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CHECKTEMPLATEVERIFY

BIP 119. Jeremy Rubin.

BIP 119, CHECKTEMPLATEVERIFY. A new opcode, OP_CHECKTEMPLATEVERIFY, to be activated as a change to the semantics of OP_NOP4.

Status in the source: Draft. A reading of the public specification, not a copy of it and not a certification.

CHECKTEMPLATEVERIFY is worth reading for the rule it actually adds: A new opcode, OP_CHECKTEMPLATEVERIFY, to be activated as a change to the semantics of OP_NOP4.
Evidence
Primary paper
Re-measured
No
Assumptions
3
Records linked
4

Code

Source, repository, and what is actually hosted

  • BIP 119https://bips.dev/119
  • The source URL is not a GitHub repository. No code remote is invented.
  • Full text is not mirrored. Public availability is not a licence to copy the file onto this site.
  • Add to a reading list

01 Claim ledger

What the paper is allowed to say

Each row is a sentence already in the study. The status is the same on every row: a model claim, not a live measurement.

  1. Claim 01 · Paper model

    The rule

    A new opcode, OP_CHECKTEMPLATEVERIFY, to be activated as a change to the semantics of OP_NOP4.

  2. Claim 02 · Paper model

    What was already failing

    Bitcoin script and block validity only change when a soft fork says an old pattern is now invalid. Without that rule, every node is free to accept what this document wants to reject.

  3. Claim 03 · Paper model

    What the number does not mean

    The source marks this draft. It is not a live consensus rule just because it has a number.

  4. Claim 04 · Paper model

    What a builder should be able to point at

    An implementation either constrains OP_CHECKTEMPLATEVERIFY, OP_NOP4, NOP or it is a different design.

02 Three cuts

Observation, model, falsifier

A desk does not stop at the summary. Each claim is cut three ways, using only this study's own assumptions and checks. Nothing here is a new figure.

  1. 01 The rule

    Observation

    What the study says

    A new opcode, OP_CHECKTEMPLATEVERIFY, to be activated as a change to the semantics of OP_NOP4.

    Model

    What has to hold

    The object that has to change is OP_CHECKTEMPLATEVERIFY, OP_NOP4, NOP. A neighbouring document with a similar name is not this one.

    Falsifier

    What would retire it

    Which bytes become invalid under CHECKTEMPLATEVERIFY, and which old transactions stay valid?

  2. 02 What was already failing

    Observation

    What the study says

    Bitcoin script and block validity only change when a soft fork says an old pattern is now invalid. Without that rule, every node is free to accept what this document wants to reject.

    Model

    What has to hold

    The object that has to change is OP_CHECKTEMPLATEVERIFY, OP_NOP4, NOP. A neighbouring document with a similar name is not this one.

    Falsifier

    What would retire it

    Which bytes become invalid under CHECKTEMPLATEVERIFY, and which old transactions stay valid?

  3. 03 What the number does not mean

    Observation

    What the study says

    The source marks this draft. It is not a live consensus rule just because it has a number.

    Model

    What has to hold

    You are implementing BIP 119 at the status the text itself states: Draft.

    Falsifier

    What would retire it

    Is enforcement in consensus, or only in the mempool policy of one client?

  4. 04 What a builder should be able to point at

    Observation

    What the study says

    An implementation either constrains OP_CHECKTEMPLATEVERIFY, OP_NOP4, NOP or it is a different design.

    Model

    What has to hold

    The object that has to change is OP_CHECKTEMPLATEVERIFY, OP_NOP4, NOP. A neighbouring document with a similar name is not this one.

    Falsifier

    What would retire it

    Which bytes become invalid under CHECKTEMPLATEVERIFY, and which old transactions stay valid?

03 Sequence

One action, as an operating tape

  1. 01Open BIP 119 and read the status line before the examples.
  2. 02Write down the rule in one sentence. A fair version of that sentence is: A new opcode, OP_CHECKTEMPLATEVERIFY, to be activated as a change to the semantics of OP_NOP4.
  3. 03Name the object that changes: OP_CHECKTEMPLATEVERIFY, OP_NOP4, NOP.
  4. 04Ask what an old client, an old contract, or an offline counterparty does. If the document is silent, the silence is part of the design.

04 Load-bearing

The argument, and where a pitch drops it

  1. What the text is allowed to settle

    The cut

    Bitcoin Improvement Proposal 119 can settle the shape of CHECKTEMPLATEVERIFY. It cannot settle whether a later client, a later fork, or a later wallet still does this.

    Why it carries weight

    If this cut is skipped, the paper's name is being used without the mechanism that makes the name mean anything.

    Where it is dropped

    A later client, parameter or reward formula is a different object from this paragraph.

  2. What this page will not pretend

    The cut

    There is no benchmark, no adoption number, and no claim that the mechanism is safe outside the assumptions written in the source.

    Why it carries weight

    If this cut is skipped, the paper's name is being used without the mechanism that makes the name mean anything.

    Where it is dropped

    The source marks this draft. It is not a live consensus rule just because it has a number.

05 Register

What has to be true

  • Model · Not re-measured

    You are implementing BIP 119 at the status the text itself states: Draft.

  • Model · Not re-measured

    The object that has to change is OP_CHECKTEMPLATEVERIFY, OP_NOP4, NOP. A neighbouring document with a similar name is not this one.

  • Cryptography · Not re-measured

    Activation is a separate mechanism from the opcode or the sighash. This page does not pick a height.

06 Divergence

What happened after the paper

The source marks this draft. It is not a live consensus rule just because it has a number. Later documents can narrow, replace, or ignore this one. Cite the number you mean.

A later client, parameter set, or reward formula is a different object. Cite this paper for the mechanism. Cite a primary release for the network. This desk has not re-run the proof.

07 Pre-mortem

What to check before you use the idea

  4. 0 of 3 marked on this browser. A mark is a reading note, not a pass, a rating, or a recommendation.

08 Anatomy

The paper, in the order a builder needs

The problem it names

Bitcoin script and block validity only change when a soft fork says an old pattern is now invalid. Without that rule, every node is free to accept what this document wants to reject.

What the design proposes

  • A new opcode, OP_CHECKTEMPLATEVERIFY, to be activated as a change to the semantics of OP_NOP4.
  • OP_CHECKTEMPLATEVERIFY uses opcode OP_NOP4 (0xb3) as a soft fork upgrade.
  • OP_CHECKTEMPLATEVERIFY does the following: There is at least one element on the stack, fail otherwise The element on the stack is 32 bytes long, NOP otherwise The DefaultCheckTemplateVerifyHash of the transaction at the current input index is equal to the element on the stack, fail otherwise The DefaultCheckTemplateVerifyHash commits to the serialized version, locktime, scriptSigs hash (if any non-null scriptSigs), number of inputs, sequences hash, number of outputs, outputs hash, and currently executing input index.

How the mechanism is specified

  • Taken from the specification, the next constraint is: OP_CHECKTEMPLATEVERIFY uses opcode OP_NOP4 (0xb3) as a soft fork upgrade.
  • Locate OP_CHECKTEMPLATEVERIFY, OP_NOP4, NOP in BIP 119 and apply it to one transaction or call.
  • Then check the failure the class of rule always has: a node that did not upgrade, a reverted call, a replayed signature, or a peer that does not speak the message.

What this page does not treat as proven

  • A soft fork does not bind a node that never upgrades. It binds the nodes that enforce the new rejection.
  • Activation is a separate mechanism from the opcode or the sighash. This page does not pick a height.
  • The source marks this draft. It is not a live consensus rule just because it has a number.

Why the desk still reads it

Use BIP 119 when a pitch says 'CHECKTEMPLATEVERIFY' without saying whether the rule is consensus, policy, or an interface. The number is the citation. The pitch is not.

09 Lexicon

Terms, opened into the record

BIP 119
The public text titled CHECKTEMPLATEVERIFY.
Draft
The document's own label for how finished the text is. It is not a market fact.

10 Repository

Every linked record on this page

Underlined words open a page that already exists: a concept, a protocol profile, a failure record, or another paper. If a word is not underlined, this desk does not have a record for it.

Protocols

This is Blockchain Lab's reading of a public design paper. It is not the paper, not a copy of it, and not an offer of tokens, equity, custody or a partnership. Later network behaviour can diverge from the text. Nothing here is investment, legal or technical advice.

Research status: Design paper. Last reviewed: 1 October 2026. This is a reading of a public paper, not investment, legal or security advice.