diff --git a/cc/config/tidy.go b/cc/config/tidy.go
index a20d556..5d53a8c 100644
--- a/cc/config/tidy.go
+++ b/cc/config/tidy.go
@@ -19,6 +19,13 @@
 	"strings"
 )
 
+// clang-tidy doesn't recognize every flag that clang does. This is unlikely to
+// be a complete list, but we can populate this with the ones we know to avoid
+// issues with clang-diagnostic-unused-command-line-argument.
+var ClangTidyUnknownCflags = sorted([]string{
+	"-Wa,%",
+})
+
 func init() {
 	// Most Android source files are not clang-tidy clean yet.
 	// Global tidy checks include only google*, performance*,
diff --git a/cc/makevars.go b/cc/makevars.go
index 5a912e1..88d4639 100644
--- a/cc/makevars.go
+++ b/cc/makevars.go
@@ -75,6 +75,7 @@
 	ctx.Strict("LLVM_OBJCOPY", "${config.ClangBin}/llvm-objcopy")
 	ctx.Strict("LLVM_STRIP", "${config.ClangBin}/llvm-strip")
 	ctx.Strict("PATH_TO_CLANG_TIDY", "${config.ClangBin}/clang-tidy")
+	ctx.Strict("CLANG_TIDY_UNKNOWN_CFLAGS", strings.Join(config.ClangTidyUnknownCflags, " "))
 	ctx.StrictSorted("CLANG_CONFIG_UNKNOWN_CFLAGS", strings.Join(config.ClangUnknownCflags, " "))
 
 	ctx.Strict("RS_LLVM_PREBUILTS_VERSION", "${config.RSClangVersion}")