feat(agent-ui): cut transcript through Nth user turn (A6.197)

packages/agent-ui/README.md

@@ -312,6 +312,7 @@ consumption of shared panel shells:
 | A6.194 | Apply session workspace target |
 | A6.195 | Remove session + next active id |
 | A6.196 | Maybe auto-derive untitled session title |
+| A6.197 | Cut transcript through Nth user turn |
 
 ### A5 complete enough
 

packages/agent-ui/src/__tests__/cutThroughNthUser.test.ts

@@ -0,0 +1,25 @@
+import { describe, expect, it } from "vitest";
+import { cutThroughNthUserMessage } from "../lib/cutThroughNthUser.js";
+
+describe("cutThroughNthUserMessage", () => {
+  const msgs = [
+    { role: "user", id: "1" },
+    { role: "assistant", id: "2" },
+    { role: "user", id: "3" },
+    { role: "assistant", id: "4" },
+  ];
+  it("keeps through Nth user", () => {
+    expect(cutThroughNthUserMessage(msgs, 1).map((m) => m.id)).toEqual(["1"]);
+    expect(cutThroughNthUserMessage(msgs, 2).map((m) => m.id)).toEqual([
+      "1",
+      "2",
+      "3",
+    ]);
+  });
+  it("noop when fewer users", () => {
+    expect(cutThroughNthUserMessage(msgs, 9)).toHaveLength(4);
+  });
+  it("empty when keep <= 0", () => {
+    expect(cutThroughNthUserMessage(msgs, 0)).toEqual([]);
+  });
+});

packages/agent-ui/src/index.ts

@@ -901,6 +901,8 @@ export {
   nextActiveIdAfterDelete,
 } from "./lib/removeSessionFromList.js";
 export { maybeDeriveSessionTitleList } from "./lib/maybeDeriveSessionTitle.js";
+export { cutThroughNthUserMessage } from "./lib/cutThroughNthUser.js";
+export type { MessageWithRole } from "./lib/cutThroughNthUser.js";
 export type { DerivableSession } from "./lib/maybeDeriveSessionTitle.js";
 export type {
   SessionWorkspaceTarget,

packages/agent-ui/src/lib/cutThroughNthUser.ts

@@ -0,0 +1,23 @@
+/**
+ * Pure rewind-window cut for chat messages (A6.197).
+ * Keep the first N user turns (inclusive) and drop everything after.
+ */
+
+export type MessageWithRole = { role: string };
+
+/**
+ * Truncate after the Nth user message. `keep <= 0` → empty.
+ * If fewer than N user messages exist, return a shallow copy of the input.
+ */
+export function cutThroughNthUserMessage<T extends MessageWithRole>(
+  messages: readonly T[],
+  keep: number,
+): T[] {
+  if (keep <= 0) return [];
+  let seen = 0;
+  for (let i = 0; i < messages.length; i++) {
+    if (messages[i]!.role === "user") seen++;
+    if (seen >= keep) return messages.slice(0, i + 1) as T[];
+  }
+  return messages.slice() as T[];
+}

src/modules/ai/store/chatStore.ts

@@ -9,6 +9,8 @@ import {
   removeSessionFromList,
   nextActiveIdAfterDelete,
   maybeDeriveSessionTitleList,
+  cutThroughNthUserMessage,
+  joinMessageTextParts,
 } from "@altai/agent-ui";
 import type { UIMessage } from "ai";
 import { native } from "../lib/native";
@@ -1117,11 +1119,9 @@ useChatStore.subscribe((state, prev) => {
 
 /** Plain-text body of a user message (text parts joined). */
 function userMessageText(m: UIMessage): string {
-  return m.parts
-    .filter((p): p is { type: "text"; text: string } => p.type === "text")
-    .map((p) => p.text)
-    .join("\n")
-    .trim();
+  return joinMessageTextParts(
+    m.parts as { type?: string; text?: string }[],
+  ).trim();
 }
 
 /**
@@ -1131,16 +1131,6 @@ function userMessageText(m: UIMessage): string {
  * (matches the backend no-op). Mirrors `TruncateAfterUserMessage` so the
  * frontend transcript stays in sync after a rewind.
  */
-function cutThroughNthUser(messages: UIMessage[], keep: number): UIMessage[] {
-  if (keep <= 0) return [];
-  let seen = 0;
-  for (let i = 0; i < messages.length; i++) {
-    if (messages[i].role === "user") seen++;
-    if (seen >= keep) return messages.slice(0, i + 1);
-  }
-  return messages.slice();
-}
-
 function workspacePathForChat(chatId?: string | null): string | undefined {
   const state = useChatStore.getState();
   const id = chatId ?? state.activeSessionId;
@@ -1198,7 +1188,7 @@ async function rewindAndResend(
     });
     return false;
   }
-  const cut = cutThroughNthUser(
+  const cut = cutThroughNthUserMessage(
     useChatStore.getState().nativeMessages,
     keepUserMessages,
   );