What
When the Read tool is used on a file that hasn't changed since a previous read in the same conversation, Claude now returns a short notice instead of re-reading the entire file, saving context window space.
Details
- Displays "Unchanged since last read" in the tool output
- The full message tells Claude to refer to the earlier Read tool result instead of re-reading
- Controlled by the
tengu_compact_line_prefix_killswitchfeature flag (enabled when killswitch is off) - Helps conserve context window tokens in long conversations with repeated file reads
Evidence
Read caching message (search for "File unchanged since last read") — kf1() at line ~163425
Strings lifted out of the shipped bundle, so the claim above can be checked against them.