During a cross-shard transaction, mongos tracks the latest statement id it has received from the client and compares it to the statement id of the first command it received to determine if snapshot errors are retryable. Currently, mongos does not actually parse the statement ids from the client and instead assumes each new command increments the id by one. Once operations inside a transaction are retryable and statement ids from the client are more meaningful, mongos should be changed to parse and use the statement id from the client instead.