Page chat history at turn boundaries

src/server/sessions/messagePaging.test.ts

@@ -0,0 +1,42 @@
+import { describe, expect, it } from "vitest";
+import { expandStartToSafeBoundary, pageMessagesAtSafeBoundary } from "./messagePaging";
+
+const user = (content: string) => ({ role: "user", content });
+const assistantText = (text: string) => ({ role: "assistant", content: [{ type: "text", text }] });
+const thinking = (text: string) => ({ role: "assistant", content: [{ type: "thinking", thinking: text }] });
+const toolCall = (name: string) => ({ role: "assistant", content: [{ type: "toolCall", name }] });
+const toolResult = (text: string) => ({ role: "toolResult", content: text });
+
+function page(start: number, total: number, messages: unknown[]) {
+  return { start, total, messages };
+}
+
+describe("message paging", () => {
+  it("returns the raw messages when paging is not requested", () => {
+    const messages = [user("hello")];
+    expect(pageMessagesAtSafeBoundary(messages)).toBe(messages);
+  });
+
+  it("uses normal bounded paging when the requested start is already a turn boundary", () => {
+    const messages = [user("a"), assistantText("b"), user("c")];
+    expect(pageMessagesAtSafeBoundary(messages, { limit: 1 })).toEqual(page(2, 3, messages.slice(2)));
+  });
+
+  it("expands a page start backward to avoid splitting a turn", () => {
+    const messages = [
+      user("prompt"),
+      thinking("plan"),
+      toolCall("read"),
+      toolResult("ok"),
+      thinking("next"),
+      assistantText("answer"),
+    ];
+
+    expect(pageMessagesAtSafeBoundary(messages, { before: 5, limit: 2 })).toEqual(page(0, 6, messages.slice(0, 5)));
+  });
+
+  it("does not split a readable assistant answer from its user prompt", () => {
+    const messages = [user("prompt"), thinking("plan"), assistantText("answer")];
+    expect(expandStartToSafeBoundary(messages, 2)).toBe(0);
+  });
+});