diff --git a/connect.c b/connect.c
index 66650ff2d3..2ada5b5161 100644
--- a/connect.c
+++ b/connect.c
@@ -460,6 +460,8 @@ struct ref **get_remote_refs(int fd_out, struct packet_reader *reader,
 			die(_("unknown object format '%s' specified by server"), hash_name);
 		reader->hash_algo = &hash_algos[hash_algo];
 		packet_write_fmt(fd_out, "object-format=%s", reader->hash_algo->name);
+	} else {
+		reader->hash_algo = &hash_algos[GIT_HASH_SHA1];
 	}
 
 	if (server_options && server_options->nr &&
diff --git a/serve.c b/serve.c
index 317256c1a4..7ab7807fef 100644
--- a/serve.c
+++ b/serve.c
@@ -22,6 +22,14 @@ static int agent_advertise(struct repository *r,
 	return 1;
 }
 
+static int object_format_advertise(struct repository *r,
+				   struct strbuf *value)
+{
+	if (value)
+		strbuf_addstr(value, r->hash_algo->name);
+	return 1;
+}
+
 struct protocol_capability {
 	/*
 	 * The name of the capability.  The server uses this name when
@@ -57,6 +65,7 @@ static struct protocol_capability capabilities[] = {
 	{ "ls-refs", always_advertise, ls_refs },
 	{ "fetch", upload_pack_advertise, upload_pack_v2 },
 	{ "server-option", always_advertise, NULL },
+	{ "object-format", object_format_advertise, NULL },
 };
 
 static void advertise_capabilities(void)
@@ -153,6 +162,22 @@ int has_capability(const struct argv_array *keys, const char *capability,
 	return 0;
 }
 
+static void check_algorithm(struct repository *r, struct argv_array *keys)
+{
+	int client = GIT_HASH_SHA1, server = hash_algo_by_ptr(r->hash_algo);
+	const char *algo_name;
+
+	if (has_capability(keys, "object-format", &algo_name)) {
+		client = hash_algo_by_name(algo_name);
+		if (client == GIT_HASH_UNKNOWN)
+			die("unknown object format '%s'", algo_name);
+	}
+
+	if (client != server)
+		die("mismatched object format: server %s; client %s\n",
+		    r->hash_algo->name, hash_algos[client].name);
+}
+
 enum request_state {
 	PROCESS_REQUEST_KEYS,
 	PROCESS_REQUEST_DONE,
@@ -223,6 +248,8 @@ static int process_request(void)
 	if (!command)
 		die("no command requested");
 
+	check_algorithm(the_repository, &keys);
+
 	command->command(the_repository, &keys, &reader);
 
 	argv_array_clear(&keys);
diff --git a/t/t5701-git-serve.sh b/t/t5701-git-serve.sh
index ffb9613885..a1f5fdc9fd 100755
--- a/t/t5701-git-serve.sh
+++ b/t/t5701-git-serve.sh
@@ -5,12 +5,17 @@ test_description='test protocol v2 server commands'
 . ./test-lib.sh
 
 test_expect_success 'test capability advertisement' '
+	test_oid_cache <<-EOF &&
+	wrong_algo sha1:sha256
+	wrong_algo sha256:sha1
+	EOF
 	cat >expect <<-EOF &&
 	version 2
 	agent=git/$(git version | cut -d" " -f3)
 	ls-refs
 	fetch=shallow
 	server-option
+	object-format=$(test_oid algo)
 	0000
 	EOF
 
@@ -45,6 +50,7 @@ test_expect_success 'request invalid capability' '
 test_expect_success 'request with no command' '
 	test-tool pkt-line pack >in <<-EOF &&
 	agent=git/test
+	object-format=$(test_oid algo)
 	0000
 	EOF
 	test_must_fail test-tool serve-v2 --stateless-rpc 2>err <in &&
@@ -54,6 +60,7 @@ test_expect_success 'request with no command' '
 test_expect_success 'request invalid command' '
 	test-tool pkt-line pack >in <<-EOF &&
 	command=foo
+	object-format=$(test_oid algo)
 	agent=git/test
 	0000
 	EOF
@@ -61,6 +68,17 @@ test_expect_success 'request invalid command' '
 	test_i18ngrep "invalid command" err
 '
 
+test_expect_success 'wrong object-format' '
+	test-tool pkt-line pack >in <<-EOF &&
+	command=fetch
+	agent=git/test
+	object-format=$(test_oid wrong_algo)
+	0000
+	EOF
+	test_must_fail test-tool serve-v2 --stateless-rpc 2>err <in &&
+	test_i18ngrep "mismatched object format" err
+'
+
 # Test the basics of ls-refs
 #
 test_expect_success 'setup some refs and tags' '
@@ -74,6 +92,7 @@ test_expect_success 'setup some refs and tags' '
 test_expect_success 'basics of ls-refs' '
 	test-tool pkt-line pack >in <<-EOF &&
 	command=ls-refs
+	object-format=$(test_oid algo)
 	0000
 	EOF
 
@@ -96,6 +115,7 @@ test_expect_success 'basics of ls-refs' '
 test_expect_success 'basic ref-prefixes' '
 	test-tool pkt-line pack >in <<-EOF &&
 	command=ls-refs
+	object-format=$(test_oid algo)
 	0001
 	ref-prefix refs/heads/master
 	ref-prefix refs/tags/one
@@ -116,6 +136,7 @@ test_expect_success 'basic ref-prefixes' '
 test_expect_success 'refs/heads prefix' '
 	test-tool pkt-line pack >in <<-EOF &&
 	command=ls-refs
+	object-format=$(test_oid algo)
 	0001
 	ref-prefix refs/heads/
 	0000
@@ -136,6 +157,7 @@ test_expect_success 'refs/heads prefix' '
 test_expect_success 'peel parameter' '
 	test-tool pkt-line pack >in <<-EOF &&
 	command=ls-refs
+	object-format=$(test_oid algo)
 	0001
 	peel
 	ref-prefix refs/tags/
@@ -157,6 +179,7 @@ test_expect_success 'peel parameter' '
 test_expect_success 'symrefs parameter' '
 	test-tool pkt-line pack >in <<-EOF &&
 	command=ls-refs
+	object-format=$(test_oid algo)
 	0001
 	symrefs
 	ref-prefix refs/heads/
@@ -178,6 +201,7 @@ test_expect_success 'symrefs parameter' '
 test_expect_success 'sending server-options' '
 	test-tool pkt-line pack >in <<-EOF &&
 	command=ls-refs
+	object-format=$(test_oid algo)
 	server-option=hello
 	server-option=world
 	0001
@@ -200,6 +224,7 @@ test_expect_success 'unexpected lines are not allowed in fetch request' '
 
 	test-tool pkt-line pack >in <<-EOF &&
 	command=fetch
+	object-format=$(test_oid algo)
 	0001
 	this-is-not-a-command
 	0000