diff --git a/src/ethereum/forks/arrow_glacier/__init__.py b/src/ethereum/forks/arrow_glacier/__init__.py
index d09621313e..6e8798e713 100644
--- a/src/ethereum/forks/arrow_glacier/__init__.py
+++ b/src/ethereum/forks/arrow_glacier/__init__.py
@@ -28,6 +28,6 @@
 [nm]: https://github.com/NethermindEth/nethermind/releases/tag/1.11.7
 """  # noqa: E501
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
-FORK_CRITERIA = ByBlockNumber(13773000)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(13773000)
diff --git a/src/ethereum/forks/berlin/__init__.py b/src/ethereum/forks/berlin/__init__.py
index 6f8f866ed6..3bc09c7878 100644
--- a/src/ethereum/forks/berlin/__init__.py
+++ b/src/ethereum/forks/berlin/__init__.py
@@ -38,6 +38,6 @@
 [oe]: https://github.com/openethereum/openethereum/releases/tag/v3.2.0
 """  # noqa: E501
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
-FORK_CRITERIA = ByBlockNumber(12244000)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(12244000)
diff --git a/src/ethereum/forks/byzantium/__init__.py b/src/ethereum/forks/byzantium/__init__.py
index 5eea245f8d..3a39f7c38b 100644
--- a/src/ethereum/forks/byzantium/__init__.py
+++ b/src/ethereum/forks/byzantium/__init__.py
@@ -45,6 +45,6 @@
 [p]: https://github.com/paritytech/parity/releases/tag/v1.7.6
 """
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
-FORK_CRITERIA = ByBlockNumber(4370000)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(4370000)
diff --git a/src/ethereum/forks/cancun/__init__.py b/src/ethereum/forks/cancun/__init__.py
index e4685c450b..fe0e897ee0 100644
--- a/src/ethereum/forks/cancun/__init__.py
+++ b/src/ethereum/forks/cancun/__init__.py
@@ -44,6 +44,6 @@
 [r]: https://github.com/paradigmxyz/reth/releases/tag/v0.1.0-alpha.19
 """  # noqa: E501
 
-from ethereum.fork_criteria import ByTimestamp
+from ethereum.fork_criteria import ByTimestamp, ForkCriteria
 
-FORK_CRITERIA = ByTimestamp(1710338135)
+FORK_CRITERIA: ForkCriteria = ByTimestamp(1710338135)
diff --git a/src/ethereum/forks/constantinople/__init__.py b/src/ethereum/forks/constantinople/__init__.py
index d3ab9f2d28..7f40cdd28b 100644
--- a/src/ethereum/forks/constantinople/__init__.py
+++ b/src/ethereum/forks/constantinople/__init__.py
@@ -46,6 +46,6 @@
 [t]: https://github.com/ethereum/trinity/releases/tag/v0.1.0-alpha.23
 """  # noqa: E501
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
-FORK_CRITERIA = ByBlockNumber(7280000)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(7280000)
diff --git a/src/ethereum/forks/dao_fork/__init__.py b/src/ethereum/forks/dao_fork/__init__.py
index bb879891ba..7516eb2de4 100644
--- a/src/ethereum/forks/dao_fork/__init__.py
+++ b/src/ethereum/forks/dao_fork/__init__.py
@@ -24,6 +24,6 @@
 [Geth 1.4.10]: https://github.com/ethereum/go-ethereum/releases/tag/v1.4.10
 """
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
-FORK_CRITERIA = ByBlockNumber(1920000)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(1920000)
diff --git a/src/ethereum/forks/dao_fork/fork.py b/src/ethereum/forks/dao_fork/fork.py
index 147ec2dc13..38eb1e61c2 100644
--- a/src/ethereum/forks/dao_fork/fork.py
+++ b/src/ethereum/forks/dao_fork/fork.py
@@ -29,6 +29,7 @@
     InvalidSenderError,
     NonceMismatchError,
 )
+from ethereum.fork_criteria import ByBlockNumber
 
 from . import FORK_CRITERIA, vm
 from .blocks import Block, Header, Log, Receipt
@@ -271,6 +272,8 @@ def validate_header(chain: BlockChain, header: Header) -> None:
     if header.parent_hash != block_parent_hash:
         raise InvalidBlock
 
+    assert isinstance(FORK_CRITERIA, ByBlockNumber)
+
     if (
         header.number >= FORK_CRITERIA.block_number
         and header.number < FORK_CRITERIA.block_number + Uint(10)
diff --git a/src/ethereum/forks/frontier/__init__.py b/src/ethereum/forks/frontier/__init__.py
index 3a7fed59eb..cc69681a3a 100644
--- a/src/ethereum/forks/frontier/__init__.py
+++ b/src/ethereum/forks/frontier/__init__.py
@@ -2,6 +2,6 @@
 Frontier is the first production-ready iteration of the Ethereum protocol.
 """
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
-FORK_CRITERIA = ByBlockNumber(0)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(0)
diff --git a/src/ethereum/forks/gray_glacier/__init__.py b/src/ethereum/forks/gray_glacier/__init__.py
index a0028106b9..c16a952fb9 100644
--- a/src/ethereum/forks/gray_glacier/__init__.py
+++ b/src/ethereum/forks/gray_glacier/__init__.py
@@ -29,6 +29,6 @@
 [n]: https://github.com/NethermindEth/nethermind/releases/tag/1.13.3
 """  # noqa: E501
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
-FORK_CRITERIA = ByBlockNumber(15050000)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(15050000)
diff --git a/src/ethereum/forks/homestead/__init__.py b/src/ethereum/forks/homestead/__init__.py
index 2d6312d696..8367f4e003 100644
--- a/src/ethereum/forks/homestead/__init__.py
+++ b/src/ethereum/forks/homestead/__init__.py
@@ -29,6 +29,6 @@
 [Geth 1.3.5]: https://github.com/ethereum/go-ethereum/releases/tag/v1.3.5
 """
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
-FORK_CRITERIA = ByBlockNumber(1150000)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(1150000)
diff --git a/src/ethereum/forks/istanbul/__init__.py b/src/ethereum/forks/istanbul/__init__.py
index fdb8b7aa8b..1b4f3b6153 100644
--- a/src/ethereum/forks/istanbul/__init__.py
+++ b/src/ethereum/forks/istanbul/__init__.py
@@ -48,6 +48,6 @@
 [t]: https://github.com/ethereum/trinity/releases/tag/v0.1.0-alpha.31
 """
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
-FORK_CRITERIA = ByBlockNumber(9069000)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(9069000)
diff --git a/src/ethereum/forks/london/__init__.py b/src/ethereum/forks/london/__init__.py
index d4ee7aed3e..95caf15ed4 100644
--- a/src/ethereum/forks/london/__init__.py
+++ b/src/ethereum/forks/london/__init__.py
@@ -43,6 +43,6 @@
 [oe]: https://github.com/openethereum/openethereum/releases/tag/v3.3.0-rc.4
 """  # noqa: E501
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
-FORK_CRITERIA = ByBlockNumber(12965000)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(12965000)
diff --git a/src/ethereum/forks/london/fork.py b/src/ethereum/forks/london/fork.py
index 14e2668987..01309a540a 100644
--- a/src/ethereum/forks/london/fork.py
+++ b/src/ethereum/forks/london/fork.py
@@ -28,6 +28,7 @@
     InvalidSenderError,
     NonceMismatchError,
 )
+from ethereum.fork_criteria import ByBlockNumber
 
 from . import FORK_CRITERIA, vm
 from .blocks import Block, Header, Log, Receipt, encode_receipt
@@ -319,6 +320,8 @@ def validate_header(chain: BlockChain, header: Header) -> None:
     if header.gas_used > header.gas_limit:
         raise InvalidBlock
 
+    assert isinstance(FORK_CRITERIA, ByBlockNumber)
+
     expected_base_fee_per_gas = INITIAL_BASE_FEE
     if header.number != FORK_CRITERIA.block_number:
         # For every block except the first, calculate the base fee per gas
diff --git a/src/ethereum/forks/muir_glacier/__init__.py b/src/ethereum/forks/muir_glacier/__init__.py
index a9ee0553f9..a722576f95 100644
--- a/src/ethereum/forks/muir_glacier/__init__.py
+++ b/src/ethereum/forks/muir_glacier/__init__.py
@@ -33,6 +33,6 @@
 [t]: https://github.com/ethereum/trinity/releases/tag/v0.1.0-alpha.34
 """
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
-FORK_CRITERIA = ByBlockNumber(9200000)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(9200000)
diff --git a/src/ethereum/forks/osaka/__init__.py b/src/ethereum/forks/osaka/__init__.py
index cdd21d54b2..c3978b993f 100644
--- a/src/ethereum/forks/osaka/__init__.py
+++ b/src/ethereum/forks/osaka/__init__.py
@@ -50,6 +50,6 @@
 [EIP-7910]: https://eips.ethereum.org/EIPS/eip-7910
 """  # noqa: E501
 
-from ethereum.fork_criteria import ByTimestamp
+from ethereum.fork_criteria import ByTimestamp, ForkCriteria
 
-FORK_CRITERIA = ByTimestamp(1764798551)
+FORK_CRITERIA: ForkCriteria = ByTimestamp(1764798551)
diff --git a/src/ethereum/forks/paris/__init__.py b/src/ethereum/forks/paris/__init__.py
index 89642fd88b..9e1f07470d 100644
--- a/src/ethereum/forks/paris/__init__.py
+++ b/src/ethereum/forks/paris/__init__.py
@@ -34,10 +34,10 @@
 [nm]: https://github.com/NethermindEth/nethermind/releases/tag/1.14.1
 """  # noqa: E501
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
 # The actual trigger for the Paris hardfork was The Merge occurring when
 # total difficulty (the sum of the all block difficulties) reached the
 # Terminal Total Difficulty value (58750000000000000000000 on Mainnet). The
 # Merge is now a historical event.
-FORK_CRITERIA = ByBlockNumber(15537394)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(15537394)
diff --git a/src/ethereum/forks/prague/__init__.py b/src/ethereum/forks/prague/__init__.py
index 16cdf6a019..9a1202050f 100644
--- a/src/ethereum/forks/prague/__init__.py
+++ b/src/ethereum/forks/prague/__init__.py
@@ -49,6 +49,6 @@
 [EIP-7549]: https://eips.ethereum.org/EIPS/eip-7549
 """  # noqa: E501
 
-from ethereum.fork_criteria import ByTimestamp
+from ethereum.fork_criteria import ByTimestamp, ForkCriteria
 
-FORK_CRITERIA = ByTimestamp(1746612311)
+FORK_CRITERIA: ForkCriteria = ByTimestamp(1746612311)
diff --git a/src/ethereum/forks/shanghai/__init__.py b/src/ethereum/forks/shanghai/__init__.py
index 34ce1fe57e..d231ca9648 100644
--- a/src/ethereum/forks/shanghai/__init__.py
+++ b/src/ethereum/forks/shanghai/__init__.py
@@ -42,6 +42,6 @@
 [js]: https://github.com/ethereumjs/ethereumjs-monorepo/releases/tag/%40ethereumjs%2Fvm%406.4.0
 """  # noqa: E501
 
-from ethereum.fork_criteria import ByTimestamp
+from ethereum.fork_criteria import ByTimestamp, ForkCriteria
 
-FORK_CRITERIA = ByTimestamp(1681338455)
+FORK_CRITERIA: ForkCriteria = ByTimestamp(1681338455)
diff --git a/src/ethereum/forks/spurious_dragon/__init__.py b/src/ethereum/forks/spurious_dragon/__init__.py
index ae19211a60..da34eee288 100644
--- a/src/ethereum/forks/spurious_dragon/__init__.py
+++ b/src/ethereum/forks/spurious_dragon/__init__.py
@@ -33,6 +33,6 @@
 [rb]: https://github.com/cryptape/ruby-ethereum/releases/tag/v0.11.0
 """
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
-FORK_CRITERIA = ByBlockNumber(2675000)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(2675000)
diff --git a/src/ethereum/forks/tangerine_whistle/__init__.py b/src/ethereum/forks/tangerine_whistle/__init__.py
index f8d67bd3ee..3b3ff486ff 100644
--- a/src/ethereum/forks/tangerine_whistle/__init__.py
+++ b/src/ethereum/forks/tangerine_whistle/__init__.py
@@ -27,6 +27,6 @@
 [p]: https://github.com/openethereum/parity-ethereum/releases/tag/v1.3.8
 """
 
-from ethereum.fork_criteria import ByBlockNumber
+from ethereum.fork_criteria import ByBlockNumber, ForkCriteria
 
-FORK_CRITERIA = ByBlockNumber(2463000)
+FORK_CRITERIA: ForkCriteria = ByBlockNumber(2463000)