From 1b70b87b8a13a3aa65424f72a253d64ea570ccc0 Mon Sep 17 00:00:00 2001
From: Damilola Edwards <damilolaedwards@gmail.com>
Date: Wed, 19 Aug 2026 10:26:51 +0100
Subject: [PATCH] Fix PreviousFork never selecting a fork at epoch 0

The search seeded its running maximum at epoch 0 and only replaced
it on a strict greater than, so a fork activating at epoch 0 could
never win. Every real chain has its genesis fork at epoch 0, so
PreviousFork returned "no previous fork" on any mainnet-shaped spec.
Changed the comparison to greater than or equal, matching how
CurrentFork already does this same search.
---
 pkg/beacon/state/fork_epoch.go      |  2 +-
 pkg/beacon/state/fork_epoch_test.go | 43 +++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/pkg/beacon/state/fork_epoch.go b/pkg/beacon/state/fork_epoch.go
index f54cce0..74655f6 100644
--- a/pkg/beacon/state/fork_epoch.go
+++ b/pkg/beacon/state/fork_epoch.go
@@ -112,7 +112,7 @@ func (f *ForkEpochs) PreviousFork(epoch phase0.Epoch) (*ForkEpoch, error) {
 	}
 
 	for _, fork := range f.Active(epoch) {
-		if fork.Active(epoch) && fork.Name != current.Name && fork.Epoch > largest.Epoch {
+		if fork.Active(epoch) && fork.Name != current.Name && fork.Epoch >= largest.Epoch {
 			found = true
 
 			largest = fork
diff --git a/pkg/beacon/state/fork_epoch_test.go b/pkg/beacon/state/fork_epoch_test.go
index 02b0153..1e022ff 100644
--- a/pkg/beacon/state/fork_epoch_test.go
+++ b/pkg/beacon/state/fork_epoch_test.go
@@ -232,6 +232,49 @@ func TestForkEpochsPreviousFork(t *testing.T) {
 				Name:  spec.DataVersionPhase0,
 			},
 		},
+		{
+			// Mainnet-shaped: genesis fork activates at epoch 0, which every
+			// real chain has. Regression test for a bug where a fork at
+			// epoch 0 could never be selected as the previous fork.
+			name: "returns genesis fork at epoch 0 as the previous fork",
+			forks: state.ForkEpochs{
+				{
+					Epoch: 0,
+					Name:  spec.DataVersionPhase0,
+				},
+				{
+					Epoch: 74240,
+					Name:  spec.DataVersionAltair,
+				},
+			},
+			epoch: 74241,
+			expected: &state.ForkEpoch{
+				Epoch: 0,
+				Name:  spec.DataVersionPhase0,
+			},
+		},
+		{
+			name: "returns the most recent previous fork across three activated forks",
+			forks: state.ForkEpochs{
+				{
+					Epoch: 0,
+					Name:  spec.DataVersionPhase0,
+				},
+				{
+					Epoch: 100,
+					Name:  spec.DataVersionAltair,
+				},
+				{
+					Epoch: 200,
+					Name:  spec.DataVersionBellatrix,
+				},
+			},
+			epoch: 250,
+			expected: &state.ForkEpoch{
+				Epoch: 100,
+				Name:  spec.DataVersionAltair,
+			},
+		},
 	}
 
 	for _, test := range tests {