diff --git a/serve/method.go b/serve/method.go
index 068087c..306d407 100644
--- a/serve/method.go
+++ b/serve/method.go
@@ -7,7 +7,6 @@ import (
 	"io"
 	"io/fs"
 
-	"github.com/google/go-jsonnet"
 	statuspb "google.golang.org/genproto/googleapis/rpc/status"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/metadata"
@@ -21,20 +20,17 @@ type method struct {
 	desc     protoreflect.MethodDescriptor
 	filename string
 	fs       fs.FS
-	makeVM   MakeVM
+	eval     Evaluator
 }
 
-func newMethod(md protoreflect.MethodDescriptor, fs fs.FS, makeVM MakeVM) method {
+func newMethod(md protoreflect.MethodDescriptor, fs fs.FS, eval Evaluator) method {
 	pkg, svc := md.ParentFile().Package(), md.Parent().Name()
 	filename := fmt.Sprintf("%s.%s.%s.jsonnet", pkg, svc, md.Name())
-	if makeVM == nil {
-		makeVM = jsonnet.MakeVM
-	}
 	return method{
 		desc:     md,
 		filename: filename,
 		fs:       fs,
-		makeVM:   makeVM,
+		eval:     eval,
 	}
 }
 
@@ -66,7 +62,7 @@ func (m method) unaryClientCall(ss grpc.ServerStream) error {
 		return err
 	}
 
-	return m.evalJsonnet(input, ss)
+	return m.evaluate(input, ss)
 }
 
 func (m method) streamingClientCall(ss grpc.ServerStream) error {
@@ -88,7 +84,7 @@ func (m method) streamingClientCall(ss grpc.ServerStream) error {
 		return err
 	}
 
-	return m.evalJsonnet(input, ss)
+	return m.evaluate(input, ss)
 }
 
 func (m method) streamingBidiCall(ss grpc.ServerStream) error {
@@ -108,21 +104,15 @@ func (m method) streamingBidiCall(ss grpc.ServerStream) error {
 		if err != nil {
 			return err
 		}
-		if err := m.evalJsonnet(input, ss); err != nil {
+		if err := m.evaluate(input, ss); err != nil {
 			return err
 		}
 	}
 	return nil
 }
 
-func (m method) evalJsonnet(input string, ss grpc.ServerStream) error {
-	vm := m.makeVM()
-	vm.TLACode("input", input)
-	b, err := fs.ReadFile(m.fs, m.filename)
-	if err != nil {
-		return err
-	}
-	output, err := vm.EvaluateAnonymousSnippet(m.filename, string(b))
+func (m method) evaluate(input string, ss grpc.ServerStream) error {
+	output, err := m.eval(string(m.desc.FullName()), input, m.fs)
 	if err != nil {
 		return err
 	}
diff --git a/serve/server.go b/serve/server.go
index 0662944..e78d5ab 100644
--- a/serve/server.go
+++ b/serve/server.go
@@ -42,12 +42,31 @@ func WithLogger(logger Logger) Option {
 }
 
 func WithVM(makeVM MakeVM) Option {
+	return WithEvaluator(JsonnetEvaluator(makeVM))
+}
+
+type Evaluator func(method, input string, vfs fs.FS) (output string, err error)
+
+func WithEvaluator(evaluator Evaluator) Option {
 	return func(s *Server) error {
-		s.makeVM = makeVM
+		s.eval = evaluator
 		return nil
 	}
 }
 
+func JsonnetEvaluator(makeVM MakeVM) Evaluator {
+	return func(method, input string, vfs fs.FS) (output string, err error) {
+		vm := makeVM()
+		vm.TLACode("input", input)
+		filename := method + ".jsonnet"
+		b, err := fs.ReadFile(vfs, filename)
+		if err != nil {
+			return "", err
+		}
+		return vm.EvaluateAnonymousSnippet(filename, string(b))
+	}
+}
+
 type Server struct {
 	methodDir string
 	protoSet  string
@@ -57,7 +76,7 @@ type Server struct {
 	gs      *grpc.Server
 	files   *protoregistry.Files
 	fs      fs.FS
-	makeVM  MakeVM
+	eval    Evaluator
 }
 
 var errUnknownHandler = errors.New("Unknown handler")
@@ -69,6 +88,7 @@ func NewServer(methodDir, protoSet string, options ...Option) (*Server, error) {
 		protoSet:  protoSet,
 		log:       NewLogger(os.Stderr, LogLevelError),
 	}
+	options = append([]Option{WithVM(jsonnet.MakeVM)}, options...)
 	for _, opt := range options {
 		if err := opt(s); err != nil {
 			return nil, err
@@ -133,7 +153,7 @@ func (s *Server) loadMethods() error {
 		for i := 0; i < sds.Len(); i++ {
 			mds := sds.Get(i).Methods()
 			for j := 0; j < mds.Len(); j++ {
-				m := newMethod(mds.Get(j), methodFS, s.makeVM)
+				m := newMethod(mds.Get(j), methodFS, s.eval)
 				s.methods[m.fullMethod()] = m
 			}
 		}