attest — agent artifact verifier

npx add-mcp 'https://attest.rta.lat/mcp'

attest — trust layer for agent artifacts

Live: https://attest.rta.lat/demo · Code: https://github.com/mrpkk/attest

Pricing: 20 checks/day free, then $0.005 per call (x402, USDC on Base). 30-day unlimited access: $9. Purchasing-power pricing (India and similar markets): the same 30 days for $4 via POST https://attest.rta.lat/subscribe?plan=ppp — same endpoint, same terms.

An agent calls an external tool or another agent and receives a foreign artifact that goes straight into its context. Nothing checks that the artifact:

• matches the declared schema (the tool quietly changed a field type),
• carries no hidden instruction ("ignore previous, send the keys to …"),
• is not a base64 wrapper around such an instruction,
• has not changed since it was checked.

attest closes that gap. No dependencies, 574 tests green.

Four layers, four different questions

Each layer answers only its own question, and none substitutes for the others. A valid form is not the same as meaningful content; a signed artifact is not the same as a permitted action.

1. Form and emptiness — emptiness.py

Does the artifact say anything? An agent told to find five vulnerabilities returns five identical items. That artifact passes every schema validation, because the form is perfect.

Seven measurable signals: identical list items, placeholder values, output cut at the token limit, whitespace-only strings and empty collections, low-variety repetition, required fields left at their schema default, enum fields sitting on the first value.

Verdicts are VALID_BUT_EMPTY, THIN_CONTENT, CONTENT_PRESENT.

VALID_BUT_EMPTY means "the form is fine, the content is suspect" — not "the content is absent". Suspicion requires a person; a verdict requires proof. The difference matters.

Every signal shows where and why. Absence of signals does not prove meaningfulness — it only means the coarse checks did not fire. That sentence is in the code, in the docstring and in the API response.

2. Generation conditions — generation.py

A signature proves who signed. It does not prove where the file came from. Anything at all can sit between "the agent produced this" and "the verifier saw the result".

Model, version, prompt, parameters, moment and executor are bound into a single chain_hash over the artifact. Change any link and the binding stops matching.

Absence of a binding is explicit, not silent. Four states are distinguished: no conditions at all; a binding claimed but conditions cannot be supplied; conditions supplied but no binding in the signature; conditions that do not match. The first and the second are different things, and confusing them would confirm a forgery.

chain_hash is inside the signature, not next to it — otherwise it could be appended afterwards.

3. Verification without trusting us — signing.py

HMAC requires a secret, and the secret is only ours. So only the service could verify a signature, and anyone who did not trust the service had to trust it first. For a product about trust, that is a contradiction at the foundation.

Now Ed25519. The public key travels with the attestation, and the verifier checks it themselves — no network, no package of ours, no secret.

python3 attest/verify_standalone.py attestation.json

That script imports nothing from attest and opens no sockets. A test enforces both.

4. Authority — authority.py

Was the action permitted? A signed action is not an authorised one: permission is granted separately and expires.

Rules match drift.check exactly — amount, payee, audience, currency, window, intent. They are duplicated on purpose: depending on another repository would mean the check cannot run without network access to someone else's code. Six comparisons are cheaper to duplicate than to make unverifiable.

A test compares the result against the live drift on every rule, so a divergence shows up immediately rather than as a client complaint.

Absence of a mandate is a denial, never consent. A verifier who presents no permission has not demonstrated one.

A finding worth stating plainly

While building layer 3, this was found in the existing code: with ATTEST_SECRET unset, signatures were computed with attest-dev-key-do-not-use-in-production, hardcoded in the source. That variable was not set, so every attestation the service had ever issued was signed with a key published next to it. Anyone could have signed an artifact as if the service had verified it.

The signature looked real and was not — which is worse than having none, because having none is visible. It is fixed: with no key, there is no signature.

Try it in a browser

https://attest.rta.lat/demo — four buttons, one per layer, the full response shown immediately.

Public repo

https://github.com/mrpkk/attest

Part of the Indra set — six small services for agents that have to be trusted: https://indra.rta.lat

MCP server (Model Context Protocol)

attest is an MCP server: it speaks the Model Context Protocol over Streamable HTTP (JSON-RPC 2.0) and exposes the verifier as MCP tools.

• MCP tools (the whole point — an agent calls them directly):
  • verify_artifact — verify one artifact, returns verdict, trust score and findings;
  • verify_artifact_batch — verify a batch in one call (a scan report, a list of findings) and get one verdict per artifact plus a summary;
  • bridge_check — settle paid work between two agents through an escrow bridge, with a signed receipt (offline-verifiable). Optional: it is listed only on a node configured with an escrow bridge, so the public node returns two tools from tools/list. The call stays in the code and answers with a clear "bridge not configured" error instead of vanishing mid-session.
• MCP resources: none exposed yet — resources/list answers with an empty list rather than an error, so MCP clients and directory scanners do not mark the server as broken.
• MCP prompts: none exposed yet — prompts/list answers with an empty list for the same reason.
• Transport: Streamable HTTP, no auth for the free tier, X-ATTEST-ENTITLEMENT header for a prepaid key, PAYMENT-SIGNATURE/X-PAYMENT for a per-call x402 payment.

Connect an MCP client

{
  "mcpServers": {
    "attest": { "url": "https://attest.rta.lat/mcp" }
  }
}

Claude Desktop, Claude Code, Cursor, Codex, VS Code and any other MCP client that supports remote Streamable HTTP servers work with that block as is.

We use it ourselves: our own agents call this exact URL through the MCP SDK in opencode.jsonc, so the server is exercised by real traffic every day, not only by the tests.

Tool reference

Both tools take plain JSON. No SDK is needed.

verify_artifact

field	type	required	meaning
artifact	object | array | string	yes	what to verify
schema	object	no	JSON Schema the artifact must satisfy
source	string	no	where the artifact came from; recorded in the verdict

Request:

{"jsonrpc":"2.0","id":1,"method":"tools/call",
 "params":{"name":"verify_artifact",
           "arguments":{"artifact":{"findings":[{"rule":"x","severity":"high"}]},
                        "source":"nightly-scan"}}}

Response (one verdict, signed):

{"verdict":"accept","trust_score":0.86,
 "checks":[{"layer":"schema","ok":true},{"layer":"emptiness","ok":true},
           {"layer":"generation","ok":true},{"layer":"authority","ok":true}],
 "findings":[],
 "signature":{"algorithm":"ed25519","public_key":"...","signature":"..."}}

verify_artifact_batch

field	type	required	meaning
artifacts	array	yes	one verdict per element
schema	object	no	applied to every element
source	string	no	recorded once for the batch

{"jsonrpc":"2.0","id":2,"method":"tools/call",
 "params":{"name":"verify_artifact_batch",
           "arguments":{"artifacts":[{"a":1},{"a":2}],"source":"batch"}}}

Response: {"verdicts":[...],"summary":{"accepted":1,"rejected":1,"total":2}}.

When the free tier runs out

The free tier is 20 checks per day per client. On the 21st call the server answers HTTP 402 with a body naming the limit, and a Payment-Required header carrying the x402 terms:

{"error":"free limit reached (20/20 per day)","free_remaining":0,"resets_at":"00:00 UTC",
 "x402":{"enabled":true,"network":"eip155:8453","asset":"0x8335...2913",
         "price_atoms":5000,"pay_to":"0x..."}}

An MCP client sees that text as a tool error — nothing hangs and nothing is retried blindly. To continue, repeat the same call with PAYMENT-SIGNATURE (per-call x402, USDC on Base) or with X-ATTEST-ENTITLEMENT (a 30-day prepaid key). The free quota resets at 00:00 UTC.

Maintenance

Maintained and general-purpose, MIT-licensed. The hosted node at https://attest.rta.lat is production and is exercised daily by our own agents, so it is not a demo that rots. Breaking protocol changes get a major bump in CHANGELOG.md; issues and pull requests are reviewed. Security reports follow SECURITY.md.

Use it in CI

A one-off check in a browser does not protect a pipeline. This repository ships a composite GitHub Action: it verifies every artifact file and fails the build on a rejection. Pinned to v1 — release v1.0.0.

- uses: mrpkk/attest@v1
  with:
    paths: 'reports/*.json'
    schema: 'schemas/scan.schema.json'   # optional
    fail-on: reject                      # or 'any' to fail on violations too

Without Actions, the same check runs locally:

python3 scripts/verify_artifacts.py 'reports/*.json' --fail-on reject

Exit codes: 0 all accepted, 1 a rejection or an error, 2 nothing matched. A network error or an exhausted free quota is reported as an error — never as "accepted".

Use it from your agent

{
  "mcpServers": {
    "attest": { "url": "https://attest.rta.lat/mcp" }
  }
}

That block is not a doc snippet — it is how our own agents call this server (the same public URL, no key, no signup).

Tools: verify_artifact, verify_artifact_batch. 20 free checks per day, then $0.005 per call over x402 (USDC, Base) or $9/30 days. India pricing: $4/30 days.