diff --git a/README.md b/README.md
index 2bf9306c6..1cb104ccf 100644
--- a/README.md
+++ b/README.md
@@ -112,7 +112,21 @@ $ scriptc coverage hello.ts
 
 ## Build WebAssembly
 
-WASI and other cross-target builds require Zig. Its bundled WASI libc produces a portable WASI Preview 1 module through the production LLVM backend:
+WASI and other cross-target builds require Zig. 
+
+Note: 
+
+`SCRIPTC_CC=zigcc` is not a command that `scriptc` runs; it's a keyword the compiler recognizes. In `packages/compiler/src/backend/native-toolchain.ts`, `resolveCc()` compares the value literally and maps it to the argv prefix `["zig", "cc"]`:
+
+```javascript
+if (cc !== "zigcc") {
+  throw new Error(`unknown SCRIPTC_CC '${cc}' (supported: clang, zigcc)`);
+}
+if (target === "") return { argv: ["zig", "cc"], ... };
+```
+Only the exact strings `clang` or `zigcc` are accepted; `scriptc` then spawns `zig cc` itself. The only prerequisite is having `zig` on `PATH`.
+
+Its bundled WASI libc produces a portable WASI Preview 1 module through the production LLVM backend:
 
 ```console
 $ SCRIPTC_CC=zigcc SCRIPTC_TARGET=wasm32-wasi scriptc build hello.ts --no-keep-c -o hello.wasm >/dev/null