diff --git a/CLAUDE.md b/CLAUDE.md
index 1c2a3be..1bb785b 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -27,6 +27,10 @@ Never use normal cargo ... commands, use the just ... commands instead.
 - `just fix` — auto-apply cargo/clippy fixes, then format.
 - `just clean` — wipe the build cache.
 
+## Protocol policy
+
+There is only one wire version — the current one. No legacy peers, no compatibility shims, no fallback paths for older builds. Anyone who wants to interop must run the current build; everyone else is out. Do not add backward-compat code, `#[serde(other)]` escape hatches, or "what if an old peer sends X" defenses.
+
 ## General info
 
 See `README.md`.