diff --git a/src/features/team-mode/team-tasklist/claim.test.ts b/src/features/team-mode/team-tasklist/claim.test.ts
new file mode 100644
index 000000000..9b41abff8
--- /dev/null
+++ b/src/features/team-mode/team-tasklist/claim.test.ts
@@ -0,0 +1,99 @@
+/// <reference types="bun-types" />
+
+import { expect, test } from "bun:test"
+import { writeFile } from "node:fs/promises"
+import path from "node:path"
+
+import { getTasksDir, resolveBaseDir } from "../team-registry"
+import { claimTask, AlreadyClaimedError, BlockedByError } from "./claim"
+import { createTask } from "./store"
+import { createTaskInput, createTasklistFixture } from "./test-support"
+import { updateTaskStatus } from "./update"
+
+test("claimTask allows exactly one concurrent claimant", async () => {
+  // given
+  const fixture = await createTasklistFixture()
+
+  try {
+    const task = await createTask(fixture.teamRunId, createTaskInput(), fixture.config)
+
+    // when
+    const claimResults = await Promise.allSettled([
+      claimTask(fixture.teamRunId, task.id, "member-a", fixture.config),
+      claimTask(fixture.teamRunId, task.id, "member-b", fixture.config),
+    ])
+
+    const successfulClaims = claimResults.filter((result) => result.status === "fulfilled")
+    const failedClaims = claimResults.filter((result) => result.status === "rejected")
+
+    // then
+    expect(successfulClaims).toHaveLength(1)
+    expect(failedClaims).toHaveLength(1)
+    expect(failedClaims[0]?.status).toBe("rejected")
+    if (failedClaims[0]?.status === "rejected") {
+      expect(failedClaims[0].reason).toBeInstanceOf(AlreadyClaimedError)
+    }
+  } finally {
+    await fixture.cleanup()
+  }
+})
+
+test("claimTask rejects blocked tasks until blockers complete", async () => {
+  // given
+  const fixture = await createTasklistFixture()
+
+  try {
+    const blockerTask = await createTask(fixture.teamRunId, createTaskInput({ subject: "blocker" }), fixture.config)
+    const blockedTask = await createTask(
+      fixture.teamRunId,
+      createTaskInput({ subject: "blocked", blockedBy: [blockerTask.id] }),
+      fixture.config,
+    )
+
+    // when
+    let blockedError: unknown = null
+    try {
+      await claimTask(fixture.teamRunId, blockedTask.id, "member-a", fixture.config)
+    } catch (error) {
+      blockedError = error
+    }
+
+    // then
+    expect(blockedError).toBeInstanceOf(BlockedByError)
+
+    // given
+    await claimTask(fixture.teamRunId, blockerTask.id, "member-b", fixture.config)
+    await updateTaskStatus(fixture.teamRunId, blockerTask.id, "in_progress", "member-b", fixture.config)
+    await updateTaskStatus(fixture.teamRunId, blockerTask.id, "completed", "member-b", fixture.config)
+
+    // when
+    const claimedTask = await claimTask(fixture.teamRunId, blockedTask.id, "member-a", fixture.config)
+
+    // then
+    expect(claimedTask.status).toBe("claimed")
+    expect(claimedTask.owner).toBe("member-a")
+  } finally {
+    await fixture.cleanup()
+  }
+})
+
+test("claimTask reaps a stale claim lock before claiming", async () => {
+  // given
+  const fixture = await createTasklistFixture()
+
+    try {
+      const task = await createTask(fixture.teamRunId, createTaskInput(), fixture.config)
+      const tasksDirectory = getTasksDir(resolveBaseDir(fixture.config), fixture.teamRunId)
+      const staleLockPath = path.join(tasksDirectory, "claims", `${task.id}.lock`)
+      await writeFile(staleLockPath, `member-z\n999999\n${Date.now() - 600_000}\n`)
+
+    // when
+    const claimedTask = await claimTask(fixture.teamRunId, task.id, "member-a", fixture.config)
+
+    // then
+    expect(claimedTask.status).toBe("claimed")
+    expect(claimedTask.owner).toBe("member-a")
+  } finally {
+    await fixture.cleanup()
+  }
+})
diff --git a/src/features/team-mode/team-tasklist/claim.ts b/src/features/team-mode/team-tasklist/claim.ts
new file mode 100644
index 000000000..986b83916
--- /dev/null
+++ b/src/features/team-mode/team-tasklist/claim.ts
@@ -0,0 +1,98 @@
+import { access, mkdir } from "node:fs/promises"
+import path from "node:path"
+
+import type { TeamModeConfig } from "../../../config/schema/team-mode"
+import { getTasksDir, resolveBaseDir } from "../team-registry"
+import { atomicWrite, detectStaleLock, reapStaleLock, withLock } from "../team-state-store/locks"
+import { TaskSchema } from "../types"
+import type { Task } from "../types"
+import { canClaim } from "./dependencies"
+import { getTask } from "./get"
+import { listTasks } from "./list"
+
+const CLAIM_STALE_AFTER_MS = 300_000
+
+async function lockExists(lockPath: string): Promise<boolean> {
+  try {
+    await access(lockPath)
+    return true
+  } catch {
+    return false
+  }
+}
+
+function getBlockingTaskIds(task: Task, allTasks: Task[]): string[] {
+  return task.blockedBy.filter((blockerId) => {
+    const blockerTask = allTasks.find((candidateTask) => candidateTask.id === blockerId)
+    return blockerTask !== undefined && blockerTask.status !== "completed"
+  })
+}
+
+export class AlreadyClaimedError extends Error {
+  constructor(message = "already_claimed") {
+    super(message)
+    this.name = "AlreadyClaimedError"
+  }
+}
+
+export class BlockedByError extends Error {
+  constructor(public readonly blockers: string[]) {
+    super(`blocked by ${blockers.join(",")}`)
+    this.name = "BlockedByError"
+  }
+}
+
+export async function claimTask(
+  teamRunId: string,
+  taskId: string,
+  memberName: string,
+  config: TeamModeConfig,
+): Promise<Task> {
+  const baseDirectory = resolveBaseDir(config)
+  const tasksDirectory = getTasksDir(baseDirectory, teamRunId)
+  const claimsDirectory = path.join(tasksDirectory, "claims")
+  const taskPath = path.join(tasksDirectory, `${taskId}.json`)
+  const claimLockPath = path.join(claimsDirectory, `${taskId}.lock`)
+
+  await mkdir(claimsDirectory, { recursive: true, mode: 0o700 })
+
+  const task = await getTask(teamRunId, taskId, config)
+  if (task.status !== "pending") {
+    throw new AlreadyClaimedError()
+  }
+
+  const allTasks = await listTasks(teamRunId, config)
+  if (!canClaim(task, allTasks)) {
+    throw new BlockedByError(getBlockingTaskIds(task, allTasks))
+  }
+
+  if (await detectStaleLock(claimLockPath, CLAIM_STALE_AFTER_MS)) {
+    await reapStaleLock(claimLockPath)
+  } else if (await lockExists(claimLockPath)) {
+    throw new AlreadyClaimedError()
+  }
+
+  return withLock(claimLockPath, async () => {
+    const refreshedTask = await getTask(teamRunId, taskId, config)
+    if (refreshedTask.status !== "pending") {
+      throw new AlreadyClaimedError()
+    }
+
+    const refreshedTasks = await listTasks(teamRunId, config)
+    if (!canClaim(refreshedTask, refreshedTasks)) {
+      throw new BlockedByError(getBlockingTaskIds(refreshedTask, refreshedTasks))
+    }
+
+    const now = Date.now()
+    const updatedTask = TaskSchema.parse({
+      ...refreshedTask,
+      status: "claimed",
+      owner: memberName,
+      claimedAt: now,
+      updatedAt: now,
+    })
+
+    await atomicWrite(taskPath, `${JSON.stringify(updatedTask, null, 2)}\n`)
+    return updatedTask
+  }, { ownerTag: memberName, staleAfterMs: CLAIM_STALE_AFTER_MS })
+}