“sccache (a compile cache that skips compiles it has seen): 0% hits across worktrees, the folder path is part of its key. Same folder: 100% hits, 37s down to 16s”

The isolation cost is concrete here. Every mitigation tried trades one failure for another: a shared build folder gave "phantom compile errors from stale build info", so each branch now compiles from scratch; sccache hits 0% across worktrees; mbx, which shares build output, gave "wrong paths in 11 of 28 runs, dropped". Next plan: a fixed pool of checkout folders so paths repeat and the cache hits. A separate reply from Doug Tarr says "all my agents do all day is manage disk usage because shared worktrees don't work with sccache". Fits the isolation watch item (setup, teardown and cleanup cost). Untested plan, early measurements.