diff --git a/cardano-api/internal/Cardano/Api/Genesis.hs b/cardano-api/internal/Cardano/Api/Genesis.hs
index 3f6ff7e34c..6d9b9abf9c 100644
--- a/cardano-api/internal/Cardano/Api/Genesis.hs
+++ b/cardano-api/internal/Cardano/Api/Genesis.hs
@@ -341,8 +341,13 @@ decodeAlonzoGenesis (Just era) genesisBs = modifyError ("Cannot decode era-sensi
 
 -- | Some reasonable starting defaults for constructing a 'AlonzoGenesis'.
 -- Based on https://github.com/IntersectMBO/cardano-node/blob/master/cardano-testnet/src/Testnet/Defaults.hs
-alonzoGenesisDefaults :: AlonzoGenesis
-alonzoGenesisDefaults =
+-- The era determines Plutus V2 cost model parameters:
+-- * Conway: 185
+-- * <= Babbage: 175
+alonzoGenesisDefaults
+  :: CardanoEra era
+  -> AlonzoGenesis
+alonzoGenesisDefaults era =
   AlonzoGenesis
     { agPrices =
         Prices
@@ -717,15 +722,21 @@ alonzoGenesisDefaults =
       , 38887044
       , 32947
       , 10
-      , -- New Conway costmodel parameters below
-        1292075
-      , 24469
-      , 74
-      , 0
-      , 1
-      , 936157
-      , 49601
-      , 237
-      , 0
-      , 1
       ]
+        <> defaultV2CostModelNewConwayParams
+
+    -- New Conway cost model parameters
+    defaultV2CostModelNewConwayParams =
+      monoidForEraInEon @ConwayEraOnwards era $
+        const
+          [ 1292075
+          , 24469
+          , 74
+          , 0
+          , 1
+          , 936157
+          , 49601
+          , 237
+          , 0
+          , 1
+          ]
diff --git a/cardano-api/internal/Cardano/Api/LedgerState.hs b/cardano-api/internal/Cardano/Api/LedgerState.hs
index 07eea2b58e..aa94454621 100644
--- a/cardano-api/internal/Cardano/Api/LedgerState.hs
+++ b/cardano-api/internal/Cardano/Api/LedgerState.hs
@@ -1358,6 +1358,8 @@ shelleyPraosNonce genesisHash =
 readCardanoGenesisConfig
   :: MonadIOTransError GenesisConfigError t m
   => Maybe (CardanoEra era)
+  -- ^ Provide era witness to read Alonzo Genesis in an era-sensitive manner (see
+  -- 'Cardano.Api.Genesis.decodeAlonzGenesis' for more details)
   -> NodeConfig
   -> t m GenesisConfig
 readCardanoGenesisConfig mEra enc = do
@@ -1442,6 +1444,8 @@ readShelleyGenesisConfig enc = do
 readAlonzoGenesisConfig
   :: MonadIOTransError GenesisConfigError t m
   => Maybe (CardanoEra era)
+  -- ^ Provide era witness to read Alonzo Genesis in an era-sensitive manner (see
+  -- 'Cardano.Api.Genesis.decodeAlonzGenesis' for more details)
   -> NodeConfig
   -> t m AlonzoGenesis
 readAlonzoGenesisConfig mEra enc = do
@@ -1523,6 +1527,8 @@ readAlonzoGenesis
   :: forall m t era
    . MonadIOTransError AlonzoGenesisError t m
   => Maybe (CardanoEra era)
+  -- ^ Provide era witness to read Alonzo Genesis in an era-sensitive manner (see
+  -- 'Cardano.Api.Genesis.decodeAlonzGenesis' for more details)
   -> File AlonzoGenesis 'In
   -> GenesisHashAlonzo
   -> t m AlonzoGenesis
diff --git a/cardano-api/test/cardano-api-test/Test/Cardano/Api/Genesis.hs b/cardano-api/test/cardano-api-test/Test/Cardano/Api/Genesis.hs
index 5f3d24662f..54ef3dd39e 100644
--- a/cardano-api/test/cardano-api-test/Test/Cardano/Api/Genesis.hs
+++ b/cardano-api/test/cardano-api-test/Test/Cardano/Api/Genesis.hs
@@ -62,11 +62,20 @@ prop_reading_plutus_v2_era_sensitive_costmodel era cmf = H.propertyOnce $ do
         else last10CostModelValues === last10CostModelCorrectValues
     else length v2costModelValues === 175
 
+  -- Make sure that our just read genesis is CBOR encoding roundtripping
   aeo <- H.nothingFail $ maybeEon @AlonzoEraOnwards @era
   let allCostModelsBs = encodeCborInEraCostModels aeo allCostModels
   allCostModels' <- H.leftFail $ decodeCborInEraCostModels aeo allCostModelsBs
+  H.note_ "Check that read genesis is CBOR encoding roundtripping"
   allCostModels' === allCostModels
 
+  -- Yeah, let's check the default one if it's roundtripping as well
+  let defaultCostModels = L.agCostModels $ alonzoGenesisDefaults era
+      defaultCostModelsBs = encodeCborInEraCostModels aeo defaultCostModels
+  defaultCostModels' <- H.leftFail $ decodeCborInEraCostModels aeo defaultCostModelsBs
+  H.note_ "Check that the default genesis is CBOR encoding roundtripping"
+  defaultCostModels' === defaultCostModels
+
 -- | Test reading and decoding of AlonzoGenesis with cost models - an era independent test
 prop_reading_plutus_v2_costmodel
   :: PlutusV2CostModelFormat