BIP 342 (Tapscript)

BIP-342 defines Tapscript - the version of Bitcoin Script used by Taproot outputs when they're spent via the script-path branch (as opposed to the cooperative key-path branch). Activated as a soft fork in November 2021 alongside BIP-340 and BIP-341.

Tapscript is mostly the same Bitcoin Script you already know, but with several improvements:

• New signature opcodes. OP_CHECKSIGADD replaces OP_CHECKMULTISIG (which is disabled in Tapscript, along with OP_CHECKMULTISIGVERIFY: executing either fails the script) with a cleaner per-signature accumulator pattern that keeps batch verification possible.
• Schnorr signatures for OP_CHECKSIG within Tapscript, matching the rest of Taproot's signature scheme.
• Disabled and reserved opcodes. Only OP_CHECKMULTISIG and OP_CHECKMULTISIGVERIFY are disabled in Tapscript. A block of otherwise unused opcode values became OP_SUCCESS opcodes, which make a script unconditionally valid today and are reserved so a future soft fork can give them new meaning.
• Versioning. Tapscript is "leaf version 0xC0" in Taproot; future leaf versions can introduce more opcodes (or different rules) without requiring another full protocol upgrade. This is the future-proofing piece.
• Cleaner resource accounting. Tapscript meters resource usage differently from legacy script, with cleaner upper bounds on validation cost.

The versioning aspect is the under-discussed win. Adding a new opcode to legacy Bitcoin Script meant repurposing one of the reserved OP_NOP opcodes, which can only read the stack - cumbersome. Tapscript reserves a set of OP_SUCCESS opcodes that a soft fork can give any meaning, including writing to the stack, and a new leaf version can introduce a different opcode set entirely while leaving existing leaf versions untouched. Future capability extensions (covenants, new signature schemes, etc.) can use this slot.

For most users, Tapscript is invisible. When you spend a Taproot output cooperatively (the "key-path" spend), Tapscript isn't involved - just a Schnorr signature. Tapscript only comes into play if you need to use the script-path branch (an alternative branch in a MAST). Even then, your wallet handles it.

See Taproot for the broader upgrade and Bitcoin Script for the legacy scripting language Tapscript extends.

See How Taproot Actually Works for what Tapscript changed and the upgrade hooks it left open.