we should fit truncation in 20KB

Author: RulaKhaled

packages/core/src/tracing/ai/messageTruncation.ts

@@ -62,15 +62,15 @@ const jsonBytes = (value: unknown): number => {
 };
 
 /**
- * Truncate a string to fit within maxBytes when encoded as UTF-8.
+ * Truncate a string to fit within maxBytes (inclusive) when encoded as UTF-8.
  * Uses binary search for efficiency with multi-byte characters.
  *
  * @param text - The string to truncate
- * @param maxBytes - Maximum byte length (UTF-8 encoded)
- * @returns Truncated string that fits within maxBytes
+ * @param maxBytes - Maximum byte length (inclusive, UTF-8 encoded)
+ * @returns Truncated string whose UTF-8 byte length is at most maxBytes
  */
 function truncateTextByBytes(text: string, maxBytes: number): string {
-  if (utf8Bytes(text) < maxBytes) {
+  if (utf8Bytes(text) <= maxBytes) {
     return text;
   }
 
@@ -83,7 +83,7 @@ function truncateTextByBytes(text: string, maxBytes: number): string {
     const candidate = text.slice(0, mid);
     const byteSize = utf8Bytes(candidate);
 
-    if (byteSize < maxBytes) {
+    if (byteSize <= maxBytes) {
       bestFit = candidate;
       low = mid + 1;
     } else {

packages/core/test/lib/tracing/ai-message-truncation.test.ts

@@ -428,7 +428,7 @@ describe('message truncation utilities', () => {
     it('keeps only the last message with truncation when it does not fit the limit', () => {
       const messages = [{ content: `1 ${humongous}` }, { content: `2 ${humongous}` }, { content: `3 ${humongous}` }];
       const result = truncateGenAiMessages(messages);
-      const truncLen = 20_000 - 2 - JSON.stringify({ content: '' }).length - 1;
+      const truncLen = 20_000 - 2 - JSON.stringify({ content: '' }).length;
       expect(result).toStrictEqual([{ content: `3 ${humongous}`.substring(0, truncLen) }]);
     });
 
@@ -450,7 +450,7 @@ describe('message truncation utilities', () => {
     it('truncates if the message content string will not fit', () => {
       const messages = [{ content: `2 ${humongous}` }];
       const result = truncateGenAiMessages(messages);
-      const truncLen = 20_000 - 2 - JSON.stringify({ content: '' }).length - 1;
+      const truncLen = 20_000 - 2 - JSON.stringify({ content: '' }).length;
       expect(result).toStrictEqual([{ content: `2 ${humongous}`.substring(0, truncLen) }]);
     });
 
@@ -491,8 +491,7 @@ describe('message truncation utilities', () => {
         2 -
         JSON.stringify({
           parts: ['', { some_other_field: 'no text here', text: '' }],
-        }).length -
-        1;
+        }).length;
 
       expect(result).toStrictEqual([
         {
@@ -513,8 +512,7 @@ describe('message truncation utilities', () => {
         2 -
         JSON.stringify({
           parts: [{ text: '' }],
-        }).length -
-        1;
+        }).length;
       expect(result).toStrictEqual([
         {
           parts: [