Own seed: "you have entered an invalid recovery seed"

I don’t want to use the recovery seed suggested by my Trezor T, for reasons I don’t want to go into. Instead I generate my own 12 words from the BIP English set.

My understanding was that this seed ought to be enough to create a new wallet: I enter them into my Trezor, and start using the corresponding (new, unique) wallet.

Not so!

After I enter my 12-word seed into a factory-reset Trezor T as part of “Recovery from seed”, I get the error you have entered an invalid recovery seed and cannot continue.

Are the 12 seed words constrained in some way = not full entropy? Is there a checksum? What’s the algo for seed validity.

Trezor T, Suite 22.2.3, latest firmware as of today.

OK, I found the algorithm: Python.

There really is a checksum. The last word cannot be arbitrary = not all 12 words are fully random.

I confirm that following that checksum algo works. Trezor T accepts my self-generated recovery seed to create a new wallet.

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I would just like to note that generating seed by hand and then entering it into a computer to generate the checksum is overall less secure than generating it on a dedicated device.