Merge pull request #363 from danroc/wildcard

Chain ID Wildcard

Author: bumblefudge

CAIPs/caip-363.md

@@ -0,0 +1,106 @@
+---
+caip: CAIP-363
+title: Chain ID Wildcard
+author: Daniel Rocha (@danroc)
+discussions-to: https://github.com/ChainAgnostic/CAIPs/discussions/364
+status: Draft
+type: Standard
+created: 2025-07-01
+requires: CAIP-2, CAIP-10
+---
+
+## Simple Summary
+
+This CAIP extends CAIP-2 and CAIP-10 by reserving the `_` character as a
+wildcard reference for "all chain IDs" within a CAIP-2 namespace. This enables
+wallets and applications to represent account identity across all chains of a
+given namespace.
+
+This CAIP makes no assumptions about how the address is derived or whether it
+is valid across all chains in the namespace. The use of `_` simply denotes
+intent to refer to the same address across chains, without implying
+compatibility or derivation logic.
+
+## Abstract
+
+By reserving the `_` wildcard in CAIP-2-compliant chain ID definitions, and
+supporting it within CAIP-10 account identifiers, this CAIP enables an address
+to represent a multichain account. For example, `eip155:_:<address>` signifies
+the given address across all chains in the `eip155` namespace.
+
+## Motivation
+
+Wallets and identity systems increasingly need to represent multichain
+accounts. The lack of a standard for denoting an address across all chains in a
+namespace hinders interoperability. Reserving `_` as a wildcard chain ID solves
+this by enabling a consistent and extensible mechanism for such expressions.
+
+## Specification
+
+### CAIP-2 Extension
+
+In addition to the existing CAIP-2 specification:
+
+> A new reserved chain ID value `_` MUST be supported in all namespaces to
+> indicate "all chains" in that namespace.
+
+Thus, the chain ID format becomes:
+
+```text
+<namespace>:<reference | _>
+```
+
+Where `_` is interpreted as "all chain references within this namespace."
+
+### CAIP-10 Extension
+
+Extend the CAIP-10 account ID to support the wildcard chain reference:
+
+```text
+<namespace>:_:<account-address>
+```
+
+For example:
+
+- `eip155:_:0x59f3...d0c3` represents the same address across all `eip155`
+  chains.
+
+- `solana:_:DAXa...bx77` represents a Solana account across all Solana chains
+  (e.g., Mainnet, Testnet, and Devnet).
+
+### Reserved Character
+
+A single `_` character is reserved solely for the wildcard chain reference and
+MUST NOT be designated as a valid reference or account identifier in any future
+[CAIP-2] profile.
+
+## Rationale
+
+The `_` character is not currently a valid chain ID reference in any known
+namespace, minimizing the risk of collision. It is also supported by the CAIP-2
+grammar, ensuring that it can be parsed correctly without breaking existing
+implementations.
+
+## Backwards Compatibility
+
+This CAIP is backwards compatible. Existing implementations that do not support
+`_` will simply reject these identifiers as invalid, while updated systems can
+handle them appropriately.
+
+## Reference Implementation
+
+No code changes are required in CAIP-2 and CAIP-10, but supporting
+implementations (e.g., wallets, dapps) should treat `_` as matching any chain
+reference within the specified namespace.
+
+## Copyright
+
+Copyright and related rights waived via CC0.
+
+## References
+
+- [CAIP-2]
+- [CAIP-10]
+
+[CAIP-2]: https://chainagnostic.org/CAIPs/caip-2
+[CAIP-10]: https://chainagnostic.org/CAIPs/caip-10