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BIP-110: Clarify P2A exemption in rule 3 requires an empty witness stack #2225

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murchandamus merged 3 commits into from
Aug 9, 2026
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BIP-110: Clarify P2A exemption in rule 3 requires an empty witness stack #2225

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ae98a6e
BIP-110: Add Defined witness versions subsection clarifying the rule …
dathonohm Jul 29, 2026
2dfb201
BIP-110: Add rule 3 test vectors for the P2A empty-witness requirement
dathonohm Jul 29, 2026
b01048e
bip110: Adopt John Moffett’s description of BIP433 relationship
murchandamus Aug 9, 2026
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11 changes: 10 additions & 1 deletion bip-0110.mediawiki
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Expand Up @@ -24,7 +24,7 @@ Blocks during a temporary, one-year deployment are checked with these additional

# New output scriptPubKeys exceeding 34 bytes are invalid, unless the first opcode is OP_RETURN, in which case up to 83 bytes are valid.
# OP_PUSHDATA* payloads and [[#script-argument-witness-items|script argument witness items]] exceeding 256 bytes are invalid, except for the redeemScript push in BIP16 scriptSigs.
# Spending undefined witness (or Tapleaf) versions (ie, not Witness v0/BIP 141, Taproot/BIP 341, or P2A) is invalid. (Creating outputs with undefined witness versions is still valid.)
# Spending undefined witness (or Tapleaf) versions (ie, anything not listed under [[#defined-witness-versions|Defined witness versions]] below) is invalid. (Creating outputs with undefined witness versions is still valid.)
# Witness stacks with a Taproot annex are invalid.
# Taproot control blocks larger than 257 bytes (a merkle tree with 128 script leaves) are invalid.
# Tapscripts including OP_SUCCESS* opcodes anywhere (even unexecuted) are invalid.
Expand All @@ -37,6 +37,15 @@ This grandfathering ensures that no existing coins can be frozen or rendered uns
The new rules apply only to UTXOs created at or after the activation height.
Once the softfork expires, UTXOs of all heights are once again unrestricted.

<span id="defined-witness-versions"></span>
===Defined witness versions===

For the purposes of rule 3, these are the witness versions (and the one Tapleaf version) that are defined as of this BIP:

* '''Witness v0''' with a 20-byte program (P2WPKH) or a 32-byte program (P2WSH), as defined in [[bip-0141.mediawiki|BIP 141]]. Other v0 program lengths are already invalid under BIP 141 itself.
* '''Witness v1''' with a 32-byte program (Taproot/P2TR), as defined in [[bip-0341.mediawiki|BIP 341]]. In a script path spend, the only defined Tapleaf version is 0xc0 (Tapscript), as defined in [[bip-0342.mediawiki|BIP 342]].
* '''Witness v1''' with the 2-byte program 0x4e73 (P2A), and only when spent with an empty witness stack, as defined in [[bip-0433.mediawiki#specification|BIP 433]].
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@jarolrod jarolrod Jul 29, 2026

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I'm all for appeasing the author, as long as it's a clear win for documentations spec, integrity, and a win for implementers of this bip.

But the author is stating here, as written, that bip433 states a p2a must be spent with an empty witness, which is incorrect.

All attempts around the 8th rule, point to this being a clear isolatable consensus rule imposed by bip110

@dathonohm dathonohm Jul 29, 2026

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But the author is stating here, as written, that bip433 states a p2a must be spent with an empty witness

That is indeed what it says:

"P2A inputs are considered standard by Bitcoin Core for spending if no witness data is attached."
Screenshot 2026-07-29 at 15 25 06

@jarolrod jarolrod Jul 29, 2026

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"Consensus meaning is unchanged"

@dathonohm dathonohm Jul 30, 2026

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We've been over this; "Consensus meaning is unchanged" just means that P2A has no consensus meaning at all. That isn't relevant to BIP-110.

@delcin-raj delcin-raj Jul 30, 2026

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P2A output type specifically states that no witness data is attached.
If witness data is attached to the above specification, can it still be considered P2A? @jarolrod

@john-moffett john-moffett Jul 30, 2026 •
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@delcin-raj

P2A output type specifically states that no witness data is attached.

The output type is defined independently of the spend. It's a template, just like P2MS or P2PKH. Spending a P2A with witness data is currently consensus valid. The current relay policy, however, restricts it to an empty witness.

If witness data is attached to the above specification, can it still be considered P2A?

Undoubtedly, as BIP 433 is unequivocal in its definition of P2A: it's the output type, not the separate policy restricting its spend. In Bitcoin Core, IsPayToAnchor only looks at the scriptPubKey. It's like asking whether a 6-of-12 P2MS is still considered a P2MS despite it not relaying in Bitcoin Core due to policy. The answer is yes.

Edit: to address @dathonohm 's objection about "P2A has no consensus meaning at all". That's not true. The script pattern had existing consensus meaning before P2A (anyone can spend even with a non-empty witness), just like P2MS's script pattern had existing consensus meaning before BIP11 made it a standard template (and also didn't change any consensus rules).

@john-moffett john-moffett Jul 30, 2026

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@delcin-raj Moreover, this PR itself also characterizes P2A by the output type, not the spending witness status.

"description": "Spend a PRE-activation P2A output with a non-empty witness stack (grandfathered, exempt)",

@jonatack jonatack Aug 5, 2026 •
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But the author (dathonohm) is stating here, as written, that bip433 states a p2a must be spent with an empty witness

That is indeed what it says:

"P2A inputs are considered standard by Bitcoin Core for spending if no witness data is attached."

@dathonohm Your statement is not equivalent to the text you've quoted in BIP 433. @john-moffett is correct and proposes a minimum viable clarification on this.


===GetBlockTemplate===

This deployment uses the GBT deployment name "reduced_data" as defined in [[bip-0009.mediawiki#getblocktemplate_changes|BIP 9]]. During mandatory signaling, the deployment's bit is included in "vbrequired".
Expand Down
88 changes: 84 additions & 4 deletions bip-0110/test-vectors.json
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Expand Up @@ -281,6 +281,66 @@
"expected": "valid",
"reject_reason": null
},
{
"rule": 3,
"name": "p2a_empty_witness_valid",
"description": "Spend a P2A output with an empty witness stack",
"reduced_data_active": true,
"spent_utxo": "post-activation",
"spent_outputs": [
{
"prevout": {
"txid": "110000000000000000000000000000000000000000000000000000000000000f",
"vout": 0
},
"scriptPubKey": "51024e73",
"amount": 100000
}
],
"tx": "02000000010f0000000000000000000000000000000000000000000000000000000000001100000000000000000001b882010000000000015100000000",
"expected": "valid",
"reject_reason": null
},
{
"rule": 3,
"name": "p2a_empty_item_invalid",
"description": "Spend a P2A output with one empty witness item (non-empty stack)",
"reduced_data_active": true,
"spent_utxo": "post-activation",
"spent_outputs": [
{
"prevout": {
"txid": "1100000000000000000000000000000000000000000000000000000000000010",
"vout": 0
},
"scriptPubKey": "51024e73",
"amount": 100000
}
],
"tx": "02000000000101100000000000000000000000000000000000000000000000000000000000001100000000000000000001b8820100000000000151010000000000",
"expected": "invalid",
"reject_reason": "script-verify-flag-failed (Witness version reserved for soft-fork upgrades)"
},
{
"rule": 3,
"name": "p2a_one_byte_item_invalid",
"description": "Spend a P2A output with one one-byte witness item",
"reduced_data_active": true,
"spent_utxo": "post-activation",
"spent_outputs": [
{
"prevout": {
"txid": "1100000000000000000000000000000000000000000000000000000000000011",
"vout": 0
},
"scriptPubKey": "51024e73",
"amount": 100000
}
],
"tx": "02000000000101110000000000000000000000000000000000000000000000000000000000001100000000000000000001b882010000000000015101010100000000",
"expected": "invalid",
"reject_reason": "script-verify-flag-failed (Witness version reserved for soft-fork upgrades)"
},
{
"rule": 2,
"name": "grandfathered_witness_item_257_valid",
Expand All @@ -290,14 +350,34 @@
"spent_outputs": [
{
"prevout": {
"txid": "110000000000000000000000000000000000000000000000000000000000000f",
"txid": "1100000000000000000000000000000000000000000000000000000000000012",
"vout": 0
},
"scriptPubKey": "002033198a9bfef674ebddb9ffaa52928017b8472791e54c609cb95f278ac6b1e349",
"amount": 100000
}
],
"tx": "020000000001010f0000000000000000000000000000000000000000000000000000000000001100000000000000000001b882010000000000015102fd0101424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424202755100000000",
"tx": "02000000000101120000000000000000000000000000000000000000000000000000000000001100000000000000000001b882010000000000015102fd0101424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424202755100000000",
"expected": "valid",
"reject_reason": null
},
{
"rule": 3,
"name": "grandfathered_p2a_nonempty_witness_valid",
"description": "Spend a PRE-activation P2A output with a non-empty witness stack (grandfathered, exempt)",
"reduced_data_active": true,
"spent_utxo": "pre-activation",
"spent_outputs": [
{
"prevout": {
"txid": "1100000000000000000000000000000000000000000000000000000000000013",
"vout": 0
},
"scriptPubKey": "51024e73",
"amount": 100000
}
],
"tx": "02000000000101130000000000000000000000000000000000000000000000000000000000001100000000000000000001b882010000000000015101010100000000",
"expected": "valid",
"reject_reason": null
},
Expand All @@ -310,14 +390,14 @@
"spent_outputs": [
{
"prevout": {
"txid": "1100000000000000000000000000000000000000000000000000000000000010",
"txid": "1100000000000000000000000000000000000000000000000000000000000014",
"vout": 0
},
"scriptPubKey": "00204ae81572f06e1b88fd5ced7a1a000945432e83e1551e6f721ee9c00b8cc33260",
"amount": 100000
}
],
"tx": "02000000000101100000000000000000000000000000000000000000000000000000000000001100000000000000000001b88201000000000023510000000000000000000000000000000000000000000000000000000000000000000001015100000000",
"tx": "02000000000101140000000000000000000000000000000000000000000000000000000000001100000000000000000001b88201000000000023510000000000000000000000000000000000000000000000000000000000000000000001015100000000",
"expected": "invalid",
"reject_reason": "bad-txns-vout-script-toolarge"
}
Expand Down
26 changes: 25 additions & 1 deletion bip-0110/test-vectors.py
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Original file line number Diff line number Diff line change
Expand Up @@ -13,7 +13,8 @@
Coverage:
Rule 1: output scriptPubKey size (<=34, or OP_RETURN <=83) [per-output]
Rule 2: push / script-argument witness item size (<=256)
Rule 3: spending undefined witness / Tapleaf versions
Rule 3: spending undefined witness / Tapleaf versions (the P2A exemption
covers only spends with an empty witness stack)
Rule 4: Taproot annex
Rule 5: Taproot control block size (<=257)
Rule 6: OP_SUCCESS* in tapscript
Expand Down Expand Up @@ -255,17 +256,20 @@ def run_test(self):
# P2WSH(OP_DROP OP_TRUE) lets us inject an arbitrary witness data element (rule 2)
wsh_drop = CScript([OP_DROP, OP_TRUE])
wsh_drop_spk = script_to_p2wsh_script(wsh_drop)
p2a_spk = CScript([OP_1, bytes.fromhex("4e73")])

# Fund "old" UTXOs BEFORE activation so they are grandfathered (exempt
# from the per-input script rules). Done first, at a low height.
self.log.info("Funding pre-activation (grandfathered) UTXOs...")
old_drop_o, old_drop_a, old_drop_h = self.fund(wsh_drop_spk)
old_true_o, old_true_a, old_true_h = self.fund(wsh_true_spk)
old_p2a_o, old_p2a_a, old_p2a_h = self.fund(p2a_spk)

self.log.info("Activating REDUCED_DATA...")
self.activate()
assert old_drop_h < self.activation_height
assert old_true_h < self.activation_height
assert old_p2a_h < self.activation_height
self.generate(self.wallet, 20) # spendable coins for funding "new" UTXOs

self.log.info("=== Rule 1: output scriptPubKey size (per-output) ===")
Expand Down Expand Up @@ -365,6 +369,21 @@ def run_test(self):
self.check(7, "witness_v0_op_if_valid", "OP_IF in a witness v0 script is valid (rule is tapscript-only)",
[(wsh_if_spk, a)], self.spend(o, a, [bytes(wsh_if)]), True, "post-activation")

# The rule 3 exemption for P2A covers only spends with an empty witness
# stack; any witness item makes the spend invalid (cf. BIP 433).
self.log.info("=== Rule 3: P2A witness stack must be empty ===")
o, a = self.fund(p2a_spk)[0:2]
self.check(3, "p2a_empty_witness_valid", "Spend a P2A output with an empty witness stack",
[(p2a_spk, a)], self.spend(o, a, []), True, "post-activation")
o, a = self.fund(p2a_spk)[0:2]
self.check(3, "p2a_empty_item_invalid",
"Spend a P2A output with one empty witness item (non-empty stack)",
[(p2a_spk, a)], self.spend(o, a, [b'']), False, "post-activation")
o, a = self.fund(p2a_spk)[0:2]
self.check(3, "p2a_one_byte_item_invalid",
"Spend a P2A output with one one-byte witness item",
[(p2a_spk, a)], self.spend(o, a, [b'\x01']), False, "post-activation")

self.log.info("=== Grandfathering: pre-activation UTXOs ===")
# An input spending a pre-activation UTXO is exempt from the per-input
# script rules: a 257-byte witness item is accepted here (cf. rule 2).
Expand All @@ -373,6 +392,11 @@ def run_test(self):
[(wsh_drop_spk, old_drop_a)],
self.spend(old_drop_o, old_drop_a, [b'\x42' * 257, bytes(wsh_drop)]),
True, "pre-activation")
self.check(3, "grandfathered_p2a_nonempty_witness_valid",
"Spend a PRE-activation P2A output with a non-empty witness stack (grandfathered, exempt)",
[(p2a_spk, old_p2a_a)],
self.spend(old_p2a_o, old_p2a_a, [b'\x01']),
True, "pre-activation")
# But the per-output rule 1 still applies even when spending an old UTXO:
# creating an oversized output is rejected regardless of input age.
self.check(1, "grandfathered_input_oversized_output_invalid",
Expand Down