diff --git a/src/build/roc/Builtin.roc b/src/build/roc/Builtin.roc
index 240d848264d..28c98c296b3 100644
--- a/src/build/roc/Builtin.roc
+++ b/src/build/roc/Builtin.roc
@@ -411,9 +411,7 @@ Builtin :: [].{
 
 			json_dec_digits_are_zero : Str -> Bool
 			json_dec_digits_are_zero = |digits| {
-				bytes = Str.to_utf8(digits)
-
-				for byte in bytes {
+				for byte in Str.iter_utf8(digits) {
 					if byte != 48 {
 						return False
 					}
@@ -424,16 +422,16 @@ Builtin :: [].{
 
 			trim_json_dec_leading_zeros : Str, I64 -> { digits : Str, digits_len : U64, point : I64 }
 			trim_json_dec_leading_zeros = |digits, point| {
-				bytes = Str.to_utf8(digits)
-				len = List.len(bytes)
+				len = Str.count_utf8_bytes(digits)
 				var $index = 0
 
-				while $index < len and list_get_unsafe(bytes, $index) == 48 {
+				while $index < len and str_get_utf8_byte_unsafe(digits, $index) == 48 {
 					$index = $index + 1
 				}
 
 				{
-					digits: Str.from_utf8_lossy(List.drop_first(bytes, $index)),
+					# the dropped prefix is ASCII zeros, so the cut is a UTF-8 boundary
+					digits: str_drop_first_bytes_unsafe(digits, $index),
 					digits_len: len - $index,
 					point: point - $index.to_i64_wrap(),
 				}
@@ -459,9 +457,9 @@ Builtin :: [].{
 						zero_count = point_u64 - digits_len
 						Ok(Str.concat(sign, Str.concat(digits, Str.repeat("0", zero_count))))
 					} else {
-						bytes = Str.to_utf8(digits)
-						before = Str.from_utf8_lossy(List.take_first(bytes, point_u64))
-						after = Str.from_utf8_lossy(List.drop_first(bytes, point_u64))
+						# digits holds only ASCII digits, so any cut is a UTF-8 boundary
+						before = str_substring_unsafe(digits, 0, point_u64)
+						after = str_drop_first_bytes_unsafe(digits, point_u64)
 
 						Ok(Str.concat(sign, Str.concat(before, Str.concat(".", after))))
 					}
@@ -1059,8 +1057,7 @@ Builtin :: [].{
 
 			is_json_number : Str -> Bool
 			is_json_number = |value| {
-				bytes = Str.to_utf8(value)
-				len = List.len(bytes)
+				len = Str.count_utf8_bytes(value)
 
 				if len == 0 {
 					return False
@@ -1068,7 +1065,7 @@ Builtin :: [].{
 
 				var $index = 0
 
-				first = list_get_unsafe(bytes, $index)
+				first = str_get_utf8_byte_unsafe(value, $index)
 
 				if first == 45 {
 					$index = $index + 1
@@ -1078,7 +1075,7 @@ Builtin :: [].{
 					}
 				}
 
-				int_first = list_get_unsafe(bytes, $index)
+				int_first = str_get_utf8_byte_unsafe(value, $index)
 
 				if int_first == 48 {
 					$index = $index + 1
@@ -1086,7 +1083,7 @@ Builtin :: [].{
 					$index = $index + 1
 
 					while $index < len {
-						byte = list_get_unsafe(bytes, $index)
+						byte = str_get_utf8_byte_unsafe(value, $index)
 
 						if Json.is_json_digit(byte) {
 							$index = $index + 1
@@ -1099,7 +1096,7 @@ Builtin :: [].{
 				}
 
 				if $index < len {
-					byte = list_get_unsafe(bytes, $index)
+					byte = str_get_utf8_byte_unsafe(value, $index)
 
 					if byte == 46 {
 						$index = $index + 1
@@ -1108,14 +1105,14 @@ Builtin :: [].{
 							return False
 						}
 
-						first_fraction_byte = list_get_unsafe(bytes, $index)
+						first_fraction_byte = str_get_utf8_byte_unsafe(value, $index)
 
 						if !Json.is_json_digit(first_fraction_byte) {
 							return False
 						}
 
 						while $index < len {
-							fraction_byte = list_get_unsafe(bytes, $index)
+							fraction_byte = str_get_utf8_byte_unsafe(value, $index)
 
 							if Json.is_json_digit(fraction_byte) {
 								$index = $index + 1
@@ -1127,7 +1124,7 @@ Builtin :: [].{
 				}
 
 				if $index < len {
-					byte = list_get_unsafe(bytes, $index)
+					byte = str_get_utf8_byte_unsafe(value, $index)
 
 					if Json.is_json_exponent_marker(byte) {
 						$index = $index + 1
@@ -1136,7 +1133,7 @@ Builtin :: [].{
 							return False
 						}
 
-						exponent_first_byte = list_get_unsafe(bytes, $index)
+						exponent_first_byte = str_get_utf8_byte_unsafe(value, $index)
 
 						if Json.is_json_sign(exponent_first_byte) {
 							$index = $index + 1
@@ -1146,14 +1143,14 @@ Builtin :: [].{
 							}
 						}
 
-						first_exponent_digit = list_get_unsafe(bytes, $index)
+						first_exponent_digit = str_get_utf8_byte_unsafe(value, $index)
 
 						if !Json.is_json_digit(first_exponent_digit) {
 							return False
 						}
 
 						while $index < len {
-							exponent_byte = list_get_unsafe(bytes, $index)
+							exponent_byte = str_get_utf8_byte_unsafe(value, $index)
 
 							if Json.is_json_digit(exponent_byte) {
 								$index = $index + 1
diff --git a/test/alloc-count/app.roc b/test/alloc-count/app.roc
index b7f63124847..d98ca35ee8d 100644
--- a/test/alloc-count/app.roc
+++ b/test/alloc-count/app.roc
@@ -99,6 +99,15 @@ check_json_parse_allocs! = |input| {
 	skip_allocs = Host.alloc_count!() - skip_before
 	expect skip_allocs == 0
 	expect skipped == Ok({ a: 7 })
+
+	# a skipped numeric field: scalar validation must not allocate either
+	num_doc = Str.concat("{\"z\":", Str.concat(Str.repeat("7", 1 + Str.count_utf8_bytes(input) % 3), ",\"a\":1}"))
+	skip_num_before = Host.alloc_count!()
+	skipped_num : Try({ a : U64 }, Json.ParseErr)
+	skipped_num = Json.parse(num_doc)
+	skip_num_allocs = Host.alloc_count!() - skip_num_before
+	expect skip_num_allocs == 0
+	expect skipped_num == Ok({ a: 1 })
 	{}
 }