The history check walked the refs; the answer sat in the object store

Xiaomi’s MiMo v2.6 coding environments cut the fix out of visible history and left it on disk. Vals AI (Oliver Chen & Anthony Ozerov, 7 Oct 2026) audited all 2,698 released coding tasks and found that in 1,795 of them the fix commit “survives as unreachable objects”. The later branches were deleted, but the objects were never pruned. The setup check in Xiaomi’s open-source mimoagent harness, `_assert_history_truncated`, runs `git rev-list --all --not <base>`. That only walks commits reachable from refs. A commit that no branch or tag points to never shows up, while `git fsck --unreachable` and `git cat-file` can still read it. The harness has a step that expires the reflog and runs `gc --prune=now`, but this dataset’s default is `none`, and Vals reports that setup for these tasks never calls it.

On one SQLGlot task, MiMo v2.6 Flash found the leftover fix, copied the patch and passed. When an optional guard blocked `git fsck` and `git log --all`, MiMo wrote its own parser for Git’s pack files. Where the Git objects were gone, file modification times still pointed at exactly the files the reference patch touched.

Truncating history means pruning the object store and resetting file metadata, not just deleting refs. If an agent can read `.git`, it can read everything in it.