diff --git a/scripts/check-wasm.sh b/scripts/check-wasm.sh
index d353465..b566d60 100755
--- a/scripts/check-wasm.sh
+++ b/scripts/check-wasm.sh
@@ -19,6 +19,32 @@ if ! swift_command sdk list | grep -qx "$SDK_ID"; then
   exit 2
 fi
 
+# The open-source swift.org toolchain ships the WebAssembly codegen backend; the
+# Xcode-bundled Apple toolchain does not. Cross-compiling to wasm through the Xcode
+# toolchain therefore aborts deep inside LLVM ("No available targets are compatible
+# with triple wasm32-...") with a multi-megabyte stack dump that looks like a code
+# bug but is purely a wrong-toolchain setup. When we are not going through swiftly,
+# detect that case up front and fail with an actionable message instead.
+#
+# Discriminate by the compiler's self-reported build tag, not its path: both
+# toolchains print "Apple Swift version X", but the Xcode/Apple build tags it
+# "(swiftlang-...)" while a swift.org toolchain tags it "(swift-...-RELEASE/SNAPSHOT)".
+# The self-report is authoritative however `swift` is symlinked or shimmed (a path
+# check misses a /usr/local/bin/swift that links into Xcode). 'swift
+# -print-target-info -target wasm32-...' is NOT a usable probe: it succeeds even
+# without the backend, because the frontend knows the triple and only codegen is gone.
+if ! command -v swiftly >/dev/null 2>&1; then
+  if swift --version 2>/dev/null | grep -q '(swiftlang-'; then
+    echo "wasm: the active Swift is the Xcode/Apple toolchain, which has no WebAssembly" >&2
+    echo "wasm: codegen backend, so a wasm cross-compile cannot work here and would crash" >&2
+    echo "wasm: inside LLVM rather than fail cleanly." >&2
+    echo "wasm: install swiftly and the swift.org toolchain that matches '$SWIFT_SELECTOR'" >&2
+    echo "wasm: (it ships the wasm backend), or put a swift.org toolchain first as 'swift'," >&2
+    echo "wasm: then re-run. See https://www.swift.org/install/ and https://swiftwasm.org." >&2
+    exit 2
+  fi
+fi
+
 swift_command build --swift-sdk "$SDK_ID"
 swift_command build -c release --swift-sdk "$SDK_ID"