https://github.com/mongodb/specifications/blob/master/source/transactions/transactions.rst#constructing-the-first-command-within-a-transaction says:

> When constructing the first command within a transaction, drivers MUST add the lsid, txnNumber, startTransaction, and autocommit fields.

It seems to me that this only happens for write commands, because tests added in https://github.com/mongodb/specifications/pull/364 / https://jira.mongodb.org/browse/DRIVERS-550 which do start_transaction -> count_documents -> commit_transaction never change session state from starting_transaction and thus do not send a commit to the server.

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Collection#count_documents does not seem to properly start transactions when passed a session in the STARTING_TRANSACTION state. This should be fixed.