PreCompact
A Claude Code hook event, read out of the shipped bundle. No mined inventory carries it: everything below comes from what this site has written about it.
Before context compaction
Anthropic's own wording. Read off Hooks reference, captured 2026-10-22.
In the changelogs
8Releases whose published page names PreCompact.
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v2.1.290
PreCompacthooks are now told when compaction was automaticThe PreCompact hook now receives a trigger of auto for automatic compaction instead of always manual
found in this entry's text, named in this entry
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v2.1.229
Hooks are handed a session object instead of reading globals
Hooks now get an explicit session with id, working directory and project root instead of guessing from globals.
found in this entry's text
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v2.1.228
Subagents doing delegated observation no longer trigger compact hooks
Observation subagents no longer fire your PostCompact hooks when they compact.
named in this entry, found in this entry's text
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v2.1.124
update-configskill prompt rewrittenfound in this entry's text
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v2.1.105
Reactive Compaction [Gradual Rollout]
found in this entry's text
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v2.1.105
PreCompact Hook Support
found in this entry's text
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v2.1.21
Enhanced Hook System for SDK Integration
found in this entry's text
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v1.0.48
New Features
found in this entry's text
First cited
2The earliest release whose published evidence quoted PreCompact was v2.1.21, 28 Jan 2026. That is the oldest release this project wrote about it, so it is a floor on the name's age and not the release that introduced it. It is not a presence reading: which builds carried the name is the table below.
Presence across releases
0The miner has never read a build containing this name. It is here because a changelog entry of ours names it, which is evidence that it existed and not evidence of which releases carried it. The inventory starts at v2.1.138 and everything older than that was never mined.
Read out of the published npm bundle release by release, and out of Anthropic's own documentation as this site captured it. Nothing on this page is a description anybody here wrote about what the hook event does. All hook events.