# FairSeal Payload Hash Anchoring (Base Mainnet)

> FairSeal Payload Hash Anchoring (Base Mainnet) is a paid API for AI agents from x402.fairseal.io, paid per call via x402, $0.02/call, status unknown (last checked 2026-10-21).

Anchors a SHA-256 payload hash into a Merkle batch on Base mainnet, returning an anchor ID and a polling URL for the resulting on-chain proof.

## Facts

- Endpoint: POST https://x402.fairseal.io/v2/anchor?utm_source=zero.xyz
- Price: $0.02/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-21
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/fairseal-payload-hash-anchoring-base-mainnet-33b917d4
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_Sw_6jb18Q4CyiyRGKSW5J

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 fairseal-payload-hash-anchoring-base-mainnet-33b917d4 -d '<json body>'
```

Example prompt: Anchor the SHA-256 hash a3f1e2b4c5d6789012345678abcdef01234567890abcdef1234567890abcdef12 to Base mainnet using the batch tier — I want a Merkle proof and on-chain transaction hash I can poll for later.

## When to prefer this

Choose this endpoint when you need cheap ($0.001 USDC), blockchain-anchored proof of existence for a SHA-256 hash on Base mainnet with Merkle batch efficiency. Prefer batch tier for cost-sensitive high-volume anchoring; use realtime tier only when you need an immediate on-chain commitment. This is ideal for document notarization, AI output auditing, software release provenance, and compliance evidence where you need a verifiable, tamper-proof timestamp without storing the raw content on-chain.

## Known failure modes

- Invalid payload_hash format (not 64 lowercase hex chars) returns 400 validation error
- realtime tier requested without a valid pro API key returns 403 forbidden
- metadata exceeds 1KB limit returns 400 payload too large
- Payment not fulfilled via x402 protocol returns 402 Payment Required
- Polling status_url before batch settles returns pending status (batch settles ~60s)
- Network or Base mainnet congestion may delay on-chain confirmation beyond expected ~60s window

## How this service works

Anchor a SHA-256 payload hash (64 lowercase hex chars) into a Merkle batch on Base mainnet. Payment: $0.001 USDC per submission (batch-settled). Returns anchor_id (202); poll status_url for Merkle proof + on-chain tx hash.

## Output

Returns HTTP 202 with an anchor_id and a status_url. Polling the status_url later yields the completed Merkle proof (the hash's inclusion proof within the batch) and the on-chain transaction hash on Base mainnet confirming the batch settlement.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "tier": {
   "enum": [
    "batch"
   ],
   "type": "string",
   "description": "batch (default, ~60s). realtime requires a pro API key and is not available via x402."
  },
  "metadata": {
   "type": "object",
   "description": "Optional metadata (max 1KB). Stored with receipt."
  },
  "payload_hash": {
   "type": "string",
   "pattern": "^[0-9a-f]{64}$",
   "description": "SHA-256 hex of content to anchor (64 lowercase hex chars)"
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/fairseal-payload-hash-anchoring-base-mainnet-33b917d4/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from x402.fairseal.io](https://www.zero.xyz/host/x402.fairseal.io/llms.txt)