feat(FN-1253): implement task checkout leasing end-to-end

- Add checkout lease types and conflict error exports, plus DB schema v20 migration for checkedOutBy/checkedOutAt
- Persist checkout lease fields in TaskStore and add AgentStore checkout/release/force-release/get-holder operations
- Add dashboard checkout API routes for acquire/release/force-release/status with explicit 409 conflict and 403 holder enforcement
- Enforce checkout ownership in heartbeat execution with graceful checkout_conflict exits when another agent holds the lease
- Expand core and dashboard test coverage for schema, store behavior, API routes, and leasing workflows, and document leasing behavior in AGENTS.md

packages/core/src/__tests__/task-documents.test.ts

@@ -51,7 +51,7 @@ describe("TaskStore task documents", () => {
 
     expect(tableNames.has("task_documents")).toBe(true);
     expect(tableNames.has("task_document_revisions")).toBe(true);
-    expect(db.getSchemaVersion()).toBe(19);
+    expect(db.getSchemaVersion()).toBe(20);
 
     const index = db
       .prepare(

packages/core/src/agent-store.test.ts

@@ -12,12 +12,13 @@
  */
 import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
 import { AgentStore } from "./agent-store.js";
+import { TaskStore } from "./store.js";
 import { rm } from "node:fs/promises";
 import { join } from "node:path";
 import { mkdtempSync, existsSync, writeFileSync, readFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { createHash } from "node:crypto";
-import type { AgentCapability, AgentState } from "./types.js";
+import { CheckoutConflictError, type AgentCapability, type AgentState } from "./types.js";
 
 function makeTmpDir(): string {
   return mkdtempSync(join(tmpdir(), "kb-agent-store-test-"));
@@ -1385,6 +1386,111 @@ describe("AgentStore", () => {
     });
   });
 
+  describe("checkout leasing", () => {
+    let taskStore: TaskStore;
+    let holderId: string;
+    let otherAgentId: string;
+    let taskId: string;
+
+    beforeEach(async () => {
+      taskStore = new TaskStore(rootDir);
+      await taskStore.init();
+
+      store = new AgentStore({ rootDir, taskStore });
+      await store.init();
+
+      const holder = await store.createAgent({ name: "Checkout Holder", role: "executor" });
+      const other = await store.createAgent({ name: "Checkout Other", role: "executor" });
+      const task = await taskStore.createTask({ description: "Task for checkout leasing tests" });
+
+      holderId = holder.id;
+      otherAgentId = other.id;
+      taskId = task.id;
+    });
+
+    it("checkoutTask acquires a lease and stamps checkedOutAt", async () => {
+      const updated = await store.checkoutTask(holderId, taskId);
+
+      expect(updated.checkedOutBy).toBe(holderId);
+      expect(updated.checkedOutAt).toBeDefined();
+
+      const persisted = await taskStore.getTask(taskId);
+      expect(persisted?.checkedOutBy).toBe(holderId);
+      expect(persisted?.checkedOutAt).toBeDefined();
+    });
+
+    it("checkoutTask is idempotent when the same agent re-checks out", async () => {
+      const first = await store.checkoutTask(holderId, taskId);
+      const second = await store.checkoutTask(holderId, taskId);
+
+      expect(second.checkedOutBy).toBe(holderId);
+      expect(second.checkedOutAt).toBe(first.checkedOutAt);
+    });
+
+    it("checkoutTask throws CheckoutConflictError when already held by another agent", async () => {
+      await store.checkoutTask(holderId, taskId);
+
+      try {
+        await store.checkoutTask(otherAgentId, taskId);
+        throw new Error("Expected checkout conflict");
+      } catch (error) {
+        expect(error).toBeInstanceOf(CheckoutConflictError);
+        const conflict = error as CheckoutConflictError;
+        expect(conflict.taskId).toBe(taskId);
+        expect(conflict.currentHolderId).toBe(holderId);
+        expect(conflict.requestedById).toBe(otherAgentId);
+      }
+    });
+
+    it("checkoutTask throws when agent is missing", async () => {
+      await expect(store.checkoutTask("agent-missing", taskId)).rejects.toThrow("Agent agent-missing not found");
+    });
+
+    it("checkoutTask throws when task is missing", async () => {
+      await expect(store.checkoutTask(holderId, "FN-404")).rejects.toThrow("Task FN-404 not found");
+    });
+
+    it("releaseTask clears checkedOutBy and checkedOutAt for the holder", async () => {
+      await store.checkoutTask(holderId, taskId);
+
+      const released = await store.releaseTask(holderId, taskId);
+      expect(released.checkedOutBy).toBeUndefined();
+      expect(released.checkedOutAt).toBeUndefined();
+
+      const persisted = await taskStore.getTask(taskId);
+      expect(persisted?.checkedOutBy).toBeUndefined();
+      expect(persisted?.checkedOutAt).toBeUndefined();
+    });
+
+    it("releaseTask throws for a non-holder agent", async () => {
+      await store.checkoutTask(holderId, taskId);
+
+      await expect(store.releaseTask(otherAgentId, taskId)).rejects.toThrow("Cannot release: not the checkout holder");
+    });
+
+    it("releaseTask is idempotent when task is already released", async () => {
+      const released = await store.releaseTask(holderId, taskId);
+
+      expect(released.checkedOutBy).toBeUndefined();
+      expect(released.checkedOutAt).toBeUndefined();
+    });
+
+    it("forceReleaseTask clears checkout regardless of holder", async () => {
+      await store.checkoutTask(holderId, taskId);
+
+      const released = await store.forceReleaseTask(taskId);
+      expect(released.checkedOutBy).toBeUndefined();
+      expect(released.checkedOutAt).toBeUndefined();
+    });
+
+    it("getCheckedOutBy returns holder ID when checked out and undefined otherwise", async () => {
+      expect(await store.getCheckedOutBy(taskId)).toBeUndefined();
+
+      await store.checkoutTask(holderId, taskId);
+      expect(await store.getCheckedOutBy(taskId)).toBe(holderId);
+    });
+  });
+
   // ── resetAgent ────────────────────────────────────────────────────
 
   describe("resetAgent", () => {

packages/core/src/agent-store.ts

@@ -40,8 +40,10 @@ import type {
   AgentRating,
   AgentRatingSummary,
   AgentRatingInput,
+  Task,
 } from "./types.js";
-import { AGENT_VALID_TRANSITIONS, agentToConfigSnapshot, diffConfigSnapshots } from "./types.js";
+import { AGENT_VALID_TRANSITIONS, agentToConfigSnapshot, diffConfigSnapshots, CheckoutConflictError } from "./types.js";
+import type { TaskStore } from "./store.js";
 import { computeAccessState } from "./agent-permissions.js";
 import { Database } from "./db.js";
 
@@ -76,8 +78,10 @@ type TypedEventEmitter<Events extends Record<string, unknown[]>> = {
 
 /** Options for AgentStore constructor */
 export interface AgentStoreOptions {
-  /** Root directory for fn data (default: .fusion) */
+  /** Root directory for kb data (default: .fusion) */
   rootDir?: string;
+  /** Optional TaskStore for checkout/release operations */
+  taskStore?: TaskStore;
 }
 
 /** Agent data as stored on disk */
@@ -119,11 +123,13 @@ export class AgentStore extends EventEmitter {
   private agentsDir: string;
   private locks: Map<string, AgentLock> = new Map();
   private _db: Database | null = null;
+  private taskStore?: TaskStore;
 
   constructor(options: AgentStoreOptions = {}) {
     super();
     this.rootDir = options.rootDir ?? ".fusion";
     this.agentsDir = join(this.rootDir, "agents");
+    this.taskStore = options.taskStore;
   }
 
   private get db(): Database {
@@ -813,6 +819,93 @@ export class AgentStore extends EventEmitter {
     });
   }
 
+  /**
+   * Acquire a checkout lease for a task.
+   * Throws CheckoutConflictError when another agent already holds the lease.
+   */
+  async checkoutTask(agentId: string, taskId: string): Promise<Task> {
+    if (!this.taskStore) {
+      throw new Error("TaskStore not configured for checkout operations");
+    }
+
+    const agent = await this.getAgent(agentId);
+    if (!agent) {
+      throw new Error(`Agent ${agentId} not found`);
+    }
+
+    const task = await this.taskStore.getTask(taskId);
+    if (!task) {
+      throw new Error(`Task ${taskId} not found`);
+    }
+
+    if (task.checkedOutBy && task.checkedOutBy !== agentId) {
+      throw new CheckoutConflictError(taskId, task.checkedOutBy, agentId);
+    }
+
+    if (task.checkedOutBy === agentId) {
+      return task;
+    }
+
+    const updated = await this.taskStore.updateTask(taskId, { checkedOutBy: agentId });
+    await this.taskStore.logEntry(taskId, `Checked out by agent ${agentId}`);
+    return updated;
+  }
+
+  /**
+   * Release a checkout lease for a task.
+   */
+  async releaseTask(agentId: string, taskId: string): Promise<Task> {
+    if (!this.taskStore) {
+      throw new Error("TaskStore not configured for checkout operations");
+    }
+
+    const task = await this.taskStore.getTask(taskId);
+    if (!task) {
+      throw new Error(`Task ${taskId} not found`);
+    }
+
+    if (task.checkedOutBy && task.checkedOutBy !== agentId) {
+      throw new Error("Cannot release: not the checkout holder");
+    }
+
+    if (!task.checkedOutBy) {
+      return task;
+    }
+
+    const updated = await this.taskStore.updateTask(taskId, { checkedOutBy: null });
+    await this.taskStore.logEntry(taskId, `Released by agent ${agentId}`);
+    return updated;
+  }
+
+  /**
+   * Force release a task checkout lease regardless of holder.
+   */
+  async forceReleaseTask(taskId: string): Promise<Task> {
+    if (!this.taskStore) {
+      throw new Error("TaskStore not configured for checkout operations");
+    }
+
+    const updated = await this.taskStore.updateTask(taskId, { checkedOutBy: null });
+    await this.taskStore.logEntry(taskId, "Checkout force-released");
+    return updated;
+  }
+
+  /**
+   * Get the current checkout lease holder for a task.
+   */
+  async getCheckedOutBy(taskId: string): Promise<string | undefined> {
+    if (!this.taskStore) {
+      throw new Error("TaskStore not configured for checkout operations");
+    }
+
+    const task = await this.taskStore.getTask(taskId);
+    if (!task) {
+      throw new Error(`Task ${taskId} not found`);
+    }
+
+    return task.checkedOutBy;
+  }
+
   /**
    * Reset budget token usage counters for an agent.
    * @param agentId - The agent ID

packages/core/src/db.test.ts

@@ -106,7 +106,7 @@ describe("Database", () => {
     });
 
     it("seeds schema version", () => {
-      expect(db.getSchemaVersion()).toBe(19);
+      expect(db.getSchemaVersion()).toBe(20);
     });
 
     it("seeds lastModified", () => {
@@ -129,7 +129,7 @@ describe("Database", () => {
 
     it("is idempotent - calling init() twice does not fail", () => {
       expect(() => db.init()).not.toThrow();
-      expect(db.getSchemaVersion()).toBe(19);
+      expect(db.getSchemaVersion()).toBe(20);
     });
 
     it("does not overwrite existing config on re-init", () => {
@@ -736,7 +736,7 @@ describe("schema migrations", () => {
     db.init();
 
     // Verify version bumped to 5 (includes v1→v2, v2→v3, v3→v4, and v4→v5 migrations)
-    expect(db.getSchemaVersion()).toBe(19);
+    expect(db.getSchemaVersion()).toBe(20);
 
     // Verify new columns exist and existing data is intact
     const cols = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
@@ -761,11 +761,11 @@ describe("schema migrations", () => {
     const db = new Database(kbDir);
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(19);
+    expect(db.getSchemaVersion()).toBe(20);
 
     // Re-init should not fail
     db.init();
-    expect(db.getSchemaVersion()).toBe(19);
+    expect(db.getSchemaVersion()).toBe(20);
 
     db.close();
   });
@@ -781,7 +781,7 @@ describe("schema migrations", () => {
 
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(19);
+    expect(db.getSchemaVersion()).toBe(20);
 
     const tables = db.prepare("SELECT name FROM sqlite_master WHERE type='table' AND name = 'agentRatings'").all() as Array<{ name: string }>;
     expect(tables).toEqual([{ name: "agentRatings" }]);
@@ -805,7 +805,7 @@ describe("schema migrations", () => {
 
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(19);
+    expect(db.getSchemaVersion()).toBe(20);
 
     const tables = db.prepare("SELECT name FROM sqlite_master WHERE type='table' AND name = 'mission_events'").all() as Array<{ name: string }>;
     expect(tables).toEqual([{ name: "mission_events" }]);
@@ -909,7 +909,7 @@ describe("schema migrations", () => {
     db.init();
 
     // Verify version bumped to 5
-    expect(db.getSchemaVersion()).toBe(19);
+    expect(db.getSchemaVersion()).toBe(20);
 
     // Verify new columns exist and existing data is intact
     const cols = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
@@ -1119,7 +1119,7 @@ describe("createDatabase factory", () => {
     const db = createDatabase(kbDir);
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(19);
+    expect(db.getSchemaVersion()).toBe(20);
     expect(db.getLastModified()).toBeGreaterThan(0);
 
     db.close();

packages/core/src/db.ts

@@ -59,7 +59,7 @@ export function fromJson<T>(json: string | null | undefined): T | undefined {
 
 // ── Schema Definition ────────────────────────────────────────────────
 
-const SCHEMA_VERSION = 19;
+const SCHEMA_VERSION = 20;
 
 function normalizeTaskComments(
   steeringComments: SteeringComment[] | undefined,
@@ -742,6 +742,13 @@ export class Database {
         this.db.exec("CREATE INDEX IF NOT EXISTS idxAiSessionsLock ON ai_sessions(lockedByTab)");
       });
     }
+
+    if (version < 20) {
+      this.applyMigration(20, () => {
+        this.addColumnIfMissing("tasks", "checkedOutBy", "TEXT");
+        this.addColumnIfMissing("tasks", "checkedOutAt", "TEXT");
+      });
+    }
   }
 
   /**

packages/core/src/index.ts

@@ -1,5 +1,5 @@
-export { COLUMNS, COLUMN_LABELS, COLUMN_DESCRIPTIONS, VALID_TRANSITIONS, DEFAULT_SETTINGS, DEFAULT_GLOBAL_SETTINGS, DEFAULT_PROJECT_SETTINGS, GLOBAL_SETTINGS_KEYS, PROJECT_SETTINGS_KEYS, THINKING_LEVELS, THEME_MODES, COLOR_THEMES, WORKFLOW_STEP_TEMPLATES, AGENT_PERMISSIONS, agentToConfigSnapshot, diffConfigSnapshots } from "./types.js";
-export type { Column, IssueInfo, IssueState, PrInfo, PrStatus, Task, TaskAttachment, TaskComment, TaskCommentInput, TaskDocument, TaskDocumentRevision, TaskDocumentCreateInput, TaskCreateInput, TaskDetail, AgentLogEntry, AgentLogType, AgentRole, BoardConfig, MergeDetails, MergeResult, Settings, GlobalSettings, ProjectSettings, SettingsScope, TaskStep, StepStatus, TaskLogEntry, ActivityLogEntry, ActivityEventType, ThinkingLevel, ThemeMode, ColorTheme, PlanningQuestion, PlanningSummary, PlanningResponse, PlanningQuestionType, ArchivedTaskEntry, BatchStatusRequest, BatchStatusResponse, BatchStatusEntry, BatchStatusResult, ModelPreset, WorkflowStep, WorkflowStepMode, WorkflowStepPhase, WorkflowStepInput, WorkflowStepResult, WorkflowStepTemplate, Agent, OrgTreeNode, AgentState, AgentDetail, AgentCreateInput, AgentUpdateInput, AgentApiKey, AgentApiKeyCreateResult, AgentCapability, AgentPromptTemplate, AgentPromptsConfig, AgentPermission, TaskAssignSource, AgentAccessState, AgentHeartbeatConfig, AgentBudgetConfig, AgentBudgetStatus, InstructionsBundleConfig, MessageResponseMode, AgentHeartbeatEvent, AgentHeartbeatRun, HeartbeatInvocationSource, AgentTaskSession, AgentRating, AgentRatingSummary, AgentRatingInput, AgentConfigSnapshot, RevisionFieldDiff, AgentConfigRevision, AgentStats, ReflectionTrigger, ReflectionMetrics, AgentReflection, AgentPerformanceSummary, NtfyNotificationEvent, SteeringComment, ParticipantType, MessageType, Message, MessageCreateInput, MessageFilter, Mailbox } from "./types.js";
+export { COLUMNS, COLUMN_LABELS, COLUMN_DESCRIPTIONS, VALID_TRANSITIONS, DEFAULT_SETTINGS, DEFAULT_GLOBAL_SETTINGS, DEFAULT_PROJECT_SETTINGS, GLOBAL_SETTINGS_KEYS, PROJECT_SETTINGS_KEYS, THINKING_LEVELS, THEME_MODES, COLOR_THEMES, WORKFLOW_STEP_TEMPLATES, AGENT_PERMISSIONS, agentToConfigSnapshot, diffConfigSnapshots, CheckoutConflictError } from "./types.js";
+export type { Column, IssueInfo, IssueState, PrInfo, PrStatus, Task, TaskAttachment, TaskComment, TaskCommentInput, TaskDocument, TaskDocumentRevision, TaskDocumentCreateInput, TaskCreateInput, TaskDetail, AgentLogEntry, AgentLogType, AgentRole, BoardConfig, MergeDetails, MergeResult, Settings, GlobalSettings, ProjectSettings, SettingsScope, TaskStep, StepStatus, TaskLogEntry, ActivityLogEntry, ActivityEventType, ThinkingLevel, ThemeMode, ColorTheme, PlanningQuestion, PlanningSummary, PlanningResponse, PlanningQuestionType, ArchivedTaskEntry, BatchStatusRequest, BatchStatusResponse, BatchStatusEntry, BatchStatusResult, ModelPreset, WorkflowStep, WorkflowStepMode, WorkflowStepPhase, WorkflowStepInput, WorkflowStepResult, WorkflowStepTemplate, Agent, OrgTreeNode, AgentState, AgentDetail, AgentCreateInput, AgentUpdateInput, AgentApiKey, AgentApiKeyCreateResult, AgentCapability, AgentPromptTemplate, AgentPromptsConfig, AgentPermission, TaskAssignSource, AgentAccessState, AgentHeartbeatConfig, AgentBudgetConfig, AgentBudgetStatus, InstructionsBundleConfig, MessageResponseMode, AgentHeartbeatEvent, AgentHeartbeatRun, HeartbeatInvocationSource, AgentTaskSession, AgentRating, AgentRatingSummary, AgentRatingInput, AgentConfigSnapshot, RevisionFieldDiff, AgentConfigRevision, AgentStats, ReflectionTrigger, ReflectionMetrics, AgentReflection, AgentPerformanceSummary, NtfyNotificationEvent, SteeringComment, ParticipantType, MessageType, Message, MessageCreateInput, MessageFilter, Mailbox, CheckoutLease } from "./types.js";
 export { AGENT_VALID_TRANSITIONS } from "./types.js";
 export {
   BUILTIN_AGENT_PROMPTS,

packages/core/src/store.ts

@@ -230,6 +230,8 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
       missionId: row.missionId || undefined,
       sliceId: row.sliceId || undefined,
       assignedAgentId: row.assignedAgentId || undefined,
+      checkedOutBy: row.checkedOutBy || undefined,
+      checkedOutAt: row.checkedOutAt || undefined,
     };
   }
 
@@ -279,10 +281,10 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
         summary, thinkingLevel, createdAt, updatedAt, columnMovedAt,
         dependencies, steps, log, attachments, steeringComments,
         comments, workflowStepResults, prInfo, issueInfo, mergeDetails,
-        breakIntoSubtasks, enabledWorkflowSteps, modifiedFiles, missionId, sliceId, assignedAgentId
+        breakIntoSubtasks, enabledWorkflowSteps, modifiedFiles, missionId, sliceId, assignedAgentId, checkedOutBy, checkedOutAt
       ) VALUES (
         ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?,
-        ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?
+        ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?
       )
     `).run(
       task.id,
@@ -332,6 +334,8 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
       task.missionId ?? null,
       task.sliceId ?? null,
       task.assignedAgentId ?? null,
+      task.checkedOutBy ?? null,
+      task.checkedOutAt ?? null,
     );
     this.db.bumpLastModified();
   }
@@ -1247,7 +1251,7 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
 
   async updateTask(
     id: string,
-    updates: { title?: string; description?: string; prompt?: string; worktree?: string | null; status?: string | null; dependencies?: string[]; blockedBy?: string | null; assignedAgentId?: string | null; paused?: boolean; baseBranch?: string | null; branch?: string | null; baseCommitSha?: string | null; size?: "S" | "M" | "L"; reviewLevel?: number; mergeRetries?: number; stuckKillCount?: number | null; recoveryRetryCount?: number | null; nextRecoveryAt?: string | null; enabledWorkflowSteps?: string[]; modelProvider?: string | null; modelId?: string | null; validatorModelProvider?: string | null; validatorModelId?: string | null; planningModelProvider?: string | null; planningModelId?: string | null; thinkingLevel?: string | null; error?: string | null; summary?: string | null; sessionFile?: string | null; workflowStepResults?: import("./types.js").WorkflowStepResult[] | null; mergeDetails?: import("./types.js").MergeDetails | null; modifiedFiles?: string[] | null; missionId?: string | null; sliceId?: string | null },
+    updates: { title?: string; description?: string; prompt?: string; worktree?: string | null; status?: string | null; dependencies?: string[]; blockedBy?: string | null; assignedAgentId?: string | null; checkedOutBy?: string | null; checkedOutAt?: string | null; paused?: boolean; baseBranch?: string | null; branch?: string | null; baseCommitSha?: string | null; size?: "S" | "M" | "L"; reviewLevel?: number; mergeRetries?: number; stuckKillCount?: number | null; recoveryRetryCount?: number | null; nextRecoveryAt?: string | null; enabledWorkflowSteps?: string[]; modelProvider?: string | null; modelId?: string | null; validatorModelProvider?: string | null; validatorModelId?: string | null; planningModelProvider?: string | null; planningModelId?: string | null; thinkingLevel?: string | null; error?: string | null; summary?: string | null; sessionFile?: string | null; workflowStepResults?: import("./types.js").WorkflowStepResult[] | null; mergeDetails?: import("./types.js").MergeDetails | null; modifiedFiles?: string[] | null; missionId?: string | null; sliceId?: string | null },
   ): Promise<Task> {
     return this.withTaskLock(id, async () => {
       // Validate that task doesn't depend on itself
@@ -1303,6 +1307,14 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
       } else if (updates.assignedAgentId !== undefined) {
         task.assignedAgentId = updates.assignedAgentId;
       }
+      if (updates.checkedOutBy === null) {
+        task.checkedOutBy = undefined;
+        task.checkedOutAt = undefined;
+      } else if (updates.checkedOutBy !== undefined) {
+        task.checkedOutBy = updates.checkedOutBy;
+        // Auto-set checkedOutAt when acquiring a lease (use provided value or generate timestamp)
+        task.checkedOutAt = updates.checkedOutAt ?? new Date().toISOString();
+      }
       if (updates.paused !== undefined) task.paused = updates.paused || undefined;
       if (updates.baseBranch === null) {
         task.baseBranch = undefined;

packages/core/src/types.ts

@@ -540,6 +540,26 @@ export interface MergeDetails {
   autoResolvedCount?: number;
 }
 
+/** Represents an agent's checkout lease on a task. */
+export interface CheckoutLease {
+  /** The agent ID that holds the lease */
+  agentId: string;
+  /** ISO-8601 timestamp when the lease was acquired */
+  checkedOutAt: string;
+}
+
+/** Thrown when a checkout is attempted on a task already checked out by another agent. */
+export class CheckoutConflictError extends Error {
+  constructor(
+    public readonly taskId: string,
+    public readonly currentHolderId: string,
+    public readonly requestedById: string,
+  ) {
+    super(`Task ${taskId} is already checked out by agent ${currentHolderId}`);
+    this.name = "CheckoutConflictError";
+  }
+}
+
 export interface Task {
   id: string;
   title?: string;
@@ -639,6 +659,10 @@ export interface Task {
   thinkingLevel?: ThinkingLevel;
   /** Explicitly assigned agent ID for task-agent linking. Distinct from Agent.taskId active execution state. */
   assignedAgentId?: string;
+  /** Agent ID currently holding the checkout lease for this task. Undefined when no active lease. */
+  checkedOutBy?: string;
+  /** ISO-8601 timestamp when the checkout lease was acquired. */
+  checkedOutAt?: string;
   /** Path to the persisted agent session file, enabling pause/resume without
    *  losing conversation context. Set when execution starts; cleared on
    *  completion or terminal failure. */

packages/dashboard/src/routes.test.ts

@@ -2202,6 +2202,230 @@ describe("PATCH /tasks/:id/assign and GET /agents/:id/tasks", () => {
   }, 30_000);
 });
 
+describe("Task checkout routes", () => {
+  let tempDir: string;
+  let fusionDir: string;
+  let store: TaskStore;
+  let agentAId: string;
+  let agentBId: string;
+  let taskState: TaskDetail;
+
+  beforeEach(async () => {
+    tempDir = mkdtempSync(join(tmpdir(), "kb-routes-task-checkout-"));
+    fusionDir = join(tempDir, ".fusion");
+    mkdirSync(fusionDir, { recursive: true });
+
+    const { AgentStore } = await import("@fusion/core");
+    const agentStore = new AgentStore({ rootDir: fusionDir });
+    await agentStore.init();
+
+    const agentA = await agentStore.createAgent({
+      name: "Checkout Agent A",
+      role: "executor",
+    });
+    const agentB = await agentStore.createAgent({
+      name: "Checkout Agent B",
+      role: "executor",
+    });
+
+    agentAId = agentA.id;
+    agentBId = agentB.id;
+
+    taskState = {
+      ...FAKE_TASK_DETAIL,
+      id: "FN-300",
+      checkedOutBy: undefined,
+      checkedOutAt: undefined,
+    };
+
+    store = createMockStore({
+      getFusionDir: vi.fn().mockReturnValue(fusionDir),
+      getTask: vi.fn().mockImplementation(async (id: string) => {
+        if (id !== taskState.id) {
+          return null;
+        }
+        return { ...taskState };
+      }),
+      updateTask: vi.fn().mockImplementation(async (id: string, updates: { checkedOutBy?: string | null; checkedOutAt?: string | null }) => {
+        if (id !== taskState.id) {
+          throw new Error(`Task ${id} not found`);
+        }
+
+        if (updates.checkedOutBy === null) {
+          taskState.checkedOutBy = undefined;
+          taskState.checkedOutAt = undefined;
+        } else if (updates.checkedOutBy !== undefined) {
+          taskState.checkedOutBy = updates.checkedOutBy;
+          taskState.checkedOutAt = updates.checkedOutAt ?? new Date().toISOString();
+        }
+
+        return { ...taskState };
+      }),
+      logEntry: vi.fn().mockResolvedValue(undefined),
+    } as any);
+  }, 30_000);
+
+  afterEach(() => {
+    rmSync(tempDir, { recursive: true, force: true });
+  });
+
+  function buildApp() {
+    const app = express();
+    app.use(express.json());
+    app.use("/api", createApiRoutes(store));
+    return app;
+  }
+
+  it("POST /tasks/:id/checkout — returns 200 on success", async () => {
+    const res = await REQUEST(
+      buildApp(),
+      "POST",
+      `/api/tasks/${taskState.id}/checkout`,
+      JSON.stringify({ agentId: agentAId }),
+      { "Content-Type": "application/json" },
+    );
+
+    expect(res.status).toBe(200);
+    expect(res.body.checkedOutBy).toBe(agentAId);
+    expect(res.body.checkedOutAt).toBeTruthy();
+  }, 20_000);
+
+  it("POST /tasks/:id/checkout — returns 409 on conflict", async () => {
+    await REQUEST(
+      buildApp(),
+      "POST",
+      `/api/tasks/${taskState.id}/checkout`,
+      JSON.stringify({ agentId: agentAId }),
+      { "Content-Type": "application/json" },
+    );
+
+    const res = await REQUEST(
+      buildApp(),
+      "POST",
+      `/api/tasks/${taskState.id}/checkout`,
+      JSON.stringify({ agentId: agentBId }),
+      { "Content-Type": "application/json" },
+    );
+
+    expect(res.status).toBe(409);
+    expect(res.body.error).toBe("Task is already checked out");
+    expect(res.body.currentHolder).toBe(agentAId);
+    expect(res.body.taskId).toBe(taskState.id);
+  }, 20_000);
+
+  it("POST /tasks/:id/checkout — returns 400 when agentId is missing", async () => {
+    const res = await REQUEST(
+      buildApp(),
+      "POST",
+      `/api/tasks/${taskState.id}/checkout`,
+      JSON.stringify({}),
+      { "Content-Type": "application/json" },
+    );
+
+    expect(res.status).toBe(400);
+    expect(res.body.error).toBe("agentId is required");
+  }, 20_000);
+
+  it("POST /tasks/:id/release — returns 200 on success", async () => {
+    await REQUEST(
+      buildApp(),
+      "POST",
+      `/api/tasks/${taskState.id}/checkout`,
+      JSON.stringify({ agentId: agentAId }),
+      { "Content-Type": "application/json" },
+    );
+
+    const res = await REQUEST(
+      buildApp(),
+      "POST",
+      `/api/tasks/${taskState.id}/release`,
+      JSON.stringify({ agentId: agentAId }),
+      { "Content-Type": "application/json" },
+    );
+
+    expect(res.status).toBe(200);
+    expect(res.body.checkedOutBy).toBeUndefined();
+
+    const statusRes = await GET(buildApp(), `/api/tasks/${taskState.id}/checkout`);
+    expect(statusRes.status).toBe(200);
+    expect(statusRes.body.checkedOutBy).toBeNull();
+    expect(statusRes.body.checkedOutAt).toBeNull();
+  }, 20_000);
+
+  it("POST /tasks/:id/release — returns 403 for wrong holder", async () => {
+    await REQUEST(
+      buildApp(),
+      "POST",
+      `/api/tasks/${taskState.id}/checkout`,
+      JSON.stringify({ agentId: agentAId }),
+      { "Content-Type": "application/json" },
+    );
+
+    const res = await REQUEST(
+      buildApp(),
+      "POST",
+      `/api/tasks/${taskState.id}/release`,
+      JSON.stringify({ agentId: agentBId }),
+      { "Content-Type": "application/json" },
+    );
+
+    expect(res.status).toBe(403);
+    expect(res.body.error).toBe("Not the checkout holder");
+  }, 20_000);
+
+  it("POST /tasks/:id/release — returns 400 when agentId is missing", async () => {
+    const res = await REQUEST(
+      buildApp(),
+      "POST",
+      `/api/tasks/${taskState.id}/release`,
+      JSON.stringify({}),
+      { "Content-Type": "application/json" },
+    );
+
+    expect(res.status).toBe(400);
+    expect(res.body.error).toBe("agentId is required");
+  }, 20_000);
+
+  it("GET /tasks/:id/checkout — returns checkout status", async () => {
+    await REQUEST(
+      buildApp(),
+      "POST",
+      `/api/tasks/${taskState.id}/checkout`,
+      JSON.stringify({ agentId: agentAId }),
+      { "Content-Type": "application/json" },
+    );
+
+    const res = await GET(buildApp(), `/api/tasks/${taskState.id}/checkout`);
+
+    expect(res.status).toBe(200);
+    expect(res.body.checkedOutBy).toBe(agentAId);
+    expect(res.body.checkedOutAt).toBeTruthy();
+  }, 20_000);
+
+  it("POST /tasks/:id/force-release — clears active checkout", async () => {
+    await REQUEST(
+      buildApp(),
+      "POST",
+      `/api/tasks/${taskState.id}/checkout`,
+      JSON.stringify({ agentId: agentAId }),
+      { "Content-Type": "application/json" },
+    );
+
+    const res = await REQUEST(
+      buildApp(),
+      "POST",
+      `/api/tasks/${taskState.id}/force-release`,
+    );
+
+    expect(res.status).toBe(200);
+
+    const statusRes = await GET(buildApp(), `/api/tasks/${taskState.id}/checkout`);
+    expect(statusRes.status).toBe(200);
+    expect(statusRes.body.checkedOutBy).toBeNull();
+    expect(statusRes.body.checkedOutAt).toBeNull();
+  }, 20_000);
+});
+
 describe("Attachment routes", () => {
   const FAKE_ATTACHMENT: TaskAttachment = {
     filename: "1234-screenshot.png",

packages/dashboard/src/routes.ts

@@ -2891,6 +2891,120 @@ export function createApiRoutes(store: TaskStore, options?: ServerOptions): Rout
     }
   });
 
+  // Acquire checkout lease for a task
+  router.post("/tasks/:id/checkout", async (req, res) => {
+    try {
+      const { agentId } = req.body ?? {};
+      if (typeof agentId !== "string" || agentId.trim().length === 0) {
+        throw badRequest("agentId is required");
+      }
+
+      const scopedStore = await getScopedStore(req);
+      const { AgentStore } = await import("@fusion/core");
+      const agentStore = new AgentStore({
+        rootDir: scopedStore.getFusionDir(),
+        taskStore: scopedStore,
+      });
+      await agentStore.init();
+
+      const task = await agentStore.checkoutTask(agentId, req.params.id);
+      res.json(task);
+    } catch (err: any) {
+      if (err instanceof ApiError) {
+        throw err;
+      }
+      if (err?.name === "CheckoutConflictError") {
+        res.status(409).json({
+          error: "Task is already checked out",
+          currentHolder: err.currentHolderId,
+          taskId: err.taskId,
+        });
+        return;
+      }
+      if (err?.message?.includes("not found")) {
+        throw notFound(err.message);
+      }
+      rethrowAsApiError(err);
+    }
+  });
+
+  // Release checkout lease for a task
+  router.post("/tasks/:id/release", async (req, res) => {
+    try {
+      const { agentId } = req.body ?? {};
+      if (typeof agentId !== "string" || agentId.trim().length === 0) {
+        throw badRequest("agentId is required");
+      }
+
+      const scopedStore = await getScopedStore(req);
+      const { AgentStore } = await import("@fusion/core");
+      const agentStore = new AgentStore({
+        rootDir: scopedStore.getFusionDir(),
+        taskStore: scopedStore,
+      });
+      await agentStore.init();
+
+      const task = await agentStore.releaseTask(agentId, req.params.id);
+      res.json(task);
+    } catch (err: any) {
+      if (err instanceof ApiError) {
+        throw err;
+      }
+      if (err?.message?.includes("not the checkout holder")) {
+        throw new ApiError(403, "Not the checkout holder");
+      }
+      if (err?.message?.includes("not found")) {
+        throw notFound(err.message);
+      }
+      rethrowAsApiError(err);
+    }
+  });
+
+  // Force release checkout lease for a task
+  router.post("/tasks/:id/force-release", async (req, res) => {
+    try {
+      const scopedStore = await getScopedStore(req);
+      const { AgentStore } = await import("@fusion/core");
+      const agentStore = new AgentStore({
+        rootDir: scopedStore.getFusionDir(),
+        taskStore: scopedStore,
+      });
+      await agentStore.init();
+
+      const task = await agentStore.forceReleaseTask(req.params.id);
+      res.json(task);
+    } catch (err: any) {
+      if (err instanceof ApiError) {
+        throw err;
+      }
+      if (err?.message?.includes("not found")) {
+        throw notFound(err.message);
+      }
+      rethrowAsApiError(err);
+    }
+  });
+
+  // Get checkout lease state for a task
+  router.get("/tasks/:id/checkout", async (req, res) => {
+    try {
+      const scopedStore = await getScopedStore(req);
+      const task = await scopedStore.getTask(req.params.id);
+      if (!task) {
+        throw notFound("Task not found");
+      }
+
+      res.json({
+        checkedOutBy: task.checkedOutBy ?? null,
+        checkedOutAt: task.checkedOutAt ?? null,
+      });
+    } catch (err: any) {
+      if (err instanceof ApiError) {
+        throw err;
+      }
+      rethrowAsApiError(err);
+    }
+  });
+
   // Delete task
   router.delete("/tasks/:id", async (req, res) => {
     try {

packages/engine/src/agent-heartbeat.ts

@@ -703,6 +703,25 @@ export class HeartbeatMonitor {
           return (await this.store.getRunDetail(agentId, run.id))!;
         }
 
+        // Checkout enforcement: agent must hold the lease to work on this task.
+        // The heartbeat only validates existing checkout state — it does NOT attempt
+        // to acquire a checkout itself. The calling system (scheduler, API trigger)
+        // is responsible for checking out the task before the heartbeat starts.
+        if (taskDetail.checkedOutBy && taskDetail.checkedOutBy !== agentId) {
+          heartbeatLog.warn(
+            `Agent ${agentId} does not hold checkout for ${taskId} (held by ${taskDetail.checkedOutBy}) — graceful exit`
+          );
+          await this.completeRun(agentId, run.id, {
+            status: "completed",
+            resultJson: {
+              reason: "checkout_conflict",
+              taskId,
+              checkedOutBy: taskDetail.checkedOutBy,
+            },
+          });
+          return (await this.store.getRunDetail(agentId, run.id))!;
+        }
+
         // Track usage via callbacks
         const STDOUT_EXCERPT_LIMIT = 4000;
         let outputLength = 0;