What is the HST file format?
.hst files store chat history archives created by Yahoo! Messenger, the instant messaging client that operated from 1998 until its discontinuation on July 17, 2018. The format is sometimes called the StoredIM format; file names typically follow the pattern <account_number>.hst - for example, 001053309244.hst.
The files hold offline and stored messages with timestamps, sender and recipient identifiers, message text, smiley codes, and font formatting attributes such as typeface, bold/italic/underline, size and colour. The format is proprietary and entirely undocumented - Yahoo never published a public specification.
Later versions of Yahoo Messenger switched to .dat files using XOR obfuscation in place of .hst. Neither format is supported by mainstream tools; third-party utilities such as Yahoo Message Archive Decoder (YMAD) can parse both variants.
Because Yahoo! Messenger reached end-of-life in 2018, .hst files are effectively legacy archives. Users were given until December 1, 2018 to download their chat history from Yahoo's servers. No supported client can create new .hst files today.
Security & safety
RISK: LOWHST files are passive data archives - they are not executable and pose no direct malware risk. The main risk is privacy: they contain complete conversation logs which may include personal information. Treat them as sensitive personal data. Do not upload them to unknown online services. Since Yahoo Messenger is EOL, avoid any 'Yahoo HST opener' download that is not from a reputable source - such tools may bundle adware.
Format details
in a nutshell- Generic History/Log file - Many programs use .hst as a generic history or log file extension (Forte Agent newsreader, terminal emulators, trading software) - these are unrelated to Yahoo Messenger.
- Hastings Network Simulator route file - Some networking tools use .hst for route/history tables - completely different binary structure.
Programs that open HST files
Windows2 apps
Yahoo Message Archive Decoder (YMAD)
Free Open the .hst file (or point YMAD at the StoredIM folder) to read, browse and export archived Yahoo conversations.
Yahoo! Instant Messenger (historical reference)
Free Yahoo Messenger was discontinued July 17, 2018 - do not download or use. Listed for historical reference only; no current download source exists. Technical details
deep spec| Developer | Yahoo, Inc. |
| Encoding | Binary (proprietary, undocumented) |
| Byte order | Little-endian (Windows x86 application) |
| Encryption | None in .hst; later .dat variant used XOR obfuscation |
| File naming pattern | <account_number>.hst - e.g. 001053309244.hst |
| Typical file size | A few KB to a few MB per account, depending on chat volume |
| Content stored | Timestamps, sender/recipient IDs, message text, smiley codes, font attributes (typeface, size, colour, bold/italic) |
| Platform | Windows (primary); macOS (Yahoo Messenger existed for Mac) |
| Service status | Yahoo! Messenger discontinued July 17, 2018; format frozen |
| Successor format | Yahoo Messenger .dat archive (XOR-encoded), also now obsolete |
| Third-party reading | Requires archive tools such as Yahoo Message Archive Decoder (YMAD) |
| Released | Late 1990s with Yahoo! Messenger (service launched 1998) |
| Latest version | Service discontinued July 17, 2018; format is frozen |
HST conversions
Community Q&A
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