COption vs Option vs OptionalNonZeroPubkey

Solana has three different ways to encode an optional value, and they're not interchangeable. SPL's COption is 4 bytes of tag, Borsh's Option is 1, and Token-2022's OptionalNonZeroPubkey is 0 — the single most common source of off-by-N decoder bugs.

Encoding concept

What it is

“This field is optional” is encoded three different ways on Solana depending on which serialization the program uses. Confusing them is the most common reason a hand-written decoder drifts off by a few bytes and reads garbage.

Why it exists

Different layers adopted different conventions: SPL Token uses its own Pack format with a fixed-width tag for alignment, Borsh (Anchor, most app programs) uses a compact 1-byte tag, and Token-2022 extensions optimize pubkey-options to zero overhead. They all coexist, sometimes in the same account.

Byte layout

Encoding	Where	None	Some(value)	Total for `Option<Pubkey>`
`COption<T>` (SPL `Pack`)	SPL Token base layout	4-byte tag `00 00 00 00` + zeroed value space	4-byte tag `01 00 00 00` + value	36 bytes (4 + 32)
`Option<T>` (Borsh)	Anchor, Metaplex, app programs	1-byte tag `00`	1-byte tag `01` + value	33 bytes (1 + 32)
`OptionalNonZeroPubkey`	Token-2022 extensions	32 all-zero bytes	the 32-byte pubkey	32 bytes (no tag)

The key differences:

COption reserves space for the value even when None. SPL Token’s Mint.freeze_authority is always 36 bytes on disk whether set or not — the layout is fixed-size.
Borsh Option omits the value when None. A None is a single 00 byte; the value bytes aren’t present at all. This makes Borsh structs variable-length.
OptionalNonZeroPubkey has no tag at all. All-zero pubkey is the None sentinel. Zero overhead, but you can’t represent “Some(all-zero pubkey)” — fine for pubkeys, which are never all-zero in practice.

Where you see it

COption: Mint and Token Account authorities.
Borsh Option: Metaplex metadata fields, Anchor account fields.
OptionalNonZeroPubkey: nearly every Token-2022 extension authority.

Common gotchas

A Token-2022 mint can contain all three. The base layout uses COption (4-byte tags), an embedded Borsh field could use Option (1-byte), and the extensions use OptionalNonZeroPubkey (0 tag). Decoding one section with the wrong convention slides everything after it.
COption is little-endian 4 bytes, value 0 or 1. Don’t read it as a single byte — bytes 1–3 being zero is what makes a naive 1-byte read accidentally work for None and then break for Some.
Borsh None changes the struct’s size. You can’t compute a Borsh struct’s byte size from its type alone if it has Options — the size depends on the values.
All-zero pubkey means None for OptionalNonZeroPubkey. Never treat the system program (also all-1s in base58, but all-zero in bytes is the default Pubkey) — the all-zero 32 bytes is the None marker, not a real address.