ACH Standard Entry Class code

ACK SEC code: ACH Payment Acknowledgment

Zero-dollar acknowledgment of a CCD credit, sent by the RDFI.

What ACK is for

An optional zero-dollar entry acknowledging that a CCD credit was received.

Sent back to the Originator of a CCD credit.

Authorization

Agreed between the trading partners; no money moves.

A summary in general terms: the Nacha Operating Rules set the exact requirements, and Regulation E adds its own for consumer accounts.

Debits, credits and addenda

Zero-dollar entries only.

Addenda on a forward entry: type 05, up to 1 per entry. Returns (addenda type 99) and Notifications of Change (type 98) follow their own layouts whatever the SEC code.

Returns

The general return timeframes apply: 2 banking days after settlement for most codes, with the account-type rules of the entry’s Receiver.

NOC codes and return codes have their own pages.

Common mix-ups

ACK acknowledges a CCD credit; ATX acknowledges a CTX credit.

Where ACK sits in a NACHA file

The SEC code is a batch-level field: positions 51–53 of the Company/Batch Header record (type 5), so every entry in the batch shares it, and a file that mixes SEC codes puts them in separate batches. The batch header also carries the Company Name, Company Identification and Company Entry Description that Receivers see on their statements.

To read a ACK batch record by record, open the file in NACHA Navigator, a free sister site that parses NACHA files in your browser without uploading them.

Look up ACK from code or an AI agent

REST · GET · no key

curl -s 'https://payopskit.com/api/v1/codes/ACK?set=ach-sec'

MCP · tools/call · lookup_payment_code

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "lookup_payment_code",
    "arguments": {
      "code": "ACK",
      "set": "ach-sec"
    }
  }
}

Response (JSON)

{
  "code": "ACK",
  "set": "ach-sec",
  "found": true,
  "matches": [
    {
      "set": "ach-sec",
      "code": "ACK",
      "name": "ACH Payment Acknowledgment",
      "description": "Zero-dollar acknowledgment of a CCD credit, sent by the RDFI.",
      "details": {
        "direction": "zero",
        "addendaTypes": "05",
        "minAddenda": 0,
        "maxAddenda": 1
      }
    }
  ]
}

The MCP server is at /mcp (Streamable HTTP, no key); the API and MCP docs show how to connect Claude, Cursor or VS Code.

Source: Nacha Operating Rules, Appendix Three (Standard Entry Class codes). The code’s name is the Rules’ own; the rest is this site’s plain-language summary, informational and not a substitute for the Rules or your ODFI agreement.