diff --git a/tools/INDEX.md b/tools/INDEX.md
index 0d5bcd5..899eff7 100644
--- a/tools/INDEX.md
+++ b/tools/INDEX.md
@@ -49,3 +49,14 @@ the skill, and holds no Docker socket and no `ygg` ssh alias. So:
   cap (set `CARGO_BUILD_JOBS`, and `-j1`; point `CARGO_TARGET_DIR` and `TMPDIR` at a disk with
   room, since cargo-mutants copies the tree to `TMPDIR` and a small run took 2.3 GB), and its survivors are not confirmed on
   the full suite. Run survivors again unfiltered before reporting them MISSED.
+
+  Learned on CultLib's Rust runtime (Libby, 38 mutants: 32 caught, 5 unviable, 1
+  missed, 7 min):
+  - Add `--in-place` when tests read files by the repo's original path (CultLib's
+    vector tests): the copy-mode baseline fails (15 failures). `--in-place`
+    conflicts with `-j`, so drop `-j1`; mutants then run one at a time.
+  - Offline resolution needs the lockfile's exact crate versions in the cargo
+    registry (one checkout wanted `cc` 1.11.0, which was missing); fetch it
+    first or the baseline fails before any mutant runs.
+  - Launch as the harness's background run. A `nohup ... &` from the shell died
+    silently.