Why can't I bumpfee the 25th tx in an unconfirmed chain?

Question

I have a chain of 25 unconfirmed transactions. There's a default mempool limit of 25 chained unconfirmed transactions, so my change output of the 25th transaction is not usable until some of the transactions have confirmed.

To make it confirm, I thought I'd bumpfee the last (25th) tx.

$ bitcoin-cli bumpfee <txid> '{"fee_rate": 410}'
error code: -4
error message:
Unable to create transaction. Transaction has too long of a mempool chain

Why do I get this error? I'm not making the chain longer, I'm just trying to replace the last tx.

Answer 1

In the mempool validation logic, the maximum chain size checks are performed without first removing the transaction to be replaced. That is, if there is a chain of 25 unconfirmed transactions such as:

txA <-- txB <-- txC <-- ... <-- txY

And a candidate for replacing txY (txY') is submitted, the mempool limits will be checked with the following topology:

txA <-- txB <-- txC <-- ... <-- txY
                            \-- txY'

txA will therefore fail the descendants count limit check (since it's now 26, accounting for itself) [0].

The wallet logic when creating a transaction (not only for fee bumps) mimics this logic (1, 2). This is why the bumpfee command for the 25th transaction in an unconfirmed chain fails.

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[0]: This ignores the RBF carveout for simplicity.