Agent Character Storage (file format)

Agent Character Storage
	Screenshot of Robby's ACS loaded in DeskBot.
Name	Agent Character File
Extension	.ACS
Initial release	September 8th, 1997 (28 years ago)
Operating system	Microsoft Windows
Predecessor	Actor Character Table

Not to be confused with Agent Character File.

An Agent Character Storage (ACS) (also known as an Agent Character File) is a file format used by Microsoft Agent for storing Microsoft Agent characters. It can be opened with any program that utilizes Microsoft Agent technology, and by web pages on Internet Explorer.

Usage

ACS files do not just contain metadata about the character, but also contain the entirety of the Agent Character themselves. They are the most common format users experience when using Microsoft Agent. They can be downloaded off of websites, used in Microsoft Agent software, and also even run on web pages that use Internet Explorer. However, they cannot be loaded through the internet.

Version differences

Microsoft Agent 1.5 and 2.0 ACS files are fundamentally different in how they store character data, even more so than the differences between Microsoft Actor 1.0 and 2.0 files.

Structure

A 1.5 ACS file is actually an OLE/Compound File container, meaning that Microsoft Agent 1.5 files are more similar to the older .DOC/.PPT/.XLS document formats than a Microsoft Agent 2.0 ACS file. Inside it are separate streams such as a 1.5 .ACF (commonly named char.acf) and multiple 1.5 .AAF animation files (for example anim1.aaf). The ACS simply bundles these web-style character files into a single installable package.

Because 1.5 ACS files are OLE containers around 1.5 ACF/AAF data, and 2.0 ACS files are flat binaries with a different layout and compression, tools written only for the 2.0 specification (such as the Microsoft Agent Decompiler) cannot directly decompile 1.5 characters. The Microsoft Agent 2.0 runtime is designed to load both 1.5 and 2.0 characters for backward compatibility, but this compatibility is provided by the Agent 2.0 engine itself and does not mean the underlying file formats are interchangeable or easily converted.

In 2025, a proof-of-concept decompiler for uncompressed Microsoft Agent 1.5 files was developed and made available on TMAFE.COM.

Animation & audio differences

In 1.5, animation data and sounds remain in the AAF streams, which use an older, undocumented 1.5-specific layout. Some 1.5 characters may store relatively "raw" image/audio blocks, while others keep these assets in a more heavily compressed form inside the AAFs.

In 2.0, animation frames, regions and audio are stored directly in the ACS file using the 2.0 compression scheme described in the specification, with images referenced through ACSIMAGEINFO entries and audio through ACSAUDIOINFO entries.

Differences from ACF

While ACFs rely off of ACA or AAF files for animations, ACS files do not require any external file, as all the needed data to load the character is contained in the file.
Unlike ACF files, ACS files can only be shown if the file is locally installed on the system, and are not able to be loaded through the internet.
Since ACS files are meant to be uses locally on the system for applications, that makes them much easier to download than ACFs.

Trivia

ACS files was not the default file type for compiling in the 1.5 version of MACE, with ACF being the default instead. However, ACS became the default type in the 2.0 version of MACE.
When compiling an ACS file, the file name shown for it in the Save File dialog is "Agent Character Storage".
- However, when right clicked, the name for the file type is as "Agent Character File", with the ACF file type name being "Agent Character File (HTTP Format)".
ACS files compiled in MACE 1.5 were unable to be decompiled with the Microsoft Agent Decompiler due to 1.5 characters being compiled differently from 2.0 characters.
While ACS files needed to be installed on the system in order to be loaded on websites, there were still many websites such as the Microsoft Agent Ring and Microsoft Agent Theatre that loaded Microsoft Agent characters using ACS files.