TERM_DEF // SCRIPT_AUTHORIZATION / OP_CAT_BIP_347
OP_CAT (BIP
347)
347)
OP_CAT (BIP 347). Concatenates the top two stack items; proposed reactivation enabling new covenant designs.
This page sits in the Script & Authorization section — The stack VM that decides whether a UTXO is spendable — opcodes, templates, and modern script trees. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
This page sits in the Script & Authorization section — The stack VM that decides whether a UTXO is spendable — opcodes, templates, and modern script trees. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
WHAT_OP_CAT_BIP_347_IS
OP_CAT (BIP 347) — at a glance
SCRIPT
Why it exists
DESIGN
OP_CAT exists because Byte-level operations were originally part of Script but most were disabled in 2010 after exploits; the surviving ones support specific Tapscript patterns.
HOW_IT_WORKS
Mechanism
HOW IT WORKS
Every UTXO is locked by a scriptPubKey — the output's locking script. To spend it, you provide a scriptSig (or witness) containing data that satisfies the lock. The node concatenates them, runs the combined script on a stack machine, and accepts the spend if and only if execution finishes with a single truthy value on the stack. OP_CAT (BIP 347) contributes a specific stack effect within that process — opcodes either push, pop, copy, hash, branch, or verify, and they do so left-to-right deterministically.
1. The script is parsed into a sequence of opcodes and push-data items.
2. Execution starts with an empty stack and an empty alt-stack.
3. Each opcode runs in order — push opcodes add to the stack, others consume the top items and may push results.
4. When OP_CAT (BIP 347) is reached, it performs its specific stack effect (see below).
5. Final state: a single non-zero (truthy) value on top → the spend is authorised. Anything else (empty stack, false, error) → the script fails and the tx is rejected.
WORKED_EXAMPLE
OP_CAT — stack effect + canonical use
EXAMPLE
Opcode : OP_CAT
Family : BITWISE
Stack effect: a b → a||b
Added : BIP-347 (draft)
Behaviour : Concatenate two stack items. Disabled in 2010; re-enabling proposed for Tapscript.
Open /playground in another tab and search for OP_CAT.
Drag the opcode in, watch the stack visualisation step through it
against any combination of inputs you choose.
KEY_PROPERTIES
FAMILY
Belongs to the BITWISE family of Script opcodes — siblings share validation rules and historical evolution.
STACK EFFECT
Exactly a b → a||b. Every full node enforces the same effect, byte-for-byte.
ACTIVE
Active on mainnet today; every spend that uses it is being validated by every full node.
COMMON_PITFALLS
Things that catch people out
PITFALLS
- Opcode semantics are consensus — verify against the latest Bitcoin Core source before relying on any subtle behaviour.
RELATED_CONCEPTS
Other terms from Script & Authorization — click any to read its page:
TERMINOLOGY_INDEX
TERMINOLOGY
OP_CAT (BIP 347)
Concatenates the top two stack items; proposed reactivation enabling new covenant designs.
Script
Bitcoin's purpose-built stack language; every locking and unlocking script is a script program.
Stack
The single LIFO data structure all script execution operates on; no variables, no registers.
Locking Script
The scriptPubKey on an output; specifies the spending conditions.
Unlocking Script
The scriptSig/witness in an input; provides the data that satisfies the lock.
Redeem Script
Witness Script
In P2WSH, the script whose SHA256 is committed in the output; placed at the end of the witness stack.
Script Execution
Sequential stepping through opcodes, mutating the stack until either a valid truthy stack remains or the script fails.