diff --git a/configs/README.md b/configs/README.md
index be3c60e6f..353cd35db 100644
--- a/configs/README.md
+++ b/configs/README.md
@@ -15,7 +15,7 @@ Over time, the need to sync an older state may be deprecated.
 In this case, the prefix on the new constant may be removed, and the old constant will keep a special name before completely being removed.
 
 A previous iteration of forking made use of "timelines", but this collides with the definitions used in the spec (constants for special forking slots, etc.), and was not integrated sufficiently in any of the spec tools or implementations.
-Instead, the config essentially doubles as fork definition now, e.g. changing the value for `PHASE_1_GENESIS_SLOT` changes the fork.
+Instead, the config essentially doubles as fork definition now, e.g. changing the value for `PHASE_1_FORK_SLOT` changes the fork.
 
 Another reason to prefer forking through constants is the ability to program a forking moment based on context, instead of being limited to a static slot number.
 
diff --git a/configs/mainnet/phase1.yaml b/configs/mainnet/phase1.yaml
index 8c0800957..a9c7f89cc 100644
--- a/configs/mainnet/phase1.yaml
+++ b/configs/mainnet/phase1.yaml
@@ -5,7 +5,7 @@
 # ---------------------------------------------------------------
 PHASE_1_FORK_VERSION: 0x01000000
 # [STUB]
-PHASE_1_GENESIS_SLOT: 32
+PHASE_1_FORK_SLOT: 0
 INITIAL_ACTIVE_SHARDS: 64
 
 
diff --git a/configs/minimal/phase1.yaml b/configs/minimal/phase1.yaml
index 7fbc7e5a3..09e288900 100644
--- a/configs/minimal/phase1.yaml
+++ b/configs/minimal/phase1.yaml
@@ -5,8 +5,8 @@
 # ---------------------------------------------------------------
 # [customized] for testnet distinction
 PHASE_1_FORK_VERSION: 0x01000001
-# [customized] for testing
-PHASE_1_GENESIS_SLOT: 8
+# [STUB]
+PHASE_1_FORK_SLOT: 0
 # [customized] reduced for testing
 INITIAL_ACTIVE_SHARDS: 2
 
diff --git a/specs/phase1/beacon-chain.md b/specs/phase1/beacon-chain.md
index 9fa04336c..b64c31a57 100644
--- a/specs/phase1/beacon-chain.md
+++ b/specs/phase1/beacon-chain.md
@@ -760,20 +760,24 @@ def validate_attestation(state: BeaconState, attestation: Attestation) -> None:
     committee = get_beacon_committee(state, data.slot, data.index)
     assert len(attestation.aggregation_bits) == len(committee)
 
-    if attestation.data.target.epoch == get_current_epoch(state):
-        assert attestation.data.source == state.current_justified_checkpoint
+    if data.target.epoch == get_current_epoch(state):
+        assert data.source == state.current_justified_checkpoint
     else:
-        assert attestation.data.source == state.previous_justified_checkpoint
+        assert data.source == state.previous_justified_checkpoint
 
     # Type 1: on-time attestations
-    if is_on_time_attestation(state, attestation.data):
+    if is_on_time_attestation(state, data):
         # Correct parent block root
         assert data.beacon_block_root == get_block_root_at_slot(state, compute_previous_slot(state.slot))
         # Correct shard number
-        shard = compute_shard_from_committee_index(state, attestation.data.index, attestation.data.slot)
-        assert attestation.data.shard == shard
-        # On-time attestations should have a non-empty shard transition root
-        assert attestation.data.shard_transition_root != hash_tree_root(ShardTransition())
+        shard = compute_shard_from_committee_index(state, data.index, data.slot)
+        assert data.shard == shard
+        # NOTE: We currently set `PHASE_1_FORK_SLOT` to `GENESIS_SLOT` for test vectors.
+        if data.slot > GENESIS_SLOT:
+            # On-time attestations should have a non-empty shard transition root
+            assert data.shard_transition_root != hash_tree_root(ShardTransition())
+        else:
+            assert data.shard_transition_root == hash_tree_root(ShardTransition())
     # Type 2: no shard transition
     else:
         # Ensure delayed attestation
@@ -811,7 +815,7 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
 ```python
 def apply_shard_transition(state: BeaconState, shard: Shard, transition: ShardTransition) -> None:
     # TODO: only need to check it once when phase 1 starts
-    assert state.slot > PHASE_1_GENESIS_SLOT
+    assert state.slot > PHASE_1_FORK_SLOT
 
     # Correct data root count
     offset_slots = get_offset_slots(state, shard)
@@ -976,8 +980,11 @@ def verify_empty_shard_transition(state: BeaconState, shard_transitions: Sequenc
 def process_shard_transitions(state: BeaconState,
                               shard_transitions: Sequence[ShardTransition],
                               attestations: Sequence[Attestation]) -> None:
-    # Process crosslinks
-    process_crosslinks(state, shard_transitions, attestations)
+    # NOTE: We currently set `PHASE_1_FORK_SLOT` to `GENESIS_SLOT` for test vectors.
+    if compute_previous_slot(state.slot) > GENESIS_SLOT:
+        # Process crosslinks
+        process_crosslinks(state, shard_transitions, attestations)
+
     # Verify the empty proposal shard states
     assert verify_empty_shard_transition(state, shard_transitions)
 ```
diff --git a/specs/phase1/phase1-fork.md b/specs/phase1/phase1-fork.md
index e83e9ef4a..e3949aca8 100644
--- a/specs/phase1/phase1-fork.md
+++ b/specs/phase1/phase1-fork.md
@@ -35,18 +35,18 @@ Warning: this configuration is not definitive.
 | Name | Value |
 | - | - |
 | `PHASE_1_FORK_VERSION` | `Version('0x01000000')` |
-| `PHASE_1_GENESIS_SLOT` | `2**5` **TBD** |
+| `PHASE_1_FORK_SLOT` | `Slot(0)` **TBD** |
 | `INITIAL_ACTIVE_SHARDS` | `2**6` (= 64) |
 
 ## Fork to Phase 1
 
 ### Fork trigger
 
-TBD. Social consensus, along with state conditions such as epoch boundary, finality, deposits, active validator count, etc. may be part of the decision process to trigger the fork. For now we assume the condition will be triggered at slot `PHASE_1_GENESIS_SLOT`, where `PHASE_1_GENESIS_SLOT % SLOTS_PER_EPOCH == 0`.
+TBD. Social consensus, along with state conditions such as epoch boundary, finality, deposits, active validator count, etc. may be part of the decision process to trigger the fork. For now we assume the condition will be triggered at slot `PHASE_1_FORK_SLOT`, where `PHASE_1_FORK_SLOT % SLOTS_PER_EPOCH == 0`.
 
 ### Upgrading the state
 
-After `process_slots` of Phase 0 finishes, if `state.slot == PHASE_1_GENESIS_SLOT`, an irregular state change is made to upgrade to Phase 1.
+After `process_slots` of Phase 0 finishes, if `state.slot == PHASE_1_FORK_SLOT`, an irregular state change is made to upgrade to Phase 1.
 
 ```python
 def upgrade_to_phase1(pre: phase0.BeaconState) -> BeaconState:
@@ -102,7 +102,7 @@ def upgrade_to_phase1(pre: phase0.BeaconState) -> BeaconState:
         current_epoch_start_shard=Shard(0),
         shard_states=List[ShardState, MAX_SHARDS](
             ShardState(
-                slot=pre.slot,
+                slot=compute_previous_slot(pre.slot),
                 gasprice=MIN_GASPRICE,
                 latest_block_root=Root(),
             ) for i in range(INITIAL_ACTIVE_SHARDS)
diff --git a/specs/phase1/shard-fork-choice.md b/specs/phase1/shard-fork-choice.md
index 8416009d7..bdc6ab8f4 100644
--- a/specs/phase1/shard-fork-choice.md
+++ b/specs/phase1/shard-fork-choice.md
@@ -48,7 +48,7 @@ def get_forkchoice_shard_store(anchor_state: BeaconState, shard: Shard) -> Shard
         shard=shard,
         signed_blocks={
             anchor_state.shard_states[shard].latest_block_root: SignedShardBlock(
-                message=ShardBlock(slot=anchor_state.slot, shard=shard)
+                message=ShardBlock(slot=compute_previous_slot(anchor_state.slot), shard=shard)
             )
         },
         block_states={anchor_state.shard_states[shard].latest_block_root: anchor_state.copy().shard_states[shard]},
diff --git a/specs/phase1/validator.md b/specs/phase1/validator.md
index ad2fa09e3..8f54e56c0 100644
--- a/specs/phase1/validator.md
+++ b/specs/phase1/validator.md
@@ -31,7 +31,7 @@
     - [`FullAttestation`](#fullattestation)
     - [Timing](#timing)
     - [Attestation data](#attestation-data)
-      - [Head shard root](#head-shard-root)
+      - [Shard head root](#shard-head-root)
       - [Shard transition](#shard-transition)
     - [Construct attestation](#construct-attestation)
   - [Attestation Aggregation](#attestation-aggregation)
@@ -267,9 +267,9 @@ A validator should create and broadcast the `attestation` to the associated atte
 
 *Note*: We assume that the fork choice only follows branches with valid `offset_slots` with respect to the most recent beacon state shard transition for the queried shard.
 
-##### Head shard root
+##### Shard head root
 
-Set `attestation_data.shard_head_root = hash_tree_root(shard_head_block)`.
+If `attestation_data.slot == GENESIS_SLOT`, set `attestation_data.shard_head_root = Root()`. Otherwise, set `attestation_data.shard_head_root = hash_tree_root(shard_head_block)`.
 
 ##### Shard transition
 
@@ -310,6 +310,10 @@ def get_shard_transition_fields(
 def get_shard_transition(beacon_state: BeaconState,
                          shard: Shard,
                          shard_blocks: Sequence[SignedShardBlock]) -> ShardTransition:
+    # NOTE: We currently set `PHASE_1_FORK_SLOT` to `GENESIS_SLOT` for test vectors.
+    if beacon_state.slot == GENESIS_SLOT:
+        return ShardTransition()
+
     offset_slots = compute_offset_slots(
         get_latest_slot_for_shard(beacon_state, shard),
         Slot(beacon_state.slot + 1),
diff --git a/tests/core/pyspec/eth2spec/test/context.py b/tests/core/pyspec/eth2spec/test/context.py
index 0d953cf24..68696ea0a 100644
--- a/tests/core/pyspec/eth2spec/test/context.py
+++ b/tests/core/pyspec/eth2spec/test/context.py
@@ -63,9 +63,6 @@ def _prepare_state(balances_fn: Callable[[Any], Sequence[int]], threshold_fn: Ca
         # TODO: instead of upgrading a test phase0 genesis state we can also write a phase1 state helper.
         # Decide based on performance/consistency results later.
         state = phases[PHASE1].upgrade_to_phase1(state)
-        # Shard state slot must lag behind BeaconState slot by at least 1
-        # Will handle this more elegantly with fork mechanics
-        spec.process_slots(state, state.slot + 1)
 
     return state
 
diff --git a/tests/core/pyspec/eth2spec/test/helpers/attestations.py b/tests/core/pyspec/eth2spec/test/helpers/attestations.py
index 3445f9ff6..ae90fc808 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/attestations.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/attestations.py
@@ -69,7 +69,7 @@ def build_attestation_data(spec, state, slot, index, shard=None, shard_transitio
         source_epoch = state.current_justified_checkpoint.epoch
         source_root = state.current_justified_checkpoint.root
 
-    attestation_data = spec.AttestationData(
+    data = spec.AttestationData(
         slot=slot,
         index=index,
         beacon_block_root=block_root,
@@ -79,23 +79,27 @@ def build_attestation_data(spec, state, slot, index, shard=None, shard_transitio
 
     if spec.fork == PHASE1:
         if shard is None:
-            shard = spec.compute_shard_from_committee_index(state, attestation_data.index, attestation_data.slot)
-        attestation_data.shard = shard
+            shard = spec.compute_shard_from_committee_index(state, data.index, data.slot)
+        data.shard = shard
 
         if shard_transition is not None:
             last_offset_index = len(shard_transition.shard_data_roots) - 1
-            attestation_data.shard_head_root = shard_transition.shard_states[last_offset_index].latest_block_root
-            attestation_data.shard_transition_root = shard_transition.hash_tree_root()
+            data.shard_head_root = shard_transition.shard_states[last_offset_index].latest_block_root
+            data.shard_transition_root = shard_transition.hash_tree_root()
         else:
             if on_time:
-                shard_transition = spec.get_shard_transition(state, shard, shard_blocks=[])
-                last_offset_index = len(shard_transition.shard_data_roots) - 1
-                attestation_data.shard_head_root = shard_transition.shard_states[last_offset_index].latest_block_root
-                attestation_data.shard_transition_root = shard_transition.hash_tree_root()
+                if data.slot == spec.GENESIS_SLOT:
+                    data.shard_head_root = spec.Root()
+                    data.shard_transition_root = spec.ShardTransition().hash_tree_root()
+                else:
+                    shard_transition = spec.get_shard_transition(state, shard, shard_blocks=[])
+                    last_offset_index = len(shard_transition.shard_data_roots) - 1
+                    data.shard_head_root = shard_transition.shard_states[last_offset_index].latest_block_root
+                    data.shard_transition_root = shard_transition.hash_tree_root()
             else:
-                attestation_data.shard_head_root = state.shard_states[shard].latest_block_root
-                attestation_data.shard_transition_root = spec.Root()
-    return attestation_data
+                data.shard_head_root = state.shard_states[shard].latest_block_root
+                data.shard_transition_root = spec.Root()
+    return data
 
 
 def get_valid_on_time_attestation(spec, state, slot=None, index=None, shard_transition=None, signed=False):
diff --git a/tests/core/pyspec/eth2spec/test/helpers/state.py b/tests/core/pyspec/eth2spec/test/helpers/state.py
index c70a47742..f53d8f711 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/state.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/state.py
@@ -42,12 +42,10 @@ def transition_to_slot_via_block(spec, state, slot):
 
 def transition_to_valid_shard_slot(spec, state):
     """
-    Transition to slot `spec.PHASE_1_GENESIS_SLOT + 1` and fork at `spec.PHASE_1_GENESIS_SLOT`.
+    Transition to slot `spec.PHASE_1_FORK_SLOT + 1` and fork at `spec.PHASE_1_FORK_SLOT`.
     """
-    transition_to(spec, state, spec.PHASE_1_GENESIS_SLOT)
-    state = spec.upgrade_to_phase1(state)  # `upgrade_to_phase1` is a pure function
+    transition_to(spec, state, spec.PHASE_1_FORK_SLOT)
     next_slot(spec, state)
-    return state
 
 
 def next_epoch(spec, state):
diff --git a/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_attester_slashing.py b/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_attester_slashing.py
index 192e0390d..5f26ba5f7 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_attester_slashing.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_attester_slashing.py
@@ -1,6 +1,5 @@
 from eth2spec.test.context import (
-    PHASE0,
-    spec_state_test, expect_assertion_error, always_bls, with_all_phases, with_phases
+    spec_state_test, expect_assertion_error, always_bls, with_all_phases
 )
 from eth2spec.test.helpers.attestations import sign_indexed_attestation
 from eth2spec.test.helpers.attester_slashings import get_valid_attester_slashing, \
@@ -197,7 +196,7 @@ def test_participants_already_slashed(spec, state):
 # Some of the following tests are phase0 only: phase 1 lists participants with bitfields instead of index list.
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 @always_bls
 def test_att1_high_index(spec, state):
@@ -210,7 +209,7 @@ def test_att1_high_index(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 @always_bls
 def test_att2_high_index(spec, state):
@@ -223,7 +222,7 @@ def test_att2_high_index(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 @always_bls
 def test_att1_empty_indices(spec, state):
@@ -235,7 +234,7 @@ def test_att1_empty_indices(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 @always_bls
 def test_att2_empty_indices(spec, state):
@@ -247,7 +246,7 @@ def test_att2_empty_indices(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 @always_bls
 def test_all_empty_indices(spec, state):
@@ -262,7 +261,7 @@ def test_all_empty_indices(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 @always_bls
 def test_att1_bad_extra_index(spec, state):
@@ -278,7 +277,7 @@ def test_att1_bad_extra_index(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 @always_bls
 def test_att1_bad_replaced_index(spec, state):
@@ -294,7 +293,7 @@ def test_att1_bad_replaced_index(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 @always_bls
 def test_att2_bad_extra_index(spec, state):
@@ -310,7 +309,7 @@ def test_att2_bad_extra_index(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 @always_bls
 def test_att2_bad_replaced_index(spec, state):
@@ -326,7 +325,7 @@ def test_att2_bad_replaced_index(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 @always_bls
 def test_att1_duplicate_index_normal_signed(spec, state):
@@ -346,7 +345,7 @@ def test_att1_duplicate_index_normal_signed(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 @always_bls
 def test_att2_duplicate_index_normal_signed(spec, state):
@@ -366,7 +365,7 @@ def test_att2_duplicate_index_normal_signed(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 @always_bls
 def test_att1_duplicate_index_double_signed(spec, state):
@@ -381,7 +380,7 @@ def test_att1_duplicate_index_double_signed(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 @always_bls
 def test_att2_duplicate_index_double_signed(spec, state):
@@ -396,7 +395,7 @@ def test_att2_duplicate_index_double_signed(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 def test_unsorted_att_1(spec, state):
     attester_slashing = get_valid_attester_slashing(spec, state, signed_1=False, signed_2=True)
@@ -409,7 +408,7 @@ def test_unsorted_att_1(spec, state):
     yield from run_attester_slashing_processing(spec, state, attester_slashing, False)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 def test_unsorted_att_2(spec, state):
     attester_slashing = get_valid_attester_slashing(spec, state, signed_1=True, signed_2=False)
diff --git a/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_rewards_and_penalties.py b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_rewards_and_penalties.py
index 72bdd3e92..0137ddca4 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_rewards_and_penalties.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/epoch_processing/test_process_rewards_and_penalties.py
@@ -1,7 +1,6 @@
 from eth2spec.test.context import (
-    PHASE0,
     spec_state_test, spec_test,
-    with_all_phases, with_phases, single_phase,
+    with_all_phases, single_phase,
     with_custom_state,
     zero_activation_threshold,
     misc_balances, low_single_balance,
@@ -25,7 +24,7 @@ def run_process_rewards_and_penalties(spec, state):
     yield from run_epoch_processing_with(spec, state, 'process_rewards_and_penalties')
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 def test_genesis_epoch_no_attestations_no_penalties(spec, state):
     pre_state = state.copy()
@@ -38,7 +37,7 @@ def test_genesis_epoch_no_attestations_no_penalties(spec, state):
         assert state.balances[index] == pre_state.balances[index]
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 def test_genesis_epoch_full_attestations_no_rewards(spec, state):
     attestations = []
diff --git a/tests/core/pyspec/eth2spec/test/phase0/finality/test_finality.py b/tests/core/pyspec/eth2spec/test/phase0/finality/test_finality.py
index d2d3d4405..c414f645e 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/finality/test_finality.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/finality/test_finality.py
@@ -1,4 +1,4 @@
-from eth2spec.test.context import PHASE0, spec_state_test, with_all_phases, with_phases
+from eth2spec.test.context import spec_state_test, with_all_phases
 from eth2spec.test.helpers.state import next_epoch_via_block
 from eth2spec.test.helpers.attestations import next_epoch_with_attestations
 
@@ -28,7 +28,7 @@ def check_finality(spec,
         assert state.finalized_checkpoint == prev_state.finalized_checkpoint
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 def test_finality_no_updates_at_genesis(spec, state):
     assert spec.get_current_epoch(state) == spec.GENESIS_EPOCH
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 c183865a5..f0028b1fe 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py
@@ -506,7 +506,7 @@ def test_duplicate_attester_slashing(spec, state):
 
 # All AttesterSlashing tests should be adopted for Phase 1 but helper support is not yet there
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 def test_multiple_attester_slashings_no_overlap(spec, state):
     # Skip test if config cannot handle multiple AttesterSlashings per block
@@ -547,7 +547,7 @@ def test_multiple_attester_slashings_no_overlap(spec, state):
     check_attester_slashing_effect(spec, pre_state, state, full_indices)
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 def test_multiple_attester_slashings_partial_overlap(spec, state):
     # Skip test if config cannot handle multiple AttesterSlashings per block
@@ -762,7 +762,7 @@ def prepare_signed_exits(spec, state, indices):
 # exceeding the minimal-config randao mixes memory size.
 # Applies to all voluntary-exit sanity block tests.
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 def test_voluntary_exit(spec, state):
     validator_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
@@ -790,7 +790,7 @@ def test_voluntary_exit(spec, state):
     assert state.validators[validator_index].exit_epoch < spec.FAR_FUTURE_EPOCH
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 def test_double_validator_exit_same_block(spec, state):
     validator_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
@@ -811,7 +811,7 @@ def test_double_validator_exit_same_block(spec, state):
     yield 'post', None
 
 
-@with_phases([PHASE0])
+@with_all_phases
 @spec_state_test
 def test_multiple_different_validator_exits_same_block(spec, state):
     validator_indices = [
diff --git a/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_chunk_challenge.py b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_chunk_challenge.py
index 4ef1d667a..395bcc2fd 100644
--- a/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_chunk_challenge.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_chunk_challenge.py
@@ -6,7 +6,7 @@ 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
+from eth2spec.test.helpers.state import transition_to, transition_to_valid_shard_slot
 from eth2spec.test.context import (
     PHASE0,
     with_all_phases_except,
@@ -68,7 +68,8 @@ def run_custody_chunk_response_processing(spec, state, custody_response, valid=T
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_challenge_appended(spec, state):
-    transition_to(spec, state, state.slot + 1)
+    transition_to_valid_shard_slot(spec, state)
+    transition_to(spec, state, state.slot + 1)  # Make len(offset_slots) == 1
     shard = 0
     offset_slots = spec.get_offset_slots(state, shard)
     shard_transition = get_sample_shard_transition(spec, state.slot, [2**15 // 3] * len(offset_slots))
@@ -89,7 +90,8 @@ def test_challenge_appended(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_challenge_empty_element_replaced(spec, state):
-    transition_to(spec, state, state.slot + 1)
+    transition_to_valid_shard_slot(spec, state)
+    transition_to(spec, state, state.slot + 1)  # Make len(offset_slots) == 1
     shard = 0
     offset_slots = spec.get_offset_slots(state, shard)
     shard_transition = get_sample_shard_transition(spec, state.slot, [2**15 // 3] * len(offset_slots))
@@ -112,7 +114,8 @@ def test_challenge_empty_element_replaced(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_duplicate_challenge(spec, state):
-    transition_to(spec, state, state.slot + 1)
+    transition_to_valid_shard_slot(spec, state)
+    transition_to(spec, state, state.slot + 1)  # Make len(offset_slots) == 1
     shard = 0
     offset_slots = spec.get_offset_slots(state, shard)
     shard_transition = get_sample_shard_transition(spec, state.slot, [2**15 // 3] * len(offset_slots))
@@ -135,7 +138,8 @@ def test_duplicate_challenge(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_second_challenge(spec, state):
-    transition_to(spec, state, state.slot + 1)
+    transition_to_valid_shard_slot(spec, state)
+    transition_to(spec, state, state.slot + 1)  # Make len(offset_slots) == 1
     shard = 0
     offset_slots = spec.get_offset_slots(state, shard)
     shard_transition = get_sample_shard_transition(spec, state.slot, [2**15 // 3] * len(offset_slots))
@@ -160,6 +164,7 @@ def test_second_challenge(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_multiple_epochs_custody(spec, state):
+    transition_to_valid_shard_slot(spec, state)
     transition_to(spec, state, state.slot + spec.SLOTS_PER_EPOCH * 3)
 
     shard = 0
@@ -182,6 +187,7 @@ def test_multiple_epochs_custody(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_many_epochs_custody(spec, state):
+    transition_to_valid_shard_slot(spec, state)
     transition_to(spec, state, state.slot + spec.SLOTS_PER_EPOCH * 20)
 
     shard = 0
@@ -204,6 +210,7 @@ def test_many_epochs_custody(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_off_chain_attestation(spec, state):
+    transition_to_valid_shard_slot(spec, state)
     transition_to(spec, state, state.slot + spec.SLOTS_PER_EPOCH)
 
     shard = 0
@@ -222,6 +229,7 @@ def test_off_chain_attestation(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_custody_response(spec, state):
+    transition_to_valid_shard_slot(spec, state)
     transition_to(spec, state, state.slot + spec.SLOTS_PER_EPOCH)
 
     shard = 0
@@ -251,6 +259,7 @@ def test_custody_response(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_custody_response_multiple_epochs(spec, state):
+    transition_to_valid_shard_slot(spec, state)
     transition_to(spec, state, state.slot + spec.SLOTS_PER_EPOCH * 3)
 
     shard = 0
@@ -280,6 +289,7 @@ def test_custody_response_multiple_epochs(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_custody_response_many_epochs(spec, state):
+    transition_to_valid_shard_slot(spec, state)
     transition_to(spec, state, state.slot + spec.SLOTS_PER_EPOCH * 20)
 
     shard = 0
diff --git a/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_custody_slashing.py b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_custody_slashing.py
index 07cd76996..23a60cd22 100644
--- a/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_custody_slashing.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_custody_slashing.py
@@ -63,6 +63,7 @@ def run_standard_custody_slashing_test(spec,
                                        slashing_message_data=None,
                                        correct=True,
                                        valid=True):
+    transition_to(spec, state, state.slot + 1)  # Make len(offset_slots) == 1
     if shard_lateness is None:
         shard_lateness = spec.SLOTS_PER_EPOCH
     transition_to(spec, state, state.slot + shard_lateness)
diff --git a/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_shard_transition.py b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_shard_transition.py
index 866831686..2bb569eac 100644
--- a/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_shard_transition.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/block_processing/test_process_shard_transition.py
@@ -22,7 +22,7 @@ from eth2spec.test.helpers.state import transition_to, transition_to_valid_shard
 
 
 def get_initial_env(spec, state, target_len_offset_slot):
-    state = transition_to_valid_shard_slot(spec, state)
+    transition_to_valid_shard_slot(spec, state)
     committee_index = spec.CommitteeIndex(0)
     target_shard_slot = state.slot + target_len_offset_slot - 1
     shard = spec.compute_shard_from_committee_index(state, committee_index, target_shard_slot)
diff --git a/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_challenge_deadlines.py b/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_challenge_deadlines.py
index 675b0d8da..828ace91c 100644
--- a/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_challenge_deadlines.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_challenge_deadlines.py
@@ -5,7 +5,7 @@ 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
+from eth2spec.test.helpers.state import transition_to, transition_to_valid_shard_slot
 from eth2spec.test.context import (
     PHASE0,
     with_all_phases_except,
@@ -26,7 +26,8 @@ def run_process_challenge_deadlines(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_validator_slashed_after_chunk_challenge(spec, state):
-    transition_to(spec, state, state.slot + 1)
+    transition_to_valid_shard_slot(spec, state)
+    transition_to(spec, state, state.slot + 1)  # Make len(offset_slots) == 1
     shard = 0
     offset_slots = spec.get_offset_slots(state, shard)
     shard_transition = get_sample_shard_transition(spec, state.slot, [2**15 // 3] * len(offset_slots))
diff --git a/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_custody_final_updates.py b/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_custody_final_updates.py
index 93fea19cf..0541411da 100644
--- a/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_custody_final_updates.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/epoch_processing/test_process_custody_final_updates.py
@@ -10,7 +10,7 @@ from eth2spec.test.helpers.custody import (
 from eth2spec.test.helpers.attestations import (
     get_valid_on_time_attestation,
 )
-from eth2spec.test.helpers.state import next_epoch_via_block, transition_to
+from eth2spec.test.helpers.state import next_epoch_via_block, transition_to, transition_to_valid_shard_slot
 from eth2spec.test.context import (
     with_all_phases_except,
     spec_state_test,
@@ -32,6 +32,8 @@ def run_process_custody_final_updates(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_validator_withdrawal_delay(spec, state):
+    transition_to_valid_shard_slot(spec, state)
+    transition_to(spec, state, state.slot + 1)  # Make len(offset_slots) == 1
     spec.initiate_validator_exit(state, 0)
     assert state.validators[0].withdrawable_epoch < spec.FAR_FUTURE_EPOCH
 
@@ -43,6 +45,8 @@ def test_validator_withdrawal_delay(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_validator_withdrawal_reenable_after_custody_reveal(spec, state):
+    transition_to_valid_shard_slot(spec, state)
+    transition_to(spec, state, state.slot + 1)  # Make len(offset_slots) == 1
     spec.initiate_validator_exit(state, 0)
     assert state.validators[0].withdrawable_epoch < spec.FAR_FUTURE_EPOCH
 
@@ -66,7 +70,8 @@ def test_validator_withdrawal_reenable_after_custody_reveal(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_validator_withdrawal_suspend_after_chunk_challenge(spec, state):
-    transition_to(spec, state, state.slot + 1)
+    transition_to_valid_shard_slot(spec, state)
+    transition_to(spec, state, state.slot + 1)  # Make len(offset_slots) == 1
     shard = 0
     offset_slots = spec.get_offset_slots(state, shard)
     shard_transition = get_sample_shard_transition(spec, state.slot, [2**15 // 3] * len(offset_slots))
@@ -114,7 +119,8 @@ def test_validator_withdrawal_suspend_after_chunk_challenge(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_validator_withdrawal_resume_after_chunk_challenge_response(spec, state):
-    transition_to(spec, state, state.slot + 1)
+    transition_to_valid_shard_slot(spec, state)
+    transition_to(spec, state, state.slot + 1)  # Make len(offset_slots) == 1
     shard = 0
     offset_slots = spec.get_offset_slots(state, shard)
     shard_transition = get_sample_shard_transition(spec, state.slot, [2**15 // 3] * len(offset_slots))
diff --git a/tests/core/pyspec/eth2spec/test/phase1/sanity/test_blocks.py b/tests/core/pyspec/eth2spec/test/phase1/sanity/test_blocks.py
index f69cd4793..5fe3394db 100644
--- a/tests/core/pyspec/eth2spec/test/phase1/sanity/test_blocks.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/sanity/test_blocks.py
@@ -105,7 +105,7 @@ def test_process_beacon_block_with_normal_shard_transition(spec, state):
         # skip
         return
 
-    state = transition_to_valid_shard_slot(spec, state)
+    transition_to_valid_shard_slot(spec, state)
 
     target_len_offset_slot = 1
     committee_index = spec.CommitteeIndex(0)
@@ -123,7 +123,7 @@ def test_process_beacon_block_with_empty_proposal_transition(spec, state):
         # skip
         return
 
-    state = transition_to_valid_shard_slot(spec, state)
+    transition_to_valid_shard_slot(spec, state)
 
     target_len_offset_slot = 1
     committee_index = spec.CommitteeIndex(0)
@@ -146,7 +146,7 @@ def test_with_shard_transition_with_custody_challenge_and_response(spec, state):
         # skip
         return
 
-    state = transition_to_valid_shard_slot(spec, state)
+    transition_to_valid_shard_slot(spec, state)
 
     # build shard block
     shard = 0
@@ -179,7 +179,7 @@ def test_with_shard_transition_with_custody_challenge_and_response(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_custody_key_reveal(spec, state):
-    state = transition_to_valid_shard_slot(spec, state)
+    transition_to_valid_shard_slot(spec, state)
     transition_to(spec, state, state.slot + spec.EPOCHS_PER_CUSTODY_PERIOD * spec.SLOTS_PER_EPOCH)
 
     block = build_empty_block(spec, state, slot=state.slot + 1)
@@ -192,7 +192,7 @@ def test_custody_key_reveal(spec, state):
 @with_all_phases_except([PHASE0])
 @spec_state_test
 def test_early_derived_secret_reveal(spec, state):
-    state = transition_to_valid_shard_slot(spec, state)
+    transition_to_valid_shard_slot(spec, state)
     block = build_empty_block(spec, state, slot=state.slot + 1)
     early_derived_secret_reveal = get_valid_early_derived_secret_reveal(spec, state)
     block.body.early_derived_secret_reveals = [early_derived_secret_reveal]
@@ -208,7 +208,7 @@ def test_custody_slashing(spec, state):
         # skip
         return
 
-    state = transition_to_valid_shard_slot(spec, state)
+    transition_to_valid_shard_slot(spec, state)
 
     # Build shard block
     shard = 0
diff --git a/tests/core/pyspec/eth2spec/test/phase1/sanity/test_shard_blocks.py b/tests/core/pyspec/eth2spec/test/phase1/sanity/test_shard_blocks.py
index 041b882ab..440d0dd8d 100644
--- a/tests/core/pyspec/eth2spec/test/phase1/sanity/test_shard_blocks.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/sanity/test_shard_blocks.py
@@ -51,10 +51,12 @@ def test_valid_shard_block(spec, state):
         # skip
         return
 
-    beacon_state = transition_to_valid_shard_slot(spec, state)
+    beacon_state = state.copy()
+    transition_to_valid_shard_slot(spec, beacon_state)
+
     shard = 0
     shard_state = beacon_state.shard_states[shard]
-    signed_shard_block = build_shard_block(spec, beacon_state, shard, slot=beacon_state.slot, signed=True)
+    signed_shard_block = build_shard_block(spec, state, shard, slot=beacon_state.slot, signed=True)
 
     yield from run_shard_blocks(spec, shard_state, signed_shard_block, beacon_state)
 
@@ -71,7 +73,9 @@ def test_invalid_shard_parent_root(spec, state):
         # skip
         return
 
-    beacon_state = transition_to_valid_shard_slot(spec, state)
+    beacon_state = state.copy()
+    transition_to_valid_shard_slot(spec, beacon_state)
+
     shard = 0
     shard_state = beacon_state.shard_states[shard]
     signed_shard_block = build_shard_block(spec, beacon_state, shard, slot=beacon_state.slot, signed=True)
@@ -88,7 +92,8 @@ def test_invalid_beacon_parent_root(spec, state):
         # skip
         return
 
-    beacon_state = transition_to_valid_shard_slot(spec, state)
+    beacon_state = state.copy()
+    transition_to_valid_shard_slot(spec, beacon_state)
     shard = 0
     shard_state = beacon_state.shard_states[shard]
     signed_shard_block = build_shard_block(spec, beacon_state, shard, slot=beacon_state.slot, signed=True)
@@ -105,7 +110,8 @@ def test_invalid_slot(spec, state):
         # skip
         return
 
-    beacon_state = transition_to_valid_shard_slot(spec, state)
+    beacon_state = state.copy()
+    transition_to_valid_shard_slot(spec, beacon_state)
     shard = 0
     shard_state = beacon_state.shard_states[shard]
     signed_shard_block = build_shard_block(spec, beacon_state, shard, slot=beacon_state.slot, signed=True)
@@ -123,7 +129,8 @@ def test_invalid_proposer_index(spec, state):
         # skip
         return
 
-    beacon_state = transition_to_valid_shard_slot(spec, state)
+    beacon_state = state.copy()
+    transition_to_valid_shard_slot(spec, beacon_state)
     shard = 0
     shard_state = beacon_state.shard_states[shard]
     signed_shard_block = build_shard_block(spec, beacon_state, shard, slot=beacon_state.slot, signed=True)
@@ -147,7 +154,8 @@ def test_out_of_bound_offset(spec, state):
         # skip
         return
 
-    beacon_state = transition_to_valid_shard_slot(spec, state)
+    beacon_state = state.copy()
+    transition_to_valid_shard_slot(spec, beacon_state)
     shard = 0
     slot = (
         beacon_state.shard_states[shard].slot
@@ -170,7 +178,8 @@ def test_invalid_offset(spec, state):
         # skip
         return
 
-    beacon_state = transition_to_valid_shard_slot(spec, state)
+    beacon_state = state.copy()
+    transition_to_valid_shard_slot(spec, beacon_state)
     # 4 is not in `SHARD_BLOCK_OFFSETS`
     shard = 0
     slot = beacon_state.shard_states[shard].slot + 4
@@ -191,7 +200,8 @@ def test_empty_block_body(spec, state):
         # skip
         return
 
-    beacon_state = transition_to_valid_shard_slot(spec, state)
+    beacon_state = state.copy()
+    transition_to_valid_shard_slot(spec, beacon_state)
     shard = 0
     shard_state = beacon_state.shard_states[shard]
     signed_shard_block = build_shard_block(spec, beacon_state, shard, slot=beacon_state.slot, body=b'', signed=True)
@@ -212,7 +222,8 @@ def test_invalid_signature(spec, state):
         # skip
         return
 
-    beacon_state = transition_to_valid_shard_slot(spec, state)
+    beacon_state = state.copy()
+    transition_to_valid_shard_slot(spec, beacon_state)
     shard = 0
     shard_state = beacon_state.shard_states[shard]
     signed_shard_block = build_shard_block(spec, beacon_state, shard, slot=beacon_state.slot, signed=False)
@@ -233,7 +244,8 @@ def test_max_offset(spec, state):
         # skip
         return
 
-    beacon_state = transition_to_valid_shard_slot(spec, state)
+    beacon_state = state.copy()
+    transition_to_valid_shard_slot(spec, beacon_state)
     shard = 0
     slot = beacon_state.shard_states[shard].slot + spec.SHARD_BLOCK_OFFSETS[spec.MAX_SHARD_BLOCKS_PER_ATTESTATION - 1]
     transition_to(spec, beacon_state, slot)
@@ -253,7 +265,8 @@ def test_pending_shard_parent_block(spec, state):
         return
 
     # Block N
-    beacon_state = transition_to_valid_shard_slot(spec, state)
+    beacon_state = state.copy()
+    transition_to_valid_shard_slot(spec, beacon_state)
     shard = 0
     shard_state = beacon_state.shard_states[shard]
     signed_shard_block_1 = build_shard_block(spec, beacon_state, shard, slot=beacon_state.slot, signed=True)
diff --git a/tests/core/pyspec/eth2spec/test/phase1/unittests/fork_choice/test_on_shard_head.py b/tests/core/pyspec/eth2spec/test/phase1/unittests/fork_choice/test_on_shard_head.py
index a87a85917..f38f1aef1 100644
--- a/tests/core/pyspec/eth2spec/test/phase1/unittests/fork_choice/test_on_shard_head.py
+++ b/tests/core/pyspec/eth2spec/test/phase1/unittests/fork_choice/test_on_shard_head.py
@@ -8,7 +8,10 @@ from eth2spec.test.helpers.shard_block import (
     get_committee_index_of_shard,
 )
 from eth2spec.test.helpers.fork_choice import add_block_to_store, get_anchor_root
-from eth2spec.test.helpers.state import state_transition_and_sign_block
+from eth2spec.test.helpers.state import (
+    state_transition_and_sign_block,
+    transition_to_valid_shard_slot,
+)
 from eth2spec.test.helpers.block import build_empty_block
 
 
@@ -102,21 +105,18 @@ def apply_shard_and_beacon(spec, state, store, shard_store, shard_blocks_buffer)
 @spec_state_test
 @never_bls  # Set to never_bls for testing `check_pending_shard_blocks`
 def test_basic(spec, state):
-    spec.PHASE_1_GENESIS_SLOT = 0  # NOTE: mock genesis slot here
-    state = spec.upgrade_to_phase1(state)
-    shard = spec.Shard(1)
+    transition_to_valid_shard_slot(spec, state)
 
     # Initialization
     store = spec.get_forkchoice_store(state)
     anchor_root = get_anchor_root(spec, state)
     assert spec.get_head(store) == anchor_root
 
+    shard = spec.Shard(1)
     shard_store = spec.get_forkchoice_shard_store(state, shard)
     shard_head_root = spec.get_shard_head(store, shard_store)
     assert shard_head_root == state.shard_states[shard].latest_block_root
-    assert shard_store.block_states[shard_head_root].slot == 1
     assert shard_store.block_states[shard_head_root] == state.shard_states[shard]
-
     # For mainnet config, it's possible that only one committee of `shard` per epoch.
     # we set this counter to test more rounds.
     shard_committee_counter = 2