diff --git a/recipes/dev/llvm21/recipe.toml b/recipes/dev/llvm21/recipe.toml
index c210b8f07b..7a5492fb3b 100644
--- a/recipes/dev/llvm21/recipe.toml
+++ b/recipes/dev/llvm21/recipe.toml
@@ -68,14 +68,20 @@ set(ENV{CFLAGS} "")
 set(ENV{CXXFLAGS} "")
 NATIVE_EOF
 
+# amd64-only (see local/AGENTS.md): the CPU target is X86 (= amd64/x86_64).
+# AMDGPU is the GPU codegen backend Mesa's radeonsi/radv (AMD GPU) drivers
+# require via `dependency('llvm', modules: [... amdgpu ...])` — without it the
+# host llvm-config the mesa build reads reports amdgpu(missing) and mesa fails
+# to configure. AMDGPU is a GPU backend, not a second CPU platform, so it does
+# not violate amd64-only.
 case "${ARCH}" in
-    x86 | x86_64) LLVM_TARGETS_TO_BUILD="X86";;
-    aarch64) LLVM_TARGETS_TO_BUILD="AArch64";;
-    riscv64gc) LLVM_TARGETS_TO_BUILD="RISCV";;
+    x86 | x86_64) LLVM_TARGETS_TO_BUILD="X86;AMDGPU";;
+    aarch64) LLVM_TARGETS_TO_BUILD="AArch64;AMDGPU";;
+    riscv64gc) LLVM_TARGETS_TO_BUILD="RISCV;AMDGPU";;
 esac
 
 if [ "${COOKBOOK_HOST_SYSROOT}" = "/usr" ]; then
-    LLVM_TARGETS_TO_BUILD="X86;AArch64;RISCV"
+    LLVM_TARGETS_TO_BUILD="X86;AMDGPU"
 fi
 
 # This just build the LLVM library and tools just enough for Rust, to build the rest of LLVM see