# Fletcher Checksum (Integrity Checksums)

> Fletcher Checksum (Integrity Checksums) is a paid API for AI agents from checksum.openverbs.com, paid per call via x402, $0.002/call, status unknown (last checked 2026-09-15).

Computes a Fletcher-16 or Fletcher-32 checksum over an input payload for data integrity verification.

## Facts

- Endpoint: POST https://checksum.openverbs.com/v1/fletcher
- Price: $0.002/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-15
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/fletcher-checksum-integrity-checksums-18b5705c
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_sscUX146XoTbITIwShua5

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 fletcher-checksum-integrity-checksums-18b5705c -d '<json body>'
```

Example prompt: Compute a Fletcher-32 checksum over this UTF-8 string so I can verify data integrity: 'Hello, world! This is my payload to checksum.'

## When to prefer this

Use this endpoint when you need a fast, lightweight, non-cryptographic checksum for error detection — particularly the Fletcher algorithm specifically. Prefer this over CRC or Adler endpoints when your downstream system expects Fletcher-compatible checksums. Suitable for data integrity checks on text, hex, or base64-encoded binary payloads up to 5MB.

## Known failure modes

- Payload exceeds 5MB maximum size — request rejected
- Invalid encoding value (not utf8, hex, or base64) — validation error
- Base64 or hex data that is malformed/unparseable — decoding error
- bits value not 16 or 32 — schema validation failure
- Missing required 'data' field — 400 error

## How this service works

Fletcher checksum over the input payload, in 16-bit (default) or 32-bit width. Returns the checksum as an unsigned integer and hex. Non-cryptographic.

## Output

Returns the computed Fletcher checksum value (16-bit or 32-bit integer) for the provided input payload, calculated according to the Fletcher algorithm over the decoded data.

## 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": {
     "type": "object",
     "required": [
      "data"
     ],
     "properties": {
      "bits": {
       "enum": [
        16,
        32
       ],
       "type": "integer",
       "description": "Checksum width in bits. Default 16."
      },
      "data": {
       "type": "string",
       "maxLength": 5000000,
       "description": "Input payload."
      },
      "encoding": {
       "enum": [
        "utf8",
        "hex",
        "base64"
       ],
       "type": "string",
       "description": "How to decode `data`. Default utf8."
      }
     },
     "additionalProperties": false
    },
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "POST"
     ],
     "type": "string"
    },
    "bodyType": {
     "enum": [
      "json",
      "form-data",
      "text"
     ],
     "type": "string"
    }
   },
   "additionalProperties": false
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/fletcher-checksum-integrity-checksums-18b5705c/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from checksum.openverbs.com](https://www.zero.xyz/host/checksum.openverbs.com/llms.txt)