diff --git a/configs/mainnet/altair.yaml b/configs/mainnet/altair.yaml
index f30e9fcdc5..1bebafe907 100644
--- a/configs/mainnet/altair.yaml
+++ b/configs/mainnet/altair.yaml
@@ -17,7 +17,7 @@ PROPORTIONAL_SLASHING_MULTIPLIER_ALTAIR: 2
 # 2**10 (= 1,024)
 SYNC_COMMITTEE_SIZE: 1024
 # 2**6 (= 64)
-SYNC_SUBCOMMITTEE_SIZE: 64
+SYNC_PUBKEYS_PER_AGGREGATE: 64
 # 2**2 (= 4)
 INACTIVITY_SCORE_BIAS: 4
 
diff --git a/configs/minimal/altair.yaml b/configs/minimal/altair.yaml
index afdaf9eb5b..f10e10bb97 100644
--- a/configs/minimal/altair.yaml
+++ b/configs/minimal/altair.yaml
@@ -17,7 +17,7 @@ PROPORTIONAL_SLASHING_MULTIPLIER_ALTAIR: 2
 # [customized]
 SYNC_COMMITTEE_SIZE: 32
 # [customized]
-SYNC_SUBCOMMITTEE_SIZE: 16
+SYNC_PUBKEYS_PER_AGGREGATE: 16
 # 2**2 (= 4)
 INACTIVITY_SCORE_BIAS: 4
 
@@ -35,7 +35,8 @@ DOMAIN_SYNC_COMMITTEE: 0x07000000
 
 # Fork
 # ---------------------------------------------------------------
-ALTAIR_FORK_VERSION: 0x01000000
+# Highest byte set to 0x01 to avoid collisions with mainnet versioning
+ALTAIR_FORK_VERSION: 0x01000001
 # [customized]
 ALTAIR_FORK_SLOT: 0
 
diff --git a/setup.py b/setup.py
index ef6c65289d..9de8ef9111 100644
--- a/setup.py
+++ b/setup.py
@@ -537,6 +537,7 @@ def finalize_options(self):
                     specs/phase0/weak-subjectivity.md
                     specs/altair/beacon-chain.md
                     specs/altair/fork.md
+                    specs/altair/validator.md
                     specs/altair/sync-protocol.md
                 """
             else:
diff --git a/specs/altair/beacon-chain.md b/specs/altair/beacon-chain.md
index 58e82364ce..a004f3fb7d 100644
--- a/specs/altair/beacon-chain.md
+++ b/specs/altair/beacon-chain.md
@@ -117,7 +117,7 @@ This patch updates a few configuration values to move penalty parameters toward
 | Name | Value |
 | - | - |
 | `SYNC_COMMITTEE_SIZE` | `uint64(2**10)` (= 1,024) |
-| `SYNC_SUBCOMMITTEE_SIZE` | `uint64(2**6)` (= 64) |
+| `SYNC_PUBKEYS_PER_AGGREGATE` | `uint64(2**6)` (= 64) |
 | `INACTIVITY_SCORE_BIAS` | `uint64(4)` |
 
 ### Time parameters
@@ -210,7 +210,7 @@ class SyncAggregate(Container):
 ```python
 class SyncCommittee(Container):
     pubkeys: Vector[BLSPubkey, SYNC_COMMITTEE_SIZE]
-    pubkey_aggregates: Vector[BLSPubkey, SYNC_COMMITTEE_SIZE // SYNC_SUBCOMMITTEE_SIZE]
+    pubkey_aggregates: Vector[BLSPubkey, SYNC_COMMITTEE_SIZE // SYNC_PUBKEYS_PER_AGGREGATE]
 ```
 
 ## Helper functions
@@ -294,8 +294,8 @@ def get_sync_committee(state: BeaconState, epoch: Epoch) -> SyncCommittee:
     """
     indices = get_sync_committee_indices(state, epoch)
     pubkeys = [state.validators[index].pubkey for index in indices]
-    subcommitees = [pubkeys[i:i + SYNC_SUBCOMMITTEE_SIZE] for i in range(0, len(pubkeys), SYNC_SUBCOMMITTEE_SIZE)]
-    pubkey_aggregates = [bls.AggregatePKs(subcommitee) for subcommitee in subcommitees]
+    partition = [pubkeys[i:i + SYNC_PUBKEYS_PER_AGGREGATE] for i in range(0, len(pubkeys), SYNC_PUBKEYS_PER_AGGREGATE)]
+    pubkey_aggregates = [bls.AggregatePKs(preaggregate) for preaggregate in partition]
     return SyncCommittee(pubkeys=pubkeys, pubkey_aggregates=pubkey_aggregates)
 ```
 
@@ -575,7 +575,7 @@ def process_epoch(state: BeaconState) -> None:
 
 #### Justification and finalization
 
-*Note*: The function `process_justification_and_finalization` is modified with `matching_target_attestations` replaced by `matching_target_indices`.
+*Note*: The function `process_justification_and_finalization` is modified to adapt to the new participation records.
 
 ```python
 def process_justification_and_finalization(state: BeaconState) -> None:
@@ -583,40 +583,12 @@ def process_justification_and_finalization(state: BeaconState) -> None:
     # Skip FFG updates in the first two epochs to avoid corner cases that might result in modifying this stub.
     if get_current_epoch(state) <= GENESIS_EPOCH + 1:
         return
-    previous_epoch = get_previous_epoch(state)
-    current_epoch = get_current_epoch(state)
-    old_previous_justified_checkpoint = state.previous_justified_checkpoint
-    old_current_justified_checkpoint = state.current_justified_checkpoint
-
-    # Process justifications
-    state.previous_justified_checkpoint = state.current_justified_checkpoint
-    state.justification_bits[1:] = state.justification_bits[:JUSTIFICATION_BITS_LENGTH - 1]
-    state.justification_bits[0] = 0b0
-    matching_target_indices = get_unslashed_participating_indices(state, TIMELY_TARGET_FLAG_INDEX, previous_epoch)
-    if get_total_balance(state, matching_target_indices) * 3 >= get_total_active_balance(state) * 2:
-        state.current_justified_checkpoint = Checkpoint(epoch=previous_epoch,
-                                                        root=get_block_root(state, previous_epoch))
-        state.justification_bits[1] = 0b1
-    matching_target_indices = get_unslashed_participating_indices(state, TIMELY_TARGET_FLAG_INDEX, current_epoch)
-    if get_total_balance(state, matching_target_indices) * 3 >= get_total_active_balance(state) * 2:
-        state.current_justified_checkpoint = Checkpoint(epoch=current_epoch,
-                                                        root=get_block_root(state, current_epoch))
-        state.justification_bits[0] = 0b1
-
-    # Process finalizations
-    bits = state.justification_bits
-    # The 2nd/3rd/4th most recent epochs are justified, the 2nd using the 4th as source
-    if all(bits[1:4]) and old_previous_justified_checkpoint.epoch + 3 == current_epoch:
-        state.finalized_checkpoint = old_previous_justified_checkpoint
-    # The 2nd/3rd most recent epochs are justified, the 2nd using the 3rd as source
-    if all(bits[1:3]) and old_previous_justified_checkpoint.epoch + 2 == current_epoch:
-        state.finalized_checkpoint = old_previous_justified_checkpoint
-    # The 1st/2nd/3rd most recent epochs are justified, the 1st using the 3rd as source
-    if all(bits[0:3]) and old_current_justified_checkpoint.epoch + 2 == current_epoch:
-        state.finalized_checkpoint = old_current_justified_checkpoint
-    # The 1st/2nd most recent epochs are justified, the 1st using the 2nd as source
-    if all(bits[0:2]) and old_current_justified_checkpoint.epoch + 1 == current_epoch:
-        state.finalized_checkpoint = old_current_justified_checkpoint
+    previous_indices = get_unslashed_participating_indices(state, TIMELY_TARGET_FLAG_INDEX, get_previous_epoch(state))
+    current_indices = get_unslashed_participating_indices(state, TIMELY_TARGET_FLAG_INDEX, get_current_epoch(state))
+    total_active_balance = get_total_active_balance(state)
+    previous_target_balance = get_total_balance(state, previous_indices)
+    current_target_balance = get_total_balance(state, current_indices)
+    weigh_justification_and_finalization(state, total_active_balance, previous_target_balance, current_target_balance)
 ```
 
 #### Inactivity scores
diff --git a/specs/altair/validator.md b/specs/altair/validator.md
index 59c1ab81f2..25897b97e0 100644
--- a/specs/altair/validator.md
+++ b/specs/altair/validator.md
@@ -18,6 +18,7 @@ This is an accompanying document to [Ethereum 2.0 Altair -- The Beacon Chain](./
   - [`SyncCommitteeContribution`](#synccommitteecontribution)
   - [`ContributionAndProof`](#contributionandproof)
   - [`SignedContributionAndProof`](#signedcontributionandproof)
+  - [`SyncCommitteeSigningData`](#synccommitteesigningdata)
 - [Validator assignments](#validator-assignments)
   - [Sync Committee](#sync-committee)
   - [Lookahead](#lookahead)
@@ -125,6 +126,14 @@ class SignedContributionAndProof(Container):
     signature: BLSSignature
 ```
 
+### `SyncCommitteeSigningData`
+
+```python
+class SyncCommitteeSigningData(Container):
+    slot: Slot
+    subcommittee_index: uint64
+```
+
 ## Validator assignments
 
 A validator determines beacon committee assignments and beacon block proposal duties as defined in the Phase 0 document.
@@ -137,7 +146,9 @@ This function is a predicate indicating the presence or absence of the validator
 ```python
 def compute_sync_committee_period(epoch: Epoch) -> uint64:
     return epoch // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
+```
 
+```python
 def is_assigned_to_sync_committee(state: BeaconState,
                                   epoch: Epoch,
                                   validator_index: ValidatorIndex) -> bool:
@@ -280,9 +291,10 @@ This function returns multiple subnets if a given validator index is included mu
 ```python
 def compute_subnets_for_sync_committee(state: BeaconState, validator_index: ValidatorIndex) -> Sequence[uint64]:
     target_pubkey = state.validators[validator_index].pubkey
-    sync_committee_indices = [index for index, pubkey in enumerate(state.current_sync_committee.pubkeys) if pubkey == target_pubkey]
+    sync_committee_indices = [index for index, pubkey in enumerate(state.current_sync_committee.pubkeys)
+                              if pubkey == target_pubkey]
     return [
-        uint64(index // (SYNC_COMMITEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT))
+        uint64(index // (SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT))
         for index in sync_committee_indices
     ]
 ```
@@ -301,13 +313,8 @@ A validator is selected to aggregate based on the computation in `is_sync_commit
 The signature function takes a `BeaconState` with the relevant sync committees for the queried `slot` (i.e. `state.slot` is within the span covered by the current or next sync committee period), the `subcommittee_index` equal to the `subnet_id`, and the `privkey` is the BLS private key associated with the validator.
 
 ```python
-class SyncCommitteeSigningData(Container):
-    slot: Slot
-    subcommittee_index: uint64
-```
-
-```python
-def get_sync_committee_slot_signature(state: BeaconState, slot: Slot, subcommittee_index: uint64, privkey: int) -> BLSSignature:
+def get_sync_committee_slot_signature(state: BeaconState, slot: Slot,
+                                      subcommittee_index: uint64, privkey: int) -> BLSSignature:
     domain = get_domain(state, DOMAIN_SYNC_COMMITTEE_SELECTION_PROOF, compute_epoch_at_slot(slot))
     signing_data = SyncCommitteeSigningData(
         slot=slot,
diff --git a/specs/phase0/beacon-chain.md b/specs/phase0/beacon-chain.md
index 9fcd118523..e56a8ea223 100644
--- a/specs/phase0/beacon-chain.md
+++ b/specs/phase0/beacon-chain.md
@@ -1320,7 +1320,19 @@ def process_justification_and_finalization(state: BeaconState) -> None:
     # Skip FFG updates in the first two epochs to avoid corner cases that might result in modifying this stub.
     if get_current_epoch(state) <= GENESIS_EPOCH + 1:
         return
+    previous_attestations = get_matching_target_attestations(state, get_previous_epoch(state))
+    current_attestations = get_matching_target_attestations(state, get_current_epoch(state))
+    total_active_balance = get_total_active_balance(state)
+    previous_target_balance = get_attesting_balance(state, previous_attestations)
+    current_target_balance = get_attesting_balance(state, current_attestations)
+    weigh_justification_and_finalization(state, total_active_balance, previous_target_balance, current_target_balance)
+```
 
+```python
+def weigh_justification_and_finalization(state: BeaconState,
+                                         total_active_balance: Gwei,
+                                         previous_epoch_target_balance: Gwei,
+                                         current_epoch_target_balance: Gwei) -> None:
     previous_epoch = get_previous_epoch(state)
     current_epoch = get_current_epoch(state)
     old_previous_justified_checkpoint = state.previous_justified_checkpoint
@@ -1330,13 +1342,11 @@ def process_justification_and_finalization(state: BeaconState) -> None:
     state.previous_justified_checkpoint = state.current_justified_checkpoint
     state.justification_bits[1:] = state.justification_bits[:JUSTIFICATION_BITS_LENGTH - 1]
     state.justification_bits[0] = 0b0
-    matching_target_attestations = get_matching_target_attestations(state, previous_epoch)  # Previous epoch
-    if get_attesting_balance(state, matching_target_attestations) * 3 >= get_total_active_balance(state) * 2:
+    if previous_epoch_target_balance * 3 >= total_active_balance * 2:
         state.current_justified_checkpoint = Checkpoint(epoch=previous_epoch,
                                                         root=get_block_root(state, previous_epoch))
         state.justification_bits[1] = 0b1
-    matching_target_attestations = get_matching_target_attestations(state, current_epoch)  # Current epoch
-    if get_attesting_balance(state, matching_target_attestations) * 3 >= get_total_active_balance(state) * 2:
+    if current_epoch_target_balance * 3 >= total_active_balance * 2:
         state.current_justified_checkpoint = Checkpoint(epoch=current_epoch,
                                                         root=get_block_root(state, current_epoch))
         state.justification_bits[0] = 0b1
diff --git a/specs/phase0/validator.md b/specs/phase0/validator.md
index 281078d209..1767d3d446 100644
--- a/specs/phase0/validator.md
+++ b/specs/phase0/validator.md
@@ -12,6 +12,10 @@ This is an accompanying document to [Ethereum 2.0 Phase 0 -- The Beacon Chain](.
 - [Prerequisites](#prerequisites)
 - [Constants](#constants)
   - [Misc](#misc)
+- [Containers](#containers)
+  - [`Eth1Block`](#eth1block)
+  - [`AggregateAndProof`](#aggregateandproof)
+  - [`SignedAggregateAndProof`](#signedaggregateandproof)
 - [Becoming a validator](#becoming-a-validator)
   - [Initialization](#initialization)
     - [BLS public key](#bls-public-key)
@@ -33,7 +37,6 @@ This is an accompanying document to [Ethereum 2.0 Phase 0 -- The Beacon Chain](.
     - [Constructing the `BeaconBlockBody`](#constructing-the-beaconblockbody)
       - [Randao reveal](#randao-reveal)
       - [Eth1 Data](#eth1-data)
-        - [`Eth1Block`](#eth1block)
         - [`get_eth1_data`](#get_eth1_data)
       - [Proposer slashings](#proposer-slashings)
       - [Attester slashings](#attester-slashings)
@@ -60,8 +63,6 @@ This is an accompanying document to [Ethereum 2.0 Phase 0 -- The Beacon Chain](.
       - [Aggregation bits](#aggregation-bits-1)
       - [Aggregate signature](#aggregate-signature-1)
     - [Broadcast aggregate](#broadcast-aggregate)
-      - [`AggregateAndProof`](#aggregateandproof)
-      - [`SignedAggregateAndProof`](#signedaggregateandproof)
 - [Phase 0 attestation subnet stability](#phase-0-attestation-subnet-stability)
 - [How to avoid slashing](#how-to-avoid-slashing)
   - [Proposer slashing](#proposer-slashing)
@@ -92,6 +93,35 @@ All terminology, constants, functions, and protocol mechanics defined in the [Ph
 | `EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION` | `2**8` (= 256) | epochs | ~27 hours |
 | `ATTESTATION_SUBNET_COUNT` | `64` | The number of attestation subnets used in the gossipsub protocol. |
 
+## Containers
+
+### `Eth1Block`
+
+```python
+class Eth1Block(Container):
+    timestamp: uint64
+    deposit_root: Root
+    deposit_count: uint64
+    # All other eth1 block fields
+```
+
+### `AggregateAndProof`
+
+```python
+class AggregateAndProof(Container):
+    aggregator_index: ValidatorIndex
+    aggregate: Attestation
+    selection_proof: BLSSignature
+```
+
+### `SignedAggregateAndProof`
+
+```python
+class SignedAggregateAndProof(Container):
+    message: AggregateAndProof
+    signature: BLSSignature
+```
+
 ## Becoming a validator
 
 ### Initialization
@@ -302,20 +332,10 @@ If over half of the block proposers in the current Eth1 voting period vote for t
 `eth1_data` then `state.eth1_data` updates immediately allowing new deposits to be processed.
 Each deposit in `block.body.deposits` must verify against `state.eth1_data.eth1_deposit_root`.
 
-###### `Eth1Block`
+###### `get_eth1_data`
 
 Let `Eth1Block` be an abstract object representing Eth1 blocks with the `timestamp` and depost contract data available.
 
-```python
-class Eth1Block(Container):
-    timestamp: uint64
-    deposit_root: Root
-    deposit_count: uint64
-    # All other eth1 block fields
-```
-
-###### `get_eth1_data`
-
 Let `get_eth1_data(block: Eth1Block) -> Eth1Data` be the function that returns the Eth1 data for a given Eth1 block.
 
 An honest block proposer sets `block.body.eth1_data = get_eth1_vote(state, eth1_chain)` where:
@@ -581,23 +601,6 @@ def get_aggregate_and_proof_signature(state: BeaconState,
     return bls.Sign(privkey, signing_root)
 ```
 
-##### `AggregateAndProof`
-
-```python
-class AggregateAndProof(Container):
-    aggregator_index: ValidatorIndex
-    aggregate: Attestation
-    selection_proof: BLSSignature
-```
-
-##### `SignedAggregateAndProof`
-
-```python
-class SignedAggregateAndProof(Container):
-    message: AggregateAndProof
-    signature: BLSSignature
-```
-
 ## Phase 0 attestation subnet stability
 
 Because Phase 0 does not have shards and thus does not have Shard Committees, there is no stable backbone to the attestation subnets (`beacon_attestation_{subnet_id}`). To provide this stability, each validator must:
diff --git a/tests/core/pyspec/eth2spec/test/context.py b/tests/core/pyspec/eth2spec/test/context.py
index ddb01adcb6..caf44e983d 100644
--- a/tests/core/pyspec/eth2spec/test/context.py
+++ b/tests/core/pyspec/eth2spec/test/context.py
@@ -97,8 +97,7 @@ def deco(fn):
 
         def entry(*args, spec: Spec, phases: SpecForks, **kw):
             # make a key for the state
-            # genesis fork version separates configs during test-generation runtime.
-            key = (spec.fork, spec.GENESIS_FORK_VERSION, spec.__file__, balances_fn, threshold_fn)
+            key = (spec.fork, spec.CONFIG_NAME, spec.__file__, balances_fn, threshold_fn)
             global _custom_state_cache_dict
             if key not in _custom_state_cache_dict:
                 state = _prepare_state(balances_fn, threshold_fn, spec, phases)
diff --git a/tests/core/pyspec/eth2spec/test/helpers/genesis.py b/tests/core/pyspec/eth2spec/test/helpers/genesis.py
index 46bc62fe53..14fd9aa474 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/genesis.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/genesis.py
@@ -28,6 +28,11 @@ def create_genesis_state(spec, validator_balances, activation_threshold):
             deposit_count=len(validator_balances),
             block_hash=eth1_block_hash,
         ),
+        fork=spec.Fork(
+            previous_version=spec.GENESIS_FORK_VERSION,
+            current_version=spec.GENESIS_FORK_VERSION,
+            epoch=spec.GENESIS_EPOCH,
+        ),
         latest_block_header=spec.BeaconBlockHeader(body_root=spec.hash_tree_root(spec.BeaconBlockBody())),
         randao_mixes=[eth1_block_hash] * spec.EPOCHS_PER_HISTORICAL_VECTOR,
     )
diff --git a/tests/core/pyspec/eth2spec/test/phase0/genesis/test_initialization.py b/tests/core/pyspec/eth2spec/test/phase0/genesis/test_initialization.py
index 1ec40c56fc..41fbd77262 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/genesis/test_initialization.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/genesis/test_initialization.py
@@ -16,6 +16,13 @@ def get_post_altair_description(spec):
     return f"Although it's not phase 0, we may use {spec.fork} spec to start testnets."
 
 
+def eth1_init_data(eth1_block_hash, eth1_timestamp):
+    yield 'eth1', {
+        'eth1_block_hash': '0x' + eth1_block_hash.hex(),
+        'eth1_timestamp': int(eth1_timestamp),
+    }
+
+
 @with_all_phases
 @spec_test
 @single_phase
@@ -35,8 +42,7 @@ def test_initialize_beacon_state_from_eth1(spec):
     eth1_block_hash = b'\x12' * 32
     eth1_timestamp = spec.MIN_GENESIS_TIME
 
-    yield 'eth1_block_hash', eth1_block_hash
-    yield 'eth1_timestamp', eth1_timestamp
+    yield from eth1_init_data(eth1_block_hash, eth1_timestamp)
     yield 'deposits', deposits
 
     # initialize beacon_state
@@ -79,8 +85,7 @@ def test_initialize_beacon_state_some_small_balances(spec):
     eth1_block_hash = b'\x12' * 32
     eth1_timestamp = spec.MIN_GENESIS_TIME
 
-    yield 'eth1_block_hash', eth1_block_hash
-    yield 'eth1_timestamp', eth1_timestamp
+    yield from eth1_init_data(eth1_block_hash, eth1_timestamp)
     yield 'deposits', deposits
 
     # initialize beacon_state
@@ -136,8 +141,7 @@ def test_initialize_beacon_state_one_topup_activation(spec):
     eth1_block_hash = b'\x13' * 32
     eth1_timestamp = spec.MIN_GENESIS_TIME
 
-    yield 'eth1_block_hash', eth1_block_hash
-    yield 'eth1_timestamp', eth1_timestamp
+    yield from eth1_init_data(eth1_block_hash, eth1_timestamp)
     yield 'deposits', deposits
 
     # initialize beacon_state
@@ -165,8 +169,7 @@ def test_initialize_beacon_state_random_invalid_genesis(spec):
     eth1_block_hash = b'\x14' * 32
     eth1_timestamp = spec.MIN_GENESIS_TIME + 1
 
-    yield 'eth1_block_hash', eth1_block_hash
-    yield 'eth1_timestamp', eth1_timestamp
+    yield from eth1_init_data(eth1_block_hash, eth1_timestamp)
     yield 'deposits', deposits
 
     # initialize beacon_state
@@ -205,8 +208,7 @@ def test_initialize_beacon_state_random_valid_genesis(spec):
     eth1_block_hash = b'\x15' * 32
     eth1_timestamp = spec.MIN_GENESIS_TIME + 2
 
-    yield 'eth1_block_hash', eth1_block_hash
-    yield 'eth1_timestamp', eth1_timestamp
+    yield from eth1_init_data(eth1_block_hash, eth1_timestamp)
     yield 'deposits', deposits
 
     # initialize beacon_state
diff --git a/tests/core/pyspec/eth2spec/test/phase0/sanity/test_slots.py b/tests/core/pyspec/eth2spec/test/phase0/sanity/test_slots.py
index dd4f302ae1..198ada6b90 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/sanity/test_slots.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/sanity/test_slots.py
@@ -10,7 +10,7 @@ def test_slots_1(spec, state):
     yield 'pre', state
 
     slots = 1
-    yield 'slots', slots
+    yield 'slots', int(slots)
     spec.process_slots(state, state.slot + slots)
 
     yield 'post', state
@@ -23,7 +23,7 @@ def test_slots_1(spec, state):
 def test_slots_2(spec, state):
     yield 'pre', state
     slots = 2
-    yield 'slots', slots
+    yield 'slots', int(slots)
     spec.process_slots(state, state.slot + slots)
     yield 'post', state
 
@@ -33,7 +33,7 @@ def test_slots_2(spec, state):
 def test_empty_epoch(spec, state):
     yield 'pre', state
     slots = spec.SLOTS_PER_EPOCH
-    yield 'slots', slots
+    yield 'slots', int(slots)
     spec.process_slots(state, state.slot + slots)
     yield 'post', state
 
@@ -43,7 +43,7 @@ def test_empty_epoch(spec, state):
 def test_double_empty_epoch(spec, state):
     yield 'pre', state
     slots = spec.SLOTS_PER_EPOCH * 2
-    yield 'slots', slots
+    yield 'slots', int(slots)
     spec.process_slots(state, state.slot + slots)
     yield 'post', state
 
@@ -55,6 +55,6 @@ def test_over_epoch_boundary(spec, state):
         spec.process_slots(state, state.slot + (spec.SLOTS_PER_EPOCH // 2))
     yield 'pre', state
     slots = spec.SLOTS_PER_EPOCH
-    yield 'slots', slots
+    yield 'slots', int(slots)
     spec.process_slots(state, state.slot + slots)
     yield 'post', state
diff --git a/tests/formats/genesis/initialization.md b/tests/formats/genesis/initialization.md
index 7a791dcfa3..73630de51c 100644
--- a/tests/formats/genesis/initialization.md
+++ b/tests/formats/genesis/initialization.md
@@ -4,13 +4,13 @@ Tests the initialization of a genesis state based on Eth1 data.
 
 ## Test case format
 
-### `eth1_block_hash.ssz_snappy`
+### `eth1.yaml`
 
-An SSZ-snappy encoded root of the Eth1 block.
-
-### `eth1_timestamp.yaml`
+```yaml
+eth1_block_hash: Bytes32  -- A `Bytes32` hex encoded, with prefix 0x. The root of the Eth1 block. E.g. "0x4242424242424242424242424242424242424242424242424242424242424242"
+eth1_timestamp: int       -- An integer. The timestamp of the block, in seconds.
+```
 
-An integer. The timestamp of the block, in seconds.
 
 ### `meta.yaml`