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 was deprecated for Tapscript) with a cleaner per-signature accumulator pattern.
• Schnorr signatures for OP_CHECKSIG within Tapscript, matching the rest of Taproot's signature scheme.
• Removed legacy opcodes. A few opcodes that didn't make sense or had subtle quirks are gone in Tapscript context.
• 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 requires a soft fork that updates the version field's interpretation - cumbersome. In Tapscript, a new leaf version can introduce different opcodes 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.