# SignalHarness EIP-191 Message Hash

> SignalHarness EIP-191 Message Hash is a paid API for AI agents from signalharness.ai, paid per call via x402, $0.005/call, status unknown (last checked 2026-09-13).

Computes the EIP-191 Ethereum personal sign message hash for a given input string

## Facts

- Endpoint: POST https://signalharness.ai/api/agent/services/message_hash_eip191/invoke
- Price: $0.005/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-13
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/signalharness-eip-191-message-hash-972e76a5
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_0bToKKn0kMhQpNcYhc6Zk

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 signalharness-eip-191-message-hash-972e76a5 -d '<json body>'
```

Example prompt: Can you compute the EIP-191 personal sign hash for this message: 'Authorize wallet connection for user 0xAbC123 at timestamp 1720000000'?

## When to prefer this

Choose this endpoint when you need a deterministic, auditable EIP-191 personal_sign message hash computed as a paid, receipted API call with on-chain payment provenance on Base — useful when you need cryptographic evidence of what was hashed and by whom, rather than computing locally. Prefer local computation for high-volume or latency-sensitive use cases; use this when auditability, receipts, or agent-native x402 payment rails are required.

## Known failure modes

- Input string too short (below minLength of 2) — validation error returned
- Input string exceeds 65536 characters — request rejected
- Malformed JSON body — HTTP 400
- Payment failure or insufficient USDC balance — HTTP 402 with payment details
- Service unavailable — HTTP 5xx with error message

## How this service works

Explore 330 pay-per-call x402 API services and 27 agent-native digital products, with Base USDC pricing, secure Polar checkout, and free discovery.

## Output

Returns the EIP-191-prefixed keccak256 hash (message_hash) of the supplied input string, along with a list of any warnings about the input, the evidence_scope indicating data provenance, a replay-detection flag, and a full execution receipt including payment ID, network (Base), amount charged in atomic USDC units, and result SHA-256 for integrity verification.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "message_hash_request": {
   "type": "string",
   "maxLength": 65536,
   "minLength": 2
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "replay": false,
  "result": {
   "warnings": [
    "Hashing does not sign or prove authorship."
   ],
   "service_id": "message_hash_eip191",
   "message_hash": "{\"encoding\":\"utf8\",\"hash\":\"0x43c3ce76740c0da4b23274e62f830220acfc4cb8d66dc3c373c6f58db8620442\"}",
   "evidence_scope": "caller_supplied_data"
  },
  "status": "succeeded",
  "receipt": {
   "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
   "usage": [],
   "status": "succeeded",
   "network": "eip155:8453",
   "artifacts": [],
   "endedAtMs": 0,
   "latencyMs": 0,
   "paymentId": "example-payment",
   "receiptId": "example-receipt",
   "requestId": "example-request",
   "serviceId": "message_hash_eip191",
   "executionId": "example-execution",
   "startedAtMs": 0,
   "amountAtomic": "5000",
   "resultSha256": "8f11719349cff4ac11654fa2d877a1171776a9cc3eee1fc4857fac9b56d1829f",
   "serviceVersion": "1.0.0",
   "settlementReference": "0x0000000000000000000000000000000000000000000000000000000000000000"
  },
  "artifacts": [],
  "requestId": "example-request",
  "executionId": "example-execution"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/signalharness-eip-191-message-hash-972e76a5/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from signalharness.ai](https://www.zero.xyz/host/signalharness.ai/llms.txt)