feat(FN-4153): complete Step 4 — block ephemeral spawn when disabled

Fusion-Task-Id: FN-4153
Fusion-Task-Lineage: 6ec09467-c7a0-4a9d-9a09-16d5ef6f7a86

packages/engine/src/ephemeral-worker-manager.ts

@@ -19,7 +19,7 @@
  * lookup on creation, the on-disk fallback on completion, and the startup
  * sweep here close that gap.
  */
-import type { AgentStore, AgentState, Agent, TaskStore, Task } from "@fusion/core";
+import type { AgentStore, AgentState, Agent, TaskStore, Task, Settings } from "@fusion/core";
 import { isEphemeralAgent } from "@fusion/core";
 
 export interface TaskOwner {
@@ -43,6 +43,7 @@ export interface EphemeralWorkerManagerOptions {
    * don't race the executor on the same agentId.
    */
   isDeletionPendingExternal?: (agentId: string) => boolean;
+  getSettings?: () => Promise<Pick<Settings, "ephemeralAgentsEnabled">>;
 }
 
 const TERMINAL_TASK_COLUMNS = new Set<Task["column"]>(["done", "archived"]);
@@ -52,6 +53,7 @@ export class EphemeralWorkerManager {
   private readonly taskStore: TaskStore;
   private readonly log: EphemeralWorkerLogger;
   private readonly isDeletionPendingExternal: (agentId: string) => boolean;
+  private readonly getSettings: () => Promise<Pick<Settings, "ephemeralAgentsEnabled">>;
 
   /** taskId → owner. In-memory only; on-disk fallback covers restart gaps. */
   private readonly taskAgentMap = new Map<string, TaskOwner>();
@@ -65,6 +67,7 @@ export class EphemeralWorkerManager {
     this.taskStore = options.taskStore;
     this.log = options.logger;
     this.isDeletionPendingExternal = options.isDeletionPendingExternal ?? (() => false);
+    this.getSettings = options.getSettings ?? (async () => ({ ephemeralAgentsEnabled: true }));
   }
 
   // ── public surface ───────────────────────────────────────────────────────
@@ -125,6 +128,14 @@ export class EphemeralWorkerManager {
         }
       }
 
+      const settings = await this.getSettings();
+      if (settings.ephemeralAgentsEnabled === false) {
+        this.log.warn(
+          `Task ${task.id} has no permanent agent assignment; ephemeralAgentsEnabled=false — refusing to spawn ephemeral worker`,
+        );
+        return null;
+      }
+
       const agent = await this.agentStore.createAgent({
         name: `executor-${task.id}`,
         role: "executor",

packages/engine/src/runtimes/__tests__/in-process-runtime.test.ts

@@ -754,6 +754,30 @@ describe("InProcessRuntime", () => {
       expect(assignTaskSpy.mock.invocationCallOrder[0]).toBeLessThan(updateStateSpy.mock.invocationCallOrder[0]);
     }, 30000);
 
+    it("does not spawn runtime task-worker agents when ephemeral agents are disabled", async () => {
+      mockTaskStoreSettings.ephemeralAgentsEnabled = false;
+      await runtime.start();
+
+      const store = getAgentStore(runtime);
+      const createAgentSpy = vi.spyOn(store, "createAgent");
+      const warnSpy = vi.spyOn(runtimeLog, "warn");
+
+      const executorOptions = mockExecutorCtor.mock.calls.at(-1)?.[0] as {
+        onStart?: (task: Task, worktreePath: string) => void;
+      };
+      executorOptions.onStart?.({ id: "FN-1663" } as Task, join(testDir, "worktree-FN-1663"));
+
+      await vi.waitFor(() => {
+        expect(warnSpy).toHaveBeenCalledWith(
+          expect.stringContaining("Task FN-1663 has no permanent agent assignment; ephemeralAgentsEnabled=false"),
+        );
+      });
+
+      const agents = await store.listAgents({ includeEphemeral: true });
+      expect(agents.some((agent: Agent) => agent.name === "executor-FN-1663")).toBe(false);
+      expect(createAgentSpy).not.toHaveBeenCalledWith(expect.objectContaining({ name: "executor-FN-1663" }));
+    }, 30000);
+
     it("falls back to runtime task-worker when assignedAgentId points to ephemeral agent", async () => {
       await runtime.start();
 

packages/engine/src/runtimes/in-process-runtime.ts

@@ -530,6 +530,10 @@ export class InProcessRuntime
             taskStore: this.taskStore,
             logger: runtimeLog,
             isDeletionPendingExternal: (agentId) => this.executor?.isEphemeralDeletionPending(agentId) ?? false,
+            getSettings: async () => {
+              const settings = await this.taskStore.getSettings();
+              return { ephemeralAgentsEnabled: settings.ephemeralAgentsEnabled };
+            },
           });
         }
         if (this.workerManager) {