diff --git a/src/features/background-agent/task-history.test.ts b/src/features/background-agent/task-history.test.ts
new file mode 100644
index 000000000..0afb426b7
--- /dev/null
+++ b/src/features/background-agent/task-history.test.ts
@@ -0,0 +1,170 @@
+import { describe, expect, it } from "bun:test"
+import { TaskHistory } from "./task-history"
+
+describe("TaskHistory", () => {
+  describe("record", () => {
+    it("stores an entry for a parent session", () => {
+      //#given
+      const history = new TaskHistory()
+
+      //#when
+      history.record("parent-1", { id: "t1", agent: "explore", description: "Find auth", status: "pending" })
+
+      //#then
+      const entries = history.getByParentSession("parent-1")
+      expect(entries).toHaveLength(1)
+      expect(entries[0].id).toBe("t1")
+      expect(entries[0].agent).toBe("explore")
+      expect(entries[0].status).toBe("pending")
+    })
+
+    it("ignores undefined parentSessionID", () => {
+      //#given
+      const history = new TaskHistory()
+
+      //#when
+      history.record(undefined, { id: "t1", agent: "explore", description: "Find auth", status: "pending" })
+
+      //#then
+      expect(history.getByParentSession("undefined")).toHaveLength(0)
+    })
+
+    it("upserts without clobbering undefined fields", () => {
+      //#given
+      const history = new TaskHistory()
+      history.record("parent-1", { id: "t1", agent: "explore", description: "Find auth", status: "pending", category: "quick" })
+
+      //#when
+      history.record("parent-1", { id: "t1", agent: "explore", description: "Find auth", status: "running" })
+
+      //#then
+      const entries = history.getByParentSession("parent-1")
+      expect(entries).toHaveLength(1)
+      expect(entries[0].status).toBe("running")
+      expect(entries[0].category).toBe("quick")
+    })
+
+    it("caps entries at MAX_ENTRIES_PER_PARENT (100)", () => {
+      //#given
+      const history = new TaskHistory()
+
+      //#when
+      for (let i = 0; i < 105; i++) {
+        history.record("parent-1", { id: `t${i}`, agent: "explore", description: `Task ${i}`, status: "completed" })
+      }
+
+      //#then
+      const entries = history.getByParentSession("parent-1")
+      expect(entries).toHaveLength(100)
+      expect(entries[0].id).toBe("t5")
+      expect(entries[99].id).toBe("t104")
+    })
+  })
+
+  describe("getByParentSession", () => {
+    it("returns defensive copies", () => {
+      //#given
+      const history = new TaskHistory()
+      history.record("parent-1", { id: "t1", agent: "explore", description: "Find auth", status: "pending" })
+
+      //#when
+      const entries = history.getByParentSession("parent-1")
+      entries[0].status = "completed"
+
+      //#then
+      const fresh = history.getByParentSession("parent-1")
+      expect(fresh[0].status).toBe("pending")
+    })
+
+    it("returns empty array for unknown parent", () => {
+      //#given
+      const history = new TaskHistory()
+
+      //#when
+      const entries = history.getByParentSession("nonexistent")
+
+      //#then
+      expect(entries).toHaveLength(0)
+    })
+  })
+
+  describe("clearSession", () => {
+    it("removes all entries for a parent session", () => {
+      //#given
+      const history = new TaskHistory()
+      history.record("parent-1", { id: "t1", agent: "explore", description: "Find auth", status: "pending" })
+      history.record("parent-2", { id: "t2", agent: "oracle", description: "Review", status: "running" })
+
+      //#when
+      history.clearSession("parent-1")
+
+      //#then
+      expect(history.getByParentSession("parent-1")).toHaveLength(0)
+      expect(history.getByParentSession("parent-2")).toHaveLength(1)
+    })
+  })
+
+  describe("formatForCompaction", () => {
+    it("returns null when no entries exist", () => {
+      //#given
+      const history = new TaskHistory()
+
+      //#when
+      const result = history.formatForCompaction("nonexistent")
+
+      //#then
+      expect(result).toBeNull()
+    })
+
+    it("formats entries with agent, status, and description", () => {
+      //#given
+      const history = new TaskHistory()
+      history.record("parent-1", { id: "t1", agent: "explore", description: "Find auth patterns", status: "completed" })
+
+      //#when
+      const result = history.formatForCompaction("parent-1")
+
+      //#then
+      expect(result).toContain("**explore**")
+      expect(result).toContain("(completed)")
+      expect(result).toContain("Find auth patterns")
+    })
+
+    it("includes category when present", () => {
+      //#given
+      const history = new TaskHistory()
+      history.record("parent-1", { id: "t1", agent: "explore", description: "Find auth", status: "running", category: "quick" })
+
+      //#when
+      const result = history.formatForCompaction("parent-1")
+
+      //#then
+      expect(result).toContain("[quick]")
+    })
+
+    it("includes session_id when present", () => {
+      //#given
+      const history = new TaskHistory()
+      history.record("parent-1", { id: "t1", sessionID: "ses_abc123", agent: "oracle", description: "Review arch", status: "completed" })
+
+      //#when
+      const result = history.formatForCompaction("parent-1")
+
+      //#then
+      expect(result).toContain("`ses_abc123`")
+    })
+
+    it("sanitizes newlines in description", () => {
+      //#given
+      const history = new TaskHistory()
+      history.record("parent-1", { id: "t1", agent: "explore", description: "Line1\nLine2\rLine3", status: "pending" })
+
+      //#when
+      const result = history.formatForCompaction("parent-1")
+
+      //#then
+      expect(result).not.toContain("\n\n")
+      expect(result).toContain("Line1 Line2 Line3")
+    })
+  })
+})
diff --git a/src/features/background-agent/task-history.ts b/src/features/background-agent/task-history.ts
new file mode 100644
index 000000000..333340c62
--- /dev/null
+++ b/src/features/background-agent/task-history.ts
@@ -0,0 +1,75 @@
+import type { BackgroundTaskStatus } from "./types"
+
+const MAX_ENTRIES_PER_PARENT = 100
+
+export interface TaskHistoryEntry {
+  id: string
+  sessionID?: string
+  agent: string
+  description: string
+  status: BackgroundTaskStatus
+  category?: string
+  startedAt?: Date
+  completedAt?: Date
+}
+
+export class TaskHistory {
+  private entries: Map<string, TaskHistoryEntry[]> = new Map()
+
+  record(parentSessionID: string | undefined, entry: TaskHistoryEntry): void {
+    if (!parentSessionID) return
+
+    const list = this.entries.get(parentSessionID) ?? []
+    const existing = list.findIndex((e) => e.id === entry.id)
+
+    if (existing !== -1) {
+      const current = list[existing]
+      list[existing] = {
+        ...current,
+        ...(entry.sessionID !== undefined ? { sessionID: entry.sessionID } : {}),
+        ...(entry.agent !== undefined ? { agent: entry.agent } : {}),
+        ...(entry.description !== undefined ? { description: entry.description } : {}),
+        ...(entry.status !== undefined ? { status: entry.status } : {}),
+        ...(entry.category !== undefined ? { category: entry.category } : {}),
+        ...(entry.startedAt !== undefined ? { startedAt: entry.startedAt } : {}),
+        ...(entry.completedAt !== undefined ? { completedAt: entry.completedAt } : {}),
+      }
+    } else {
+      if (list.length >= MAX_ENTRIES_PER_PARENT) {
+        list.shift()
+      }
+      list.push({ ...entry })
+    }
+
+    this.entries.set(parentSessionID, list)
+  }
+
+  getByParentSession(parentSessionID: string): TaskHistoryEntry[] {
+    const list = this.entries.get(parentSessionID)
+    if (!list) return []
+    return list.map((e) => ({ ...e }))
+  }
+
+  clearSession(parentSessionID: string): void {
+    this.entries.delete(parentSessionID)
+  }
+
+  formatForCompaction(parentSessionID: string): string | null {
+    const list = this.getByParentSession(parentSessionID)
+    if (list.length === 0) return null
+
+    const lines = list.map((e) => {
+      const desc = e.description.replace(/[\n\r]+/g, " ").trim()
+      const parts = [
+        `- **${e.agent}**`,
+        e.category ? `[${e.category}]` : null,
+        `(${e.status})`,
+        `: ${desc}`,
+        e.sessionID ? ` | session: \`${e.sessionID}\`` : null,
+      ]
+      return parts.filter(Boolean).join("")
+    })
+
+    return lines.join("\n")
+  }
+}