<p>OP_CAT was opcode <code>0x7e</code> in the original Bitcoin release’s version of Script. It was disabled in 2010 along with a number of opcodes when a series of bugs in other opcodes was discovered. Prior to it being disabled, the resulting stack item was limited and that limit then lowered further.</p>
<p>OP_CAT pops the top two values off the stack, concatenates the popped values in stack order, and then pushes the result to the top of the stack. <a href="https://github.com/bitcoin/bips/blob/master/bip-0347.mediawiki" rel="nofollow noreferrer">BIP 347</a> proposes that OP_CAT is added to Tapscript as <code>0x7e</code>, redefining <code>OP_SUCCESS126</code>. This would enable use of OP_CAT in P2TR version <code>0xc0</code> leafscripts. BIP 347 includes the later lowest limit on the resulting stack size item (520 bytes).</p>

CC BY-SA 4.0
2024-08-01T17:04:53.050Z
2024-08-01T18:20:31.373Z
2024-08-01T18:20:31.373Z
Murch
<p>What does the proposed OP_CAT opcode do? What is the history of the OP_CAT opcode?</p>

<script><taproot><opcodes><bip347-op-cat>
What is the OP_CAT proposal?
Murch
# What is the OP_CAT proposal?

(Asked by: username_0 on 2024-08-01)

What does the proposed OP\_CAT opcode do? What is the history of the OP\_CAT opcode?

**Tags:** script, taproot, opcodes, bip347-op-cat


---

## 1 answers

### Answer by username_0 on 2024-08-01. Score: 2 | Accepted answer

OP\_CAT was opcode `0x7e` in the original Bitcoin release’s version of Script. It was disabled in 2010 along with a number of opcodes when a series of bugs in other opcodes was discovered. Prior to it being disabled, the resulting stack item was limited and that limit then lowered further.

OP\_CAT pops the top two values off the stack, concatenates the popped values in stack order, and then pushes the result to the top of the stack. [BIP 347](https://github.com/bitcoin/bips/blob/master/bip-0347.mediawiki) proposes that OP\_CAT is added to Tapscript as `0x7e`, redefining `OP_SUCCESS126`. This would enable use of OP\_CAT in P2TR version `0xc0` leafscripts. BIP 347 includes the later lowest limit on the resulting stack size item (520 bytes).


---

