diff --git a/src/features/team-mode/team-tasklist/update.test.ts b/src/features/team-mode/team-tasklist/update.test.ts
new file mode 100644
index 000000000..441a7c32a
--- /dev/null
+++ b/src/features/team-mode/team-tasklist/update.test.ts
@@ -0,0 +1,112 @@
+/// <reference types="bun-types" />
+
+import { expect, test } from "bun:test"
+
+import { claimTask } from "./claim"
+import { getTask } from "./get"
+import { createTask } from "./store"
+import { createTaskInput, createTasklistFixture } from "./test-support"
+import { CrossOwnerUpdateError, InvalidTaskTransitionError, updateTaskStatus } from "./update"
+
+test("updateTaskStatus supports the one-way claim to complete flow", async () => {
+  // given
+  const fixture = await createTasklistFixture()
+
+  try {
+    const task = await createTask(fixture.teamRunId, createTaskInput(), fixture.config)
+    await claimTask(fixture.teamRunId, task.id, "member-a", fixture.config)
+
+    // when
+    await updateTaskStatus(fixture.teamRunId, task.id, "in_progress", "member-a", fixture.config)
+    const completedTask = await updateTaskStatus(fixture.teamRunId, task.id, "completed", "member-a", fixture.config)
+    const loadedTask = await getTask(fixture.teamRunId, task.id, fixture.config)
+
+    // then
+    expect(completedTask.status).toBe("completed")
+    expect(loadedTask.status).toBe("completed")
+  } finally {
+    await fixture.cleanup()
+  }
+})
+
+test("updateTaskStatus auto-claims when a member starts a pending task directly", async () => {
+  // given
+  const fixture = await createTasklistFixture()
+
+  try {
+    const task = await createTask(fixture.teamRunId, createTaskInput(), fixture.config)
+
+    // when
+    const inProgressTask = await updateTaskStatus(fixture.teamRunId, task.id, "in_progress", "member-a", fixture.config)
+    const loadedTask = await getTask(fixture.teamRunId, task.id, fixture.config)
+
+    // then
+    expect(inProgressTask.status).toBe("in_progress")
+    expect(inProgressTask.owner).toBe("member-a")
+    expect(typeof inProgressTask.claimedAt).toBe("number")
+    expect(loadedTask.status).toBe("in_progress")
+    expect(loadedTask.owner).toBe("member-a")
+    expect(typeof loadedTask.claimedAt).toBe("number")
+  } finally {
+    await fixture.cleanup()
+  }
+})
+
+test("updateTaskStatus rejects reverse transitions", async () => {
+  // given
+  const fixture = await createTasklistFixture()
+
+  try {
+    const task = await createTask(
+      fixture.teamRunId,
+      createTaskInput({ status: "completed", owner: "member-a", claimedAt: Date.now() }),
+      fixture.config,
+    )
+
+    // when
+    let thrownError: unknown = null
+    try {
+      await updateTaskStatus(fixture.teamRunId, task.id, "claimed", "member-a", fixture.config)
+    } catch (error) {
+      thrownError = error
+    }
+
+    // then
+    expect(thrownError).toBeInstanceOf(InvalidTaskTransitionError)
+    expect(thrownError).toHaveProperty("message", "no reverse transitions from completed to claimed")
+  } finally {
+    await fixture.cleanup()
+  }
+})
+
+test("updateTaskStatus rejects non-owner updates except deletion", async () => {
+  // given
+  const fixture = await createTasklistFixture()
+
+  try {
+    const task = await createTask(
+      fixture.teamRunId,
+      createTaskInput({ status: "claimed", owner: "member-a", claimedAt: Date.now() }),
+      fixture.config,
+    )
+
+    // when
+    let crossOwnerError: unknown = null
+    try {
+      await updateTaskStatus(fixture.teamRunId, task.id, "in_progress", "member-b", fixture.config)
+    } catch (error) {
+      crossOwnerError = error
+    }
+
+    // then
+    expect(crossOwnerError).toBeInstanceOf(CrossOwnerUpdateError)
+
+    // when
+    const deletedTask = await updateTaskStatus(fixture.teamRunId, task.id, "deleted", "lead-member", fixture.config)
+
+    // then
+    expect(deletedTask.status).toBe("deleted")
+  } finally {
+    await fixture.cleanup()
+  }
+})
diff --git a/src/features/team-mode/team-tasklist/update.ts b/src/features/team-mode/team-tasklist/update.ts
new file mode 100644
index 000000000..5aa4f7b04
--- /dev/null
+++ b/src/features/team-mode/team-tasklist/update.ts
@@ -0,0 +1,75 @@
+import path from "node:path"
+
+import type { TeamModeConfig } from "../../../config/schema/team-mode"
+import { getTasksDir, resolveBaseDir } from "../team-registry"
+import { atomicWrite } from "../team-state-store/locks"
+import { TaskSchema } from "../types"
+import type { Task } from "../types"
+import { claimTask } from "./claim"
+import { getTask } from "./get"
+
+const ALLOWED_TRANSITIONS: Readonly<Record<Task["status"], ReadonlyArray<Task["status"]>>> = {
+  pending: ["claimed", "deleted"],
+  claimed: ["in_progress", "deleted"],
+  in_progress: ["completed", "deleted"],
+  completed: ["deleted"],
+  deleted: [],
+}
+
+function isValidTransition(currentStatus: Task["status"], nextStatus: Task["status"]): boolean {
+  if (currentStatus === nextStatus) return true
+  return ALLOWED_TRANSITIONS[currentStatus].includes(nextStatus)
+}
+
+export class InvalidTaskTransitionError extends Error {
+  constructor(currentStatus: Task["status"], nextStatus: Task["status"]) {
+    super(`no reverse transitions from ${currentStatus} to ${nextStatus}`)
+    this.name = "InvalidTaskTransitionError"
+  }
+}
+
+export class CrossOwnerUpdateError extends Error {
+  constructor(message = "cross-owner updates are not allowed") {
+    super(message)
+    this.name = "CrossOwnerUpdateError"
+  }
+}
+
+export async function updateTaskStatus(
+  teamRunId: string,
+  taskId: string,
+  newStatus: Task["status"],
+  memberName: string,
+  config: TeamModeConfig,
+): Promise<Task> {
+  const task = await getTask(teamRunId, taskId, config)
+
+  if (task.status === newStatus) return task
+
+  if (task.status === "pending" && newStatus === "in_progress") {
+    await claimTask(teamRunId, taskId, memberName, config)
+    return updateTaskStatus(teamRunId, taskId, newStatus, memberName, config)
+  }
+
+  if (!isValidTransition(task.status, newStatus)) {
+    throw new InvalidTaskTransitionError(task.status, newStatus)
+  }
+
+  if (newStatus !== "deleted" && task.owner !== memberName) {
+    throw new CrossOwnerUpdateError()
+  }
+
+  const updatedTask = TaskSchema.parse({
+    ...task,
+    status: newStatus,
+    updatedAt: Date.now(),
+  })
+
+  const tasksDirectory = getTasksDir(resolveBaseDir(config), teamRunId)
+  await atomicWrite(
+    path.join(tasksDirectory, `${taskId}.json`),
+    `${JSON.stringify(updatedTask, null, 2)}\n`,
+  )
+
+  return updatedTask
+}