# difficulty (/docs/hyperparameters/difficulty)

`difficulty` is the proof-of-work target a miner's registration nonce must
beat to claim a UID via PoW registration. It is a raw u64: finding a valid
nonce takes about `difficulty` hash attempts on average, so the value is a
direct price in compute. Miners planning PoW registration care about it;
`u64::MAX` is the sentinel for "no nonce can ever pass" — PoW registration
effectively disabled, burned registration only.

## How it works [#how-it-works]

A PoW registration submits a seal built from a recent block hash, the
hotkey, and a nonce (`create_seal_hash` in
`pallets/subtensor/src/subnets/registration.rs`). The check is
`hash_meets_difficulty`: the seal hash times `difficulty` must not overflow
a 256-bit integer, which passes with probability roughly 1 in `difficulty`.
At `u64::MAX` essentially nothing passes.

On the current runtime this check no longer gates subnet registration: the
`register` extrinsic (`pallets/subtensor/src/macros/dispatches.rs`, call
index 6) accepts the work arguments but ignores them and routes to the
burned-registration path (`do_register`). No controller adjusts `difficulty`
anymore either — the per-block price update
(`update_registration_prices_for_networks`) only decays the burn cost. The
value stays on-chain, is reported in the metagraph, and root can still
change it. The seal check itself survives in the testnet faucet
(`do_faucet`, fixed difficulty 1,000,000).

Historically, `difficulty` walked between its floor and ceiling as the
chain steered registrations toward a target rate:

<HyperparamPowDifficulty focus="difficulty" />

## Reading and setting [#reading-and-setting]

```
btcli sudo get --netuid N --name difficulty
```

Root/governance only: `sudo_set_difficulty` in the AdminUtils pallet
requires the root origin and takes a raw u64. It is not settable by the
subnet owner via `btcli sudo set`.

## Related [#related]

[`min_difficulty`](/docs/hyperparameters/min-difficulty) ·
[`max_difficulty`](/docs/hyperparameters/max-difficulty) ·
[`network_pow_registration_allowed`](/docs/hyperparameters/network-pow-registration-allowed) ·
[`registration_allowed`](/docs/hyperparameters/registration-allowed) ·
[`max_regs_per_block`](/docs/hyperparameters/max-regs-per-block) ·
[`min_burn`](/docs/hyperparameters/min-burn) ·
[`max_burn`](/docs/hyperparameters/max-burn)