Under the hood
A new 'artifact_opening_prefetch' message type exists but nothing yet creates one
What
Claude Code now knows how to format and handle a new kind of internal attachment called artifact_opening_prefetch — it has a formatter that wraps its content as a system reminder message, and it's specifically excluded from a more generic reminder-processing path. However, no code in this release actually creates an attachment of this type; it only appears in the formatting and exclusion logic and in type lists.
Why
This appears to be groundwork for a feature related to opening artifacts (documents or content Claude generates) that isn't active yet, since nothing produces this attachment type in the current build.
e.content
preserved-thinking modified, high confidence
thinking_blocks = sum(block.type == "thinking" for block in response.content)see the edit
Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.
The writer flagged doubt
What would eventually produce an `artifact_opening_prefetch` attachment, and what it is for, is not shown in this build.
Anthropic's documentation agrees
e.content on preserved-thinking