diff --git a/configs/mainnet/altair.yaml b/configs/mainnet/altair.yaml
index 2a2552da4..475cea732 100644
--- a/configs/mainnet/altair.yaml
+++ b/configs/mainnet/altair.yaml
@@ -51,3 +51,9 @@ MIN_SYNC_COMMITTEE_PARTICIPANTS: 1
 MAX_VALID_LIGHT_CLIENT_UPDATES: 8192
 # 2**13 (=8192)
 LIGHT_CLIENT_UPDATE_TIMEOUT: 8192
+
+
+# Validator
+# ---------------------------------------------------------------
+# 2**2 (= 4)
+TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE: 4
diff --git a/configs/minimal/altair.yaml b/configs/minimal/altair.yaml
index b852e13c0..096941721 100644
--- a/configs/minimal/altair.yaml
+++ b/configs/minimal/altair.yaml
@@ -51,3 +51,8 @@ MIN_SYNC_COMMITTEE_PARTICIPANTS: 1
 MAX_VALID_LIGHT_CLIENT_UPDATES: 32
 # [customized]
 LIGHT_CLIENT_UPDATE_TIMEOUT: 32
+
+# Validator
+# ---------------------------------------------------------------
+# 2**2 (= 4)
+TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE: 4
diff --git a/setup.py b/setup.py
index 514f75c50..0a64307c0 100644
--- a/setup.py
+++ b/setup.py
@@ -695,7 +695,7 @@ setup(
         "py_ecc==5.2.0",
         "milagro_bls_binding==1.6.3",
         "dataclasses==0.6",
-        "remerkleable==0.1.18",
+        "remerkleable==0.1.19",
         "ruamel.yaml==0.16.5",
         "lru-dict==1.1.6",
     ]
diff --git a/tests/core/pyspec/eth2spec/gen_helpers/gen_from_tests/gen.py b/tests/core/pyspec/eth2spec/gen_helpers/gen_from_tests/gen.py
index c090869d5..bc63b212b 100644
--- a/tests/core/pyspec/eth2spec/gen_helpers/gen_from_tests/gen.py
+++ b/tests/core/pyspec/eth2spec/gen_helpers/gen_from_tests/gen.py
@@ -4,7 +4,8 @@ from typing import Any, Callable, Dict, Iterable, Optional
 
 from eth2spec.config import config_util
 from eth2spec.utils import bls
-from eth2spec.test.context import ALL_CONFIGS, TESTGEN_FORKS, SpecForkName, ConfigName
+from eth2spec.test.helpers.constants import ALL_CONFIGS, TESTGEN_FORKS
+from eth2spec.test.helpers.typing import SpecForkName, ConfigName
 
 from eth2spec.gen_helpers.gen_base import gen_runner
 from eth2spec.gen_helpers.gen_base.gen_typing import TestCase, TestProvider
diff --git a/tests/core/pyspec/eth2spec/test/altair/block_processing/test_process_sync_committee.py b/tests/core/pyspec/eth2spec/test/altair/block_processing/test_process_sync_committee.py
index ed9bcac9c..307d0f82d 100644
--- a/tests/core/pyspec/eth2spec/test/altair/block_processing/test_process_sync_committee.py
+++ b/tests/core/pyspec/eth2spec/test/altair/block_processing/test_process_sync_committee.py
@@ -8,12 +8,14 @@ from eth2spec.test.helpers.state import (
     state_transition_and_sign_block,
     transition_to,
 )
+from eth2spec.test.helpers.constants import (
+    PHASE0,
+    MAINNET, MINIMAL,
+)
 from eth2spec.test.helpers.sync_committee import (
     compute_aggregate_sync_committee_signature,
 )
 from eth2spec.test.context import (
-    PHASE0,
-    MAINNET, MINIMAL,
     expect_assertion_error,
     with_all_phases_except,
     with_configs,
diff --git a/tests/core/pyspec/eth2spec/test/altair/epoch_processing/test_process_sync_committee_updates.py b/tests/core/pyspec/eth2spec/test/altair/epoch_processing/test_process_sync_committee_updates.py
index 4c5ac0633..7ba2645a2 100644
--- a/tests/core/pyspec/eth2spec/test/altair/epoch_processing/test_process_sync_committee_updates.py
+++ b/tests/core/pyspec/eth2spec/test/altair/epoch_processing/test_process_sync_committee_updates.py
@@ -1,10 +1,12 @@
 from eth2spec.test.context import (
-    PHASE0,
-    MINIMAL,
     spec_state_test,
     with_all_phases_except,
     with_configs,
 )
+from eth2spec.test.helpers.constants import (
+    PHASE0,
+    MINIMAL,
+)
 from eth2spec.test.helpers.state import transition_to
 from eth2spec.test.helpers.epoch_processing import (
     run_epoch_processing_with,
diff --git a/tests/core/pyspec/eth2spec/test/altair/fork/test_fork.py b/tests/core/pyspec/eth2spec/test/altair/fork/test_fork.py
index fb056b72a..1ad39209c 100644
--- a/tests/core/pyspec/eth2spec/test/altair/fork/test_fork.py
+++ b/tests/core/pyspec/eth2spec/test/altair/fork/test_fork.py
@@ -1,6 +1,4 @@
 from eth2spec.test.context import (
-    PHASE0, ALTAIR,
-    MINIMAL,
     with_phases,
     with_custom_state,
     with_configs,
@@ -8,6 +6,10 @@ from eth2spec.test.context import (
     low_balances, misc_balances, large_validator_set,
 )
 from eth2spec.test.utils import with_meta_tags
+from eth2spec.test.helpers.constants import (
+    PHASE0, ALTAIR,
+    MINIMAL,
+)
 from eth2spec.test.helpers.state import (
     next_epoch,
     next_epoch_via_block,
diff --git a/tests/core/pyspec/eth2spec/test/altair/sanity/test_blocks.py b/tests/core/pyspec/eth2spec/test/altair/sanity/test_blocks.py
index 9f28926d4..48ab6956b 100644
--- a/tests/core/pyspec/eth2spec/test/altair/sanity/test_blocks.py
+++ b/tests/core/pyspec/eth2spec/test/altair/sanity/test_blocks.py
@@ -11,8 +11,8 @@ from eth2spec.test.helpers.block import (
 from eth2spec.test.helpers.sync_committee import (
     compute_aggregate_sync_committee_signature,
 )
+from eth2spec.test.helpers.constants import PHASE0
 from eth2spec.test.context import (
-    PHASE0,
     with_all_phases_except,
     spec_state_test,
 )
diff --git a/tests/core/pyspec/eth2spec/test/altair/unittests/test_helpers.py b/tests/core/pyspec/eth2spec/test/altair/unittests/test_helpers.py
index d89ff6d84..c837f06c3 100644
--- a/tests/core/pyspec/eth2spec/test/altair/unittests/test_helpers.py
+++ b/tests/core/pyspec/eth2spec/test/altair/unittests/test_helpers.py
@@ -1,8 +1,8 @@
 from eth2spec.test.context import (
     spec_state_test,
     with_phases,
-    ALTAIR,
 )
+from eth2spec.test.helpers.constants import ALTAIR
 from eth2spec.test.helpers.merkle import build_proof
 
 
diff --git a/tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py b/tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py
index 4c9b98e0a..9b8c35e76 100644
--- a/tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py
+++ b/tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py
@@ -1,6 +1,4 @@
 from eth2spec.test.context import (
-    ALTAIR,
-    MINIMAL,
     spec_state_test,
     with_configs,
     with_phases,
@@ -10,6 +8,10 @@ from eth2spec.test.helpers.block import (
     build_empty_block,
     build_empty_block_for_next_slot,
 )
+from eth2spec.test.helpers.constants import (
+    ALTAIR,
+    MINIMAL,
+)
 from eth2spec.test.helpers.state import (
     next_slots,
     state_transition_and_sign_block,
diff --git a/tests/core/pyspec/eth2spec/test/altair/unittests/validator/__init__.py b/tests/core/pyspec/eth2spec/test/altair/unittests/validator/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/core/pyspec/eth2spec/test/altair/unittests/validator/test_validator.py b/tests/core/pyspec/eth2spec/test/altair/unittests/validator/test_validator.py
new file mode 100644
index 000000000..244e1aa7e
--- /dev/null
+++ b/tests/core/pyspec/eth2spec/test/altair/unittests/validator/test_validator.py
@@ -0,0 +1,166 @@
+import random
+from collections import defaultdict
+from eth2spec.utils.ssz.ssz_typing import Bitvector
+from eth2spec.test.helpers.block import build_empty_block
+from eth2spec.test.helpers.keys import pubkey_to_privkey
+from eth2spec.test.helpers.state import transition_to
+from eth2spec.utils import bls
+from eth2spec.utils.bls import only_with_bls
+from eth2spec.test.context import (
+    PHASE0,
+    with_all_phases_except,
+    with_state,
+)
+
+rng = random.Random(1337)
+
+
+def ensure_assignments_in_sync_committee(
+    spec, state, epoch, sync_committee, active_pubkeys
+):
+    assert len(sync_committee.pubkeys) >= 3
+    some_pubkeys = rng.sample(sync_committee.pubkeys, 3)
+    for pubkey in some_pubkeys:
+        validator_index = active_pubkeys.index(pubkey)
+        assert spec.is_assigned_to_sync_committee(state, epoch, validator_index)
+
+
+@with_all_phases_except([PHASE0])
+@with_state
+def test_is_assigned_to_sync_committee(phases, spec, state):
+    epoch = spec.get_current_epoch(state)
+    validator_indices = spec.get_active_validator_indices(state, epoch)
+    validator_count = len(validator_indices)
+
+    query_epoch = epoch + 1
+    next_query_epoch = query_epoch + spec.EPOCHS_PER_SYNC_COMMITTEE_PERIOD
+    active_pubkeys = [state.validators[index].pubkey for index in validator_indices]
+
+    ensure_assignments_in_sync_committee(
+        spec, state, query_epoch, state.current_sync_committee, active_pubkeys
+    )
+    ensure_assignments_in_sync_committee(
+        spec, state, next_query_epoch, state.next_sync_committee, active_pubkeys
+    )
+
+    sync_committee_pubkeys = set(
+        list(state.current_sync_committee.pubkeys)
+        + list(state.next_sync_committee.pubkeys)
+    )
+    disqualified_pubkeys = set(
+        filter(lambda key: key not in sync_committee_pubkeys, active_pubkeys)
+    )
+    # NOTE: only check `disqualified_pubkeys` if SYNC_COMMITEE_SIZE < validator count
+    if disqualified_pubkeys:
+        sample_size = 3
+        assert validator_count >= sample_size
+        some_pubkeys = rng.sample(disqualified_pubkeys, sample_size)
+        for pubkey in some_pubkeys:
+            validator_index = active_pubkeys.index(pubkey)
+            is_current = spec.is_assigned_to_sync_committee(
+                state, query_epoch, validator_index
+            )
+            is_next = spec.is_assigned_to_sync_committee(
+                state, next_query_epoch, validator_index
+            )
+            is_current_or_next = is_current or is_next
+            assert not is_current_or_next
+
+
+def _get_sync_committee_signature(
+    spec,
+    state,
+    target_slot,
+    target_block_root,
+    subcommittee_index,
+    index_in_subcommittee,
+):
+    subcommittee_size = spec.SYNC_COMMITTEE_SIZE // spec.SYNC_COMMITTEE_SUBNET_COUNT
+    sync_committee_index = (
+        subcommittee_index * subcommittee_size + index_in_subcommittee
+    )
+    pubkey = state.current_sync_committee.pubkeys[sync_committee_index]
+    privkey = pubkey_to_privkey[pubkey]
+
+    domain = spec.get_domain(
+        state,
+        spec.DOMAIN_SYNC_COMMITTEE,
+    )
+    signing_data = spec.compute_signing_root(target_block_root, domain)
+    return bls.Sign(privkey, spec.hash_tree_root(signing_data))
+
+
+@only_with_bls()
+@with_all_phases_except([PHASE0])
+@with_state
+def test_process_sync_committee_contributions(phases, spec, state):
+    # skip over slots at genesis
+    transition_to(spec, state, state.slot + 3)
+
+    # build a block and attempt to assemble a sync aggregate
+    # from some sync committee contributions
+    block = build_empty_block(spec, state)
+    previous_slot = state.slot - 1
+    target_block_root = spec.get_block_root_at_slot(state, previous_slot)
+    aggregation_bits = Bitvector[
+        spec.SYNC_COMMITTEE_SIZE // spec.SYNC_COMMITTEE_SUBNET_COUNT
+    ]()
+    aggregation_index = 0
+    aggregation_bits[aggregation_index] = True
+
+    contributions = [
+        spec.SyncCommitteeContribution(
+            slot=block.slot,
+            beacon_block_root=target_block_root,
+            subcommittee_index=i,
+            aggregation_bits=aggregation_bits,
+            signature=_get_sync_committee_signature(
+                spec, state, previous_slot, target_block_root, i, aggregation_index
+            ),
+        )
+        for i in range(spec.SYNC_COMMITTEE_SUBNET_COUNT)
+    ]
+
+    # ensure the block has an empty sync aggregate...
+    empty_sync_aggregate = spec.SyncAggregate()
+    empty_sync_aggregate.sync_committee_signature = spec.G2_POINT_AT_INFINITY
+    assert block.body.sync_aggregate == empty_sync_aggregate
+    spec.process_sync_committee_contributions(block, set(contributions))
+
+    # and that after processing, it is no longer empty
+    assert len(block.body.sync_aggregate.sync_committee_bits) != 0
+    assert (
+        block.body.sync_aggregate.sync_committee_signature != spec.G2_POINT_AT_INFINITY
+    )
+    # moreover, ensure the sync aggregate is valid if the block is accepted
+    spec.process_block(state, block)
+
+
+def _validator_index_for_pubkey(state, pubkey):
+    return list(map(lambda v: v.pubkey, state.validators)).index(pubkey)
+
+
+def _subnet_for_sync_committee_index(spec, i):
+    return i // (spec.SYNC_COMMITTEE_SIZE // spec.SYNC_COMMITTEE_SUBNET_COUNT)
+
+
+@with_all_phases_except([PHASE0])
+@with_state
+def test_compute_subnets_for_sync_committee(state, spec, phases):
+    some_sync_committee_members = list(
+        (
+            _subnet_for_sync_committee_index(spec, i),
+            pubkey,
+        )
+        for i, pubkey in enumerate(state.current_sync_committee.pubkeys)
+    )
+
+    expected_subnets_by_pubkey = defaultdict(list)
+    for (subnet, pubkey) in some_sync_committee_members:
+        expected_subnets_by_pubkey[pubkey].append(subnet)
+
+    for _, pubkey in some_sync_committee_members:
+        validator_index = _validator_index_for_pubkey(state, pubkey)
+        subnets = spec.compute_subnets_for_sync_committee(state, validator_index)
+        expected_subnets = expected_subnets_by_pubkey[pubkey]
+        assert subnets == expected_subnets
diff --git a/tests/core/pyspec/eth2spec/test/context.py b/tests/core/pyspec/eth2spec/test/context.py
index 34b6e79a1..147793afe 100644
--- a/tests/core/pyspec/eth2spec/test/context.py
+++ b/tests/core/pyspec/eth2spec/test/context.py
@@ -6,11 +6,15 @@ from eth2spec.merge import spec as spec_merge
 from eth2spec.utils import bls
 
 from .exceptions import SkippedTest
+from .helpers.constants import (
+    PHASE0, ALTAIR, MERGE, SHARDING, CUSTODY_GAME, DAS,
+    ALL_PHASES,
+)
 from .helpers.genesis import create_genesis_state
 from .utils import vector_test, with_meta_tags
 
 from random import Random
-from typing import Any, Callable, NewType, Sequence, TypedDict, Protocol
+from typing import Any, Callable, Sequence, TypedDict, Protocol
 
 from lru import LRU
 
@@ -23,30 +27,6 @@ def reload_specs():
     reload(spec_merge)
 
 
-# Some of the Spec module functionality is exposed here to deal with phase-specific changes.
-
-SpecForkName = NewType("SpecForkName", str)
-ConfigName = NewType("ConfigName", str)
-
-PHASE0 = SpecForkName('phase0')
-ALTAIR = SpecForkName('altair')
-
-# Experimental phases (not included in default "ALL_PHASES"):
-MERGE = SpecForkName('merge')
-SHARDING = SpecForkName('sharding')
-CUSTODY_GAME = SpecForkName('custody_game')
-DAS = SpecForkName('das')
-
-ALL_PHASES = (PHASE0, ALTAIR)
-
-MAINNET = ConfigName('mainnet')
-MINIMAL = ConfigName('minimal')
-
-ALL_CONFIGS = (MINIMAL, MAINNET)
-
-# The forks that output to the test vectors.
-TESTGEN_FORKS = (PHASE0, ALTAIR)
-
 # TODO: currently phases are defined as python modules.
 # It would be better if they would be more well-defined interfaces for stronger typing.
 
diff --git a/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_attestation.py b/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_attestation.py
index 92633a8c5..707ac0b2e 100644
--- a/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_attestation.py
+++ b/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_attestation.py
@@ -1,9 +1,9 @@
 from eth2spec.test.context import (
-    CUSTODY_GAME,
     with_phases,
     spec_state_test,
     always_bls,
 )
+from eth2spec.test.helpers.constants import CUSTODY_GAME
 from eth2spec.test.helpers.state import transition_to
 from eth2spec.test.helpers.attestations import (
     run_attestation_processing,
diff --git a/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_chunk_challenge.py b/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_chunk_challenge.py
index 69143812f..5a939c108 100644
--- a/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_chunk_challenge.py
+++ b/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_chunk_challenge.py
@@ -6,10 +6,12 @@ from eth2spec.test.helpers.custody import (
 from eth2spec.test.helpers.attestations import (
     get_valid_on_time_attestation,
 )
-from eth2spec.test.helpers.state import transition_to, transition_to_valid_shard_slot
-from eth2spec.test.context import (
+from eth2spec.test.helpers.constants import (
     CUSTODY_GAME,
     MINIMAL,
+)
+from eth2spec.test.helpers.state import transition_to, transition_to_valid_shard_slot
+from eth2spec.test.context import (
     expect_assertion_error,
     disable_process_reveal_deadlines,
     spec_state_test,
diff --git a/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_custody_key_reveal.py b/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_custody_key_reveal.py
index a825d7c75..fa8401228 100644
--- a/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_custody_key_reveal.py
+++ b/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_custody_key_reveal.py
@@ -1,6 +1,6 @@
+from eth2spec.test.helpers.constants import CUSTODY_GAME
 from eth2spec.test.helpers.custody import get_valid_custody_key_reveal
 from eth2spec.test.context import (
-    CUSTODY_GAME,
     with_phases,
     spec_state_test,
     expect_assertion_error,
diff --git a/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_custody_slashing.py b/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_custody_slashing.py
index 12ac708aa..f554761f3 100644
--- a/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_custody_slashing.py
+++ b/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_custody_slashing.py
@@ -5,12 +5,14 @@ from eth2spec.test.helpers.custody import (
 from eth2spec.test.helpers.attestations import (
     get_valid_on_time_attestation,
 )
+from eth2spec.test.helpers.constants import (
+    CUSTODY_GAME,
+    MINIMAL,
+)
 from eth2spec.test.helpers.keys import privkeys
 from eth2spec.utils.ssz.ssz_typing import ByteList
 from eth2spec.test.helpers.state import get_balance, transition_to
 from eth2spec.test.context import (
-    MINIMAL,
-    CUSTODY_GAME,
     with_phases,
     spec_state_test,
     expect_assertion_error,
diff --git a/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_early_derived_secret_reveal.py b/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_early_derived_secret_reveal.py
index d3da6c580..904c9af4e 100644
--- a/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_early_derived_secret_reveal.py
+++ b/tests/core/pyspec/eth2spec/test/custody_game/block_processing/test_process_early_derived_secret_reveal.py
@@ -1,7 +1,7 @@
+from eth2spec.test.helpers.constants import CUSTODY_GAME
 from eth2spec.test.helpers.custody import get_valid_early_derived_secret_reveal
 from eth2spec.test.helpers.state import next_epoch_via_block, get_balance
 from eth2spec.test.context import (
-    CUSTODY_GAME,
     with_phases,
     spec_state_test,
     expect_assertion_error,
diff --git a/tests/core/pyspec/eth2spec/test/custody_game/epoch_processing/test_process_challenge_deadlines.py b/tests/core/pyspec/eth2spec/test/custody_game/epoch_processing/test_process_challenge_deadlines.py
index f0e353cec..42ff54303 100644
--- a/tests/core/pyspec/eth2spec/test/custody_game/epoch_processing/test_process_challenge_deadlines.py
+++ b/tests/core/pyspec/eth2spec/test/custody_game/epoch_processing/test_process_challenge_deadlines.py
@@ -7,13 +7,15 @@ from eth2spec.test.helpers.attestations import (
 )
 from eth2spec.test.helpers.state import transition_to, transition_to_valid_shard_slot
 from eth2spec.test.context import (
-    CUSTODY_GAME,
-    MINIMAL,
     spec_state_test,
     with_phases,
     with_configs,
 )
 from eth2spec.test.phase0.block_processing.test_process_attestation import run_attestation_processing
+from eth2spec.test.helpers.constants import (
+    CUSTODY_GAME,
+    MINIMAL,
+)
 from eth2spec.test.helpers.epoch_processing import run_epoch_processing_with
 
 from eth2spec.test.custody_game.block_processing.test_process_chunk_challenge import (
diff --git a/tests/core/pyspec/eth2spec/test/custody_game/epoch_processing/test_process_custody_final_updates.py b/tests/core/pyspec/eth2spec/test/custody_game/epoch_processing/test_process_custody_final_updates.py
index acc076bce..92c311a29 100644
--- a/tests/core/pyspec/eth2spec/test/custody_game/epoch_processing/test_process_custody_final_updates.py
+++ b/tests/core/pyspec/eth2spec/test/custody_game/epoch_processing/test_process_custody_final_updates.py
@@ -1,4 +1,4 @@
-from eth2spec.test.context import (
+from eth2spec.test.helpers.constants import (
     CUSTODY_GAME,
 )
 from eth2spec.test.helpers.custody import (
diff --git a/tests/core/pyspec/eth2spec/test/custody_game/epoch_processing/test_process_reveal_deadlines.py b/tests/core/pyspec/eth2spec/test/custody_game/epoch_processing/test_process_reveal_deadlines.py
index c82ebd8bb..aec24bca8 100644
--- a/tests/core/pyspec/eth2spec/test/custody_game/epoch_processing/test_process_reveal_deadlines.py
+++ b/tests/core/pyspec/eth2spec/test/custody_game/epoch_processing/test_process_reveal_deadlines.py
@@ -3,12 +3,14 @@ from eth2spec.test.helpers.custody import (
 )
 from eth2spec.test.helpers.state import transition_to
 from eth2spec.test.context import (
-    CUSTODY_GAME,
-    MINIMAL,
     with_phases,
     with_configs,
     spec_state_test,
 )
+from eth2spec.test.helpers.constants import (
+    CUSTODY_GAME,
+    MINIMAL,
+)
 from eth2spec.test.helpers.epoch_processing import run_epoch_processing_with
 from eth2spec.test.custody_game.block_processing.test_process_custody_key_reveal import (
     run_custody_key_reveal_processing,
diff --git a/tests/core/pyspec/eth2spec/test/custody_game/sanity/test_blocks.py b/tests/core/pyspec/eth2spec/test/custody_game/sanity/test_blocks.py
index 88dd54bda..5f5eadef3 100644
--- a/tests/core/pyspec/eth2spec/test/custody_game/sanity/test_blocks.py
+++ b/tests/core/pyspec/eth2spec/test/custody_game/sanity/test_blocks.py
@@ -1,14 +1,16 @@
 from typing import Dict, Sequence
 
 from eth2spec.test.context import (
-    CUSTODY_GAME,
-    MINIMAL,
     with_phases,
     spec_state_test,
     with_configs,
 )
 from eth2spec.test.helpers.attestations import get_valid_on_time_attestation
 from eth2spec.test.helpers.block import build_empty_block
+from eth2spec.test.helpers.constants import (
+    CUSTODY_GAME,
+    MINIMAL,
+)
 from eth2spec.test.helpers.custody import (
     get_custody_slashable_test_vector,
     get_valid_chunk_challenge,
diff --git a/tests/core/pyspec/eth2spec/test/helpers/constants.py b/tests/core/pyspec/eth2spec/test/helpers/constants.py
new file mode 100644
index 000000000..bffffe348
--- /dev/null
+++ b/tests/core/pyspec/eth2spec/test/helpers/constants.py
@@ -0,0 +1,29 @@
+from .typing import SpecForkName, ConfigName
+
+
+#
+# SpecForkName
+#
+# Some of the Spec module functionality is exposed here to deal with phase-specific changes.
+PHASE0 = SpecForkName('phase0')
+ALTAIR = SpecForkName('altair')
+
+# Experimental phases (not included in default "ALL_PHASES"):
+MERGE = SpecForkName('merge')
+SHARDING = SpecForkName('sharding')
+CUSTODY_GAME = SpecForkName('custody_game')
+DAS = SpecForkName('das')
+
+# The forks that pytest runs with.
+ALL_PHASES = (PHASE0, ALTAIR)
+# The forks that output to the test vectors.
+TESTGEN_FORKS = (PHASE0, ALTAIR)
+
+
+#
+# Config
+#
+MAINNET = ConfigName('mainnet')
+MINIMAL = ConfigName('minimal')
+
+ALL_CONFIGS = (MINIMAL, MAINNET)
diff --git a/tests/core/pyspec/eth2spec/test/helpers/typing.py b/tests/core/pyspec/eth2spec/test/helpers/typing.py
new file mode 100644
index 000000000..04578f64c
--- /dev/null
+++ b/tests/core/pyspec/eth2spec/test/helpers/typing.py
@@ -0,0 +1,4 @@
+from typing import NewType
+
+SpecForkName = NewType("SpecForkName", str)
+ConfigName = NewType("ConfigName", str)
diff --git a/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_participation_record_updates.py b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_participation_record_updates.py
index 978ef5739..c4d4332bc 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_participation_record_updates.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_participation_record_updates.py
@@ -1,4 +1,5 @@
-from eth2spec.test.context import PHASE0, spec_state_test, with_phases
+from eth2spec.test.context import spec_state_test, with_phases
+from eth2spec.test.helpers.constants import PHASE0
 from eth2spec.test.helpers.epoch_processing import (
     run_epoch_processing_with
 )
diff --git a/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py b/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py
index c598a3a7f..0822d44c2 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py
@@ -1,7 +1,6 @@
 from eth_utils import encode_hex
 
 from eth2spec.test.context import (
-    MINIMAL,
     is_post_altair,
     spec_state_test,
     with_all_phases,
@@ -9,6 +8,7 @@ from eth2spec.test.context import (
 )
 from eth2spec.test.helpers.attestations import get_valid_attestation, next_epoch_with_attestations
 from eth2spec.test.helpers.block import build_empty_block_for_next_slot
+from eth2spec.test.helpers.constants import MINIMAL
 from eth2spec.test.helpers.fork_choice import (
     tick_and_run_on_attestation,
     tick_and_run_on_block,
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 41fbd7726..be9d120b4 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/genesis/test_initialization.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/genesis/test_initialization.py
@@ -1,11 +1,11 @@
 from eth2spec.test.context import (
-    MINIMAL,
     is_post_altair,
     single_phase,
     spec_test,
     with_configs,
     with_all_phases,
 )
+from eth2spec.test.helpers.constants import MINIMAL
 from eth2spec.test.helpers.deposits import (
     prepare_full_genesis_deposits,
     prepare_random_genesis_deposits,
diff --git a/tests/core/pyspec/eth2spec/test/phase0/genesis/test_validity.py b/tests/core/pyspec/eth2spec/test/phase0/genesis/test_validity.py
index a7bcaa6d0..91148da1d 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/genesis/test_validity.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/genesis/test_validity.py
@@ -1,11 +1,11 @@
 from eth2spec.test.context import (
-    MINIMAL,
     is_post_altair,
     spec_test,
     single_phase,
     with_configs,
     with_all_phases,
 )
+from eth2spec.test.helpers.constants import MINIMAL
 from eth2spec.test.helpers.deposits import (
     prepare_full_genesis_deposits,
 )
diff --git a/tests/core/pyspec/eth2spec/test/phase0/rewards/test_basic.py b/tests/core/pyspec/eth2spec/test/phase0/rewards/test_basic.py
index 82dbccc52..511849f8f 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/rewards/test_basic.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/rewards/test_basic.py
@@ -1,4 +1,5 @@
-from eth2spec.test.context import PHASE0, with_all_phases, with_phases, spec_state_test
+from eth2spec.test.context import with_all_phases, with_phases, spec_state_test
+from eth2spec.test.helpers.constants import PHASE0
 import eth2spec.test.helpers.rewards as rewards_helpers
 
 
diff --git a/tests/core/pyspec/eth2spec/test/phase0/rewards/test_leak.py b/tests/core/pyspec/eth2spec/test/phase0/rewards/test_leak.py
index f11b2734f..0ad2491f0 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/rewards/test_leak.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/rewards/test_leak.py
@@ -1,4 +1,5 @@
-from eth2spec.test.context import PHASE0, with_all_phases, with_phases, spec_state_test
+from eth2spec.test.context import with_all_phases, with_phases, spec_state_test
+from eth2spec.test.helpers.constants import PHASE0
 from eth2spec.test.helpers.rewards import leaking
 import eth2spec.test.helpers.rewards as rewards_helpers
 
diff --git a/tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py b/tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py
index 3efe07b01..9b184b018 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py
@@ -24,9 +24,8 @@ from eth2spec.test.helpers.multi_operations import (
     run_slash_and_exit,
     run_test_full_random_operations,
 )
-
+from eth2spec.test.helpers.constants import PHASE0, MINIMAL
 from eth2spec.test.context import (
-    PHASE0, MINIMAL,
     spec_test, spec_state_test, dump_skipping_message,
     with_phases, with_all_phases, single_phase,
     expect_assertion_error, always_bls,
diff --git a/tests/core/pyspec/eth2spec/test/phase0/unittests/fork_choice/test_on_attestation.py b/tests/core/pyspec/eth2spec/test/phase0/unittests/fork_choice/test_on_attestation.py
index 4d5f1ca16..fcc5e2d21 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/unittests/fork_choice/test_on_attestation.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/unittests/fork_choice/test_on_attestation.py
@@ -1,6 +1,7 @@
-from eth2spec.test.context import PHASE0, ALTAIR, with_all_phases, spec_state_test
+from eth2spec.test.context import with_all_phases, spec_state_test
 from eth2spec.test.helpers.block import build_empty_block_for_next_slot
 from eth2spec.test.helpers.attestations import get_valid_attestation, sign_attestation
+from eth2spec.test.helpers.constants import PHASE0, ALTAIR
 from eth2spec.test.helpers.state import transition_to, state_transition_and_sign_block, next_epoch, next_slot
 from eth2spec.test.helpers.fork_choice import get_genesis_forkchoice_store
 
diff --git a/tests/core/pyspec/eth2spec/test/phase0/unittests/validator/test_validator_unittest.py b/tests/core/pyspec/eth2spec/test/phase0/unittests/validator/test_validator_unittest.py
index d793db789..fb63839d6 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/unittests/validator/test_validator_unittest.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/unittests/validator/test_validator_unittest.py
@@ -1,8 +1,8 @@
 from eth2spec.test.context import (
     spec_state_test,
     always_bls, with_phases, with_all_phases,
-    PHASE0,
 )
+from eth2spec.test.helpers.constants import PHASE0
 from eth2spec.test.helpers.attestations import build_attestation_data, get_valid_attestation
 from eth2spec.test.helpers.block import build_empty_block
 from eth2spec.test.helpers.deposits import prepare_state_and_deposit
diff --git a/tests/core/pyspec/eth2spec/test/sharding/unittests/test_get_start_shard.py b/tests/core/pyspec/eth2spec/test/sharding/unittests/test_get_start_shard.py
index 41ef754b3..c38f28b2b 100644
--- a/tests/core/pyspec/eth2spec/test/sharding/unittests/test_get_start_shard.py
+++ b/tests/core/pyspec/eth2spec/test/sharding/unittests/test_get_start_shard.py
@@ -1,8 +1,8 @@
 from eth2spec.test.context import (
-    SHARDING,
     with_phases,
     spec_state_test,
 )
+from eth2spec.test.helpers.constants import SHARDING
 from eth2spec.test.helpers.state import next_epoch
 
 
diff --git a/tests/generators/README.md b/tests/generators/README.md
index 5e12e6bfe..731184326 100644
--- a/tests/generators/README.md
+++ b/tests/generators/README.md
@@ -164,7 +164,7 @@ Another example, to generate tests from pytests:
 ```python
 from eth2spec.phase0 import spec as spec_phase0
 from eth2spec.altair import spec as spec_altair
-from eth2spec.test.context import PHASE0, ALTAIR
+from eth2spec.test.helpers.constants import PHASE0, ALTAIR
 
 from eth2spec.gen_helpers.gen_from_tests.gen import run_state_test_generators
 
diff --git a/tests/generators/bls/main.py b/tests/generators/bls/main.py
index fecc2df7d..3d67c9db3 100644
--- a/tests/generators/bls/main.py
+++ b/tests/generators/bls/main.py
@@ -12,7 +12,7 @@ from eth_utils import (
 import milagro_bls_binding as milagro_bls
 
 from eth2spec.utils import bls
-from eth2spec.test.context import PHASE0
+from eth2spec.test.helpers.constants import PHASE0
 from eth2spec.gen_helpers.gen_base import gen_runner, gen_typing
 
 
diff --git a/tests/generators/epoch_processing/main.py b/tests/generators/epoch_processing/main.py
index e3caf69fa..9efd96534 100644
--- a/tests/generators/epoch_processing/main.py
+++ b/tests/generators/epoch_processing/main.py
@@ -1,7 +1,7 @@
 from eth2spec.gen_helpers.gen_from_tests.gen import run_state_test_generators
 from eth2spec.phase0 import spec as spec_phase0
 from eth2spec.altair import spec as spec_altair
-from eth2spec.test.context import PHASE0, ALTAIR
+from eth2spec.test.helpers.constants import PHASE0, ALTAIR
 
 
 specs = (spec_phase0, spec_altair)
diff --git a/tests/generators/finality/main.py b/tests/generators/finality/main.py
index 29ab46c39..e50425da4 100644
--- a/tests/generators/finality/main.py
+++ b/tests/generators/finality/main.py
@@ -1,7 +1,7 @@
 from eth2spec.gen_helpers.gen_from_tests.gen import run_state_test_generators
 from eth2spec.phase0 import spec as spec_phase0
 from eth2spec.altair import spec as spec_altair
-from eth2spec.test.context import PHASE0, ALTAIR
+from eth2spec.test.helpers.constants import PHASE0, ALTAIR
 
 
 specs = (spec_phase0, spec_altair)
diff --git a/tests/generators/fork_choice/main.py b/tests/generators/fork_choice/main.py
index f09bbcc0a..445ee629c 100644
--- a/tests/generators/fork_choice/main.py
+++ b/tests/generators/fork_choice/main.py
@@ -1,7 +1,7 @@
 from eth2spec.gen_helpers.gen_from_tests.gen import run_state_test_generators
 from eth2spec.phase0 import spec as spec_phase0
 from eth2spec.altair import spec as spec_altair
-from eth2spec.test.context import PHASE0, ALTAIR
+from eth2spec.test.helpers.constants import PHASE0, ALTAIR
 
 
 specs = (spec_phase0, spec_altair)
diff --git a/tests/generators/forks/main.py b/tests/generators/forks/main.py
index 3bec19735..71a4a54de 100644
--- a/tests/generators/forks/main.py
+++ b/tests/generators/forks/main.py
@@ -1,7 +1,7 @@
 from importlib import reload
 from typing import Iterable
 
-from eth2spec.test.context import PHASE0, ALTAIR, MINIMAL, MAINNET
+from eth2spec.test.helpers.constants import PHASE0, ALTAIR, MINIMAL, MAINNET
 from eth2spec.config import config_util
 from eth2spec.test.altair.fork import test_fork as test_altair_forks
 from eth2spec.phase0 import spec as spec_phase0
diff --git a/tests/generators/genesis/main.py b/tests/generators/genesis/main.py
index 362677c24..3b4d045a3 100644
--- a/tests/generators/genesis/main.py
+++ b/tests/generators/genesis/main.py
@@ -1,7 +1,7 @@
 from eth2spec.gen_helpers.gen_from_tests.gen import run_state_test_generators
 from eth2spec.phase0 import spec as spec_phase0
 from eth2spec.altair import spec as spec_altair
-from eth2spec.test.context import PHASE0, ALTAIR
+from eth2spec.test.helpers.constants import PHASE0, ALTAIR
 
 
 specs = (spec_phase0, spec_altair)
diff --git a/tests/generators/operations/main.py b/tests/generators/operations/main.py
index f00c4f894..0c1b84c24 100644
--- a/tests/generators/operations/main.py
+++ b/tests/generators/operations/main.py
@@ -1,7 +1,7 @@
 from eth2spec.gen_helpers.gen_from_tests.gen import run_state_test_generators
 from eth2spec.phase0 import spec as spec_phase0
 from eth2spec.altair import spec as spec_altair
-from eth2spec.test.context import PHASE0, ALTAIR
+from eth2spec.test.helpers.constants import PHASE0, ALTAIR
 
 
 specs = (spec_phase0, spec_altair)
diff --git a/tests/generators/rewards/main.py b/tests/generators/rewards/main.py
index d8115f4cb..2a0f14863 100644
--- a/tests/generators/rewards/main.py
+++ b/tests/generators/rewards/main.py
@@ -1,7 +1,7 @@
 from eth2spec.gen_helpers.gen_from_tests.gen import run_state_test_generators
 from eth2spec.phase0 import spec as spec_phase0
 from eth2spec.altair import spec as spec_altair
-from eth2spec.test.context import PHASE0, ALTAIR
+from eth2spec.test.helpers.constants import PHASE0, ALTAIR
 
 
 specs = (spec_phase0, spec_altair)
diff --git a/tests/generators/sanity/main.py b/tests/generators/sanity/main.py
index 90f9a00df..7f7fecc67 100644
--- a/tests/generators/sanity/main.py
+++ b/tests/generators/sanity/main.py
@@ -1,6 +1,6 @@
 from eth2spec.phase0 import spec as spec_phase0
 from eth2spec.altair import spec as spec_altair
-from eth2spec.test.context import PHASE0, ALTAIR
+from eth2spec.test.helpers.constants import PHASE0, ALTAIR
 
 from eth2spec.gen_helpers.gen_from_tests.gen import run_state_test_generators
 
diff --git a/tests/generators/shuffling/main.py b/tests/generators/shuffling/main.py
index 091207bca..73b714760 100644
--- a/tests/generators/shuffling/main.py
+++ b/tests/generators/shuffling/main.py
@@ -6,7 +6,7 @@ from eth2spec.gen_helpers.gen_base import gen_runner, gen_typing
 
 from eth2spec.config import config_util
 from eth2spec.phase0 import spec as spec
-from eth2spec.test.context import PHASE0
+from eth2spec.test.helpers.constants import PHASE0
 
 
 def shuffling_case_fn(seed, count):
diff --git a/tests/generators/ssz_generic/main.py b/tests/generators/ssz_generic/main.py
index 737f8cda6..e6cd3d976 100644
--- a/tests/generators/ssz_generic/main.py
+++ b/tests/generators/ssz_generic/main.py
@@ -6,7 +6,7 @@ import ssz_bitvector
 import ssz_boolean
 import ssz_uints
 import ssz_container
-from eth2spec.test.context import PHASE0
+from eth2spec.test.helpers.constants import PHASE0
 
 
 def create_provider(handler_name: str, suite_name: str, case_maker) -> gen_typing.TestProvider:
diff --git a/tests/generators/ssz_static/main.py b/tests/generators/ssz_static/main.py
index 77a88a8cd..617fb2f02 100644
--- a/tests/generators/ssz_static/main.py
+++ b/tests/generators/ssz_static/main.py
@@ -9,7 +9,7 @@ from eth2spec.debug import random_value, encode
 from eth2spec.config import config_util
 from eth2spec.phase0 import spec as spec_phase0
 from eth2spec.altair import spec as spec_altair
-from eth2spec.test.context import ALTAIR, TESTGEN_FORKS, MINIMAL, MAINNET
+from eth2spec.test.helpers.constants import ALTAIR, TESTGEN_FORKS, MINIMAL, MAINNET
 from eth2spec.utils.ssz.ssz_typing import Container
 from eth2spec.utils.ssz.ssz_impl import (
     hash_tree_root,