Taproot 2/3 tr thresh does not work on bdk-wallet

[toskaen]

let external_descriptor = format!(
"tr({{}},thresh(2,{},{},{}))",
user_pubkey, lsp_pubkey, trustee_pubkey
);
let internal_descriptor = format!(
"tr({{}},thresh(2,{},{},{}))",
lsp_pubkey, trustee_pubkey, user_pubkey

It seems like I have to pin myself into 0.29.0 to use multi_a

why is that the case?

[artofbitcoin]

multi_a works with current bdk_wallet, no need to pin an old version. The descriptor in your snippet fails for two other reasons. I checked each variant by creating a wallet with Wallet::create(...).create_wallet_no_persist() on bdk_wallet master (miniscript 12.3):

1. thresh with bare keys is not valid miniscript. Each argument of thresh must be a miniscript fragment, not a key. thresh(2,A,B,C) fails with unexpected «<key>(0 args) while parsing Miniscript». The valid form wraps the keys: thresh(2,pk(A),s:pk(B),s:pk(C)).
2. The internal key is empty. In Rust, {{}} in format! produces a literal {}, so the descriptor starts with tr({},...). That fails with Key too short (<66 char), doesn't match any format. You need a real key there. If no single party should be able to spend through the key path, use an unspendable key such as the NUMS point from BIP 341 (50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0).

For a 2-of-3 in a tapscript leaf, multi_a is the simplest:

let external_descriptor = format!(
    "tr({NUMS},multi_a(2,{user_pubkey},{lsp_pubkey},{trustee_pubkey}))"
);
let internal_descriptor = format!(
    "tr({NUMS},multi_a(2,{lsp_pubkey},{trustee_pubkey},{user_pubkey}))"
);

Results with the same three keys:

Descriptor	Result
tr({},thresh(2,A,B,C)) (as posted)	error
tr(NUMS,thresh(2,A,B,C))	error (bare keys)
tr(NUMS,thresh(2,pk(A),s:pk(B),s:pk(C)))	OK
tr(NUMS,multi_a(2,A,B,C))	OK, with compressed or x-only keys
tr(NUMS,multi(2,A,B,C))	error: multi is segwit v0 only, multi_a is its taproot version

With fixed single keys rather than xpubs with a /* wildcard, the wallet always derives the same address for each keychain.