Flexible minimum validator set size on RCs

CHANGELOG.md

@@ -27,6 +27,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - PAH: filter `staking.reap_stash` from both `BaseCallFilter` and the XCM `SafeCallFilter` while the validator self-stake transition to higher bond. See Ref: [#1890](https://polkadot.subsquare.io/referenda/1890) ([#1159](https://github.com/polkadot-fellows/runtimes/pull/1159))
 - Collectives Polkadot: Increase the Secretary Collective salary budget ([#1172](https://github.com/polkadot-fellows/runtimes/pull/1172))
 - Align all runtimes to use the same values for e.g. `TransactionByteFee` and some other clean ups. ([#1175](https://github.com/polkadot-fellows/runtimes/pull/1175))
+- Flexible minimum validator set size on rc ([#PR1193](https://github.com/polkadot-fellows/runtimes/pull/1193))
+
 
 ## [2.2.2] 23.04.2026
 

relay/kusama/src/lib.rs

@@ -795,6 +795,15 @@ pub mod dynamic_params {
 		#[codec(index = 1)]
 		pub static BurnDestination: BurnDestinationAccount = Default::default();
 	}
+
+	/// Parameters used by `pallet-staking-async-ah-client`.
+	#[dynamic_pallet_params]
+	#[codec(index = 2)]
+	pub mod ah_client {
+		/// Minimum size of the validator set that the relay chain will accept from Asset Hub.
+		#[codec(index = 0)]
+		pub static MinimumValidatorSetSize: u32 = 100;
+	}
 }
 
 #[cfg(feature = "runtime-benchmarks")]
@@ -822,6 +831,12 @@ impl EnsureOriginWithArg<RuntimeOrigin, RuntimeParametersKey> for DynamicParamet
 			Inflation(_) => frame_system::ensure_root(origin.clone()),
 			Treasury(_) =>
 				EitherOf::<EnsureRoot<AccountId>, GeneralAdmin>::ensure_origin(origin.clone()),
+			AhClient(_) => EitherOfDiverse::<
+				// either local root or StakingAdmin, or same from OpenGov on AH
+				EitherOf<EnsureRoot<AccountId>, StakingAdmin>,
+				EnsureXcm<IsVoiceOfBody<AssetHubLocation, StakingAdminBodyId>>,
+			>::ensure_origin(origin.clone())
+			.map(|_success| ()),
 		}
 		.map_err(|_| origin)
 	}
@@ -1760,8 +1775,6 @@ impl pallet_nomination_pools::Config for Runtime {
 parameter_types! {
 	pub const DelegatedStakingPalletId: PalletId = PalletId(*b"py/dlstk");
 	pub const SlashRewardFraction: Perbill = Perbill::from_percent(1);
-	// Kusama always wants 1000 validators, we reject anything smaller than that.
-	pub storage MinimumValidatorSetSize: u32 = 1000;
 }
 
 impl pallet_delegated_staking::Config for Runtime {
@@ -1782,7 +1795,7 @@ impl pallet_staking_async_ah_client::Config for Runtime {
 	type AdminOrigin = EnsureRoot<AccountId>;
 	type SessionInterface = Session;
 	type SendToAssetHub = StakingXcmToAssetHub;
-	type MinimumValidatorSetSize = MinimumValidatorSetSize;
+	type MinimumValidatorSetSize = dynamic_params::ah_client::MinimumValidatorSetSize;
 	type UnixTime = Timestamp;
 	type PointsPerBlock = ConstU32<20>;
 	type MaxOffenceBatchSize = ConstU32<32>;

relay/polkadot/Cargo.toml

@@ -68,6 +68,7 @@ pallet-multisig = { workspace = true }
 pallet-nomination-pools = { workspace = true }
 pallet-nomination-pools-runtime-api = { workspace = true }
 pallet-offences = { workspace = true }
+pallet-parameters = { workspace = true }
 pallet-preimage = { workspace = true }
 pallet-proxy = { workspace = true }
 pallet-referenda = { workspace = true }
@@ -180,6 +181,7 @@ std = [
 	"pallet-nomination-pools/std",
 	"pallet-offences-benchmarking?/std",
 	"pallet-offences/std",
+	"pallet-parameters/std",
 	"pallet-preimage/std",
 	"pallet-proxy/std",
 	"pallet-referenda/std",
@@ -264,6 +266,7 @@ runtime-benchmarks = [
 	"pallet-nomination-pools/runtime-benchmarks",
 	"pallet-offences-benchmarking/runtime-benchmarks",
 	"pallet-offences/runtime-benchmarks",
+	"pallet-parameters/runtime-benchmarks",
 	"pallet-preimage/runtime-benchmarks",
 	"pallet-proxy/runtime-benchmarks",
 	"pallet-referenda/runtime-benchmarks",
@@ -326,6 +329,7 @@ try-runtime = [
 	"pallet-multisig/try-runtime",
 	"pallet-nomination-pools/try-runtime",
 	"pallet-offences/try-runtime",
+	"pallet-parameters/try-runtime",
 	"pallet-preimage/try-runtime",
 	"pallet-proxy/try-runtime",
 	"pallet-referenda/try-runtime",

relay/polkadot/src/lib.rs

@@ -40,14 +40,15 @@ use frame_election_provider_support::{
 };
 use frame_support::{
 	construct_runtime,
+	dynamic_params::{dynamic_pallet_params, dynamic_params},
 	genesis_builder_helper::{build_state, get_preset},
 	parameter_types,
 	traits::{
 		fungible::HoldConsideration,
 		tokens::{imbalance::ResolveTo, UnityOrOuterConversion},
-		ConstU32, ConstU8, ConstUint, Contains, EitherOf, EitherOfDiverse, FromContains, Get,
-		InstanceFilter, KeyOwnerProofSystem, LinearStoragePrice, PrivilegeCmp, ProcessMessage,
-		ProcessMessageError, WithdrawReasons,
+		ConstU32, ConstU8, ConstUint, Contains, EitherOf, EitherOfDiverse, EnsureOrigin,
+		EnsureOriginWithArg, FromContains, Get, InstanceFilter, KeyOwnerProofSystem,
+		LinearStoragePrice, PrivilegeCmp, ProcessMessage, ProcessMessageError, WithdrawReasons,
 	},
 	weights::{
 		constants::{WEIGHT_PROOF_SIZE_PER_KB, WEIGHT_REF_TIME_PER_MICROS},
@@ -1601,16 +1602,75 @@ impl pallet_delegated_staking::Config for Runtime {
 	type CoreStaking = Staking;
 }
 
+/// Dynamic params that can be adjusted at runtime.
+#[dynamic_params(RuntimeParameters, pallet_parameters::Parameters::<Runtime>)]
+pub mod dynamic_params {
+	use super::*;
+
+	/// Parameters used by `pallet-staking-async-ah-client`.
+	#[dynamic_pallet_params]
+	#[codec(index = 0)]
+	pub mod ah_client {
+		/// Minimum size of the validator set that the relay chain will accept from Asset Hub.
+		#[codec(index = 0)]
+		pub static MinimumValidatorSetSize: u32 = 250;
+	}
+}
+
+#[cfg(feature = "runtime-benchmarks")]
+impl Default for RuntimeParameters {
+	fn default() -> Self {
+		RuntimeParameters::AhClient(dynamic_params::ah_client::Parameters::MinimumValidatorSetSize(
+			dynamic_params::ah_client::MinimumValidatorSetSize,
+			Some(250),
+		))
+	}
+}
+
+/// Defines what origin can modify which dynamic parameters.
+pub struct DynamicParameterOrigin;
+impl EnsureOriginWithArg<RuntimeOrigin, RuntimeParametersKey> for DynamicParameterOrigin {
+	type Success = ();
+
+	fn try_origin(
+		origin: RuntimeOrigin,
+		key: &RuntimeParametersKey,
+	) -> Result<Self::Success, RuntimeOrigin> {
+		use crate::RuntimeParametersKey::*;
+
+		match key {
+			AhClient(_) => EitherOfDiverse::<
+				// either local root or StakingAdmin, or same from OpenGov on AH
+				EitherOf<EnsureRoot<AccountId>, StakingAdmin>,
+				EnsureXcm<IsVoiceOfBody<AssetHubLocation, StakingAdminBodyId>>,
+			>::ensure_origin(origin.clone())
+			.map(|_success| ()),
+		}
+		.map_err(|_| origin)
+	}
+
+	#[cfg(feature = "runtime-benchmarks")]
+	fn try_successful_origin(_key: &RuntimeParametersKey) -> Result<RuntimeOrigin, ()> {
+		// Provide the origin for the parameter returned by `Default`:
+		Ok(RuntimeOrigin::root())
+	}
+}
+
+impl pallet_parameters::Config for Runtime {
+	type RuntimeEvent = RuntimeEvent;
+	type RuntimeParameters = RuntimeParameters;
+	type AdminOrigin = DynamicParameterOrigin;
+	type WeightInfo = weights::pallet_parameters::WeightInfo<Runtime>;
+}
+
 impl ah_client::Config for Runtime {
 	type CurrencyBalance = Balance;
 	type AssetHubOrigin =
 		frame_support::traits::EitherOfDiverse<EnsureRoot<AccountId>, EnsureAssetHub>;
 	type AdminOrigin = EnsureRoot<AccountId>;
 	type SessionInterface = Session;
 	type SendToAssetHub = StakingXcmToAssetHub;
-	// Polkadot RC currently has 600 validators. Note: this has to be updated with AH validator
-	// count increasing.
-	type MinimumValidatorSetSize = ConstU32<600>;
+	type MinimumValidatorSetSize = dynamic_params::ah_client::MinimumValidatorSetSize;
 	type UnixTime = Timestamp;
 	type PointsPerBlock = ConstU32<20>;
 	type MaxOffenceBatchSize = ConstU32<32>;
@@ -1844,6 +1904,9 @@ construct_runtime! {
 		DelegatedStaking: pallet_delegated_staking = 41,
 		StakingAhClient: pallet_staking_async_ah_client = 42,
 
+		// Dynamic, configurable parameters.
+		Parameters: pallet_parameters = 46,
+
 		// Parachains pallets. Start indices at 50 to leave room.
 		ParachainsOrigin: parachains_origin = 50,
 		Configuration: parachains_configuration = 51,
@@ -1999,6 +2062,7 @@ mod benches {
 		[pallet_multisig, Multisig]
 		[pallet_nomination_pools, NominationPoolsBench::<Runtime>]
 		[pallet_offences, OffencesBench::<Runtime>]
+		[pallet_parameters, Parameters]
 		[pallet_preimage, Preimage]
 		[pallet_proxy, Proxy]
 		[pallet_scheduler, Scheduler]