When it comes to "how much of it has to be available to be open source", I think it may be instructive to look at encryption algorithms.
Many of them have numeric constants or initial values--NOT part of the secret key itself--which need to be known and available, both for interoperability and for the expected security-level of the algorithm. These are arguably similar to LLM weights. (Perhaps the simplest example would be the prominence of "13" in ROT13.)
Yet if someone tried saying that their encryption standard was "open source" while keeping those constants secret and/or legally-encumbered, I think a lot of people would complain that the label is incorrect or inappropriate.
When it comes to "how much of it has to be available to be open source", I think it may be instructive to look at encryption algorithms.
Many of them have numeric constants or initial values--NOT part of the secret key itself--which need to be known and available, both for interoperability and for the expected security-level of the algorithm. These are arguably similar to LLM weights. (Perhaps the simplest example would be the prominence of "13" in ROT13.)
Yet if someone tried saying that their encryption standard was "open source" while keeping those constants secret and/or legally-encumbered, I think a lot of people would complain that the label is incorrect or inappropriate.