Was the OP_SUCCESSx reservation in BIP-342 designed with specific opcode families in mind, or as a generic forward-compatibility mechanism?

External question — Bitcoin Stack Exchange Author: Aaron Zhang Original post: https://bitcoin.stackexchange.com/questions/130670 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. In Pieter Wuille's recent answer [Why did BIP-342 replace CHECKMULTISIG with a new opcode] , BIP-342's deliberate minimization of semantic changes was attributed to the expectation that "those could always be introduced with later softforks that redefine OP_SUCCESSes." I'm curious about the granularity of this reservation: Were specific opcode candidates (e.g., CHECKSIGFROMSTACK, CAT, TXHASH) already on the radar when OP_SUCCESS positions were allocated, or was the allocation purely abstract — "reserve space for unknown future use"? Was there discussion about classes of additions (introspection opcodes, signature variants, hash operations) that would or wouldn't be appropriate candidates for OP_SUCCESS redefinition vs. requiring a deeper softfork? Are there design properties an opcode SHOULD have to be a clean OP_SUCCESS redefinition (vs. requiring more invasive consensus changes)? I ask because the activation-path mechanics matter for how community discussions about future opcodes should be framed: is the conversation "which opcode" or also "which opcode reservation slot."

External answer — Bitcoin Stack Exchange Author: Murch Original post: https://bitcoin.stackexchange.com/a/130859 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. I don’t have an answer to which opcodes were on the mind of the Tapscript designers when they came up with OP_SUCCESS , but the result was that the OP_SUCCESS opcodes are designed as generic upgrade hooks. Some of the issues with the previously existing OP_NOP opcodes are that there are few of them, and because they are "no operation", unupgraded nodes would simply pop them off the stack and do nothing before moving on to the next stack operation. This means that when an OP_NOP is given more restrictive meaning with a soft fork, it would break forward-compatibility if executing them would change the stack. E.g., if a redefined OP_NOP were to evaluate some stack elements, it cannot pop them off the stack and cannot put a result on the stack, as unupgraded nodes would not evaluate such scripts in the same manner. OP_SUCCESS opcodes were implemented such that the presence of any OP_SUCCESS opcode immediately makes a script return successfully, regardless of whether the OP_SUCCESS opcode would be executed . This allows future soft forks to introduce more complex behavior, because any sort of predicate evaluation, stack manipulations etc. will be more restrictive than "always succeed" and unupgraded nodes will always agree with upgraded nodes evaluating a script as valid.