diff --git a/src/types/error_code.rs b/src/types/error_code.rs
new file mode 100644
index 0000000..6febaa8
--- /dev/null
+++ b/src/types/error_code.rs
@@ -0,0 +1,36 @@
+#[non_exhaustive]
+#[derive(Clone, Debug)]
+pub enum ErrorCode {
+    Forbidden,
+    UnknownToken,
+    MissingToken,
+    BadJson,
+    NotJson,
+    NotFound,
+    LimitExceeded,
+    Unknown,
+    UserInUse,
+    InvalidUsername,
+    Exclusive,
+}
+
+impl serde::Serialize for ErrorCode {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        serializer.serialize_str(match self {
+            ErrorCode::Forbidden => "M_FORBIDDEN",
+            ErrorCode::UnknownToken => "M_UNKNOWN_TOKEN",
+            ErrorCode::MissingToken => "M_MISSING_TOKEN",
+            ErrorCode::BadJson => "M_BAD_JSON",
+            ErrorCode::NotJson => "M_NOT_JSON",
+            ErrorCode::NotFound => "M_NOT_FOUND",
+            ErrorCode::LimitExceeded => "M_LIMIT_EXCEEDED",
+            ErrorCode::Unknown => "M_UNKNOWN",
+            ErrorCode::UserInUse => "M_USER_IN_USE",
+            ErrorCode::InvalidUsername => "M_INVALID_USERNAME",
+            ErrorCode::Exclusive => "M_EXCLUSIVE",
+        })
+    }
+}