lib/PublicInbox/Git.pm       |  7 +++++++
 lib/PublicInbox/SolverGit.pm | 11 +++++++++--

diff --git a/lib/PublicInbox/Git.pm b/lib/PublicInbox/Git.pm
index 309f80dbfcf001e51f130cddee1a14ee38303dd3..54ceaf0b8a5d6935f9101dbf52a45c4155eeb90f 100644
--- a/lib/PublicInbox/Git.pm
+++ b/lib/PublicInbox/Git.pm
@@ -94,6 +94,13 @@ 	return 0 if $self->{alt_st} eq $st;
 	$self->{alt_st} = $st; # always a true value
 }
 
+sub object_format {
+	$_[0]->{object_format} //= do {
+		my $fmt = $_[0]->qx(qw(config extensions.objectformat));
+		$fmt eq "sha256\n" ? \'sha256' : \undef;
+	}
+}
+
 sub last_check_err {
 	my ($self) = @_;
 	my $fh = $self->{err_c} or return;
diff --git a/lib/PublicInbox/SolverGit.pm b/lib/PublicInbox/SolverGit.pm
index 5d5060f453cbf58da624c64f27cb13e6ab7dc221..62b5a3439b2be772b004f13a5a8c2380a171f200 100644
--- a/lib/PublicInbox/SolverGit.pm
+++ b/lib/PublicInbox/SolverGit.pm
@@ -293,14 +293,21 @@ 	foreach ('', qw(objects refs objects/info refs/heads)) {
 		mkdir("$git_dir/$_") or die "mkdir $_: $!";
 	}
 	open my $fh, '>', "$git_dir/config" or die "open git/config: $!";
-	print $fh <<'EOF' or die "print git/config $!";
+	my $first = $self->{gits}->[0];
+	my $fmt = $first->object_format;
+	my $v = defined($$fmt) ? 1 : 0;
+	print $fh <<EOF or die "print git/config $!";
 [core]
-	repositoryFormatVersion = 0
+	repositoryFormatVersion = $v
 	filemode = true
 	bare = false
 	fsyncObjectfiles = false
 	logAllRefUpdates = false
 EOF
+	print $fh <<EOM if defined($$fmt);
+[extensions]
+	objectformat = $$fmt
+EOM
 	close $fh or die "close git/config: $!";
 
 	open $fh, '>', "$git_dir/HEAD" or die "open git/HEAD: $!";