diff --git a/README.md b/README.md
index c9e744a..4124e9c 100644
--- a/README.md
+++ b/README.md
@@ -62,7 +62,7 @@ The `actionheader` package uses only the standard library, so native HTTP server
 Configure the header name explicitly.
 Selection preserves case, trims outer spaces/tabs, and requires a nonempty HTTP token.
 Repeated values and comma-bearing values are rejected, including Gateway payload 2.0's comma-combined duplicates.
-Use `errors.Is` with `actionheader.ErrActionMissing`, `actionheader.ErrActionAmbiguous`, or `actionheader.ErrActionInvalid` to distinguish request-input failures.
+Use `errors.Is` with `actionheader.ErrMissing`, `actionheader.ErrAmbiguous`, or `actionheader.ErrInvalid` to distinguish request-input failures.
 
 Use ordinary `http.Handler` middleware to process this and other custom headers.
 The consumer owns its action registry, metadata, authorization and HTTP error responses.
diff --git a/action_gateway_test.go b/action_gateway_test.go
index 22bc0b8..173e3e9 100644
--- a/action_gateway_test.go
+++ b/action_gateway_test.go
@@ -37,12 +37,12 @@ func TestActionSelectorAcrossRequestFormats(t *testing.T) {
 	}{
 		{name: "single", values: []string{"Orders.Create"}, want: "Orders.Create"},
 		{name: "outer_whitespace", values: []string{" \tOrders.Create\t "}, want: "Orders.Create"},
-		{name: "missing", wantErr: actionheader.ErrActionMissing},
-		{name: "empty", values: []string{""}, wantErr: actionheader.ErrActionInvalid},
-		{name: "invalid", values: []string{"orders/create"}, wantErr: actionheader.ErrActionInvalid},
-		{name: "duplicates", values: []string{"read", "delete"}, wantErr: actionheader.ErrActionAmbiguous},
-		{name: "identical_duplicates", values: []string{"read", "read"}, wantErr: actionheader.ErrActionAmbiguous},
-		{name: "literal_comma", values: []string{"read,delete"}, wantErr: actionheader.ErrActionAmbiguous},
+		{name: "missing", wantErr: actionheader.ErrMissing},
+		{name: "empty", values: []string{""}, wantErr: actionheader.ErrInvalid},
+		{name: "invalid", values: []string{"orders/create"}, wantErr: actionheader.ErrInvalid},
+		{name: "duplicates", values: []string{"read", "delete"}, wantErr: actionheader.ErrAmbiguous},
+		{name: "identical_duplicates", values: []string{"read", "read"}, wantErr: actionheader.ErrAmbiguous},
+		{name: "literal_comma", values: []string{"read,delete"}, wantErr: actionheader.ErrAmbiguous},
 	} {
 		t.Run(tt.name, func(t *testing.T) {
 			for _, format := range []string{"typed_v1", "raw_rest", "raw_http_v1", "typed_v2", "raw_http_v2", "native_http"} {
diff --git a/actionheader/actionheader.go b/actionheader/actionheader.go
index 4eccef2..c9ed75f 100644
--- a/actionheader/actionheader.go
+++ b/actionheader/actionheader.go
@@ -10,17 +10,17 @@ import (
 )
 
 var (
-	// ErrActionMissing means the configured header has no represented values.
+	// ErrMissing means the configured header has no represented values.
 	// An absent header and a nil or empty value slice are all missing.
-	ErrActionMissing = errors.New("actionheader: missing action")
+	ErrMissing = errors.New("actionheader: missing action")
 
-	// ErrActionAmbiguous means the header has multiple represented values or a value containing a comma.
+	// ErrAmbiguous means the header has multiple represented values or a value containing a comma.
 	// Gateway payload 2.0 combines duplicates with commas;
 	// a literal comma cannot be distinguished from combined values.
-	ErrActionAmbiguous = errors.New("actionheader: ambiguous action")
+	ErrAmbiguous = errors.New("actionheader: ambiguous action")
 
-	// ErrActionInvalid means the single action value is empty after trimming outer spaces and tabs, or does not satisfy the HTTP token grammar.
-	ErrActionInvalid = errors.New("actionheader: invalid action")
+	// ErrInvalid means the single action value is empty after trimming outer spaces and tabs, or does not satisfy the HTTP token grammar.
+	ErrInvalid = errors.New("actionheader: invalid action")
 )
 
 // Selector selects an action from a configured request header.
@@ -35,7 +35,7 @@ type Selector struct {
 // The name is required and matched case-insensitively;
 // no default is inferred.
 // On error, the returned selector is the zero value.
-// Configuration errors are separate from [ErrActionMissing], [ErrActionAmbiguous] and [ErrActionInvalid].
+// Configuration errors are separate from [ErrMissing], [ErrAmbiguous] and [ErrInvalid].
 func NewSelector(name string) (Selector, error) {
 	if !validToken(name) {
 		return Selector{}, errors.New("actionheader: invalid header name")
@@ -51,7 +51,7 @@ func NewSelector(name string) (Selector, error) {
 // Multiplicity and comma checks take precedence over token validation.
 // Duplicates already lost by a gateway or earlier proxy cannot be detected.
 //
-// On failure Parse returns an empty string and [ErrActionMissing], [ErrActionAmbiguous] or [ErrActionInvalid], matchable with [errors.Is].
+// On failure Parse returns an empty string and [ErrMissing], [ErrAmbiguous] or [ErrInvalid], matchable with [errors.Is].
 // An unconfigured selector instead returns a configuration error.
 // Errors do not include supplied header values.
 // Parse does not modify headers or write an HTTP response.
@@ -70,19 +70,19 @@ func (s Selector) Parse(headers http.Header) (string, error) {
 			continue
 		}
 		if found || len(values) > 1 {
-			return "", ErrActionAmbiguous
+			return "", ErrAmbiguous
 		}
 		value, found = values[0], true
 	}
 	if !found {
-		return "", ErrActionMissing
+		return "", ErrMissing
 	}
 	if strings.Contains(value, ",") {
-		return "", ErrActionAmbiguous
+		return "", ErrAmbiguous
 	}
 	value = strings.Trim(value, " \t")
 	if !validToken(value) {
-		return "", ErrActionInvalid
+		return "", ErrInvalid
 	}
 	return value, nil
 }
diff --git a/actionheader/actionheader_test.go b/actionheader/actionheader_test.go
index 5afaec0..f1c73fc 100644
--- a/actionheader/actionheader_test.go
+++ b/actionheader/actionheader_test.go
@@ -22,34 +22,34 @@ func TestSelectorParse(t *testing.T) {
 		want    string
 		wantErr error
 	}{
-		{name: "nil", wantErr: actionheader.ErrActionMissing},
-		{name: "unrelated", headers: http.Header{"Else": {"orders.create"}}, wantErr: actionheader.ErrActionMissing},
-		{name: "nil_slice", headers: http.Header{"Action": nil}, wantErr: actionheader.ErrActionMissing},
-		{name: "empty_slice", headers: http.Header{"Action": {}}, wantErr: actionheader.ErrActionMissing},
-		{name: "empty", headers: http.Header{"Action": {""}}, wantErr: actionheader.ErrActionInvalid},
-		{name: "whitespace", headers: http.Header{"Action": {" \t "}}, wantErr: actionheader.ErrActionInvalid},
+		{name: "nil", wantErr: actionheader.ErrMissing},
+		{name: "unrelated", headers: http.Header{"Else": {"orders.create"}}, wantErr: actionheader.ErrMissing},
+		{name: "nil_slice", headers: http.Header{"Action": nil}, wantErr: actionheader.ErrMissing},
+		{name: "empty_slice", headers: http.Header{"Action": {}}, wantErr: actionheader.ErrMissing},
+		{name: "empty", headers: http.Header{"Action": {""}}, wantErr: actionheader.ErrInvalid},
+		{name: "whitespace", headers: http.Header{"Action": {" \t "}}, wantErr: actionheader.ErrInvalid},
 		{name: "trim_and_preserve_case", headers: http.Header{"Action": {" \tOrders.Create\t "}}, want: "Orders.Create"},
 		{name: "direct_assignment", headers: http.Header{"aCtIoN": {"orders.create"}}, want: "orders.create"},
 		{name: "empty_alias", headers: http.Header{"ACTION": {}, "action": {"orders.create"}}, want: "orders.create"},
-		{name: "conflict", headers: http.Header{"Action": {"read", "delete"}}, wantErr: actionheader.ErrActionAmbiguous},
-		{name: "identical", headers: http.Header{"Action": {"read", "read"}}, wantErr: actionheader.ErrActionAmbiguous},
-		{name: "case_aliases", headers: http.Header{"Action": {"read"}, "ACTION": {"delete"}}, wantErr: actionheader.ErrActionAmbiguous},
-		{name: "identical_aliases", headers: http.Header{"Action": {"read"}, "action": {"read"}}, wantErr: actionheader.ErrActionAmbiguous},
-		{name: "invalid_plus_alias", headers: http.Header{"Action": {""}, "action": {"read"}}, wantErr: actionheader.ErrActionAmbiguous},
-		{name: "multiple_empty", headers: http.Header{"Action": {"", ""}}, wantErr: actionheader.ErrActionAmbiguous},
-		{name: "joined", headers: http.Header{"Action": {"read,delete"}}, wantErr: actionheader.ErrActionAmbiguous},
-		{name: "identical_joined", headers: http.Header{"Action": {"read,read"}}, wantErr: actionheader.ErrActionAmbiguous},
-		{name: "comma_before_invalid", headers: http.Header{"Action": {",\r\n"}}, wantErr: actionheader.ErrActionAmbiguous},
-		{name: "space_in_token", headers: http.Header{"Action": {"orders create"}}, wantErr: actionheader.ErrActionInvalid},
-		{name: "slash", headers: http.Header{"Action": {"orders/create"}}, wantErr: actionheader.ErrActionInvalid},
-		{name: "colon", headers: http.Header{"Action": {"orders:create"}}, wantErr: actionheader.ErrActionInvalid},
-		{name: "quoted", headers: http.Header{"Action": {`"read"`}}, wantErr: actionheader.ErrActionInvalid},
-		{name: "unicode_space", headers: http.Header{"Action": {"\u00a0read"}}, wantErr: actionheader.ErrActionInvalid},
-		{name: "unicode", headers: http.Header{"Action": {"créer"}}, wantErr: actionheader.ErrActionInvalid},
-		{name: "invalid_utf8", headers: http.Header{"Action": {"\xff"}}, wantErr: actionheader.ErrActionInvalid},
-		{name: "newline", headers: http.Header{"Action": {"read\n"}}, wantErr: actionheader.ErrActionInvalid},
-		{name: "nul", headers: http.Header{"Action": {"read\x00"}}, wantErr: actionheader.ErrActionInvalid},
-		{name: "del", headers: http.Header{"Action": {"read\x7f"}}, wantErr: actionheader.ErrActionInvalid},
+		{name: "conflict", headers: http.Header{"Action": {"read", "delete"}}, wantErr: actionheader.ErrAmbiguous},
+		{name: "identical", headers: http.Header{"Action": {"read", "read"}}, wantErr: actionheader.ErrAmbiguous},
+		{name: "case_aliases", headers: http.Header{"Action": {"read"}, "ACTION": {"delete"}}, wantErr: actionheader.ErrAmbiguous},
+		{name: "identical_aliases", headers: http.Header{"Action": {"read"}, "action": {"read"}}, wantErr: actionheader.ErrAmbiguous},
+		{name: "invalid_plus_alias", headers: http.Header{"Action": {""}, "action": {"read"}}, wantErr: actionheader.ErrAmbiguous},
+		{name: "multiple_empty", headers: http.Header{"Action": {"", ""}}, wantErr: actionheader.ErrAmbiguous},
+		{name: "joined", headers: http.Header{"Action": {"read,delete"}}, wantErr: actionheader.ErrAmbiguous},
+		{name: "identical_joined", headers: http.Header{"Action": {"read,read"}}, wantErr: actionheader.ErrAmbiguous},
+		{name: "comma_before_invalid", headers: http.Header{"Action": {",\r\n"}}, wantErr: actionheader.ErrAmbiguous},
+		{name: "space_in_token", headers: http.Header{"Action": {"orders create"}}, wantErr: actionheader.ErrInvalid},
+		{name: "slash", headers: http.Header{"Action": {"orders/create"}}, wantErr: actionheader.ErrInvalid},
+		{name: "colon", headers: http.Header{"Action": {"orders:create"}}, wantErr: actionheader.ErrInvalid},
+		{name: "quoted", headers: http.Header{"Action": {`"read"`}}, wantErr: actionheader.ErrInvalid},
+		{name: "unicode_space", headers: http.Header{"Action": {"\u00a0read"}}, wantErr: actionheader.ErrInvalid},
+		{name: "unicode", headers: http.Header{"Action": {"créer"}}, wantErr: actionheader.ErrInvalid},
+		{name: "invalid_utf8", headers: http.Header{"Action": {"\xff"}}, wantErr: actionheader.ErrInvalid},
+		{name: "newline", headers: http.Header{"Action": {"read\n"}}, wantErr: actionheader.ErrInvalid},
+		{name: "nul", headers: http.Header{"Action": {"read\x00"}}, wantErr: actionheader.ErrInvalid},
+		{name: "del", headers: http.Header{"Action": {"read\x7f"}}, wantErr: actionheader.ErrInvalid},
 		{name: "all_token_punctuation", headers: http.Header{"Action": {"!#$%&'*+-.^_`|~AZaz09"}}, want: "!#$%&'*+-.^_`|~AZaz09"},
 	} {
 		t.Run(tt.name, func(t *testing.T) {
@@ -80,7 +80,7 @@ func TestSelectorConfiguration(t *testing.T) {
 			if parseErr == nil {
 				t.Fatal("zero selector Parse succeeded")
 			}
-			for _, category := range []error{actionheader.ErrActionMissing, actionheader.ErrActionAmbiguous, actionheader.ErrActionInvalid} {
+			for _, category := range []error{actionheader.ErrMissing, actionheader.ErrAmbiguous, actionheader.ErrInvalid} {
 				if errors.Is(err, category) || errors.Is(parseErr, category) {
 					t.Errorf("configuration error matched input category %v", category)
 				}
diff --git a/docs/plan.md b/docs/plan.md
index 86fdaa9..c1e84ed 100644
--- a/docs/plan.md
+++ b/docs/plan.md
@@ -377,7 +377,7 @@ Behavior, error semantics and trust boundaries do not change;
 these are placement, naming, typing and documentation changes made before the first release.
 
 - [x] E1: move the action-header selector to the standard-library-only `actionheader` package as `Selector`/`NewSelector`.
-- [ ] E2: rename the moved sentinels to `ErrMissing`, `ErrAmbiguous` and `ErrInvalid`.
+- [x] E2: rename the moved sentinels to `ErrMissing`, `ErrAmbiguous` and `ErrInvalid`.
 - [ ] E3: rename `identity.WithCaller` to `identity.NewContext`.
 - [ ] E4: type the `InvocationError` operation and limit names with exported constants.
 - [ ] E5: add `String` methods to the `identity` enumerations.