write_file creates a missing parent directory inside the root

[adityak74]

write_file refused to write into a directory that did not exist yet: resolve_for_create canonicalized the parent first, and a missing parent fails that call. A model that never shells out mkdir -p then repeats the same failing write until the repeat guard ends the attempt. In the terminal-bench-science oracle run of 2026-09-05, dots-3-note-preview did exactly that: 12 write_file calls, 12 errors, 0 files created, across six tasks whose images ship /app/data and not /app/results. The three models that scored before it all ran mkdir -p from the shell first, which is why the harness had not been caught.

The tool now creates a missing parent the way an editor does, without loosening the root check:

• .. anywhere in the path is refused before anything is made, so nothing can be created above the nearest existing ancestor.
• That ancestor is canonicalized and checked against the root before create_dir_all runs, so a symlink out of the root creates nothing on the far side.
• The final canonical check on the parent stays as it was.

Tests cover the new parent, an escaping path that must create nothing, and a symlink out of the root that must create nothing. The tool description tells the model that parent directories are created.

https://claude.ai/code/session_01ADjombyM8oH114zG9FnMxo