@@ -7,6 +7,18 @@ Note that this setting should only be set by linkgit:git-init[1] or
 linkgit:git-clone[1].  Trying to change it after initialization will not
 work and will produce hard-to-diagnose issues.
 
+extensions.compatObjectFormat::
+
+	Specify a compatitbility hash algorithm to use.  The acceptable values
+	are `sha1` and `sha256`.  The value specified must be different from the
+	value of extensions.objectFormat.  This allows client level
+	interoperability between git repositories whose objectFormat matches
+	this compatObjectFormat.  In particular when fully implemented the
+	pushes and pulls from a repository in whose objectFormat matches
+	compatObjectFormat.  As well as being able to use oids encoded in
+	compatObjectFormat in addition to oids encoded with objectFormat to
+	locally specify objects.
+
 extensions.worktreeConfig::
 	If enabled, then worktrees will load config settings from the
 	`$GIT_DIR/config.worktree` file in addition to the

@@ -194,7 +194,7 @@ int repo_init(struct repository *repo,
 		goto error;
 
 	repo_set_hash_algo(repo, format.hash_algo);
-	repo_set_compat_hash_algo(repo, GIT_HASH_UNKNOWN);
+	repo_set_compat_hash_algo(repo, format.compat_hash_algo);
 	repo->repository_format_worktree_config = format.worktree_config;
 
 	/* take ownership of format.partial_clone */

@@ -590,6 +590,25 @@ static enum extension_result handle_extension(const char *var,
 				     "extensions.objectformat", value);
 		data->hash_algo = format;
 		return EXTENSION_OK;
+	} else if (!strcmp(ext, "compatobjectformat")) {
+		struct string_list_item *item;
+		int format;
+
+		if (!value)
+			return config_error_nonbool(var);
+		format = hash_algo_by_name(value);
+		if (format == GIT_HASH_UNKNOWN)
+			return error(_("invalid value for '%s': '%s'"),
+				     "extensions.compatobjectformat", value);
+		/* For now only support compatObjectFormat being specified once. */
+		for_each_string_list_item(item, &data->v1_only_extensions) {
+			if (!strcmp(item->string, "compatobjectformat"))
+				return error(_("'%s' already specified as '%s'"),
+					"extensions.compatobjectformat",
+					hash_algos[data->compat_hash_algo].name);
+		}
+		data->compat_hash_algo = format;
+		return EXTENSION_OK;
 	}
 	return EXTENSION_UNKNOWN;
 }

@@ -1565,7 +1584,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		if (startup_info->have_repository) {
 			repo_set_hash_algo(the_repository, repo_fmt.hash_algo);
 			repo_set_compat_hash_algo(the_repository,
-						  GIT_HASH_UNKNOWN);
+						  repo_fmt.compat_hash_algo);
 			the_repository->repository_format_worktree_config =
 				repo_fmt.worktree_config;
 			/* take ownership of repo_fmt.partial_clone */

@@ -1659,7 +1678,7 @@ void check_repository_format(struct repository_format *fmt)
 	check_repository_format_gently(get_git_dir(), fmt, NULL);
 	startup_info->have_repository = 1;
 	repo_set_hash_algo(the_repository, fmt->hash_algo);
-	repo_set_compat_hash_algo(the_repository, GIT_HASH_UNKNOWN);
+	repo_set_compat_hash_algo(the_repository, fmt->compat_hash_algo);
 	the_repository->repository_format_worktree_config =
 		fmt->worktree_config;
 	the_repository->repository_format_partial_clone =

@@ -86,6 +86,7 @@ struct repository_format {
 	int worktree_config;
 	int is_bare;
 	int hash_algo;
+	int compat_hash_algo;
 	int sparse_index;
 	char *work_tree;
 	struct string_list unknown_extensions;