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CCTP V2 does not use the V1 nonce as a wider version of the same counter. V1 uses a sequential uint64; V2 uses an attestation-time bytes32 nonce. Circle’s V2 source-message format initially contains an empty nonce, so treating that source-side placeholder as the final arrival identifier can make a valid transfer appear missing. Match transfers using version-aware message and receipt data instead.
How V1 and V2 nonces differ
| Behavior | CCTP V1 | CCTP V2 |
|---|---|---|
| Type and width | uint64 |
bytes32 |
| Assignment | Auto-incrementing integer | Computed off-chain at attestation time from multiple transaction and event inputs |
| Source message nonce | The sequential nonce is part of the V1 message semantics. | The serialized message uses an empty nonce before the operational nonce is assigned. |
| Destination behavior | Uses a nonce for destination-chain replay protection. | Checks nonce availability, marks the nonce used, and emits it as bytes32 in MessageReceived. |
Circle’s V2 message source code notes the type change directly: “Nonce is now bytes32 (vs. uint64).” The important difference is not just the field’s width. The versions differ in how the nonce is assigned and what the source message contains before attestation. See Circle’s MessageV2.sol and the CCTP V2 white paper.
Why a V2 arrival checker can report a valid transfer as missing
Circle’s V2 serializer writes an empty nonce into the nonce field of the emitted message. The V2 nonce is assigned off-chain at attestation time. A checker that reads the raw source message and stores that empty field as the final arrival key can later compare the wrong value with the attestation-backed V2 message or destination receipt.
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This is a likely join failure when an implementation treats the source placeholder as final; it does not establish that every indexer or API returns identical message bytes or fields. The exact fields and persistence approach depend on the integration.
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How to match a CCTP V2 source message to destination receipt
- Identify the message version. Decode and handle V1 and V2 according to their respective message formats; do not parse both as if they shared one nonce type or assignment rule.
- Keep the source-side V2 nonce provisional. Circle’s V2 serializer places an empty nonce in the source message before the attestation-time assignment.
- Use attestation-backed V2 data for matching. Match against the V2 message with its assigned
bytes32nonce and the destination receipt’s nonce, rather than using the empty source-message field as the final identifier. - Use the destination event as receipt evidence. The V2 transmitter validates the message and attestation, verifies that the nonce is available, marks it used, and emits
MessageReceivedwith the nonce asbytes32. See Circle’s MessageTransmitterV2.sol.
Circle’s legacy Get an attestation API reference documents legacy behavior; it is not a universal recipe for comparing V2 API fields. Use the field semantics documented for the specific V2 integration you operate.
Why V2 assigns the nonce at attestation time
In V1, an auto-incrementing nonce supports destination-chain replay protection. Circle’s V2 white paper explains that fast transfers and possible source-chain reorganizations create a different problem: a source event that is later reorganized could conflict with an already assigned sequential nonce. V2 instead computes a bytes32 nonce off-chain at attestation time from multiple transaction and event inputs. Circle describes this design as intended to improve resistance to reorganization-related conflicts and prevent duplicate assignments.
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The V2 nonce still matters on the destination. The MessageTransmitter checks whether it has already been used and records it to prevent replay; it is not a field the destination contract ignores.
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What changes—and what does not—when migrating from V1
- Maintain version-specific decoding and nonce handling. A V1
uint64counter and a V2 attestation-timebytes32identifier are semantically different. - Do not convert a V1 nonce arithmetically to derive a matching V2 nonce.
- Do not confuse the CCTP message nonce with the underlying blockchain transaction nonce; they identify different things.
- Verify behavior against the deployed contract version and the V2 integration’s documented response fields. Circle’s current source branch can differ from a deployed release.
V1-to-V2 status and deprecation timing
In an announcement dated November 14, 2025, Circle described V2 as canonical and V1 as legacy. The announcement said manual V1 phase-out would begin July 31, 2026, followed by a full contract pause at the end of the announced deprecation period. That is a published schedule, not confirmation that every chain has the same current status. Check Circle’s latest CCTP V1 deprecation announcement and chain-specific status before making a migration or availability decision.
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