compaction-thinking-blocks changedbuild-with-claude/compaction-thinking-blocks
Nearest release: v2.1.280, published under an hour before this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
Recorded here
Lines+2added
Lines−1removed
From line
11
where the diff opens
First seen
21 Sep 2026
this site's first read of the page
Recorded edits13to this page, all time
The whole hunk
from line 11, old and new numbered
/
from line 11
1111 - claude-fable-5
1212 - claude-mythos-5
1313 - claude-mythos-preview
14 - claude-opus-5-5
1415 - claude-opus-5
1516 - claude-opus-4-8
1617 - claude-opus-4-7
from line 56
5556
5657To add an instruction or change the available tools without touching `system` or `tools`, append the change to `messages`, as described in [Make changes without editing the prefix](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#replace-prefix-edits).
5758
58Mid-conversation system messages inside the summarized turns are summarized too, so their instructions and tool changes stop applying after the swap. To keep one in force, state it again in a `role: "system"` message directly after the first new `user` turn that follows the kept turns. A system message placed between the block and the kept turns breaks their thinking.
59Mid-conversation system messages inside the summarized turns are summarized too, so their text instructions stop applying after the swap. To keep one in force, state it again in a `role: "system"` message directly after the first new `user` turn that follows the kept turns. Tool changes inside those turns carry over on their own when the compaction request also carries `inline-tools-2026-09-15`: the returned block records their net effect in its `tool_changes` field, so send the block back unmodified. If the block has no `tool_changes` field, restate those tool changes the same way. A system message placed between the block and the kept turns breaks their thinking.
5960
6061## Check that the kept thinking held
6162
No line in this hunk matches that.