diff --git a/pkg/chain/chain.go b/pkg/chain/chain.go
index b04ce3b..e9fadc2 100644
--- a/pkg/chain/chain.go
+++ b/pkg/chain/chain.go
@@ -3,6 +3,7 @@ package chain
 import (
 	"errors"
 	"fmt"
+	"strconv"
 	"strings"
 )
 
@@ -75,3 +76,20 @@ func ChainFromInt(chainID uint64) (Chain, error) {
 	}
 	return "", fmt.Errorf("unknown chain ID: %d", chainID)
 }
+
+// ParseChainIDParam parses a chain_id query param. Empty means 0 ("all chains").
+// A literal 0 is rejected unless allowZero is true.
+func ParseChainIDParam(raw string, allowZero bool) (uint64, error) {
+	s := strings.TrimSpace(raw)
+	if s == "" {
+		return 0, nil
+	}
+	id, err := strconv.ParseUint(s, 10, 64)
+	if err != nil {
+		return 0, fmt.Errorf("invalid chain_id %q: %w", raw, err)
+	}
+	if id == 0 && !allowZero {
+		return 0, errors.New("chain_id must be a non-zero uint64")
+	}
+	return id, nil
+}
diff --git a/pkg/chain/chain_test.go b/pkg/chain/chain_test.go
index 3f39321..91e2b3a 100644
--- a/pkg/chain/chain_test.go
+++ b/pkg/chain/chain_test.go
@@ -70,3 +70,42 @@ func TestGenesisTime(t *testing.T) {
 	_, ok := chain.GenesisTime("unknown")
 	require.False(t, ok)
 }
+
+func TestParseChainIDParam(t *testing.T) {
+	cases := []struct {
+		raw       string
+		allowZero bool
+		want      uint64
+		wantErr   bool
+	}{
+		{"", false, 0, false},
+		{"   ", false, 0, false},
+		{"1", false, 1, false},
+		{" 1 ", false, 1, false},
+		{"18446744073709551615", false, 18446744073709551615, false},
+		{"0", false, 0, true},
+		{"00", false, 0, true},
+		{"-1", false, 0, true},
+		{"+1", false, 0, true},
+		{"abc", false, 0, true},
+		{"1.0", false, 0, true},
+		{"18446744073709551616", false, 0, true},
+		{"", true, 0, false},
+		{"0", true, 0, false},
+		{"00", true, 0, false},
+		{"1", true, 1, false},
+		{"-1", true, 0, true},
+		{"abc", true, 0, true},
+		{"18446744073709551616", true, 0, true},
+	}
+	for _, c := range cases {
+		id, err := chain.ParseChainIDParam(c.raw, c.allowZero)
+		if c.wantErr {
+			require.Error(t, err)
+			require.Equal(t, uint64(0), id)
+			continue
+		}
+		require.NoError(t, err)
+		require.Equal(t, c.want, id)
+	}
+}