diff --git a/tools/rbcrun/host.go b/tools/rbcrun/host.go
index f36553e..8cd2845 100644
--- a/tools/rbcrun/host.go
+++ b/tools/rbcrun/host.go
@@ -24,7 +24,6 @@
 	"strings"
 
 	"go.starlark.net/starlark"
-	"go.starlark.net/starlarkjson"
 	"go.starlark.net/starlarkstruct"
 )
 
@@ -60,7 +59,6 @@
 
 var sclBuiltins starlark.StringDict = starlark.StringDict{
 	"struct":   starlark.NewBuiltin("struct", starlarkstruct.Make),
-	"json": starlarkjson.Module,
 }
 
 func isSymlink(filepath string) (bool, error) {
@@ -389,6 +387,10 @@
 		return nil, nil, err
 	}
 
+	if mode == ExecutionModeScl && !strings.HasSuffix(filename, ".scl") {
+		return nil, nil, fmt.Errorf("filename must end in .scl: %s", filename)
+	}
+
 	// Add top-level file to cache for cycle detection purposes
 	moduleCache[filename] = nil
 
diff --git a/tools/rbcrun/host_test.go b/tools/rbcrun/host_test.go
index 7cfeb14..38b2923 100644
--- a/tools/rbcrun/host_test.go
+++ b/tools/rbcrun/host_test.go
@@ -143,7 +143,7 @@
 	if err := os.Chdir(filepath.Dir(dir)); err != nil {
 		t.Fatal(err)
 	}
-	vars, _, err := Run("testdata/bzl_loads_scl.bzl", nil, ExecutionModeScl, false)
+	vars, _, err := Run("testdata/bzl_loads_scl.bzl", nil, ExecutionModeRbc, false)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -160,7 +160,7 @@
 	if err := os.Chdir(filepath.Dir(dir)); err != nil {
 		t.Fatal(err)
 	}
-	vars, _, err := Run("testdata/bzl_loads_scl_2.bzl", nil, ExecutionModeScl, false)
+	vars, _, err := Run("testdata/bzl_loads_scl_2.bzl", nil, ExecutionModeRbc, false)
 	if err != nil {
 		t.Fatal(err)
 	}