diff --git a/tags.go b/tags.go
index 7719e85..41707ed 100644
--- a/tags.go
+++ b/tags.go
@@ -9,13 +9,34 @@ import (
 )
 
 var (
-	errTagSyntax      = errors.New("bad syntax for struct tag pair")
-	errTagKeySyntax   = errors.New("bad syntax for struct tag key")
-	errTagValueSyntax = errors.New("bad syntax for struct tag value")
+	// ErrTagSyntax is returned when a struct tag pair has invalid syntax.
+	ErrTagSyntax = errors.New("bad syntax for struct tag pair")
 
-	errKeyNotSet      = errors.New("tag key does not exist")
-	errTagNotExist    = errors.New("tag does not exist")
-	errTagKeyMismatch = errors.New("mismatch between key and tag.key")
+	// ErrTagKeySyntax is returned when a struct tag key has invalid syntax.
+	ErrTagKeySyntax = errors.New("bad syntax for struct tag key")
+
+	// ErrTagValueSyntax is returned when a struct tag value has invalid syntax.
+	ErrTagValueSyntax = errors.New("bad syntax for struct tag value")
+
+	// ErrKeyNotExist is returned when a tag key is empty or not set.
+	ErrKeyNotExist = errors.New("key does not exist")
+
+	// ErrKeyNotSet is an alias for ErrKeyNotExist for backward compatibility.
+	ErrKeyNotSet = ErrKeyNotExist
+
+	// ErrTagNotExist is returned when a requested tag does not exist.
+	ErrTagNotExist = errors.New("tag does not exist")
+
+	// ErrTagKeyMismatch is returned when there is a mismatch between key and tag.key.
+	ErrTagKeyMismatch = errors.New("mismatch between key and tag.key")
+
+	errTagSyntax      = ErrTagSyntax
+	errTagKeySyntax   = ErrTagKeySyntax
+	errTagValueSyntax = ErrTagValueSyntax
+	errKeyNotSet      = ErrKeyNotExist
+	errKeyNotExist    = ErrKeyNotExist
+	errTagNotExist    = ErrTagNotExist
+	errTagKeyMismatch = ErrTagKeyMismatch
 )
 
 // Tags represent a set of tags from a single struct field
@@ -69,13 +90,13 @@ func Parse(tag string) (*Tags, error) {
 		}
 
 		if i == 0 {
-			return nil, errTagKeySyntax
+			return nil, ErrTagKeySyntax
 		}
 		if i+1 >= len(tag) || tag[i] != ':' {
-			return nil, errTagSyntax
+			return nil, ErrTagSyntax
 		}
 		if tag[i+1] != '"' {
-			return nil, errTagValueSyntax
+			return nil, ErrTagValueSyntax
 		}
 
 		key := tag[:i]
@@ -90,7 +111,7 @@ func Parse(tag string) (*Tags, error) {
 			i++
 		}
 		if i >= len(tag) {
-			return nil, errTagValueSyntax
+			return nil, ErrTagValueSyntax
 		}
 
 		qvalue := tag[:i+1]
@@ -98,7 +119,7 @@ func Parse(tag string) (*Tags, error) {
 
 		value, err := strconv.Unquote(qvalue)
 		if err != nil {
-			return nil, errTagValueSyntax
+			return nil, ErrTagValueSyntax
 		}
 
 		res := strings.Split(value, ",")
@@ -135,13 +156,13 @@ func (t *Tags) Get(key string) (*Tag, error) {
 		}
 	}
 
-	return nil, errTagNotExist
+	return nil, ErrTagNotExist
 }
 
 // Set sets the given tag. If the tag key already exists it'll override it
 func (t *Tags) Set(tag *Tag) error {
 	if tag.Key == "" {
-		return errKeyNotSet
+		return ErrKeyNotExist
 	}
 
 	added := false
diff --git a/tags_test.go b/tags_test.go
index 1dc7810..497c10c 100644
--- a/tags_test.go
+++ b/tags_test.go
@@ -1,6 +1,7 @@
 package structtag
 
 import (
+	"errors"
 	"reflect"
 	"sort"
 	"strings"
@@ -422,3 +423,113 @@ func TestTags_Sort(t *testing.T) {
 		t.Errorf("string\n\twant: %#v\n\tgot : %#v", want, tags.String())
 	}
 }
+
+func TestTags_Get_TagNotExist(t *testing.T) {
+	tag := `json:"foo,omitempty" structs:"bar,omitnested"`
+
+	tags, err := Parse(tag)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	found, err := tags.Get("nonexistent")
+	if found != nil {
+		t.Fatalf("expected nil tag for nonexistent key, got: %v", found)
+	}
+	if err == nil {
+		t.Fatal("expected error for nonexistent key, got nil")
+	}
+
+	if !errors.Is(err, ErrTagNotExist) {
+		t.Errorf("errors.Is(err, ErrTagNotExist)\n\twant: true\n\tgot : false (err: %v)", err)
+	}
+
+	// Verify backward-compatible unexported alias.
+	if !errors.Is(err, errTagNotExist) {
+		t.Errorf("errors.Is(err, errTagNotExist)\n\twant: true\n\tgot : false (err: %v)", err)
+	}
+
+	if errors.Is(err, ErrKeyNotExist) {
+		t.Errorf("errors.Is(err, ErrKeyNotExist)\n\twant: false\n\tgot : true")
+	}
+}
+
+func TestTags_Set_KeyNotExist(t *testing.T) {
+	tag := `json:"foo,omitempty"`
+
+	tags, err := Parse(tag)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = tags.Set(&Tag{
+		Key:  "",
+		Name: "bar",
+	})
+	if err == nil {
+		t.Fatal("setting tag with empty key should error")
+	}
+
+	if !errors.Is(err, ErrKeyNotExist) {
+		t.Errorf("errors.Is(err, ErrKeyNotExist)\n\twant: true\n\tgot : false (err: %v)", err)
+	}
+
+	// Verify backward-compatible aliases.
+	if !errors.Is(err, ErrKeyNotSet) {
+		t.Errorf("errors.Is(err, ErrKeyNotSet)\n\twant: true\n\tgot : false (err: %v)", err)
+	}
+	if !errors.Is(err, errKeyNotSet) {
+		t.Errorf("errors.Is(err, errKeyNotSet)\n\twant: true\n\tgot : false (err: %v)", err)
+	}
+	if !errors.Is(err, errKeyNotExist) {
+		t.Errorf("errors.Is(err, errKeyNotExist)\n\twant: true\n\tgot : false (err: %v)", err)
+	}
+
+	if errors.Is(err, ErrTagNotExist) {
+		t.Errorf("errors.Is(err, ErrTagNotExist)\n\twant: false\n\tgot : true")
+	}
+}
+
+func TestSentinelErrors(t *testing.T) {
+	t.Run("ParseSyntaxErrors", func(t *testing.T) {
+		testCases := []struct {
+			name     string
+			tag      string
+			expected error
+		}{
+			{
+				name:     "ErrTagKeySyntax",
+				tag:      `:"foo"`,
+				expected: ErrTagKeySyntax,
+			},
+			{
+				name:     "ErrTagSyntax",
+				tag:      "json:",
+				expected: ErrTagSyntax,
+			},
+			{
+				name:     "ErrTagValueSyntax_Unclosed",
+				tag:      `json:"`,
+				expected: ErrTagValueSyntax,
+			},
+			{
+				name:     "ErrTagValueSyntax_NoQuotes",
+				tag:      "json:foo",
+				expected: ErrTagValueSyntax,
+			},
+		}
+
+		for _, tc := range testCases {
+			t.Run(tc.name, func(t *testing.T) {
+				_, err := Parse(tc.tag)
+				if err == nil {
+					t.Fatalf("expected error for tag %q, got nil", tc.tag)
+				}
+				if !errors.Is(err, tc.expected) {
+					t.Errorf("errors.Is(err, %v)\n\twant: true\n\tgot : false (err: %v)", tc.expected, err)
+				}
+			})
+		}
+	})
+}
+