# Anchor-X402 Signature Attestation & Dual-Chain Anchoring

> Anchor-X402 Signature Attestation & Dual-Chain Anchoring is a paid API for AI agents from api.anchor-x402.com, paid per call via x402, $0.01/call, status unknown (last checked 2026-09-15).

Verifies a cryptographic signature over (input_hash, output_hash, decision) and anchors the attestation result to both Base and Solana mainnet for tamper-evident proof.

## Facts

- Endpoint: POST https://api.anchor-x402.com/v1/attest
- Price: $0.01/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-15
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/api-anchor-x402-com-62ad6770
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_F7UhDyA-HFKHrOGR7-Eze

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability api-anchor-x402-com-62ad6770 -d '<json body>'
```

Example prompt: Verify this signature over my AI decision — input_hash 0xabc123, output_hash 0xdef456, decision 'approved' — and anchor the attestation to both Base and Solana so I have a tamper-proof on-chain record.

## When to prefer this

Choose this endpoint when you need cryptographic proof that a specific AI decision or computation was made from specific inputs — particularly for audit trails, AI governance, compliance, or any scenario requiring tamper-evident dual-chain notarization of a signed (input, output, decision) tuple. Prefer over a pure hash-anchor endpoint when you also need to verify a signature as part of the same atomic operation.

## Known failure modes

- Invalid or malformed signature returns signature verification failure
- Mismatched input_hash or output_hash causes attestation rejection
- Chain anchoring failure if Base or Solana RPC is degraded
- Payment not processed or insufficient USDC balance returns 402
- Malformed hash format returns 400 bad request
- Duplicate attestation of identical tuple may return conflict or idempotency error

## How this service works

Attest a decision: sign (input_hash, output_hash, decision) yourself, or omit the signature and the treasury signs — then dual-chain anchor. Free re-verify at /v1/attest/verify. $0.01 USDC.

## Output

Returns a verification result confirming signature validity, plus dual-chain anchor proofs containing transaction IDs on both Base and Solana mainnet, creating an immutable on-chain attestation record of the (input_hash, output_hash, decision) tuple.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method",
    "bodyType",
    "body"
   ],
   "properties": {
    "body": {
     "required": [
      "input_hash",
      "output_hash",
      "decision"
     ],
     "properties": {
      "scheme": {
       "enum": [
        "eip191",
        "ed25519"
       ],
       "type": "string"
      },
      "decision": {
       "type": "string",
       "maxLength": 64,
       "description": "Free-form short label."
      },
      "signature": {
       "type": "string",
       "description": "0x-prefixed hex (eip191) or base58 (ed25519)."
      },
      "input_hash": {
       "type": "string",
       "description": "64-char hex SHA-256 of the agent's input."
      },
      "output_hash": {
       "type": "string",
       "description": "64-char hex SHA-256 of the agent's output / decision payload."
      },
      "signer_pubkey": {
       "type": "string",
       "description": "Required for ed25519. Ignored for eip191."
      }
     }
    },
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "POST",
      "PUT",
      "PATCH"
     ],
     "type": "string"
    },
    "bodyType": {
     "enum": [
      "json",
      "form-data",
      "text"
     ],
     "type": "string"
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object"
    }
   }
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "base": {
   "tx": "0x...",
   "explorer_url": "https://basescan.org/tx/0x..."
  },
  "signer": "0xFE708ED41DE893390240C95A801A49ed8F974702",
  "solana": {
   "tx": "...",
   "explorer_url": "https://solscan.io/tx/..."
  },
  "decision": "APPROVED",
  "signed_at": 1746820000,
  "merkle_root": "10ce4d38b59746febff0651a2d19d5da9a36f4e209e4333414d6d8e4abad898b",
  "signer_verified": true
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/api-anchor-x402-com-62ad6770/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.anchor-x402.com](https://www.zero.xyz/host/api.anchor-x402.com/llms.txt)