What
Hooks can return extra context text (additionalContext) that gets added to what the model sees. When a single hook's context text is too large for its allotted space, Claude Code now writes the full text to disk and passes along a reference to that file plus a truncated preview, instead of simply cutting the text short and discarding the rest.
Why
Previously oversized context was hard-truncated and the rest lost. Saving it to disk means the full content isn't discarded, even though only a preview is shown inline.
Names in the bundleadditionalContext
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Added since
A small documentation edit on Hooks reference touched a line naming additionalContext after this was published.
The `additionalContext` field passes a string from your hook into Claude's context window. Claude Code wraps the string in a [system reminder](/docs/en/glossary#system-reminder) and inserts it into the conversation at the point where the h…hooks see the edit
Confirmed since
Anthropic's documentation has since written up additionalContext, on Monitoring.
* `additional_context_chars`: Total characters of `additionalContext` returned by the matching hooks. Requires Claude Code v2.1.280 or latermonitoring-usage see the edit
One source agreesOne thing we can check says the same as this entry.
Anthropic's documentation agrees
Anthropic's documentation has since written up additionalContext, on Monitoring.