diff --git a/client/client.go b/client/client.go
index d1d0434..678153c 100644
--- a/client/client.go
+++ b/client/client.go
@@ -26,7 +26,7 @@ func isValidCredsMessage(msg string) error {
 
 // Store uses an external program to save credentials.
 func Store(program ProgramFunc, creds *credentials.Credentials) error {
-	cmd := program("store")
+	cmd := program(credentials.ActionStore)
 
 	buffer := new(bytes.Buffer)
 	if err := json.NewEncoder(buffer).Encode(creds); err != nil {
@@ -50,7 +50,7 @@ func Store(program ProgramFunc, creds *credentials.Credentials) error {
 
 // Get executes an external program to get the credentials from a native store.
 func Get(program ProgramFunc, serverURL string) (*credentials.Credentials, error) {
-	cmd := program("get")
+	cmd := program(credentials.ActionGet)
 	cmd.Input(strings.NewReader(serverURL))
 
 	out, err := cmd.Output()
@@ -81,7 +81,7 @@ func Get(program ProgramFunc, serverURL string) (*credentials.Credentials, error
 
 // Erase executes a program to remove the server credentials from the native store.
 func Erase(program ProgramFunc, serverURL string) error {
-	cmd := program("erase")
+	cmd := program(credentials.ActionErase)
 	cmd.Input(strings.NewReader(serverURL))
 	out, err := cmd.Output()
 	if err != nil {
@@ -99,7 +99,7 @@ func Erase(program ProgramFunc, serverURL string) error {
 
 // List executes a program to list server credentials in the native store.
 func List(program ProgramFunc) (map[string]string, error) {
-	cmd := program("list")
+	cmd := program(credentials.ActionList)
 	cmd.Input(strings.NewReader("unused"))
 	out, err := cmd.Output()
 	if err != nil {
diff --git a/credentials/credentials.go b/credentials/credentials.go
index b3acd83..eac5518 100644
--- a/credentials/credentials.go
+++ b/credentials/credentials.go
@@ -10,6 +10,20 @@ import (
 	"strings"
 )
 
+// Action defines the name of an action (sub-command) supported by a
+// credential-helper binary. It is an alias for "string", and mostly
+// for convenience.
+type Action = string
+
+// List of actions (sub-commands) supported by credential-helper binaries.
+const (
+	ActionStore   Action = "store"
+	ActionGet     Action = "get"
+	ActionErase   Action = "erase"
+	ActionList    Action = "list"
+	ActionVersion Action = "version"
+)
+
 // Credentials holds the information shared between docker and the credentials store.
 type Credentials struct {
 	ServerURL string
@@ -74,17 +88,17 @@ func usage() string {
 }
 
 // HandleCommand runs a helper to execute a credential action.
-func HandleCommand(helper Helper, action string, in io.Reader, out io.Writer) error {
+func HandleCommand(helper Helper, action Action, in io.Reader, out io.Writer) error {
 	switch action {
-	case "store":
+	case ActionStore:
 		return Store(helper, in)
-	case "get":
+	case ActionGet:
 		return Get(helper, in, out)
-	case "erase":
+	case ActionErase:
 		return Erase(helper, in)
-	case "list":
+	case ActionList:
 		return List(helper, out)
-	case "version":
+	case ActionVersion:
 		return PrintVersion(out)
 	default:
 		return fmt.Errorf("%s: unknown action: %s", Name, action)