diff --git a/builtin/lib.go b/builtin/lib.go
index 579393161..6f6a3b6cd 100644
--- a/builtin/lib.go
+++ b/builtin/lib.go
@@ -421,10 +421,14 @@ func get(params ...any) (out any, err error) {
 		fieldName := i.(string)
 		value := v.FieldByNameFunc(func(name string) bool {
 			field, _ := v.Type().FieldByName(name)
-			if field.Tag.Get("expr") == fieldName {
+			switch field.Tag.Get("expr") {
+			case "-":
+				return false
+			case fieldName:
 				return true
+			default:
+				return name == fieldName
 			}
-			return name == fieldName
 		})
 		if value.IsValid() {
 			return value.Interface(), nil
diff --git a/checker/nature/utils.go b/checker/nature/utils.go
index 179459874..c1551546c 100644
--- a/checker/nature/utils.go
+++ b/checker/nature/utils.go
@@ -6,11 +6,15 @@ import (
 	"github.com/expr-lang/expr/internal/deref"
 )
 
-func fieldName(field reflect.StructField) string {
-	if taggedName := field.Tag.Get("expr"); taggedName != "" {
-		return taggedName
+func fieldName(field reflect.StructField) (string, bool) {
+	switch taggedName := field.Tag.Get("expr"); taggedName {
+	case "-":
+		return "", false
+	case "":
+		return field.Name, true
+	default:
+		return taggedName, true
 	}
-	return field.Name
 }
 
 func fetchField(t reflect.Type, name string) (reflect.StructField, bool) {
@@ -23,7 +27,7 @@ func fetchField(t reflect.Type, name string) (reflect.StructField, bool) {
 	for i := 0; i < t.NumField(); i++ {
 		field := t.Field(i)
 		// Search all fields, even embedded structs.
-		if fieldName(field) == name {
+		if n, ok := fieldName(field); ok && n == name {
 			return field, true
 		}
 	}
@@ -69,7 +73,11 @@ func StructFields(t reflect.Type) map[string]Nature {
 				}
 			}
 
-			table[fieldName(f)] = Nature{
+			name, ok := fieldName(f)
+			if !ok {
+				continue
+			}
+			table[name] = Nature{
 				Type:       f.Type,
 				FieldIndex: f.Index,
 			}
diff --git a/expr_test.go b/expr_test.go
index 57d2cfbbb..32ec6dfb8 100644
--- a/expr_test.go
+++ b/expr_test.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"os"
 	"reflect"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -139,7 +140,86 @@ func ExampleEnv_tagged_field_names() {
 
 	fmt.Printf("%v", output)
 
-	// Output : Hello World
+	// Output: Hello World
+}
+
+func ExampleEnv_hidden_tagged_field_names() {
+	type Internal struct {
+		Visible string
+		Hidden  string `expr:"-"`
+	}
+	type environment struct {
+		Visible         string
+		Hidden          string   `expr:"-"`
+		HiddenInternal  Internal `expr:"-"`
+		VisibleInternal Internal
+	}
+
+	env := environment{
+		Hidden: "First level secret",
+		HiddenInternal: Internal{
+			Visible: "Second level secret",
+			Hidden:  "Also hidden",
+		},
+		VisibleInternal: Internal{
+			Visible: "Not a secret",
+			Hidden:  "Hidden too",
+		},
+	}
+
+	hiddenValues := []string{
+		`Hidden`,
+		`HiddenInternal`,
+		`HiddenInternal.Visible`,
+		`HiddenInternal.Hidden`,
+		`VisibleInternal["Hidden"]`,
+	}
+	for _, expression := range hiddenValues {
+		output, err := expr.Eval(expression, env)
+		if err == nil || !strings.Contains(err.Error(), "cannot fetch") {
+			fmt.Printf("unexpected output: %v; err: %v\n", output, err)
+			return
+		}
+		fmt.Printf("%q is hidden as expected\n", expression)
+	}
+
+	visibleValues := []string{
+		`Visible`,
+		`VisibleInternal`,
+		`VisibleInternal["Visible"]`,
+	}
+	for _, expression := range visibleValues {
+		_, err := expr.Eval(expression, env)
+		if err != nil {
+			fmt.Printf("unexpected error: %v\n", err)
+			return
+		}
+		fmt.Printf("%q is visible as expected\n", expression)
+	}
+
+	testWithIn := []string{
+		`not ("Hidden" in $env)`,
+		`"Visible" in $env`,
+		`not ("Hidden" in VisibleInternal)`,
+		`"Visible" in VisibleInternal`,
+	}
+	for _, expression := range testWithIn {
+		val, err := expr.Eval(expression, env)
+		shouldBeTrue, ok := val.(bool)
+		if err != nil || !ok || !shouldBeTrue {
+			fmt.Printf("unexpected result; value: %v; error: %v\n", val, err)
+			return
+		}
+	}
+
+	// Output: "Hidden" is hidden as expected
+	// "HiddenInternal" is hidden as expected
+	// "HiddenInternal.Visible" is hidden as expected
+	// "HiddenInternal.Hidden" is hidden as expected
+	// "VisibleInternal[\"Hidden\"]" is hidden as expected
+	// "Visible" is visible as expected
+	// "VisibleInternal" is visible as expected
+	// "VisibleInternal[\"Visible\"]" is visible as expected
 }
 
 func ExampleAsKind() {
@@ -529,7 +609,7 @@ func ExamplePatch() {
 	}
 	fmt.Printf("%v", output)
 
-	// Output : Hello, you, world!
+	// Output: Hello, you, world!
 }
 
 func ExampleWithContext() {
@@ -2765,3 +2845,20 @@ func TestMemoryBudget(t *testing.T) {
 		})
 	}
 }
+
+func TestIssue807(t *testing.T) {
+	type MyStruct struct {
+		nonExported string
+	}
+	out, err := expr.Eval(` "nonExported" in $env `, MyStruct{})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	b, ok := out.(bool)
+	if !ok {
+		t.Fatalf("expected boolean type, got %T: %v", b, b)
+	}
+	if b {
+		t.Fatalf("expected 'in' operator to return false for unexported field")
+	}
+}
diff --git a/vm/runtime/runtime.go b/vm/runtime/runtime.go
index cd48a280d..fa14c4d0a 100644
--- a/vm/runtime/runtime.go
+++ b/vm/runtime/runtime.go
@@ -65,10 +65,14 @@ func Fetch(from, i any) any {
 		fieldName := i.(string)
 		value := v.FieldByNameFunc(func(name string) bool {
 			field, _ := v.Type().FieldByName(name)
-			if field.Tag.Get("expr") == fieldName {
+			switch field.Tag.Get("expr") {
+			case "-":
+				return false
+			case fieldName:
 				return true
+			default:
+				return name == fieldName
 			}
-			return name == fieldName
 		})
 		if value.IsValid() {
 			return value.Interface()
@@ -213,7 +217,11 @@ func In(needle any, array any) bool {
 		if !n.IsValid() || n.Kind() != reflect.String {
 			panic(fmt.Sprintf("cannot use %T as field name of %T", needle, array))
 		}
-		value := v.FieldByName(n.String())
+		field, ok := v.Type().FieldByName(n.String())
+		if !ok || !field.IsExported() || field.Tag.Get("expr") == "-" {
+			return false
+		}
+		value := v.FieldByIndex(field.Index)
 		if value.IsValid() {
 			return true
 		}