From 8c0528cdad40ff465b6a1dd3535de9564338f677 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 11 Mar 2026 16:18:18 +0300
Subject: [PATCH] json: encodes differently unicode strings yet decoded well
 (fixes #25103)

---
 vlib/json/json_primitives.c.v              | 63 +++++++++++++++++++++-
 vlib/json/tests/json_encode_primite_test.v | 14 +++++
 2 files changed, 75 insertions(+), 2 deletions(-)

diff --git a/vlib/json/json_primitives.c.v b/vlib/json/json_primitives.c.v
index e1e718d11..29d54c720 100644
--- a/vlib/json/json_primitives.c.v
+++ b/vlib/json/json_primitives.c.v
@@ -36,6 +36,8 @@ fn C.cJSON_CreateBool(bool) &C.cJSON
 
 fn C.cJSON_CreateString(&char) &C.cJSON
 
+fn C.cJSON_CreateRaw(&char) &C.cJSON
+
 fn C.cJSON_Parse(&char) &C.cJSON
 
 fn C.cJSON_PrintUnformatted(&C.cJSON) &char
@@ -251,12 +253,69 @@ fn encode_bool(val bool) &C.cJSON {
 
 @[markused]
 fn encode_rune(val rune) &C.cJSON {
-	return C.cJSON_CreateString(&char(val.str().str))
+	return C.cJSON_CreateRaw(&char(json_ascii_string(val.str()).str))
 }
 
 @[markused]
 fn encode_string(val string) &C.cJSON {
-	return C.cJSON_CreateString(&char(val.str))
+	return C.cJSON_CreateRaw(&char(json_ascii_string(val).str))
+}
+
+// json_ascii_string returns a quoted JSON string with non-ASCII runes escaped as `\uXXXX`.
+fn json_ascii_string(val string) string {
+	mut output := []u8{cap: val.len + 2}
+	output << `"`
+	for character in val.runes() {
+		match character {
+			`"` {
+				output << `\\`
+				output << `"`
+			}
+			`\\` {
+				output << `\\`
+				output << `\\`
+			}
+			`\b` {
+				output << `\\`
+				output << `b`
+			}
+			`\f` {
+				output << `\\`
+				output << `f`
+			}
+			`\n` {
+				output << `\\`
+				output << `n`
+			}
+			`\r` {
+				output << `\\`
+				output << `r`
+			}
+			`\t` {
+				output << `\\`
+				output << `t`
+			}
+			else {
+				if character < 0x20 || character > 0x7f {
+					if character <= 0xffff {
+						output << `\\`
+						output << `u`
+						hex_string := '${u32(character):04x}'
+						unsafe { output.push_many(hex_string.str, 4) }
+					} else {
+						unicode_point_low := u32(character) - 0x10000
+						surrogate_pair := '\\u${0xD800 + ((unicode_point_low >> 10) & 0x3FF):04X}\\u${
+							0xDC00 + (unicode_point_low & 0x3FF):04x}'
+						unsafe { output.push_many(surrogate_pair.str, surrogate_pair.len) }
+					}
+				} else {
+					output << u8(character)
+				}
+			}
+		}
+	}
+	output << `"`
+	return output.bytestr()
 }
 
 // json_float_to_raw_string uses V's float formatter so json.encode keeps exact float round-trips.
diff --git a/vlib/json/tests/json_encode_primite_test.v b/vlib/json/tests/json_encode_primite_test.v
index b68318b64..4875dc0bb 100644
--- a/vlib/json/tests/json_encode_primite_test.v
+++ b/vlib/json/tests/json_encode_primite_test.v
@@ -8,6 +8,10 @@ type MyInt = int
 type MyString = string
 type MySumType = MyString | int | string
 
+struct UnicodeString {
+	emoji string
+}
+
 fn test_alias_to_primitive() {
 	mut test := Test{
 		field: MyString('foo')
@@ -38,3 +42,13 @@ fn test_alias_to_primitive() {
 	encoded = json.encode(test3)
 	assert dump(encoded) == '1000'
 }
+
+fn test_encode_unicode_as_ascii_escape_sequences() {
+	valid_json := r'{"emoji":"\u3007"}'
+	decoded := json.decode(UnicodeString, valid_json)!
+	assert decoded.emoji == '〇'
+	assert json.encode(UnicodeString{
+		emoji: '〇'
+	}) == valid_json
+	assert json.encode('😀') == r'"\uD83D\ude00"'
+}
-- 
2.39.5