So, I noticed Android has a “NFCSnoop” format that you can extract using
adb shell dumpsys nfc
but the format is kind of unknown. I went and dug around, eventually finding the code that actually generates the format and working from there.
Turns out, Android saves a ring buffer full of all NFC interactions in NCI (so between the phone and the actual NFC chip in the phone). You can extract them with the command above.
I ended up writing a decoder to decompress and interpret the headers/data, it’s on Github.
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Oh wow you decoded that? That’s brilliant. Thanks! I tried to make sense of all that a few months ago and gave up.
Also, if you look at the log in adb shell, you can see a lot of stuff being logged in real time by the various bits of Android that handle the tag. There’s quite a few things to learn out of that too, but it’s also not documented.
I was looking into that when I implemented the cellphone-as-tabletop-reader option in SiRFIDaL, but ended up releasing a companion Tasker script to extract the information it needs. Now I’m gonna see if perhaps there’s a way to leverage your work and do away with the tasker script. Thanks!
EDIT: I like the unit test 
Oh, that unit test was the default in the Maven archetype. I didn’t bother loading it onto Jenkins so I didn’t bother writing tests.
Edit: About the program: It doesn’t decode NCI packets yet. It’s kind of clear I need to do so to make any sense of these dumps.