+/* The longest possible tag value. */
+#define NOTMUCH_TAG_MAX 200
+
+/* Add a tag to the given message.
+ *
+ * Return value:
+ *
+ * NOTMUCH_STATUS_SUCCESS: Tag successfully added to message
+ *
+ * NOTMUCH_STATUS_NULL_POINTER: The 'tag' argument is NULL
+ *
+ * NOTMUCH_STATUS_TAG_TOO_LONG: The length of 'tag' is longer than
+ * too long (exceeds NOTMUCH_TAG_MAX)
+ */
+notmuch_status_t
+notmuch_message_add_tag (notmuch_message_t *message, const char *tag);
+
+/* Remove a tag from the given message.
+ *
+ * Return value:
+ *
+ * NOTMUCH_STATUS_SUCCESS: Tag successfully added to message
+ *
+ * NOTMUCH_STATUS_NULL_POINTER: The 'tag' argument is NULL
+ *
+ * NOTMUCH_STATUS_TAG_TOO_LONG: The length of 'tag' is longer than
+ * too long (exceeds NOTMUCH_TAG_MAX)
+ */
+notmuch_status_t
+notmuch_message_remove_tag (notmuch_message_t *message, const char *tag);
+