Skip to content

MS Agent Character file (.acs)

.acs file signature | application/octet-stream

MS Agent Character (.acs) is a file format created by Microsoft for packaging animated agent characters used by the Microsoft Agent platform. It was used in desktop assistants, interactive help systems, and applications that displayed speech, animation, and scripted behavior. The format is legacy and largely obsolete, and files from untrusted sources should still be handled cautiously as with any executable-associated content.

Safe

Magic Bytes

Offset 0
C3 AB CD AB

Sources: Gary Kessler

Extension

.acs

MIME Type

application/octet-stream

Byte Offset

0

Risk Level

Safe

Validation Code

How to validate .acs files in Python

Python 
def is_acs(file_path: str) -> bool:
    """Check if file is a valid ACS by magic bytes."""
    signature = bytes([0xC3, 0xAB, 0xCD, 0xAB])
    with open(file_path, "rb") as f:
        return f.read(4) == signature

How to validate .acs files in Node.js

Node.js 
function isACS(buffer: Buffer): boolean {
  const signature = Buffer.from([0xC3, 0xAB, 0xCD, 0xAB]);
  return buffer.subarray(0, 4).equals(signature);
}

How to validate .acs files in Go

Go 
func IsACS(data []byte) bool {
    signature := []byte{0xC3, 0xAB, 0xCD, 0xAB}
    if len(data) < 4 {
        return false
    }
    return bytes.Equal(data[:4], signature)
}

API Endpoint

GET /api/v1/acs
curl https://filesignature.org/api/v1/acs

See the full API documentation for all endpoints and parameters.

Frequently Asked Questions

What is a .acs file?

A .acs file is a MS Agent Character file file. MS Agent Character (.acs) is a file format created by Microsoft for packaging animated agent characters used by the Microsoft Agent platform. It was used in desktop assistants, interactive help systems, and applications that displayed speech, animation, and scripted behavior. The format is legacy and largely obsolete, and files from untrusted sources should still be handled cautiously as with any executable-associated content.

What are the magic bytes for .acs files?

The magic bytes for MS Agent Character file files are C3 AB CD AB at byte offset 0. These bytes uniquely identify the file format regardless of the file extension.

How do I validate a .acs file?

To validate a .acs file, read the first bytes of the file and compare them against the known magic bytes (C3 AB CD AB) at offset 0. This is more reliable than checking the file extension alone, as extensions can be renamed.

What is the MIME type for .acs files?

There is no officially registered MIME type for .acs files. Systems typically use application/octet-stream as a generic fallback when handling this format.

Is it safe to open .acs files?

MS Agent Character file (.acs) files are generally safe to open. They are classified as low risk because they primarily contain data rather than executable code. However, always ensure files come from a trusted source.