diff --git a/docs/m19-m20-plan.md b/docs/m19-m20-plan.md
index 01b96ef..7b9bd26 100644
--- a/docs/m19-m20-plan.md
+++ b/docs/m19-m20-plan.md
@@ -57,6 +57,21 @@ branch is green; keep branches; push everything.
 6. **Both new processes are protocol drivers** under the manager: hello,
    supervision, restart with backoff — all inherited from M18.1 for free.
    Registration idempotence (decision 3) is what makes their restarts sound.
+7. **Firmware neutrality is the contract** (2026-07-13). The generic layer is
+   everything at and above the device-manager protocol — descriptors,
+   containment, reports, matching, supervision — and none of it may become
+   x86-specific. Discovery is one swappable process per firmware: the acpi
+   service on x86; an **fdt service** on the Raspberry Pis (claims a
+   `devicetree-blob` node, reports children from the flattened device tree —
+   pure data, no bytecode, no port grant, strictly simpler than ACPI). The
+   manager owns the tree as *data* and touches no hardware, ever — AML runs in
+   a crashable, supervised discoverer precisely so a firmware-bytecode fault
+   can never take down the supervisor. Two consequences recorded now:
+   `DeviceDescriptor`'s 8-byte `hid` cannot hold an FDT `compatible` string
+   ("brcm,bcm2835-aux-uart") — identity widens before the fdt service exists;
+   and cross-firmware surfaces are named by **domain, not firmware** (M21
+   defines a *power* protocol, not an "ACPI events" protocol — PSCI/mailbox
+   sources feed the same subscribers on ARM).
 
 ## Status