What is the ACF file format?
Files with the .acf extension are used by Valve Steam, a digital distribution platform and application store. Unlike binary cache files, .acf files are plain-text key-value files in Valve’s VDF format, making them human-readable and editable in any text editor.
Contents of .acf files
.acf files store the app manifest data for each installed Steam game or application. Steam uses these files to support downloading and updating games, and the data they contain is also used for verifying the integrity of installed software. Each file is named appmanifest_APPID.acf (for example, appmanifest_440.acf for Team Fortress 2) and lives in the Steam/steamapps/ directory. The file’s root key is AppState, with sub-fields for the app’s name, install directory, StateFlags (4 = fully installed, 1026 = update required), download size, depot data, and build ID.
What is Valve Steam?
Steam is a digital storefront where users can buy games and other software. The platform offers integrated software protection, cloud saves, a micro-payments service, and community features. Developers publish games through Steam Direct. It is because of this rich set of functions that Steam has gained huge popularity on the computer game market.
Deleting an .acf file causes Steam to lose track of the installation, though the game files themselves remain intact and can be re-linked by regenerating or repairing the manifest.
Security & safety
RISK: LOWACF files are plain text and cannot execute. Editing them incorrectly can cause Steam to mis-identify an install or trigger a full re-download, but no security risk exists. Do not download ACF files from untrusted sites - at best they could point Steam to a wrong install path; the format itself carries no executable payload.
Format details
in a nutshell- Microsoft Agent HTTP Character File - Legacy animated character download descriptor for Microsoft Agent (Windows 98/XP era); unrelated to Steam.
Programs that open ACF files
Windows2 apps
Notepad
Open-source Open the file; Notepad++ provides syntax highlighting and easier editing of the nested structure. 
macOS2 apps
Linux2 apps
Steam
Free Steam on Linux stores ACF files in ~/.steam/steam/steamapps/ or the Steam library folder. Technical details
deep spec| Format type | Plain-text key-value app manifest (Valve VDF/KeyValues format) |
| Developer | Valve Corporation |
| Encoding | Text, UTF-8 (ASCII-compatible) |
| File naming | appmanifest_APPID.acf (e.g., appmanifest_440.acf for Team Fortress 2) |
| Location | Steam/steamapps/ directory |
| Root key | AppState (required identifier for Steam to parse the file) |
| Key fields | AppID, name, installdir, StateFlags, SizeOnDisk, buildid, MountedDepots, InstalledDepots |
| StateFlags values | 4 = fully installed; 1026 = update required |
| Parsing | Human-readable; editable in any plain-text editor |
| Steam startup | Steam reads ACF files at launch to enumerate installed games and their depot manifests |
| Typical file size | 1 KB - 5 KB |
| MIME type | text/plain |
| Supported OS | Windows, macOS, Linux |
| Integrity checking | Steam validates game files via depot hashes separately; no checksum embedded in the ACF itself |
| Released | ~2003 (with Steam client) |
| Latest version | No versioned spec; format evolves with Steam client updates |
| Specification | developer.valvesoftware.com |
Community Q&A
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