From a98713d01da2cb1de55db617809f4e8fdba863cf Mon Sep 17 00:00:00 2001
From: gsxdsm <gsxdsm@users.noreply.github.com>
Date: Mon, 8 Jun 2026 22:47:51 -0700
Subject: [PATCH] fix(FN-6035): cap scheduler dispatch by global slots

---
 .../engine/src/__tests__/scheduler.test.ts    | 28 +++++++++++++++++++
 packages/engine/src/scheduler.ts              |  5 +++-
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/packages/engine/src/__tests__/scheduler.test.ts b/packages/engine/src/__tests__/scheduler.test.ts
index 0f89d9f09d..8c6004a8d7 100644
--- a/packages/engine/src/__tests__/scheduler.test.ts
+++ b/packages/engine/src/__tests__/scheduler.test.ts
@@ -1411,6 +1411,34 @@ describe("Scheduler", () => {
       expect(store.moveTask).not.toHaveBeenCalled();
     });
 
+    it("caps in-progress dispatch by the global semaphore limit even before executors acquire slots", async () => {
+      vi.mocked(existsSync).mockReturnValue(true);
+      vi.mocked(readFile).mockResolvedValue("# Task\nDo something");
+
+      const semaphore = new AgentSemaphore(3);
+      const tasks = [
+        createMockTask({ id: "FN-001", column: "in-progress" }),
+        createMockTask({ id: "FN-002", column: "in-progress" }),
+        createMockTask({ id: "FN-003", column: "in-progress" }),
+        createMockTask({ id: "FN-004", column: "todo", dependencies: [] }),
+        createMockTask({ id: "FN-005", column: "todo", dependencies: [] }),
+      ];
+
+      const store = createMockStore({
+        listTasks: vi.fn().mockResolvedValue(tasks),
+        getTask: vi.fn(async (taskId: string) => tasks.find((task) => task.id === taskId) ?? null),
+        getSettings: vi.fn().mockResolvedValue({ maxConcurrent: 5, maxWorktrees: 10 }),
+        updateTask: vi.fn().mockResolvedValue(undefined),
+        moveTask: vi.fn().mockResolvedValue(undefined),
+      });
+
+      const scheduler = new Scheduler(store, { semaphore });
+      scheduler.start();
+      await scheduler.schedule();
+
+      expect(store.moveTask).not.toHaveBeenCalled();
+    });
+
     it("respects maxWorktrees limit", async () => {
       const tasks = [
         createMockTask({ id: "FN-001", column: "in-progress" }),
diff --git a/packages/engine/src/scheduler.ts b/packages/engine/src/scheduler.ts
index c099ccfcdb..7db22e4db9 100644
--- a/packages/engine/src/scheduler.ts
+++ b/packages/engine/src/scheduler.ts
@@ -1277,7 +1277,10 @@ export class Scheduler {
       // When a semaphore is provided, factor in its available slots so we
       // don't schedule more tasks than the global limit allows.
       const semaphoreAvailable = this.options.semaphore
-        ? this.options.semaphore.availableCount
+        ? Math.min(
+          this.options.semaphore.availableCount,
+          this.options.semaphore.limit - agentSlots,
+        )
         : Infinity;
 
       const available = Math.min(