I considered a higher level interface where the caller passes a tag
name rather than a flag character, but the role of the "unread" tag is
particularly confusing with such an interface.
---
 lib/message.cc | 7 +++++++
 lib/notmuch.h  | 8 ++++++++
 2 files changed, 15 insertions(+)

diff --git a/lib/message.cc b/lib/message.cc
index 18f830dd..2efddb6c 100644
--- a/lib/message.cc
+++ b/lib/message.cc
@@ -1560,6 +1560,13 @@ _ensure_maildir_flags (notmuch_message_t *message, notmuch_bool_t force)
 	message->maildir_flags = combined_flags;
 }
 
+notmuch_bool_t
+notmuch_message_has_maildir_flag (notmuch_message_t *message, char flag)
+{
+    _ensure_maildir_flags (message, FALSE);
+    return message->maildir_flags && (strchr (message->maildir_flags, flag) != NULL);
+}
+
 notmuch_status_t
 notmuch_message_maildir_flags_to_tags (notmuch_message_t *message)
 {
diff --git a/lib/notmuch.h b/lib/notmuch.h
index f78b3473..b2c8e929 100644
--- a/lib/notmuch.h
+++ b/lib/notmuch.h
@@ -1589,6 +1589,14 @@ notmuch_status_t
 notmuch_message_maildir_flags_to_tags (notmuch_message_t *message);
 
 /**
+ * return TRUE if any filename in a maildir has the given flag, FALSE
+ * otherwise.
+ *
+ */
+notmuch_bool_t
+notmuch_message_has_maildir_flag (notmuch_message_t *message, char flag);
+
+/**
  * Rename message filename(s) to encode tags as maildir flags.
  *
  * Specifically, for each filename corresponding to this message:
-- 
2.13.2