isHex shouldn't accept odd number of nibbles

[CodeSandwich]

Check existing issues

• I checked there isn't already an issue for the bug I encountered.

Viem Version

2.48.1

Current Behavior

'isHex' accepts odd numbered nibbles, even in the strict mode, isHex("0x1") === true.

Expected Behavior

'isHex' rejects odd numbered nibbles in the scrict mode, isHex(0x1) === false.

Steps To Reproduce

No response

Link to Minimal Reproducible Example

No response

Anything else?

It really doesn't make sense to accept odd-length hex values.

Everything around EVM works with full bytes and never with half-bytes. I'm not aware of any API or tool that would accept odd-length hex values, they all reject such hex strings as malformed with the risk of weirder ones silently corrupting data by "fixing" the parity. There's no use case for handling odd-length hex values, but there are risks in accepting them as valid.

[psamuelvijay]

I'd like to work on this — will fix the regex in isHex.ts to require full bytes in strict mode.

[LeonxLJX]

I'd like to take this one — I'll follow up with a PR. (claiming via @LeonxLJX)

[LeonxLJX]

Hi! I'd like to make isHex reject odd nibble counts. Plan: reproduce, fix, add tests. May I be assigned?

[DijieDeng]

Great catch, @CodeSandwich! I've been looking at the viem codebase conventions and wanted to add a concrete suggestion for the fix.

Looking at the current isHex implementation, the fix should be straightforward: in strict mode, the regex should reject odd-length hex strings. Here's a concrete approach:

// In strict mode, the regex should enforce even-length hex (full bytes):
const strictHexRegex = /^0x[0-9a-fA-F]*$/
const hasEvenLength = (hex: string) => hex.length % 2 === 0

The rationale: EVM and all Ethereum tooling work exclusively with full bytes. An odd-length hex like 0x1 is ambiguous — does it mean 0x01 or is it malformed? The ox helper library that viem already depends on (Hex.from, Hex.toBytes, etc.) also enforces full-byte hex. So isHex in strict mode should align with the rest of the ecosystem.

Additionally, from viem's own AGENTS.md conventions:

• "Ox helpers over ad hoc conversion" — the fix should ensure isHex stays consistent with ox's hex handling
• "No mocks, ever" — the test should use real hex values, not mocked inputs

I'd also suggest adding a JSDoc note to isHex clarifying that strict mode enforces full-byte hex (even nibble count), since this is a subtle but important behavioral distinction.

Happy to help review a PR for this!