diff --git a/docs/DESIGN.md b/docs/DESIGN.md
index f4a02d8..4c5d4c5 100644
--- a/docs/DESIGN.md
+++ b/docs/DESIGN.md
@@ -1705,12 +1705,13 @@ hashes stay bit-identical.
 ## Data model
 
 The on-disk JSON-AST is what the toolchain hashes, typechecks, and
-lowers. Every node in this section is the schema mirror of an enum or
-struct in `crates/ailang-core/src/ast.rs`; whenever the two disagree,
-`ast.rs` is the source of truth. Every additive field is declared with
-`skip_serializing_if` so pre-existing fixtures keep bit-identical
-canonical-JSON hashes — that gating contract is what makes growing
-the schema cheap.
+lowers. **This section is the canonical schema.** The Rust types in
+`crates/ailang-core/src/ast.rs` are the in-memory projection of it;
+when the two disagree, this section wins, and the drift test
+`crates/ailang-core/tests/design_schema_drift.rs` fires. Every
+additive field is declared with `skip_serializing_if` so pre-existing
+fixtures keep bit-identical canonical-JSON hashes — that gating
+contract is what makes growing the schema cheap.
 
 ### Module