CHIPs · CHIP-2026-06 · withdrawn
CashTokens FT+NFT Ambiguity Fix
Proposed banning new outputs that hold both fungible tokens and a zero-byte immutable NFT, which contracts cannot tell apart from FT-only outputs. Retracted two days after publication.
Summary
Under the CashTokens introspection opcodes, a UTXO holding both a fungible token amount and an immutable NFT with a zero-byte commitment looks the same as a UTXO holding only the fungible tokens. This maintenance CHIP proposed a consensus rule forbidding new outputs with that combination. Existing ones would stay spendable but would have to be split when spent.
Motivation
The author wanted “future-proofing”: so a contract could commit to an entire transaction, using loops and introspection, without these “invisible” bits leaking through. A blockchain scan found the combination only a few times.
Current status
Withdrawn. Posted 14 June 2026 and retracted 16 June 2026. The author’s reason: the fix “is not really free.” It would push technical debt onto wallet developers, who would have to handle this edge case when merging a user’s balances into fewer UTXOs. Discussion noted that issuers can avoid the problem by never emitting null NFTs, though category-agnostic contracts like TapSwap may still be affected.
Note: this shares the “CHIP-2026-06” prefix with the unrelated OP_SIGHASH and Post-Quantum Signatures proposals.