diff --git a/docs/HOST-MATRIX.md b/docs/HOST-MATRIX.md
index 2ec78a4..ccd6b91 100644
--- a/docs/HOST-MATRIX.md
+++ b/docs/HOST-MATRIX.md
@@ -14,6 +14,10 @@ on any host fills in its own row. Source of truth for facts is the probe — not
 > and commit. **Never guess hardware, OS, or IPs** — paste what the probe reports.
 > On FreeBSD the probe synthesizes an OS-specific command map; trust its output over
 > Linux habits.
+>
+> **Disk before action:** before installing a toolchain or starting a build, check
+> real free space (`df -h /`, or the probe's `--storage`) — never estimate. Keep the
+> **Disk (free)** column current and flag any host past ~85%. See _Disk discipline_ below.
 
 ---
 
@@ -43,6 +47,23 @@ on any host fills in its own row. Source of truth for facts is the probe — not
 | **debby**   | 100.66.193.10  | Debian 13 / 6.12.90+deb13.1-amd64  | bare metal            | AMD Ryzen 7 5700U (8-core)             | 16   | 15 GiB  | 15 GiB                | nvme0n1p2 453G (23G free)    | Radeon Graphics (iGPU) | 2026-06-17 | Hermes |
 | **osa**     | 100.72.229.63  | FreeBSD 15.0-RELEASE-p10 / GENERIC | not reported by probe | Intel Core Processor (Haswell, no TSX) | 6    | 11 GiB  | not reported by probe | ZFS pool: zroot (23.9G free) | not reported by probe  | 2026-06-17 | Mevy   |
 
+### Disk discipline (check, don't guess)
+
+Disk is a first-class fact, same as OS or CPU — **measure it before you act, don't estimate.**
+
+- **Before installing a toolchain or starting a build**, run `df -h /` (Linux) or
+  `zfs list` / `df -h` (FreeBSD), or the probe's `--storage`. Confirm the headroom is
+  really there.
+- **Keep the `Disk (free)` column above current** when you add or remove anything large.
+- **Flag any host past ~85% used.** Reference footprints to budget with: Go SDK ≈ 290 MB,
+  Rust toolchain (`~/.rustup` + `~/.cargo`) ≈ 1.8 GB, a Node version ≈ 150 MB; build/module
+  caches grow on top of these.
+- **Standing watch:** `debby` runs ~95% full (23 GB free). Treat new installs/builds there
+  as a deliberate decision, not a default — prefer the host with real headroom.
+
+This is the survivability principle applied to storage: a host that silently fills up is a
+host that fails. What you guess will be wrong; what you probe will be right.
+
 ---
 
 ## 3. Per-host detail (expand as needed)