diff --git a/t/helper/test-synthesize.c b/t/helper/test-synthesize.c
index 83c40ee02a..1f28ecf0f2 100644
--- a/t/helper/test-synthesize.c
+++ b/t/helper/test-synthesize.c
@@ -246,6 +246,90 @@ static const unsigned char fast_pack_sha1_suffix[] = {
 	0xe3
 };
 
+/*
+ * SHA-256 suffix: same structure, but with 32-byte OIDs and SHA-256
+ * pack checksum (609 bytes vs 513 for SHA-1).
+ */
+static const unsigned char fast_pack_sha256_suffix[] = {
+	0xac, 0x02, 0x78, 0x01, 0x01, 0x2c, 0x00, 0xd3,
+	0xff, 0x31, 0x30, 0x30, 0x36, 0x34, 0x34, 0x20,
+	0x66, 0x69, 0x6c, 0x65, 0x00, 0x42, 0x53, 0xc1,
+	0x8a, 0x9f, 0x5e, 0xc3, 0xbb, 0x47, 0xb0, 0x83,
+	0x8a, 0x19, 0xdb, 0x31, 0xbb, 0x7b, 0x0f, 0x3b,
+	0x80, 0xa4, 0xbc, 0x2f, 0xaf, 0x72, 0x6b, 0xdb,
+	0x62, 0xaa, 0xba, 0xdd, 0xde, 0x77, 0xc6, 0x13,
+	0xeb, 0x9d, 0x0c, 0x78, 0x01, 0x01, 0xcd, 0x00,
+	0x32, 0xff, 0x74, 0x72, 0x65, 0x65, 0x20, 0x62,
+	0x36, 0x30, 0x39, 0x37, 0x37, 0x64, 0x37, 0x63,
+	0x34, 0x63, 0x32, 0x64, 0x31, 0x65, 0x63, 0x63,
+	0x33, 0x66, 0x62, 0x61, 0x31, 0x64, 0x39, 0x38,
+	0x65, 0x65, 0x31, 0x32, 0x30, 0x61, 0x64, 0x63,
+	0x32, 0x34, 0x38, 0x33, 0x34, 0x39, 0x35, 0x30,
+	0x62, 0x65, 0x34, 0x31, 0x32, 0x64, 0x39, 0x34,
+	0x63, 0x38, 0x30, 0x39, 0x34, 0x38, 0x30, 0x66,
+	0x35, 0x38, 0x62, 0x61, 0x39, 0x64, 0x61, 0x0a,
+	0x61, 0x75, 0x74, 0x68, 0x6f, 0x72, 0x20, 0x41,
+	0x20, 0x55, 0x20, 0x54, 0x68, 0x6f, 0x72, 0x20,
+	0x3c, 0x61, 0x75, 0x74, 0x68, 0x6f, 0x72, 0x40,
+	0x65, 0x78, 0x61, 0x6d, 0x70, 0x6c, 0x65, 0x2e,
+	0x63, 0x6f, 0x6d, 0x3e, 0x20, 0x31, 0x32, 0x33,
+	0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x30, 0x20,
+	0x2b, 0x30, 0x30, 0x30, 0x30, 0x0a, 0x63, 0x6f,
+	0x6d, 0x6d, 0x69, 0x74, 0x74, 0x65, 0x72, 0x20,
+	0x43, 0x20, 0x4f, 0x20, 0x4d, 0x69, 0x74, 0x74,
+	0x65, 0x72, 0x20, 0x3c, 0x63, 0x6f, 0x6d, 0x6d,
+	0x69, 0x74, 0x74, 0x65, 0x72, 0x40, 0x65, 0x78,
+	0x61, 0x6d, 0x70, 0x6c, 0x65, 0x2e, 0x63, 0x6f,
+	0x6d, 0x3e, 0x20, 0x31, 0x32, 0x33, 0x34, 0x35,
+	0x36, 0x37, 0x38, 0x39, 0x30, 0x20, 0x2b, 0x30,
+	0x30, 0x30, 0x30, 0x0a, 0x0a, 0x4c, 0x61, 0x72,
+	0x67, 0x65, 0x20, 0x62, 0x6c, 0x6f, 0x62, 0x20,
+	0x63, 0x6f, 0x6d, 0x6d, 0x69, 0x74, 0x0a, 0xb7,
+	0x80, 0x3d, 0xd7, 0x20, 0x78, 0x01, 0x01, 0x00,
+	0x00, 0xff, 0xff, 0x00, 0x00, 0x00, 0x01, 0x95,
+	0x11, 0x78, 0x01, 0x01, 0x15, 0x01, 0xea, 0xfe,
+	0x74, 0x72, 0x65, 0x65, 0x20, 0x36, 0x65, 0x66,
+	0x31, 0x39, 0x62, 0x34, 0x31, 0x32, 0x32, 0x35,
+	0x63, 0x35, 0x33, 0x36, 0x39, 0x66, 0x31, 0x63,
+	0x31, 0x30, 0x34, 0x64, 0x34, 0x35, 0x64, 0x38,
+	0x64, 0x38, 0x35, 0x65, 0x66, 0x61, 0x39, 0x62,
+	0x30, 0x35, 0x37, 0x62, 0x35, 0x33, 0x62, 0x31,
+	0x34, 0x62, 0x34, 0x62, 0x39, 0x62, 0x39, 0x33,
+	0x39, 0x64, 0x64, 0x37, 0x34, 0x64, 0x65, 0x63,
+	0x63, 0x35, 0x33, 0x32, 0x31, 0x0a, 0x70, 0x61,
+	0x72, 0x65, 0x6e, 0x74, 0x20, 0x37, 0x35, 0x62,
+	0x66, 0x30, 0x63, 0x34, 0x37, 0x61, 0x65, 0x34,
+	0x62, 0x62, 0x33, 0x30, 0x38, 0x65, 0x37, 0x63,
+	0x63, 0x32, 0x34, 0x38, 0x32, 0x65, 0x32, 0x32,
+	0x65, 0x66, 0x61, 0x65, 0x33, 0x37, 0x38, 0x37,
+	0x61, 0x39, 0x36, 0x38, 0x34, 0x38, 0x62, 0x64,
+	0x31, 0x37, 0x34, 0x39, 0x35, 0x36, 0x37, 0x31,
+	0x34, 0x37, 0x31, 0x35, 0x32, 0x34, 0x36, 0x64,
+	0x64, 0x62, 0x64, 0x35, 0x34, 0x0a, 0x61, 0x75,
+	0x74, 0x68, 0x6f, 0x72, 0x20, 0x41, 0x20, 0x55,
+	0x20, 0x54, 0x68, 0x6f, 0x72, 0x20, 0x3c, 0x61,
+	0x75, 0x74, 0x68, 0x6f, 0x72, 0x40, 0x65, 0x78,
+	0x61, 0x6d, 0x70, 0x6c, 0x65, 0x2e, 0x63, 0x6f,
+	0x6d, 0x3e, 0x20, 0x31, 0x32, 0x33, 0x34, 0x35,
+	0x36, 0x37, 0x38, 0x39, 0x30, 0x20, 0x2b, 0x30,
+	0x30, 0x30, 0x30, 0x0a, 0x63, 0x6f, 0x6d, 0x6d,
+	0x69, 0x74, 0x74, 0x65, 0x72, 0x20, 0x43, 0x20,
+	0x4f, 0x20, 0x4d, 0x69, 0x74, 0x74, 0x65, 0x72,
+	0x20, 0x3c, 0x63, 0x6f, 0x6d, 0x6d, 0x69, 0x74,
+	0x74, 0x65, 0x72, 0x40, 0x65, 0x78, 0x61, 0x6d,
+	0x70, 0x6c, 0x65, 0x2e, 0x63, 0x6f, 0x6d, 0x3e,
+	0x20, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
+	0x38, 0x39, 0x30, 0x20, 0x2b, 0x30, 0x30, 0x30,
+	0x30, 0x0a, 0x0a, 0x45, 0x6d, 0x70, 0x74, 0x79,
+	0x20, 0x74, 0x72, 0x65, 0x65, 0x20, 0x63, 0x6f,
+	0x6d, 0x6d, 0x69, 0x74, 0x0a, 0x6d, 0x6d, 0x51,
+	0x9a, 0xc9, 0x11, 0x76, 0x61, 0xa3, 0x89, 0x49,
+	0xb7, 0xa1, 0x58, 0xc6, 0x1d, 0x8c, 0x33, 0x75,
+	0x8d, 0x7e, 0x4d, 0x8e, 0x58, 0x91, 0xf8, 0x5c,
+	0x57, 0xd9, 0x89, 0x9e, 0xb8, 0xd2, 0x9a, 0xd8,
+	0xc9
+};
+
 static const struct fast_pack fast_packs[] = {
 	{
 		.format_id = GIT_SHA1_FORMAT_ID,
@@ -253,6 +337,13 @@ static const struct fast_pack fast_packs[] = {
 		.suffix_len = sizeof(fast_pack_sha1_suffix),
 		.commit_oid = "aac43daf40d0377af31aa9c798a4ae8a31b55c1d",
 	},
+	{
+		.format_id = GIT_SHA256_FORMAT_ID,
+		.suffix = fast_pack_sha256_suffix,
+		.suffix_len = sizeof(fast_pack_sha256_suffix),
+		.commit_oid = "63c46ca51267b1d45be69a044bb84b4bf0559f09"
+			      "d727f861d2ae94ddebdddbc9",
+	},
 };
 
 /*