diff --git a/src/lean_spec/subspecs/chain/__init__.py b/src/lean_spec/subspecs/chain/__init__.py
new file mode 100644
index 00000000..9ffe8101
--- /dev/null
+++ b/src/lean_spec/subspecs/chain/__init__.py
@@ -0,0 +1,7 @@
+"""Specifications for chain and consensus parameters."""
+
+from .config import DEVNET_CONFIG
+
+__all__ = [
+    "DEVNET_CONFIG",
+]
diff --git a/src/lean_spec/subspecs/chain/config.py b/src/lean_spec/subspecs/chain/config.py
new file mode 100644
index 00000000..7bb5bfbe
--- /dev/null
+++ b/src/lean_spec/subspecs/chain/config.py
@@ -0,0 +1,86 @@
+"""
+Chain and Consensus Configuration Specification
+
+This file defines the core consensus parameters and chain presets for the
+Lean Consensus Experimental Chain.
+"""
+
+from pydantic import BaseModel, ConfigDict
+from typing_extensions import Final
+
+from lean_spec.subspecs.types import BasisPoint, uint64
+
+# --- Time Parameters ---
+
+# The fixed duration of a single slot in milliseconds.
+SLOT_DURATION_MS: Final = 4000
+
+# The deadline within a slot (in basis points) for a proposer to publish a
+# block.
+#
+# Honest validators may re-org blocks published after this cutoff.
+#
+# (2500 bps = 25% of slot duration).
+PROPOSER_REORG_CUTOFF_BPS: Final = 2500
+
+# The deadline within a slot (in basis points) by which validators must
+# submit their votes.
+#
+# (5000 bps = 50% of slot duration).
+VOTE_DUE_BPS: Final = 5000
+
+# The deadline within a slot (in basis points) for achieving a fast
+# confirmation.
+#
+# (7500 bps = 75% of slot duration).
+FAST_CONFIRM_DUE_BPS: Final = 7500
+
+# The cutoff within a slot (in basis points) after which the current view is
+# considered 'frozen', preventing further changes.
+#
+# (7500 bps = 75% of slot duration).
+VIEW_FREEZE_CUTOFF_BPS: Final = 7500
+
+# --- State List Length Presets ---
+
+# The maximum number of historical block roots to store in the state.
+#
+# With a 4-second slot, this corresponds to a history
+# of approximately 12.1 days.
+HISTORICAL_ROOTS_LIMIT: Final = 2**18
+
+# The maximum number of validators that can be in the registry.
+VALIDATOR_REGISTRY_LIMIT: Final = 2**12
+
+
+class _ChainConfig(BaseModel):
+    """
+    A model holding the canonical, immutable configuration constants
+    for the chain.
+    """
+
+    # Configuration to make the model immutable.
+    model_config = ConfigDict(frozen=True, extra="forbid")
+
+    # Time Parameters
+    slot_duration_ms: uint64
+    proposer_reorg_cutoff_bps: BasisPoint
+    vote_due_bps: BasisPoint
+    fast_confirm_due_bps: BasisPoint
+    view_freeze_cutoff_bps: BasisPoint
+
+    # State List Length Presets
+    historical_roots_limit: uint64
+    validator_registry_limit: uint64
+
+
+# The Devnet Chain Configuration.
+DEVNET_CONFIG: Final = _ChainConfig(
+    slot_duration_ms=SLOT_DURATION_MS,
+    proposer_reorg_cutoff_bps=PROPOSER_REORG_CUTOFF_BPS,
+    vote_due_bps=VOTE_DUE_BPS,
+    fast_confirm_due_bps=FAST_CONFIRM_DUE_BPS,
+    view_freeze_cutoff_bps=VIEW_FREEZE_CUTOFF_BPS,
+    historical_roots_limit=HISTORICAL_ROOTS_LIMIT,
+    validator_registry_limit=VALIDATOR_REGISTRY_LIMIT,
+)
diff --git a/src/lean_spec/subspecs/types/__init__.py b/src/lean_spec/subspecs/types/__init__.py
new file mode 100644
index 00000000..57173887
--- /dev/null
+++ b/src/lean_spec/subspecs/types/__init__.py
@@ -0,0 +1,9 @@
+"""Reusable type definitions for the Lean Ethereum specification."""
+
+from .basispt import BasisPoint
+from .uint64 import uint64
+
+__all__ = [
+    "uint64",
+    "BasisPoint",
+]
diff --git a/src/lean_spec/subspecs/types/basispt.py b/src/lean_spec/subspecs/types/basispt.py
new file mode 100644
index 00000000..821a4f87
--- /dev/null
+++ b/src/lean_spec/subspecs/types/basispt.py
@@ -0,0 +1,14 @@
+"""Basis Point Type Specification."""
+
+from pydantic import Field
+from typing_extensions import Annotated
+
+from ..types import uint64
+
+# A type alias for basis points
+#
+# A basis point (bps) is 1/100th of a percent. 100% = 10,000 bps.
+BasisPoint = Annotated[
+    uint64,
+    Field(le=10000, description="A value in basis points (1/10000)."),
+]
diff --git a/src/lean_spec/subspecs/types/uint64.py b/src/lean_spec/subspecs/types/uint64.py
new file mode 100644
index 00000000..9a7abb31
--- /dev/null
+++ b/src/lean_spec/subspecs/types/uint64.py
@@ -0,0 +1,10 @@
+"""Unsigned 64-bit Integer Type Specification."""
+
+from pydantic import Field
+from typing_extensions import Annotated
+
+# The maximum value for an unsigned 64-bit integer (2**64).
+UINT64_MAX = 2**64
+
+# A type alias to represent a uint64.
+uint64 = Annotated[int, Field(ge=0, lt=UINT64_MAX)]