fix(gateway): preserve utf8-heavy chat history text

navendugoyal19 · navendugoyal19 · commit df04b818de86 · 2026-04-01T08:42:00.000+08:00
Address the non-ASCII chat.history regression by adding a byte-aware fallback to text truncation. Keep long UTF-8 replies readable instead of replacing them with the oversized placeholder, and cover the behavior with a regression test for emoji-heavy assistant text.
diff --git a/src/gateway/server-methods/chat.ts b/src/gateway/server-methods/chat.ts
@@ -96,7 +96,9 @@ type ChatAbortRequester = {
 
 const CHAT_HISTORY_TEXT_MAX_CHARS = 64_000;
 const CHAT_HISTORY_MAX_SINGLE_MESSAGE_BYTES = 128 * 1024;
+const CHAT_HISTORY_TEXT_MAX_BYTES = CHAT_HISTORY_MAX_SINGLE_MESSAGE_BYTES - 8 * 1024;
 const CHAT_HISTORY_OVERSIZED_PLACEHOLDER = "[chat.history omitted: message too large]";
+const CHAT_HISTORY_TRUNCATED_SUFFIX = "\n...(truncated)...";
 let chatHistoryPlaceholderEmitCount = 0;
 const CHANNEL_AGNOSTIC_SESSION_SCOPES = new Set([
   "main",
@@ -405,11 +407,45 @@ async function rewriteChatSendUserTurnMediaPaths(params: {
 }
 
 function truncateChatHistoryText(text: string): { text: string; truncated: boolean } {
-  if (text.length <= CHAT_HISTORY_TEXT_MAX_CHARS) {
+  if (
+    text.length <= CHAT_HISTORY_TEXT_MAX_CHARS &&
+    Buffer.byteLength(text, "utf8") <= CHAT_HISTORY_TEXT_MAX_BYTES
+  ) {
     return { text, truncated: false };
   }
+
+  let next = text;
+  let truncated = false;
+
+  if (next.length > CHAT_HISTORY_TEXT_MAX_CHARS) {
+    next = next.slice(0, CHAT_HISTORY_TEXT_MAX_CHARS);
+    truncated = true;
+  }
+
+  if (Buffer.byteLength(next, "utf8") > CHAT_HISTORY_TEXT_MAX_BYTES) {
+    const suffixBytes = Buffer.byteLength(CHAT_HISTORY_TRUNCATED_SUFFIX, "utf8");
+    const maxPrefixBytes = Math.max(0, CHAT_HISTORY_TEXT_MAX_BYTES - suffixBytes);
+    let low = 0;
+    let high = next.length;
+    while (low < high) {
+      const mid = Math.ceil((low + high) / 2);
+      const candidate = next.slice(0, mid);
+      if (Buffer.byteLength(candidate, "utf8") <= maxPrefixBytes) {
+        low = mid;
+      } else {
+        high = mid - 1;
+      }
+    }
+    next = next.slice(0, low);
+    truncated = true;
+  }
+
+  if (!truncated) {
+    return { text: next, truncated: false };
+  }
+
   return {
-    text: `${text.slice(0, CHAT_HISTORY_TEXT_MAX_CHARS)}\n...(truncated)...`,
+    text: `${next}${CHAT_HISTORY_TRUNCATED_SUFFIX}`,
     truncated: true,
   };
 }
diff --git a/src/gateway/server.chat.gateway-server-chat-b.test.ts b/src/gateway/server.chat.gateway-server-chat-b.test.ts
@@ -321,6 +321,41 @@ describe("gateway server chat", () => {
     });
   });
 
+  test("chat.history truncates UTF-8-heavy assistant text before oversized placeholder fallback", async () => {
+    await withGatewayChatHarness(async ({ ws, createSessionDir }) => {
+      const historyMaxBytes = 128 * 1024;
+      const sessionDir = await prepareMainHistoryHarness({
+        ws,
+        createSessionDir,
+        historyMaxBytes,
+      });
+
+      const emojiText = "😀".repeat(40_000);
+      await writeMainSessionTranscript(sessionDir, [
+        JSON.stringify({
+          message: {
+            role: "assistant",
+            timestamp: Date.now(),
+            content: [{ type: "text", text: emojiText }],
+          },
+        }),
+      ]);
+
+      const messages = await fetchHistoryMessages(ws);
+      expect(messages).toHaveLength(1);
+
+      const first = messages[0] as { content?: Array<{ text?: string }> };
+      const rendered = first.content?.[0]?.text ?? "";
+      expect(rendered).toContain("...(truncated)...");
+      expect(rendered).not.toContain("[chat.history omitted: message too large]");
+
+      const serialized = JSON.stringify(messages);
+      const bytes = Buffer.byteLength(serialized, "utf8");
+      expect(bytes).toBeLessThanOrEqual(historyMaxBytes);
+      expect(serialized).not.toContain("[chat.history omitted: message too large]");
+    });
+  });
+
   test("chat.history preserves usage and cost metadata for assistant messages", async () => {
     await withGatewayChatHarness(async ({ ws, createSessionDir }) => {
       await connectOk(ws);
