diff --git a/internal/validation/conversion.go b/internal/validation/conversion.go
index c313bab8..ab0d2195 100644
--- a/internal/validation/conversion.go
+++ b/internal/validation/conversion.go
@@ -14,9 +14,15 @@ func GetIntFromInterface(value interface{}) (int, bool) {
 		return v, true
 	case float64:
 		// Reject fractional or out-of-range values rather than silently truncating
-		// JSON-derived numbers (e.g. 3.9 -> 3). Bounds use platform int width so
-		// 32-bit targets (js/wasm) don't accept values that overflow int.
-		if v != math.Trunc(v) || v < math.MinInt || v > math.MaxInt {
+		// JSON-derived numbers (e.g. 3.9 -> 3). Bounds use math.MaxInt so this
+		// stays correct across platform int widths.
+		//
+		// float64(math.MaxInt) itself rounds up to 2^63 on platforms where int is
+		// 64 bits (2^63-1 isn't exactly representable), so comparing v against it
+		// directly would wrongly reject the true max int on platforms where MaxInt
+		// fits exactly in a float64 (e.g. a 32-bit int). Add 1 to get the true
+		// exclusive upper bound on either width.
+		if v != math.Trunc(v) || v < math.MinInt || v >= float64(math.MaxInt)+1 {
 			return 0, false
 		}
 		return int(v), true
diff --git a/internal/validation/conversion_test.go b/internal/validation/conversion_test.go
index 8a13ff36..812ce173 100644
--- a/internal/validation/conversion_test.go
+++ b/internal/validation/conversion_test.go
@@ -8,6 +8,11 @@ import (
 )
 
 func TestGetIntFromInterface(t *testing.T) {
+	// Derived from math.MaxInt (rather than a hardcoded 2^63) so the boundary
+	// cases stay in range regardless of platform int width.
+	nearMaxInt := float64(math.MaxInt) - 4096
+	justAboveMaxInt := float64(math.MaxInt) + 4096
+
 	tests := []struct {
 		name  string
 		value interface{}
@@ -20,6 +25,8 @@ func TestGetIntFromInterface(t *testing.T) {
 		{"float64 whole", float64(10), 10, true},
 		{"float64 fractional rejected", float64(3.9), 0, false},
 		{"float64 above int range rejected", float64(math.MaxInt) * 2, 0, false},
+		{"float64 just above max int rejected", justAboveMaxInt, 0, false},
+		{"float64 just below max int accepted", nearMaxInt, int(nearMaxInt), true},
 		{"numeric string", "123", 123, true},
 		{"negative numeric string", "-5", -5, true},
 		{"empty string", "", 0, false},