From 2cd686a669716f4d469df9cd3718c0e9a1a046b1 Mon Sep 17 00:00:00 2001
From: Lukasz Raczylo <lukasz@raczylo.com>
Date: Tue, 29 Sep 2026 12:26:15 +0100
Subject: [PATCH] perf(stream): append text deltas as JSON without an
 intermediate Marshal

---
 translate_anthropic.go      | 66 +++++++++++++++++++++++++++++++++----
 translate_anthropic_test.go | 17 ++++++++++
 2 files changed, 77 insertions(+), 6 deletions(-)

diff --git a/translate_anthropic.go b/translate_anthropic.go
index 4935b9d..672ca73 100644
--- a/translate_anthropic.go
+++ b/translate_anthropic.go
@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"strconv"
 	"strings"
+	"unicode/utf8"
 )
 
 // anthropicAPIVersion is the "anthropic-version" header value every request
@@ -776,16 +777,69 @@ func (c *contentChunker) set(id, model string, created int64) {
 }
 
 func (c *contentChunker) build(text string) []byte {
-	tb, err := json.Marshal(text)
-	if err != nil {
-		panic(fmt.Sprintf("llmgateway: stream chunk failed to marshal text: %v", err))
-	}
-	out := make([]byte, 0, len(contentChunkHead)+len(tb)+len(c.tail))
+	out := make([]byte, 0, len(contentChunkHead)+len(text)+len(text)/8+2+len(c.tail))
 	out = append(out, contentChunkHead...)
-	out = append(out, tb...)
+	out = appendJSONString(out, text)
 	return append(out, c.tail...)
 }
 
+const jsonHex = "0123456789abcdef"
+
+// appendJSONString appends s as a quoted JSON string using
+// encoding/json's default escaping (HTML characters, U+2028/U+2029 and
+// invalid UTF-8 included), so its output equals json.Marshal(s).
+func appendJSONString(dst []byte, s string) []byte {
+	dst = append(dst, '"')
+	start := 0
+	for i := 0; i < len(s); {
+		b := s[i]
+		if b < utf8.RuneSelf {
+			if b >= 0x20 && b != '"' && b != '\\' && b != '<' && b != '>' && b != '&' {
+				i++
+				continue
+			}
+			dst = append(dst, s[start:i]...)
+			switch b {
+			case '"', '\\':
+				dst = append(dst, '\\', b)
+			case '\b':
+				dst = append(dst, '\\', 'b')
+			case '\f':
+				dst = append(dst, '\\', 'f')
+			case '\n':
+				dst = append(dst, '\\', 'n')
+			case '\r':
+				dst = append(dst, '\\', 'r')
+			case '\t':
+				dst = append(dst, '\\', 't')
+			default:
+				dst = append(dst, '\\', 'u', '0', '0', jsonHex[b>>4], jsonHex[b&0xF])
+			}
+			i++
+			start = i
+			continue
+		}
+		r, size := utf8.DecodeRuneInString(s[i:])
+		if r == utf8.RuneError && size == 1 {
+			dst = append(dst, s[start:i]...)
+			dst = append(dst, "\xef\xbf\xbd"...)
+			i += size
+			start = i
+			continue
+		}
+		if r == '\u2028' || r == '\u2029' {
+			dst = append(dst, s[start:i]...)
+			dst = append(dst, '\\', 'u', '2', '0', '2', jsonHex[r&0xF])
+			i += size
+			start = i
+			continue
+		}
+		i += size
+	}
+	dst = append(dst, s[start:]...)
+	return append(dst, '"')
+}
+
 // usageChunk builds the final usage-only "chat.completion.chunk" payload —
 // empty "choices" plus a populated "usage" — mirroring the shape
 // provider_openai.go's forwardStream relays from a native OpenAI upstream
diff --git a/translate_anthropic_test.go b/translate_anthropic_test.go
index d630812..757b112 100644
--- a/translate_anthropic_test.go
+++ b/translate_anthropic_test.go
@@ -1150,3 +1150,20 @@ func TestContentChunkerMatchesChunk(t *testing.T) {
 		}
 	}
 }
+
+func TestAppendJSONStringMatchesMarshal(t *testing.T) {
+	var cases []string
+	for r := rune(0); r < 0x250; r++ {
+		cases = append(cases, string(r), "a"+string(r)+"b")
+	}
+	cases = append(cases, "\u2028", "\u2029", "x\u2028y\u2029z", "bad\xffutf8", "\xc3", "\xe2\x80", "\U0001F600 emoji", "", "<script>alert('&')</script>")
+	for _, s := range cases {
+		want, err := json.Marshal(s)
+		if err != nil {
+			t.Fatalf("Marshal(%q): %v", s, err)
+		}
+		if got := appendJSONString(nil, s); !bytes.Equal(got, want) {
+			t.Errorf("appendJSONString(%q) = %s, want %s", s, got, want)
+		}
+	}
+}