Why didn't my TX get rejected?

Question

Here is the address I'm going to talk about: https://mempool.space/testnet/address/tb1p3yyenp8ppqyze8th3pnng33l7c4sl80yx02s5achpf62v340xvpstngz04

In this address there are 3 transactions I chained together:

1. Receiving an inscription.
2. Receiving 175 sats, so that the address is able to send the received inscription.
3. Sending the inscription to itself, paying 175 sats to miners for the tx fee (so the "dust" utxo was both successfully created and spent!).

These transactions were broadcast to the public mempool.space api (this request: https://mempool.space/testnet/docs/api/rest#post-transaction).

What I don't understand is why didn't the node reject the transaction, which sent 175 sats to this address? Isn't there a dust limit, below which you are not supposed to go? Am I safe to do things like these?

Answer 1

The dust limit for P2TR used by Bitcoin Core is 330 sats, which is indeed more than 175. However, it is only a standardness rule that nodes and miners are free to ignore.

In the case of testnet, Bitcoin Core's default was historically to ignore many standardness rules, including the dust limit. This was only changed in v26 (in PR #28354), so there are likely still many nodes on testnet accepting non-standard transactions, either because they haven't upgraded yet or because they're manually overriding the new default – as is likely the case with the mempool.space testnet node.

Am I safe to do things like these?

Testnet is a safe place for testing pretty much anything. However, you might want to make sure your code still works when you set acceptnonstdtxn=0 so you don't run against standardness rules when moving to mainnet.