diff --git a/compileopts/config.go b/compileopts/config.go
index a06aa8fd..9fb6a3bb 100644
--- a/compileopts/config.go
+++ b/compileopts/config.go
@@ -287,6 +287,7 @@ func (c *Config) CFlags() []string {
 		path, _ := c.LibcPath("picolibc")
 		cflags = append(cflags,
 			"--sysroot="+path,
+			"-isystem", filepath.Join(path, "include"), // necessary for Xtensa
 			"-isystem", filepath.Join(picolibcDir, "include"),
 			"-isystem", filepath.Join(picolibcDir, "tinystdio"),
 		)