diff --git a/Cargo.lock b/Cargo.lock
index 3702b95485..a6b5f56374 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -7073,6 +7073,7 @@ dependencies = [
  "itertools",
  "lazy_static",
  "log",
+ "maplit",
  "parking_lot 0.12.1",
  "rand 0.8.5",
  "rand_xorshift",
diff --git a/beacon_node/beacon_chain/src/test_utils.rs b/beacon_node/beacon_chain/src/test_utils.rs
index 411bd7b1fd..9b62590703 100644
--- a/beacon_node/beacon_chain/src/test_utils.rs
+++ b/beacon_node/beacon_chain/src/test_utils.rs
@@ -132,8 +132,7 @@ pub fn test_spec<E: EthSpec>() -> ChainSpec {
                 FORK_NAME_ENV_VAR, e
             )
         });
-        let fork = ForkName::from_str(fork_name.as_str())
-            .unwrap_or_else(|()| panic!("unknown FORK_NAME: {}", fork_name));
+        let fork = ForkName::from_str(fork_name.as_str()).unwrap();
         fork.make_genesis_spec(E::default_spec())
     } else {
         E::default_spec()
diff --git a/beacon_node/http_api/src/lib.rs b/beacon_node/http_api/src/lib.rs
index ec34dd0663..bcd8788465 100644
--- a/beacon_node/http_api/src/lib.rs
+++ b/beacon_node/http_api/src/lib.rs
@@ -104,9 +104,9 @@ pub struct Config {
     pub listen_addr: IpAddr,
     pub listen_port: u16,
     pub allow_origin: Option<String>,
-    pub serve_legacy_spec: bool,
     pub tls_config: Option<TlsConfig>,
     pub allow_sync_stalled: bool,
+    pub spec_fork_name: Option<ForkName>,
 }
 
 impl Default for Config {
@@ -116,9 +116,9 @@ impl Default for Config {
             listen_addr: IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)),
             listen_port: 5052,
             allow_origin: None,
-            serve_legacy_spec: true,
             tls_config: None,
             allow_sync_stalled: false,
+            spec_fork_name: None,
         }
     }
 }
@@ -1534,18 +1534,15 @@ pub fn serve<T: BeaconChainTypes>(
         });
 
     // GET config/spec
-    let serve_legacy_spec = ctx.config.serve_legacy_spec;
+    let spec_fork_name = ctx.config.spec_fork_name;
     let get_config_spec = config_path
         .and(warp::path("spec"))
         .and(warp::path::end())
         .and(chain_filter.clone())
         .and_then(move |chain: Arc<BeaconChain<T>>| {
             blocking_json_task(move || {
-                let mut config_and_preset =
-                    ConfigAndPreset::from_chain_spec::<T::EthSpec>(&chain.spec);
-                if serve_legacy_spec {
-                    config_and_preset.make_backwards_compat(&chain.spec);
-                }
+                let config_and_preset =
+                    ConfigAndPreset::from_chain_spec::<T::EthSpec>(&chain.spec, spec_fork_name);
                 Ok(api_types::GenericResponse::from(config_and_preset))
             })
         });
diff --git a/beacon_node/http_api/tests/common.rs b/beacon_node/http_api/tests/common.rs
index 8f9856991f..1dd7aea923 100644
--- a/beacon_node/http_api/tests/common.rs
+++ b/beacon_node/http_api/tests/common.rs
@@ -141,9 +141,9 @@ pub async fn create_api_server_on_port<T: BeaconChainTypes>(
             listen_addr: IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)),
             listen_port: port,
             allow_origin: None,
-            serve_legacy_spec: true,
             tls_config: None,
             allow_sync_stalled: false,
+            spec_fork_name: None,
         },
         chain: Some(chain.clone()),
         network_tx: Some(network_tx),
diff --git a/beacon_node/http_api/tests/tests.rs b/beacon_node/http_api/tests/tests.rs
index fa41102292..bd25450a47 100644
--- a/beacon_node/http_api/tests/tests.rs
+++ b/beacon_node/http_api/tests/tests.rs
@@ -1253,10 +1253,13 @@ impl ApiTester {
     }
 
     pub async fn test_get_config_spec(self) -> Self {
-        let result = self.client.get_config_spec().await.unwrap().data;
-
-        let mut expected = ConfigAndPreset::from_chain_spec::<E>(&self.chain.spec);
-        expected.make_backwards_compat(&self.chain.spec);
+        let result = self
+            .client
+            .get_config_spec::<ConfigAndPresetBellatrix>()
+            .await
+            .map(|res| ConfigAndPreset::Bellatrix(res.data))
+            .unwrap();
+        let expected = ConfigAndPreset::from_chain_spec::<E>(&self.chain.spec, None);
 
         assert_eq!(result, expected);
 
diff --git a/beacon_node/src/cli.rs b/beacon_node/src/cli.rs
index 3515263878..edf79ad34f 100644
--- a/beacon_node/src/cli.rs
+++ b/beacon_node/src/cli.rs
@@ -229,8 +229,14 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
         .arg(
             Arg::with_name("http-disable-legacy-spec")
                 .long("http-disable-legacy-spec")
-                .help("Disable serving of legacy data on the /config/spec endpoint. May be \
-                       disabled by default in a future release.")
+                .hidden(true)
+        )
+        .arg(
+            Arg::with_name("http-spec-fork")
+                .long("http-spec-fork")
+                .help("Serve the spec for a specific hard fork on /eth/v1/config/spec. It should \
+                       not be necessary to set this flag.")
+                .takes_value(true)
         )
         .arg(
             Arg::with_name("http-enable-tls")
diff --git a/beacon_node/src/config.rs b/beacon_node/src/config.rs
index 6daee50de0..35d566d76e 100644
--- a/beacon_node/src/config.rs
+++ b/beacon_node/src/config.rs
@@ -116,7 +116,14 @@ pub fn get_config<E: EthSpec>(
     }
 
     if cli_args.is_present("http-disable-legacy-spec") {
-        client_config.http_api.serve_legacy_spec = false;
+        warn!(
+            log,
+            "The flag --http-disable-legacy-spec is deprecated and will be removed"
+        );
+    }
+
+    if let Some(fork_name) = clap_utils::parse_optional(cli_args, "http-spec-fork")? {
+        client_config.http_api.spec_fork_name = Some(fork_name);
     }
 
     if cli_args.is_present("http-enable-tls") {
diff --git a/common/eth2/src/lib.rs b/common/eth2/src/lib.rs
index 21608ba6dd..6317523fee 100644
--- a/common/eth2/src/lib.rs
+++ b/common/eth2/src/lib.rs
@@ -977,7 +977,9 @@ impl BeaconNodeHttpClient {
     }
 
     /// `GET config/spec`
-    pub async fn get_config_spec(&self) -> Result<GenericResponse<ConfigAndPreset>, Error> {
+    pub async fn get_config_spec<T: Serialize + DeserializeOwned>(
+        &self,
+    ) -> Result<GenericResponse<T>, Error> {
         let mut path = self.eth_path(V1)?;
 
         path.path_segments_mut()
diff --git a/common/eth2/src/lighthouse_vc/http_client.rs b/common/eth2/src/lighthouse_vc/http_client.rs
index abed4fe5e7..5f83e81aa0 100644
--- a/common/eth2/src/lighthouse_vc/http_client.rs
+++ b/common/eth2/src/lighthouse_vc/http_client.rs
@@ -354,7 +354,9 @@ impl ValidatorClientHttpClient {
     }
 
     /// `GET lighthouse/spec`
-    pub async fn get_lighthouse_spec(&self) -> Result<GenericResponse<ConfigAndPreset>, Error> {
+    pub async fn get_lighthouse_spec<T: Serialize + DeserializeOwned>(
+        &self,
+    ) -> Result<GenericResponse<T>, Error> {
         let mut path = self.server.full.clone();
 
         path.path_segments_mut()
diff --git a/common/eth2/src/mixin.rs b/common/eth2/src/mixin.rs
index 1de26961e6..a33cf8a40c 100644
--- a/common/eth2/src/mixin.rs
+++ b/common/eth2/src/mixin.rs
@@ -21,17 +21,17 @@ impl ResponseOptional for Result<Response, Error> {
 /// Trait for extracting the fork name from the headers of a response.
 pub trait ResponseForkName {
     #[allow(clippy::result_unit_err)]
-    fn fork_name_from_header(&self) -> Result<Option<ForkName>, ()>;
+    fn fork_name_from_header(&self) -> Result<Option<ForkName>, String>;
 }
 
 impl ResponseForkName for Response {
-    fn fork_name_from_header(&self) -> Result<Option<ForkName>, ()> {
+    fn fork_name_from_header(&self) -> Result<Option<ForkName>, String> {
         self.headers()
             .get(CONSENSUS_VERSION_HEADER)
             .map(|fork_name| {
                 fork_name
                     .to_str()
-                    .map_err(|_| ())
+                    .map_err(|e| e.to_string())
                     .and_then(ForkName::from_str)
             })
             .transpose()
diff --git a/consensus/types/Cargo.toml b/consensus/types/Cargo.toml
index c3e454fdfc..68fdbf7990 100644
--- a/consensus/types/Cargo.toml
+++ b/consensus/types/Cargo.toml
@@ -47,6 +47,7 @@ superstruct = "0.5.0"
 serde_json = "1.0.74"
 smallvec = "1.8.0"
 serde_with = "1.13.0"
+maplit = "1.0.2"
 
 [dev-dependencies]
 criterion = "0.3.3"
diff --git a/consensus/types/src/chain_spec.rs b/consensus/types/src/chain_spec.rs
index 3668d0524c..8d56ce2da9 100644
--- a/consensus/types/src/chain_spec.rs
+++ b/consensus/types/src/chain_spec.rs
@@ -803,6 +803,10 @@ impl Default for ChainSpec {
 }
 
 /// Exact implementation of the *config* object from the Ethereum spec (YAML/JSON).
+///
+/// Fields relevant to hard forks after Altair should be optional so that we can continue
+/// to parse Altair configs. This default approach turns out to be much simpler than trying to
+/// make `Config` a superstruct because of the hassle of deserializing an untagged enum.
 #[derive(Serialize, Deserialize, Debug, PartialEq, Clone)]
 #[serde(rename_all = "UPPERCASE")]
 pub struct Config {
@@ -813,17 +817,13 @@ pub struct Config {
     #[serde(default)]
     pub preset_base: String,
 
-    // TODO(merge): remove this default
     #[serde(default = "default_terminal_total_difficulty")]
     #[serde(with = "eth2_serde_utils::quoted_u256")]
     pub terminal_total_difficulty: Uint256,
-    // TODO(merge): remove this default
     #[serde(default = "default_terminal_block_hash")]
     pub terminal_block_hash: ExecutionBlockHash,
-    // TODO(merge): remove this default
     #[serde(default = "default_terminal_block_hash_activation_epoch")]
     pub terminal_block_hash_activation_epoch: Epoch,
-    // TODO(merge): remove this default
     #[serde(default = "default_safe_slots_to_import_optimistically")]
     #[serde(with = "eth2_serde_utils::quoted_u64")]
     pub safe_slots_to_import_optimistically: u64,
@@ -843,12 +843,10 @@ pub struct Config {
     #[serde(deserialize_with = "deserialize_fork_epoch")]
     pub altair_fork_epoch: Option<MaybeQuoted<Epoch>>,
 
-    // TODO(merge): remove this default
     #[serde(default = "default_bellatrix_fork_version")]
     #[serde(with = "eth2_serde_utils::bytes_4_hex")]
     bellatrix_fork_version: [u8; 4],
-    // TODO(merge): remove this default
-    #[serde(default = "default_bellatrix_fork_epoch")]
+    #[serde(default)]
     #[serde(serialize_with = "serialize_fork_epoch")]
     #[serde(deserialize_with = "deserialize_fork_epoch")]
     pub bellatrix_fork_epoch: Option<MaybeQuoted<Epoch>>,
@@ -890,10 +888,6 @@ fn default_bellatrix_fork_version() -> [u8; 4] {
     [0xff, 0xff, 0xff, 0xff]
 }
 
-fn default_bellatrix_fork_epoch() -> Option<MaybeQuoted<Epoch>> {
-    None
-}
-
 /// Placeholder value: 2^256-2^10 (115792089237316195423570985008687907853269984665640564039457584007913129638912).
 ///
 /// Taken from https://github.com/ethereum/consensus-specs/blob/d5e4828aecafaf1c57ef67a5f23c4ae7b08c5137/configs/mainnet.yaml#L15-L16
@@ -1335,10 +1329,7 @@ mod yaml_tests {
             default_safe_slots_to_import_optimistically()
         );
 
-        assert_eq!(
-            chain_spec.bellatrix_fork_epoch,
-            default_bellatrix_fork_epoch()
-        );
+        assert_eq!(chain_spec.bellatrix_fork_epoch, None);
 
         assert_eq!(
             chain_spec.bellatrix_fork_version,
diff --git a/consensus/types/src/config_and_preset.rs b/consensus/types/src/config_and_preset.rs
index 8b3a753bd5..e624afe2db 100644
--- a/consensus/types/src/config_and_preset.rs
+++ b/consensus/types/src/config_and_preset.rs
@@ -1,12 +1,21 @@
-use crate::{AltairPreset, BasePreset, BellatrixPreset, ChainSpec, Config, EthSpec};
+use crate::{
+    consts::altair, AltairPreset, BasePreset, BellatrixPreset, ChainSpec, Config, EthSpec, ForkName,
+};
+use maplit::hashmap;
 use serde_derive::{Deserialize, Serialize};
 use serde_json::Value;
 use std::collections::HashMap;
+use superstruct::superstruct;
 
 /// Fusion of a runtime-config with the compile-time preset values.
 ///
 /// Mostly useful for the API.
+#[superstruct(
+    variants(Altair, Bellatrix),
+    variant_attributes(derive(Serialize, Deserialize, Debug, PartialEq, Clone))
+)]
 #[derive(Serialize, Deserialize, Debug, PartialEq, Clone)]
+#[serde(untagged)]
 pub struct ConfigAndPreset {
     #[serde(flatten)]
     pub config: Config,
@@ -15,80 +24,75 @@ pub struct ConfigAndPreset {
     pub base_preset: BasePreset,
     #[serde(flatten)]
     pub altair_preset: AltairPreset,
-    // TODO(merge): re-enable
-    // #[serde(flatten)]
-    // pub bellatrix_preset: BellatrixPreset,
+    #[superstruct(only(Bellatrix))]
+    #[serde(flatten)]
+    pub bellatrix_preset: BellatrixPreset,
     /// The `extra_fields` map allows us to gracefully decode fields intended for future hard forks.
     #[serde(flatten)]
     pub extra_fields: HashMap<String, Value>,
 }
 
 impl ConfigAndPreset {
-    pub fn from_chain_spec<T: EthSpec>(spec: &ChainSpec) -> Self {
+    pub fn from_chain_spec<T: EthSpec>(spec: &ChainSpec, fork_name: Option<ForkName>) -> Self {
         let config = Config::from_chain_spec::<T>(spec);
         let base_preset = BasePreset::from_chain_spec::<T>(spec);
         let altair_preset = AltairPreset::from_chain_spec::<T>(spec);
-        // TODO(merge): re-enable
-        let _bellatrix_preset = BellatrixPreset::from_chain_spec::<T>(spec);
-        let extra_fields = HashMap::new();
+        let extra_fields = get_extra_fields(spec);
 
-        Self {
-            config,
-            base_preset,
-            altair_preset,
-            extra_fields,
+        if spec.bellatrix_fork_epoch.is_some()
+            || fork_name == None
+            || fork_name == Some(ForkName::Merge)
+        {
+            let bellatrix_preset = BellatrixPreset::from_chain_spec::<T>(spec);
+
+            ConfigAndPreset::Bellatrix(ConfigAndPresetBellatrix {
+                config,
+                base_preset,
+                altair_preset,
+                bellatrix_preset,
+                extra_fields,
+            })
+        } else {
+            ConfigAndPreset::Altair(ConfigAndPresetAltair {
+                config,
+                base_preset,
+                altair_preset,
+                extra_fields,
+            })
         }
     }
+}
 
-    /// Add fields that were previously part of the config but are now constants.
-    pub fn make_backwards_compat(&mut self, spec: &ChainSpec) {
-        let hex_string = |value: &[u8]| format!("0x{}", hex::encode(&value));
-        let u32_hex = |v: u32| hex_string(&v.to_le_bytes());
-        let u8_hex = |v: u8| hex_string(&v.to_le_bytes());
-        let fields = vec![
-            (
-                "bls_withdrawal_prefix",
-                u8_hex(spec.bls_withdrawal_prefix_byte),
-            ),
-            (
-                "domain_beacon_proposer",
-                u32_hex(spec.domain_beacon_proposer),
-            ),
-            (
-                "domain_beacon_attester",
-                u32_hex(spec.domain_beacon_attester),
-            ),
-            ("domain_randao", u32_hex(spec.domain_randao)),
-            ("domain_deposit", u32_hex(spec.domain_deposit)),
-            ("domain_voluntary_exit", u32_hex(spec.domain_voluntary_exit)),
-            (
-                "domain_selection_proof",
-                u32_hex(spec.domain_selection_proof),
-            ),
-            (
-                "domain_aggregate_and_proof",
-                u32_hex(spec.domain_aggregate_and_proof),
-            ),
-            (
-                "domain_application_mask",
-                u32_hex(spec.domain_application_mask),
-            ),
-            (
-                "target_aggregators_per_committee",
-                spec.target_aggregators_per_committee.to_string(),
-            ),
-            (
-                "random_subnets_per_validator",
-                spec.random_subnets_per_validator.to_string(),
-            ),
-            (
-                "epochs_per_random_subnet_subscription",
-                spec.epochs_per_random_subnet_subscription.to_string(),
-            ),
-        ];
-        for (key, value) in fields {
-            self.extra_fields.insert(key.to_uppercase(), value.into());
-        }
+/// Get a hashmap of constants to add to the `PresetAndConfig`
+pub fn get_extra_fields(spec: &ChainSpec) -> HashMap<String, Value> {
+    let hex_string = |value: &[u8]| format!("0x{}", hex::encode(&value)).into();
+    let u32_hex = |v: u32| hex_string(&v.to_le_bytes());
+    let u8_hex = |v: u8| hex_string(&v.to_le_bytes());
+    hashmap! {
+        "bls_withdrawal_prefix".to_uppercase() => u8_hex(spec.bls_withdrawal_prefix_byte),
+        "domain_beacon_proposer".to_uppercase() => u32_hex(spec.domain_beacon_proposer),
+        "domain_beacon_attester".to_uppercase() => u32_hex(spec.domain_beacon_attester),
+        "domain_randao".to_uppercase()=> u32_hex(spec.domain_randao),
+        "domain_deposit".to_uppercase()=> u32_hex(spec.domain_deposit),
+        "domain_voluntary_exit".to_uppercase() => u32_hex(spec.domain_voluntary_exit),
+        "domain_selection_proof".to_uppercase() => u32_hex(spec.domain_selection_proof),
+        "domain_aggregate_and_proof".to_uppercase() => u32_hex(spec.domain_aggregate_and_proof),
+        "domain_application_mask".to_uppercase()=> u32_hex(spec.domain_application_mask),
+        "target_aggregators_per_committee".to_uppercase() =>
+            spec.target_aggregators_per_committee.to_string().into(),
+        "random_subnets_per_validator".to_uppercase() =>
+            spec.random_subnets_per_validator.to_string().into(),
+        "epochs_per_random_subnet_subscription".to_uppercase() =>
+            spec.epochs_per_random_subnet_subscription.to_string().into(),
+        "domain_contribution_and_proof".to_uppercase() =>
+            u32_hex(spec.domain_contribution_and_proof),
+        "domain_sync_committee".to_uppercase() => u32_hex(spec.domain_sync_committee),
+        "domain_sync_committee_selection_proof".to_uppercase() =>
+            u32_hex(spec.domain_sync_committee_selection_proof),
+        "sync_committee_subnet_count".to_uppercase() =>
+            altair::SYNC_COMMITTEE_SUBNET_COUNT.to_string().into(),
+        "target_aggregators_per_sync_subcommittee".to_uppercase() =>
+            altair::TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE.to_string().into(),
     }
 }
 
@@ -108,15 +112,16 @@ mod test {
             .open(tmp_file.as_ref())
             .expect("error opening file");
         let mainnet_spec = ChainSpec::mainnet();
-        let mut yamlconfig = ConfigAndPreset::from_chain_spec::<MainnetEthSpec>(&mainnet_spec);
+        let mut yamlconfig =
+            ConfigAndPreset::from_chain_spec::<MainnetEthSpec>(&mainnet_spec, None);
         let (k1, v1) = ("SAMPLE_HARDFORK_KEY1", "123456789");
         let (k2, v2) = ("SAMPLE_HARDFORK_KEY2", "987654321");
         let (k3, v3) = ("SAMPLE_HARDFORK_KEY3", 32);
         let (k4, v4) = ("SAMPLE_HARDFORK_KEY4", Value::Null);
-        yamlconfig.extra_fields.insert(k1.into(), v1.into());
-        yamlconfig.extra_fields.insert(k2.into(), v2.into());
-        yamlconfig.extra_fields.insert(k3.into(), v3.into());
-        yamlconfig.extra_fields.insert(k4.into(), v4);
+        yamlconfig.extra_fields_mut().insert(k1.into(), v1.into());
+        yamlconfig.extra_fields_mut().insert(k2.into(), v2.into());
+        yamlconfig.extra_fields_mut().insert(k3.into(), v3.into());
+        yamlconfig.extra_fields_mut().insert(k4.into(), v4);
 
         serde_yaml::to_writer(writer, &yamlconfig).expect("failed to write or serialize");
 
@@ -125,8 +130,8 @@ mod test {
             .write(false)
             .open(tmp_file.as_ref())
             .expect("error while opening the file");
-        let from: ConfigAndPreset =
+        let from: ConfigAndPresetBellatrix =
             serde_yaml::from_reader(reader).expect("error while deserializing");
-        assert_eq!(from, yamlconfig);
+        assert_eq!(ConfigAndPreset::Bellatrix(from), yamlconfig);
     }
 }
diff --git a/consensus/types/src/fork_name.rs b/consensus/types/src/fork_name.rs
index 4a2e762087..e97b08309b 100644
--- a/consensus/types/src/fork_name.rs
+++ b/consensus/types/src/fork_name.rs
@@ -106,14 +106,14 @@ macro_rules! map_fork_name_with {
 }
 
 impl FromStr for ForkName {
-    type Err = ();
+    type Err = String;
 
-    fn from_str(fork_name: &str) -> Result<Self, ()> {
+    fn from_str(fork_name: &str) -> Result<Self, String> {
         Ok(match fork_name.to_lowercase().as_ref() {
             "phase0" | "base" => ForkName::Base,
             "altair" => ForkName::Altair,
             "bellatrix" | "merge" => ForkName::Merge,
-            _ => return Err(()),
+            _ => return Err(format!("unknown fork name: {}", fork_name)),
         })
     }
 }
@@ -138,7 +138,7 @@ impl TryFrom<String> for ForkName {
     type Error = String;
 
     fn try_from(s: String) -> Result<Self, Self::Error> {
-        Self::from_str(&s).map_err(|()| format!("Invalid fork name: {}", s))
+        Self::from_str(&s)
     }
 }
 
@@ -178,8 +178,8 @@ mod test {
         assert_eq!(ForkName::from_str("AlTaIr"), Ok(ForkName::Altair));
         assert_eq!(ForkName::from_str("altair"), Ok(ForkName::Altair));
 
-        assert_eq!(ForkName::from_str("NO_NAME"), Err(()));
-        assert_eq!(ForkName::from_str("no_name"), Err(()));
+        assert!(ForkName::from_str("NO_NAME").is_err());
+        assert!(ForkName::from_str("no_name").is_err());
     }
 
     #[test]
diff --git a/consensus/types/src/lib.rs b/consensus/types/src/lib.rs
index 7823ec223c..f05012c0b7 100644
--- a/consensus/types/src/lib.rs
+++ b/consensus/types/src/lib.rs
@@ -110,7 +110,9 @@ pub use crate::beacon_committee::{BeaconCommittee, OwnedBeaconCommittee};
 pub use crate::beacon_state::{BeaconTreeHashCache, Error as BeaconStateError, *};
 pub use crate::chain_spec::{ChainSpec, Config, Domain};
 pub use crate::checkpoint::Checkpoint;
-pub use crate::config_and_preset::ConfigAndPreset;
+pub use crate::config_and_preset::{
+    ConfigAndPreset, ConfigAndPresetAltair, ConfigAndPresetBellatrix,
+};
 pub use crate::contribution_and_proof::ContributionAndProof;
 pub use crate::deposit::{Deposit, DEPOSIT_TREE_DEPTH};
 pub use crate::deposit_data::DepositData;
diff --git a/lighthouse/tests/beacon_node.rs b/lighthouse/tests/beacon_node.rs
index 0236ba6589..9d952e5cc5 100644
--- a/lighthouse/tests/beacon_node.rs
+++ b/lighthouse/tests/beacon_node.rs
@@ -11,7 +11,7 @@ use std::process::Command;
 use std::str::FromStr;
 use std::string::ToString;
 use tempfile::TempDir;
-use types::{Address, Checkpoint, Epoch, ExecutionBlockHash, Hash256, MainnetEthSpec};
+use types::{Address, Checkpoint, Epoch, ExecutionBlockHash, ForkName, Hash256, MainnetEthSpec};
 use unused_port::{unused_tcp_port, unused_udp_port};
 
 const DEFAULT_ETH1_ENDPOINT: &str = "http://localhost:8545/";
@@ -949,6 +949,21 @@ fn http_tls_flags() {
         });
 }
 
+#[test]
+fn http_spec_fork_default() {
+    CommandLineTest::new()
+        .run_with_zero_port()
+        .with_config(|config| assert_eq!(config.http_api.spec_fork_name, None));
+}
+
+#[test]
+fn http_spec_fork_override() {
+    CommandLineTest::new()
+        .flag("http-spec-fork", Some("altair"))
+        .run_with_zero_port()
+        .with_config(|config| assert_eq!(config.http_api.spec_fork_name, Some(ForkName::Altair)));
+}
+
 // Tests for Metrics flags.
 #[test]
 fn metrics_flag() {
diff --git a/validator_client/src/beacon_node_fallback.rs b/validator_client/src/beacon_node_fallback.rs
index d4f7c6c874..0b808e71bb 100644
--- a/validator_client/src/beacon_node_fallback.rs
+++ b/validator_client/src/beacon_node_fallback.rs
@@ -7,7 +7,7 @@ use crate::http_metrics::metrics::{inc_counter_vec, ENDPOINT_ERRORS, ENDPOINT_RE
 use environment::RuntimeContext;
 use eth2::BeaconNodeHttpClient;
 use futures::future;
-use slog::{debug, error, info, warn, Logger};
+use slog::{error, info, warn, Logger};
 use slot_clock::SlotClock;
 use std::fmt;
 use std::fmt::Debug;
@@ -16,7 +16,7 @@ use std::marker::PhantomData;
 use std::sync::Arc;
 use std::time::Duration;
 use tokio::{sync::RwLock, time::sleep};
-use types::{ChainSpec, EthSpec};
+use types::{ChainSpec, Config, EthSpec};
 
 /// The number of seconds *prior* to slot start that we will try and update the state of fallback
 /// nodes.
@@ -213,9 +213,9 @@ impl<E: EthSpec> CandidateBeaconNode<E> {
 
     /// Checks if the node has the correct specification.
     async fn is_compatible(&self, spec: &ChainSpec, log: &Logger) -> Result<(), CandidateError> {
-        let config_and_preset = self
+        let config = self
             .beacon_node
-            .get_config_spec()
+            .get_config_spec::<Config>()
             .await
             .map_err(|e| {
                 error!(
@@ -228,25 +228,15 @@ impl<E: EthSpec> CandidateBeaconNode<E> {
             })?
             .data;
 
-        let beacon_node_spec =
-            ChainSpec::from_config::<E>(&config_and_preset.config).ok_or_else(|| {
-                error!(
-                    log,
-                    "The minimal/mainnet spec type of the beacon node does not match the validator \
-                    client. See the --network command.";
-                    "endpoint" => %self.beacon_node,
-                );
-                CandidateError::Incompatible
-            })?;
-
-        if !config_and_preset.extra_fields.is_empty() {
-            debug!(
+        let beacon_node_spec = ChainSpec::from_config::<E>(&config).ok_or_else(|| {
+            error!(
                 log,
-                "Beacon spec includes unknown fields";
+                "The minimal/mainnet spec type of the beacon node does not match the validator \
+                client. See the --network command.";
                 "endpoint" => %self.beacon_node,
-                "fields" => ?config_and_preset.extra_fields,
             );
-        }
+            CandidateError::Incompatible
+        })?;
 
         if beacon_node_spec.genesis_fork_version != spec.genesis_fork_version {
             error!(
diff --git a/validator_client/src/http_api/mod.rs b/validator_client/src/http_api/mod.rs
index a5d8d0e71c..1e48e86c05 100644
--- a/validator_client/src/http_api/mod.rs
+++ b/validator_client/src/http_api/mod.rs
@@ -217,8 +217,7 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
         .and(signer.clone())
         .and_then(|spec: Arc<_>, signer| {
             blocking_signed_json_task(signer, move || {
-                let mut config = ConfigAndPreset::from_chain_spec::<E>(&spec);
-                config.make_backwards_compat(&spec);
+                let config = ConfigAndPreset::from_chain_spec::<E>(&spec, None);
                 Ok(api_types::GenericResponse::from(config))
             })
         });
diff --git a/validator_client/src/http_api/tests.rs b/validator_client/src/http_api/tests.rs
index e67a82634c..b121dda5b1 100644
--- a/validator_client/src/http_api/tests.rs
+++ b/validator_client/src/http_api/tests.rs
@@ -208,10 +208,13 @@ impl ApiTester {
     }
 
     pub async fn test_get_lighthouse_spec(self) -> Self {
-        let result = self.client.get_lighthouse_spec().await.unwrap().data;
-
-        let mut expected = ConfigAndPreset::from_chain_spec::<E>(&E::default_spec());
-        expected.make_backwards_compat(&E::default_spec());
+        let result = self
+            .client
+            .get_lighthouse_spec::<ConfigAndPresetBellatrix>()
+            .await
+            .map(|res| ConfigAndPreset::Bellatrix(res.data))
+            .unwrap();
+        let expected = ConfigAndPreset::from_chain_spec::<E>(&E::default_spec(), None);
 
         assert_eq!(result, expected);
 
@@ -623,7 +626,9 @@ fn routes_with_invalid_auth() {
             .await
             .test_with_invalid_auth(|client| async move { client.get_lighthouse_health().await })
             .await
-            .test_with_invalid_auth(|client| async move { client.get_lighthouse_spec().await })
+            .test_with_invalid_auth(|client| async move {
+                client.get_lighthouse_spec::<types::Config>().await
+            })
             .await
             .test_with_invalid_auth(
                 |client| async move { client.get_lighthouse_validators().await },