diff --git a/packages/queue/src/index.ts b/packages/queue/src/index.ts
index 9998ac0..1a005ff 100644
--- a/packages/queue/src/index.ts
+++ b/packages/queue/src/index.ts
@@ -15,10 +15,22 @@ export type {
 export {
   RedisTaskRepository,
   TaskAlreadyExistsError,
+  TaskAtomicConflictError,
   TaskNotFoundError,
+  TaskOwnershipError,
   TaskSerializationError,
+  TaskTransitionError,
+} from './task-repository.js';
+export type {
+  ClaimTaskInput,
+  CompleteTaskInput,
+  FailTaskInput,
+  HeartbeatTaskInput,
+  RedisTaskClient,
+  RedisTaskRepositoryOptions,
+  RedisTaskTransaction,
+  ReleaseTaskInput,
 } from './task-repository.js';
-export type { RedisTaskClient, RedisTaskRepositoryOptions } from './task-repository.js';
 export {
   TASK_LANES,
   TASK_PRIORITIES,
diff --git a/packages/queue/src/task-repository.ts b/packages/queue/src/task-repository.ts
index b31f8b8..30d504a 100644
--- a/packages/queue/src/task-repository.ts
+++ b/packages/queue/src/task-repository.ts
@@ -16,6 +16,7 @@ const PRIORITY_SET = new Set<TaskPriority>(TASK_PRIORITIES);
 const LANE_SET = new Set<TaskLane>(TASK_LANES);
 
 const DEFAULT_KEY_PREFIX = 'mosaic:queue';
+const MAX_ATOMIC_RETRIES = 8;
 
 interface RepositoryKeys {
   readonly taskIds: string;
@@ -27,6 +28,15 @@ export interface RedisTaskClient {
   set(key: string, value: string, mode?: 'NX' | 'XX'): Promise<'OK' | null>;
   smembers(key: string): Promise<string[]>;
   sadd(key: string, member: string): Promise<number>;
+  watch(...keys: string[]): Promise<'OK'>;
+  unwatch(): Promise<'OK'>;
+  multi(): RedisTaskTransaction;
+}
+
+export interface RedisTaskTransaction {
+  set(key: string, value: string, mode?: 'NX' | 'XX'): RedisTaskTransaction;
+  sadd(key: string, member: string): RedisTaskTransaction;
+  exec(): Promise<readonly (readonly [Error | null, unknown])[] | null>;
 }
 
 export interface RedisTaskRepositoryOptions {
@@ -35,6 +45,30 @@ export interface RedisTaskRepositoryOptions {
   readonly now?: () => number;
 }
 
+export interface ClaimTaskInput {
+  readonly agentId: string;
+  readonly ttlSeconds: number;
+}
+
+export interface ReleaseTaskInput {
+  readonly agentId?: string;
+}
+
+export interface HeartbeatTaskInput {
+  readonly agentId?: string;
+  readonly ttlSeconds?: number;
+}
+
+export interface CompleteTaskInput {
+  readonly agentId?: string;
+  readonly summary?: string;
+}
+
+export interface FailTaskInput {
+  readonly agentId?: string;
+  readonly reason: string;
+}
+
 export class TaskAlreadyExistsError extends Error {
   public constructor(taskId: string) {
     super(`Task ${taskId} already exists.`);
@@ -56,6 +90,29 @@ export class TaskSerializationError extends Error {
   }
 }
 
+export class TaskTransitionError extends Error {
+  public constructor(taskId: string, status: TaskStatus, action: string) {
+    super(`Task ${taskId} cannot transition from ${status} via ${action}.`);
+    this.name = 'TaskTransitionError';
+  }
+}
+
+export class TaskOwnershipError extends Error {
+  public constructor(taskId: string, expectedAgentId: string, actualAgentId: string) {
+    super(
+      `Task ${taskId} is owned by ${actualAgentId}, not ${expectedAgentId}.`,
+    );
+    this.name = 'TaskOwnershipError';
+  }
+}
+
+export class TaskAtomicConflictError extends Error {
+  public constructor(taskId: string) {
+    super(`Task ${taskId} could not be updated atomically after multiple retries.`);
+    this.name = 'TaskAtomicConflictError';
+  }
+}
+
 export class RedisTaskRepository {
   private readonly client: RedisTaskClient;
   private readonly keys: RepositoryKeys;
@@ -157,6 +214,158 @@ export class RedisTaskRepository {
 
     return updated;
   }
+
+  public async claim(taskId: string, input: ClaimTaskInput): Promise<Task> {
+    if (input.ttlSeconds <= 0) {
+      throw new Error(`Task ${taskId} claim ttl must be greater than 0 seconds.`);
+    }
+
+    return this.mutateTaskAtomically(taskId, (existing, now) => {
+      if (!canClaimTask(existing, now)) {
+        throw new TaskTransitionError(taskId, existing.status, 'claim');
+      }
+
+      const base = withoutCompletionAndFailureFields(withoutClaimFields(existing));
+
+      return {
+        ...base,
+        status: 'claimed',
+        claimedBy: input.agentId,
+        claimedAt: now,
+        claimTTL: input.ttlSeconds,
+        updatedAt: now,
+      };
+    });
+  }
+
+  public async release(taskId: string, input: ReleaseTaskInput = {}): Promise<Task> {
+    return this.mutateTaskAtomically(taskId, (existing, now) => {
+      if (!isClaimedLikeStatus(existing.status)) {
+        throw new TaskTransitionError(taskId, existing.status, 'release');
+      }
+
+      assertTaskOwnership(taskId, existing, input.agentId);
+
+      const base = withoutClaimFields(existing);
+
+      return {
+        ...base,
+        status: 'pending',
+        updatedAt: now,
+      };
+    });
+  }
+
+  public async heartbeat(
+    taskId: string,
+    input: HeartbeatTaskInput = {},
+  ): Promise<Task> {
+    return this.mutateTaskAtomically(taskId, (existing, now) => {
+      if (!isClaimedLikeStatus(existing.status)) {
+        throw new TaskTransitionError(taskId, existing.status, 'heartbeat');
+      }
+
+      if (isClaimExpired(existing, now)) {
+        throw new TaskTransitionError(taskId, existing.status, 'heartbeat');
+      }
+
+      assertTaskOwnership(taskId, existing, input.agentId);
+
+      const ttl = input.ttlSeconds ?? existing.claimTTL;
+
+      if (ttl === undefined || ttl <= 0) {
+        throw new TaskTransitionError(taskId, existing.status, 'heartbeat');
+      }
+
+      return {
+        ...existing,
+        claimedAt: now,
+        claimTTL: ttl,
+        updatedAt: now,
+      };
+    });
+  }
+
+  public async complete(
+    taskId: string,
+    input: CompleteTaskInput = {},
+  ): Promise<Task> {
+    return this.mutateTaskAtomically(taskId, (existing, now) => {
+      if (!isClaimedLikeStatus(existing.status)) {
+        throw new TaskTransitionError(taskId, existing.status, 'complete');
+      }
+
+      assertTaskOwnership(taskId, existing, input.agentId);
+
+      const base = withoutCompletionAndFailureFields(withoutClaimFields(existing));
+
+      return {
+        ...base,
+        status: 'completed',
+        completedAt: now,
+        ...(input.summary === undefined ? {} : { completionSummary: input.summary }),
+        updatedAt: now,
+      };
+    });
+  }
+
+  public async fail(taskId: string, input: FailTaskInput): Promise<Task> {
+    return this.mutateTaskAtomically(taskId, (existing, now) => {
+      if (!isClaimedLikeStatus(existing.status)) {
+        throw new TaskTransitionError(taskId, existing.status, 'fail');
+      }
+
+      assertTaskOwnership(taskId, existing, input.agentId);
+
+      const base = withoutCompletionAndFailureFields(withoutClaimFields(existing));
+
+      return {
+        ...base,
+        status: 'failed',
+        failedAt: now,
+        failureReason: input.reason,
+        retryCount: existing.retryCount + 1,
+        updatedAt: now,
+      };
+    });
+  }
+
+  private async mutateTaskAtomically(
+    taskId: string,
+    mutation: (existing: Task, now: number) => Task,
+  ): Promise<Task> {
+    const taskKey = this.keys.task(taskId);
+
+    for (let attempt = 0; attempt < MAX_ATOMIC_RETRIES; attempt += 1) {
+      await this.client.watch(taskKey);
+
+      try {
+        const raw = await this.client.get(taskKey);
+
+        if (raw === null) {
+          throw new TaskNotFoundError(taskId);
+        }
+
+        const existing = deserializeTask(taskId, raw);
+        const updated = mutation(existing, this.now());
+
+        const transaction = this.client.multi();
+        transaction.set(taskKey, JSON.stringify(updated), 'XX');
+        transaction.sadd(this.keys.taskIds, taskId);
+        const execResult = await transaction.exec();
+
+        if (execResult === null) {
+          continue;
+        }
+
+        return updated;
+      } finally {
+        await this.client.unwatch();
+      }
+    }
+
+    throw new TaskAtomicConflictError(taskId);
+  }
 }
 
 function matchesFilters(task: Task, filters: TaskListFilters): boolean {
@@ -175,6 +384,69 @@ function matchesFilters(task: Task, filters: TaskListFilters): boolean {
   return true;
 }
 
+function canClaimTask(task: Task, now: number): boolean {
+  if (task.status === 'pending') {
+    return true;
+  }
+
+  if (!isClaimedLikeStatus(task.status)) {
+    return false;
+  }
+
+  return isClaimExpired(task, now);
+}
+
+function isClaimedLikeStatus(status: TaskStatus): boolean {
+  return status === 'claimed' || status === 'in-progress';
+}
+
+function isClaimExpired(task: Task, now: number): boolean {
+  if (task.claimedAt === undefined || task.claimTTL === undefined) {
+    return false;
+  }
+
+  return task.claimedAt + task.claimTTL * 1000 <= now;
+}
+
+function assertTaskOwnership(
+  taskId: string,
+  task: Task,
+  expectedAgentId: string | undefined,
+): void {
+  if (expectedAgentId === undefined || task.claimedBy === undefined) {
+    return;
+  }
+
+  if (task.claimedBy !== expectedAgentId) {
+    throw new TaskOwnershipError(taskId, expectedAgentId, task.claimedBy);
+  }
+}
+
+type TaskWithoutClaimFields = Omit<Task, 'claimedBy' | 'claimedAt' | 'claimTTL'>;
+type TaskWithoutCompletionAndFailureFields = Omit<
+  Task,
+  'completedAt' | 'failedAt' | 'failureReason' | 'completionSummary'
+>;
+
+function withoutClaimFields(task: Task): TaskWithoutClaimFields {
+  const draft: Partial<Task> = { ...task };
+  delete draft.claimedBy;
+  delete draft.claimedAt;
+  delete draft.claimTTL;
+  return draft as TaskWithoutClaimFields;
+}
+
+function withoutCompletionAndFailureFields(
+  task: TaskWithoutClaimFields,
+): TaskWithoutCompletionAndFailureFields {
+  const draft: Partial<TaskWithoutClaimFields> = { ...task };
+  delete draft.completedAt;
+  delete draft.failedAt;
+  delete draft.failureReason;
+  delete draft.completionSummary;
+  return draft as TaskWithoutCompletionAndFailureFields;
+}
+
 function deserializeTask(taskId: string, raw: string): Task {
   let parsed: unknown;
 
diff --git a/packages/queue/tests/task-atomic.test.ts b/packages/queue/tests/task-atomic.test.ts
new file mode 100644
index 0000000..f931f27
--- /dev/null
+++ b/packages/queue/tests/task-atomic.test.ts
@@ -0,0 +1,330 @@
+import { describe, expect, it } from 'vitest';
+
+import {
+  RedisTaskRepository,
+  TaskTransitionError,
+  type RedisTaskClient,
+  type RedisTaskTransaction,
+} from '../src/task-repository.js';
+
+type QueuedOperation =
+  | {
+      readonly type: 'set';
+      readonly key: string;
+      readonly value: string;
+      readonly mode?: 'NX' | 'XX';
+    }
+  | {
+      readonly type: 'sadd';
+      readonly key: string;
+      readonly member: string;
+    };
+
+class InMemoryRedisBackend {
+  public readonly kv = new Map<string, string>();
+  public readonly sets = new Map<string, Set<string>>();
+  public readonly revisions = new Map<string, number>();
+
+  public getRevision(key: string): number {
+    return this.revisions.get(key) ?? 0;
+  }
+
+  public bumpRevision(key: string): void {
+    this.revisions.set(key, this.getRevision(key) + 1);
+  }
+}
+
+class InMemoryRedisTransaction implements RedisTaskTransaction {
+  private readonly operations: QueuedOperation[] = [];
+
+  public constructor(
+    private readonly backend: InMemoryRedisBackend,
+    private readonly watchedRevisions: ReadonlyMap<string, number>,
+  ) {}
+
+  public set(key: string, value: string, mode?: 'NX' | 'XX'): RedisTaskTransaction {
+    this.operations.push({
+      type: 'set',
+      key,
+      value,
+      mode,
+    });
+    return this;
+  }
+
+  public sadd(key: string, member: string): RedisTaskTransaction {
+    this.operations.push({
+      type: 'sadd',
+      key,
+      member,
+    });
+    return this;
+  }
+
+  public exec(): Promise<readonly (readonly [Error | null, unknown])[] | null> {
+    for (const [key, revision] of this.watchedRevisions.entries()) {
+      if (this.backend.getRevision(key) !== revision) {
+        return Promise.resolve(null);
+      }
+    }
+
+    const results: (readonly [Error | null, unknown])[] = [];
+
+    for (const operation of this.operations) {
+      if (operation.type === 'set') {
+        const exists = this.backend.kv.has(operation.key);
+        if (operation.mode === 'NX' && exists) {
+          results.push([null, null]);
+          continue;
+        }
+
+        if (operation.mode === 'XX' && !exists) {
+          results.push([null, null]);
+          continue;
+        }
+
+        this.backend.kv.set(operation.key, operation.value);
+        this.backend.bumpRevision(operation.key);
+        results.push([null, 'OK']);
+        continue;
+      }
+
+      const set = this.backend.sets.get(operation.key) ?? new Set<string>();
+      const before = set.size;
+
+      set.add(operation.member);
+      this.backend.sets.set(operation.key, set);
+      this.backend.bumpRevision(operation.key);
+      results.push([null, set.size === before ? 0 : 1]);
+    }
+
+    return Promise.resolve(results);
+  }
+}
+
+class InMemoryAtomicRedisClient implements RedisTaskClient {
+  private watchedRevisions = new Map<string, number>();
+
+  public constructor(private readonly backend: InMemoryRedisBackend) {}
+
+  public get(key: string): Promise<string | null> {
+    return Promise.resolve(this.backend.kv.get(key) ?? null);
+  }
+
+  public set(
+    key: string,
+    value: string,
+    mode?: 'NX' | 'XX',
+  ): Promise<'OK' | null> {
+    const exists = this.backend.kv.has(key);
+
+    if (mode === 'NX' && exists) {
+      return Promise.resolve(null);
+    }
+
+    if (mode === 'XX' && !exists) {
+      return Promise.resolve(null);
+    }
+
+    this.backend.kv.set(key, value);
+    this.backend.bumpRevision(key);
+
+    return Promise.resolve('OK');
+  }
+
+  public smembers(key: string): Promise<string[]> {
+    return Promise.resolve([...(this.backend.sets.get(key) ?? new Set<string>())]);
+  }
+
+  public sadd(key: string, member: string): Promise<number> {
+    const values = this.backend.sets.get(key) ?? new Set<string>();
+    const before = values.size;
+
+    values.add(member);
+    this.backend.sets.set(key, values);
+    this.backend.bumpRevision(key);
+
+    return Promise.resolve(values.size === before ? 0 : 1);
+  }
+
+  public watch(...keys: string[]): Promise<'OK'> {
+    this.watchedRevisions = new Map(
+      keys.map((key) => [key, this.backend.getRevision(key)]),
+    );
+    return Promise.resolve('OK');
+  }
+
+  public unwatch(): Promise<'OK'> {
+    this.watchedRevisions.clear();
+    return Promise.resolve('OK');
+  }
+
+  public multi(): RedisTaskTransaction {
+    const watchedSnapshot = new Map(this.watchedRevisions);
+    this.watchedRevisions.clear();
+    return new InMemoryRedisTransaction(this.backend, watchedSnapshot);
+  }
+}
+
+function createRepositoryPair(now: () => number): [RedisTaskRepository, RedisTaskRepository] {
+  const backend = new InMemoryRedisBackend();
+
+  return [
+    new RedisTaskRepository({
+      client: new InMemoryAtomicRedisClient(backend),
+      now,
+    }),
+    new RedisTaskRepository({
+      client: new InMemoryAtomicRedisClient(backend),
+      now,
+    }),
+  ];
+}
+
+describe('RedisTaskRepository atomic transitions', () => {
+  it('claims a pending task once and blocks concurrent double-claim', async () => {
+    let timestamp = 1_700_000_000_000;
+    const now = (): number => timestamp;
+    const [repositoryA, repositoryB] = createRepositoryPair(now);
+
+    await repositoryA.create({
+      project: 'queue',
+      mission: 'phase1',
+      taskId: 'MQ-004',
+      title: 'Atomic claim',
+    });
+
+    const [claimA, claimB] = await Promise.allSettled([
+      repositoryA.claim('MQ-004', { agentId: 'agent-a', ttlSeconds: 60 }),
+      repositoryB.claim('MQ-004', { agentId: 'agent-b', ttlSeconds: 60 }),
+    ]);
+
+    const fulfilled = [claimA, claimB].filter((result) => result.status === 'fulfilled');
+    const rejected = [claimA, claimB].filter((result) => result.status === 'rejected');
+
+    expect(fulfilled).toHaveLength(1);
+    expect(rejected).toHaveLength(1);
+  });
+
+  it('allows claim takeover after TTL expiry', async () => {
+    let timestamp = 1_700_000_000_000;
+    const now = (): number => timestamp;
+    const [repositoryA, repositoryB] = createRepositoryPair(now);
+
+    await repositoryA.create({
+      project: 'queue',
+      mission: 'phase1',
+      taskId: 'MQ-004-EXP',
+      title: 'TTL expiry',
+    });
+
+    await repositoryA.claim('MQ-004-EXP', {
+      agentId: 'agent-a',
+      ttlSeconds: 1,
+    });
+
+    timestamp += 2_000;
+
+    const takeover = await repositoryB.claim('MQ-004-EXP', {
+      agentId: 'agent-b',
+      ttlSeconds: 60,
+    });
+
+    expect(takeover.claimedBy).toBe('agent-b');
+  });
+
+  it('releases a claimed task back to pending', async () => {
+    const [repository] = createRepositoryPair(() => 1_700_000_000_000);
+
+    await repository.create({
+      project: 'queue',
+      mission: 'phase1',
+      taskId: 'MQ-004-REL',
+      title: 'Release test',
+    });
+
+    await repository.claim('MQ-004-REL', {
+      agentId: 'agent-a',
+      ttlSeconds: 60,
+    });
+
+    const released = await repository.release('MQ-004-REL', {
+      agentId: 'agent-a',
+    });
+
+    expect(released.status).toBe('pending');
+    expect(released.claimedBy).toBeUndefined();
+    expect(released.claimedAt).toBeUndefined();
+  });
+
+  it('heartbeats, completes, and fails with valid transitions', async () => {
+    let timestamp = 1_700_000_000_000;
+    const now = (): number => timestamp;
+    const [repository] = createRepositoryPair(now);
+
+    await repository.create({
+      project: 'queue',
+      mission: 'phase1',
+      taskId: 'MQ-004-HCF',
+      title: 'Transition test',
+    });
+
+    await repository.claim('MQ-004-HCF', {
+      agentId: 'agent-a',
+      ttlSeconds: 60,
+    });
+
+    timestamp += 1_000;
+    const heartbeat = await repository.heartbeat('MQ-004-HCF', {
+      agentId: 'agent-a',
+      ttlSeconds: 120,
+    });
+    expect(heartbeat.claimTTL).toBe(120);
+    expect(heartbeat.claimedAt).toBe(1_700_000_001_000);
+
+    const completed = await repository.complete('MQ-004-HCF', {
+      agentId: 'agent-a',
+      summary: 'done',
+    });
+    expect(completed.status).toBe('completed');
+    expect(completed.completionSummary).toBe('done');
+
+    await repository.create({
+      project: 'queue',
+      mission: 'phase1',
+      taskId: 'MQ-004-FAIL',
+      title: 'Failure test',
+    });
+
+    await repository.claim('MQ-004-FAIL', {
+      agentId: 'agent-a',
+      ttlSeconds: 60,
+    });
+
+    const failed = await repository.fail('MQ-004-FAIL', {
+      agentId: 'agent-a',
+      reason: 'boom',
+    });
+
+    expect(failed.status).toBe('failed');
+    expect(failed.failureReason).toBe('boom');
+    expect(failed.retryCount).toBe(1);
+  });
+
+  it('rejects invalid transitions', async () => {
+    const [repository] = createRepositoryPair(() => 1_700_000_000_000);
+
+    await repository.create({
+      project: 'queue',
+      mission: 'phase1',
+      taskId: 'MQ-004-INV',
+      title: 'Invalid transitions',
+    });
+
+    await expect(
+      repository.complete('MQ-004-INV', {
+        agentId: 'agent-a',
+      }),
+    ).rejects.toBeInstanceOf(TaskTransitionError);
+  });
+});
diff --git a/packages/queue/tests/task-repository.test.ts b/packages/queue/tests/task-repository.test.ts
index 180b7df..226d546 100644
--- a/packages/queue/tests/task-repository.test.ts
+++ b/packages/queue/tests/task-repository.test.ts
@@ -4,8 +4,23 @@ import {
   RedisTaskRepository,
   TaskAlreadyExistsError,
   type RedisTaskClient,
+  type RedisTaskTransaction,
 } from '../src/task-repository.js';
 
+class NoopRedisTransaction implements RedisTaskTransaction {
+  public set(): RedisTaskTransaction {
+    return this;
+  }
+
+  public sadd(): RedisTaskTransaction {
+    return this;
+  }
+
+  public exec(): Promise<readonly (readonly [Error | null, unknown])[] | null> {
+    return Promise.resolve([]);
+  }
+}
+
 class InMemoryRedisClient implements RedisTaskClient {
   private readonly kv = new Map<string, string>();
   private readonly sets = new Map<string, Set<string>>();
@@ -46,6 +61,18 @@ class InMemoryRedisClient implements RedisTaskClient {
 
     return Promise.resolve(values.size === beforeSize ? 0 : 1);
   }
+
+  public watch(): Promise<'OK'> {
+    return Promise.resolve('OK');
+  }
+
+  public unwatch(): Promise<'OK'> {
+    return Promise.resolve('OK');
+  }
+
+  public multi(): RedisTaskTransaction {
+    return new NoopRedisTransaction();
+  }
 }
 
 describe('RedisTaskRepository CRUD', () => {