MANIFEST       |  1 +
 t/content_id.t | 24 ++++++++++++++++++++++++

diff --git a/MANIFEST b/MANIFEST
index 85e850314526a7b64ce5fb317368509eb50fdb8d..1df27f26ed81f558b952a8eda26a6f6c716cd02f 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -128,6 +128,7 @@ t/check-www-inbox.perl
 t/common.perl
 t/config.t
 t/config_limiter.t
+t/content_id.t
 t/emergency.t
 t/fail-bin/spamc
 t/feed.t
diff --git a/t/content_id.t b/t/content_id.t
new file mode 100644
index 0000000000000000000000000000000000000000..c0ae6ecde27a47bd440ebd5c93a456182365400e
--- /dev/null
+++ b/t/content_id.t
@@ -0,0 +1,24 @@
+# Copyright (C) 2018 all contributors <meta@public-inbox.org>
+# License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
+use strict;
+use warnings;
+use Test::More;
+use PublicInbox::ContentId qw(content_id);
+use Email::MIME;
+
+my $mime = Email::MIME->create(
+	header => [
+		From => 'a@example.com',
+		To => 'b@example.com',
+		'Content-Type' => 'text/plain',
+		Subject => 'this is a subject',
+		'Message-ID' => '<a@example.com>',
+		Date => 'Fri, 02 Oct 1993 00:00:00 +0000',
+	],
+	body => "hello world\n",
+);
+
+my $res = content_id($mime);
+like($res, qr/\ASHA-256:[a-f0-9]{64}\z/, 'cid in format expected');
+
+done_testing();