fix: add session cursor for tool output

src/shared/index.ts

@@ -22,3 +22,4 @@ export * from "./permission-compat"
 export * from "./external-plugin-detector"
 export * from "./zip-extractor"
 export * from "./agent-variant"
+export * from "./session-cursor"

src/shared/session-cursor.test.ts

@@ -0,0 +1,53 @@
+import { beforeEach, describe, expect, it } from "bun:test"
+import { getNewMessages, resetMessageCursor } from "./session-cursor"
+
+describe("getNewMessages", () => {
+  const sessionID = "session-123"
+
+  const buildMessage = (id: string, created: number) => ({
+    info: { id, time: { created } },
+  })
+
+  beforeEach(() => {
+    resetMessageCursor(sessionID)
+  })
+
+  it("returns all messages on first read and none on repeat", () => {
+    // #given
+    const messages = [buildMessage("m1", 1), buildMessage("m2", 2)]
+
+    // #when
+    const first = getNewMessages(sessionID, messages)
+    const second = getNewMessages(sessionID, messages)
+
+    // #then
+    expect(first).toEqual(messages)
+    expect(second).toEqual([])
+  })
+
+  it("returns only new messages after cursor advances", () => {
+    // #given
+    const messages = [buildMessage("m1", 1), buildMessage("m2", 2)]
+    getNewMessages(sessionID, messages)
+    const extended = [...messages, buildMessage("m3", 3)]
+
+    // #when
+    const next = getNewMessages(sessionID, extended)
+
+    // #then
+    expect(next).toEqual([extended[2]])
+  })
+
+  it("resets when message history shrinks", () => {
+    // #given
+    const messages = [buildMessage("m1", 1), buildMessage("m2", 2)]
+    getNewMessages(sessionID, messages)
+    const shorter = [buildMessage("n1", 1)]
+
+    // #when
+    const next = getNewMessages(sessionID, shorter)
+
+    // #then
+    expect(next).toEqual(shorter)
+  })
+})

src/shared/session-cursor.ts

@@ -0,0 +1,84 @@
+type MessageTime =
+  | { created?: number | string }
+  | number
+  | string
+  | undefined
+
+type MessageInfo = {
+  id?: string
+  time?: MessageTime
+}
+
+export type CursorMessage = {
+  info?: MessageInfo
+}
+
+interface CursorState {
+  lastKey?: string
+  lastCount: number
+}
+
+const sessionCursors = new Map<string, CursorState>()
+
+function buildMessageKey(message: CursorMessage, index: number): string {
+  const id = message.info?.id
+  if (id) return `id:${id}`
+
+  const time = message.info?.time
+  if (typeof time === "number" || typeof time === "string") {
+    return `t:${time}:${index}`
+  }
+
+  const created = time?.created
+  if (typeof created === "number") {
+    return `t:${created}:${index}`
+  }
+  if (typeof created === "string") {
+    return `t:${created}:${index}`
+  }
+
+  return `i:${index}`
+}
+
+export function getNewMessages<T extends CursorMessage>(
+  sessionID: string | undefined,
+  messages: T[]
+): T[] {
+  if (!sessionID) return messages
+
+  const keys = messages.map((message, index) => buildMessageKey(message, index))
+  const cursor = sessionCursors.get(sessionID)
+  let startIndex = 0
+
+  if (cursor) {
+    if (cursor.lastCount > messages.length) {
+      startIndex = 0
+    } else if (cursor.lastKey) {
+      const lastIndex = keys.lastIndexOf(cursor.lastKey)
+      if (lastIndex >= 0) {
+        startIndex = lastIndex + 1
+      } else if (cursor.lastCount <= messages.length) {
+        startIndex = cursor.lastCount
+      }
+    }
+  }
+
+  if (messages.length === 0) {
+    sessionCursors.delete(sessionID)
+  } else {
+    sessionCursors.set(sessionID, {
+      lastKey: keys[keys.length - 1],
+      lastCount: messages.length,
+    })
+  }
+
+  return messages.slice(startIndex)
+}
+
+export function resetMessageCursor(sessionID?: string): void {
+  if (sessionID) {
+    sessionCursors.delete(sessionID)
+    return
+  }
+  sessionCursors.clear()
+}