diff --git a/cel/cel_test.go b/cel/cel_test.go
index 042cc93e..5bcffa25 100644
--- a/cel/cel_test.go
+++ b/cel/cel_test.go
@@ -15,6 +15,7 @@
 package cel
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"io/ioutil"
@@ -446,9 +447,37 @@ func TestCrossTypeNumericComparisons(t *testing.T) {
 	}
 }
 
+func TestExtendStdlibFunction(t *testing.T) {
+	e, err := NewEnv(
+		Function(overloads.Contains,
+			MemberOverload("bytes_contains_bytes", []*Type{BytesType, BytesType}, BoolType,
+				BinaryBinding(func(bstr, bsub ref.Val) ref.Val {
+					return types.Bool(bytes.Contains([]byte(bstr.(types.Bytes)), []byte(bsub.(types.Bytes))))
+				}))),
+	)
+	if err != nil {
+		t.Fatalf("NewEnv() failed: %v", err)
+	}
+	ast, iss := e.Compile(`b'string'.contains(b'tri') && 'string'.contains('tri')`)
+	if iss.Err() != nil {
+		t.Fatalf("Compile() failed: %v", iss.Err())
+	}
+	prg, err := e.Program(ast)
+	if err != nil {
+		t.Fatalf("Progarm(ast) failed: %v", err)
+	}
+	out, _, err := prg.Eval(NoVars())
+	if err != nil {
+		t.Fatalf("contains check errored: %v", err)
+	}
+	if out != types.True {
+		t.Errorf("contains check got %v, wanted true", out)
+	}
+}
+
 func TestCustomTypes(t *testing.T) {
 	reg := types.NewEmptyRegistry()
-	e, _ := NewEnv(
+	e, err := NewEnv(
 		CustomTypeAdapter(reg),
 		CustomTypeProvider(reg),
 		Container("google.api.expr.v1alpha1"),
@@ -459,6 +488,9 @@ func TestCustomTypes(t *testing.T) {
 			types.StringType),
 		Variable("expr", ObjectType("google.api.expr.v1alpha1.Expr")),
 	)
+	if err != nil {
+		t.Fatalf("NewEnv() failed: %v", err)
+	}
 
 	ast, iss := e.Compile(`
 		expr == Expr{id: 2,
diff --git a/cel/library.go b/cel/library.go
index 10037988..8de7b40a 100644
--- a/cel/library.go
+++ b/cel/library.go
@@ -109,29 +109,28 @@ func (stdLibrary) LibraryName() string {
 // CompileOptions returns options for the standard CEL function declarations and macros.
 func (stdLibrary) CompileOptions() []EnvOption {
 	return []EnvOption{
+		func(e *Env) (*Env, error) {
+			var err error
+			for _, fn := range stdlib.Functions() {
+				existing, found := e.functions[fn.Name]
+				if found {
+					fn, err = existing.Merge(fn)
+					if err != nil {
+						return nil, err
+					}
+				}
+				e.functions[fn.Name] = fn
+			}
+			return e, nil
+		},
 		Declarations(stdlib.TypeExprDecls()...),
-		Declarations(stdlib.FunctionExprDecls()...),
 		Macros(StandardMacros...),
 	}
 }
 
 // ProgramOptions returns function implementations for the standard CEL functions.
 func (stdLibrary) ProgramOptions() []ProgramOption {
-	return []ProgramOption{
-		func(p *prog) (*prog, error) {
-			for _, fn := range stdlib.Functions() {
-				bindings, err := fn.Bindings()
-				if err != nil {
-					return nil, err
-				}
-				err = p.dispatcher.Add(bindings...)
-				if err != nil {
-					return nil, err
-				}
-			}
-			return p, nil
-		},
-	}
+	return []ProgramOption{}
 }
 
 // OptionalTypes enable support for optional syntax and types in CEL.