I know there is gap600, but it's proprietary, closed source and most likely expensive. If there is an open source free solution then please let me know.
1 Answer
Zero-confirmation transactions are inherently unsafe to be considered as completed payments. It is not possible to create a tool that could give a truly reliable indication of whether or not an unconfirmed transaction will in fact confirm (or not). If it were possible to reliably predict such a thing, we wouldn't need a blockchain record in the first place.
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Gemini has its own tool that does this. Gap600 is being used by many big businesses. There are ways to give you probabilities of your tx being double spent. One way for example is how far your tx has been propagated on the network. Commented Jul 26, 2020 at 5:57
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2@hashtables I don't doubt that someone has created/is selling such a tool, but it won't be perfectly reliable, which is all I meant to point out in my answer. A tx can propagate to every node in the network, but then still be made invalid by the confirmation of a different transaction which spends one (or more) of the same inputs.– chytrikCommented Jul 26, 2020 at 6:08