diff --git a/configs/mainnet.yaml b/configs/mainnet.yaml
index 77d7a136f..74896c90f 100644
--- a/configs/mainnet.yaml
+++ b/configs/mainnet.yaml
@@ -7,8 +7,10 @@ PRESET_BASE: 'mainnet'
# ---------------------------------------------------------------
# TBD, 2**256-2**10 is a placeholder
TERMINAL_TOTAL_DIFFICULTY: 115792089237316195423570985008687907853269984665640564039457584007913129638912
-# By default, don't use this param
+# By default, don't use these params
TERMINAL_BLOCK_HASH: 0x0000000000000000000000000000000000000000000000000000000000000000
+TBH_ACTIVATION_EPOCH: 18446744073709551615
+
# Genesis
diff --git a/configs/minimal.yaml b/configs/minimal.yaml
index a8b30fd54..35e321e84 100644
--- a/configs/minimal.yaml
+++ b/configs/minimal.yaml
@@ -7,8 +7,10 @@ PRESET_BASE: 'minimal'
# ---------------------------------------------------------------
# TBD, 2**256-2**10 is a placeholder
TERMINAL_TOTAL_DIFFICULTY: 115792089237316195423570985008687907853269984665640564039457584007913129638912
-# By default, don't use this param
+# By default, don't use these params
TERMINAL_BLOCK_HASH: 0x0000000000000000000000000000000000000000000000000000000000000000
+TBH_ACTIVATION_EPOCH: 18446744073709551615
+
# Genesis
diff --git a/specs/merge/beacon-chain.md b/specs/merge/beacon-chain.md
index f5977db5e..12919b474 100644
--- a/specs/merge/beacon-chain.md
+++ b/specs/merge/beacon-chain.md
@@ -74,7 +74,8 @@ This patch adds transaction execution to the beacon chain as part of the Merge f
| Name | Value |
| - | - |
| `TERMINAL_TOTAL_DIFFICULTY` | **TBD** |
-| `TERMINAL_BLOCK_HASH` | `Hash32('0x0000000000000000000000000000000000000000000000000000000000000000')` |
+| `TERMINAL_BLOCK_HASH` | `Hash32()` |
+| `TBH_ACTIVATION_EPOCH` | `FAR_FUTURE_EPOCH` |
## Containers
diff --git a/specs/merge/client-settings.md b/specs/merge/client-settings.md
index 4a4b38d3e..63ddfe5ae 100644
--- a/specs/merge/client-settings.md
+++ b/specs/merge/client-settings.md
@@ -18,11 +18,12 @@ This document specifies configurable settings that clients must implement for th
To coordinate manual overrides to [`TERMINAL_TOTAL_DIFFICULTY`](./beacon-chain.md#Transition-settings) parameter, clients must provide `--terminal-total-difficulty-override` as a configurable setting. The value provided by this setting must take precedence over pre-configured `TERMINAL_TOTAL_DIFFICULTY` parameter. Clients should accept the setting as a decimal value (i.e., *not* hexadecimal).
-Except under exceptional scenarios, this setting is expected to not be used. Sufficient warning to the user about this exceptional configurable setting should be provided.
+Except under exceptional scenarios, this setting is not expected to be used. Sufficient warning to the user about this exceptional configurable setting should be provided.
### Override terminal block hash
-To allow for transition coordination around a specific PoW block, clients must also provide `--terminal-block-hash-override` as a configurable setting.
-The value provided by this setting takes precedence over the pre-configured `TERMINAL_BLOCK_HASH` parameter.
+To allow for transition coordination around a specific PoW block, clients must also provide `--terminal-block-hash-override` and `--terminal-block-hash-epoch-override` as configurable settings.
+* The value provided by `--terminal-block-hash-override` takes precedence over the pre-configured `TERMINAL_BLOCK_HASH` parameter.
+* The value provided by `--terminal-block-hash-epoch-override` takes precedence over the pre-configured `TBH_ACTIVATION_EPOCH` parameter.
-Except under exceptional scenarios, this setting is expected to not be used. Sufficient warning to the user about this exceptional configurable setting should be provided.
+Except under exceptional scenarios, these settings are not expected to be used. Sufficient warning to the user about this exceptional configurable setting should be provided.
diff --git a/specs/merge/fork-choice.md b/specs/merge/fork-choice.md
index ea4a5588a..f1f5e265c 100644
--- a/specs/merge/fork-choice.md
+++ b/specs/merge/fork-choice.md
@@ -74,8 +74,8 @@ Used by fork-choice handler, `on_block`.
```python
def is_valid_terminal_pow_block(block: PowBlock, parent: PowBlock) -> bool:
- if block.block_hash == TERMINAL_BLOCK_HASH:
- return True
+ if TERMINAL_BLOCK_HASH != Hash32():
+ return block.block_hash == TERMINAL_BLOCK_HASH
is_total_difficulty_reached = block.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
is_parent_total_difficulty_valid = parent.total_difficulty < TERMINAL_TOTAL_DIFFICULTY
@@ -113,6 +113,8 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
pow_block = get_pow_block(block.body.execution_payload.parent_hash)
pow_parent = get_pow_block(pow_block.parent_hash)
assert is_valid_terminal_pow_block(pow_block, pow_parent)
+ if TERMINAL_BLOCK_HASH != Hash32():
+ assert compute_epoch_at_slot(block.slot) >= TBH_ACTIVATION_EPOCH
# Add new block to the store
store.blocks[hash_tree_root(block)] = block
diff --git a/specs/merge/validator.md b/specs/merge/validator.md
index 2de4ef6b1..f15f62f3d 100644
--- a/specs/merge/validator.md
+++ b/specs/merge/validator.md
@@ -127,6 +127,11 @@ def prepare_execution_payload(state: BeaconState,
fee_recipient: ExecutionAddress,
execution_engine: ExecutionEngine) -> Optional[PayloadId]:
if not is_merge_complete(state):
+ is_tbh_override_set = TERMINAL_BLOCK_HASH != Hash32()
+ if is_tbh_override_set and get_current_epoch(state.slot) < TBH_ACTIVATION_EPOCH:
+ # TBH is set but activation epoch is not yet reached, no prepare payload call is needed
+ return None
+
terminal_pow_block = get_terminal_pow_block(pow_chain)
if terminal_pow_block is None:
# Pre-merge, no prepare payload call is needed