The Write tool now advises: "For appending to an existing file, prefer shell redirection via Bash (e.g. echo "..." >> file) over rewriting the whole file." The old mode:'append' parameter has been removed from the Write tool schema.
Evidence
Write tool description change (search for "For appending to an existing file, prefer shell redirection")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
