.
The agent skill tersign-evidence ships at tersignhq/skills.
The Live Record
- Ledger + dashboard — public verify page: https://tersign.ai/verify
- Census — hash-chained observations across the live x402 seller catalog, probed autonomously; the numbers are served live, never quoted stale: https://prober.tersign.ai/v1/prober/stats
- Conformance — RFC 8785 (JCS) canonical serialization, keccak256 digests, and the public two-sided vector suite (canonical bytes, number domain, content address, chain continuity, completeness, anchored existence, phase separation, offer binding, independence — every criterion carrying both an accepting and an adversarial vector): tersignhq/evidence-record-conformance. Reproduce the bytes and your implementation is conformant — in any language.
- Standards — the
compliance-fieldsextension — a typed compliance-record schema plus four evaluator-side disqualifications (independence, completeness/existence, economic-phase separation, and commitment scope — an independence claim reaches exactly as far as the record's own commitments), each executable as a two-sided conformance vector — is under review upstream (x402-foundation/x402#2853) and referenced in the x402 TSC's evidence-record charter agenda (tsc#4). The merged offer-receipt spec already carries post-session verification guidance — signer authorization evaluated as ofissuedAt, with mutable-source rotation handled explicitly (#2811, merged); the completeness, independence, existence and phase disqualifications are the open extension's normative core.
Machine Surfaces
Full URLs, readable without auth. If you are an agent, start here.
Contributing — read this before opening a pull request
tersignhq/tersign-js is a publish snapshot, not the development repository. Each release
overwrites src/, test/ and the packaged manifests wholesale from an upstream working tree, so
a pull request opened here cannot be merged in any durable way — the next release would silently
erase it. That is a property of the pipeline, not an assessment of the change.
Contributions are wanted; the mechanics just have to route around that:
- Bugs, questions, spec disagreements — open an issue here. Issues are read, and they are the right surface for anything that does not need to touch this tree.
- Code changes — open the issue first with the diff or a description. The change is applied upstream preserving your authorship, released, and the issue closed with a link to the version that carries your work. Credit follows commit authorship, not a merge badge.
- Conformance disagreements — the two-sided vector suite at tersignhq/evidence-record-conformance takes pull requests normally, and is the better venue for "this criterion is wrong" or "here is the case it misses." External vectors have been merged there.
If a maintainer ever asks you to reopen work elsewhere because of this, that is why.
