fix(core): enforce agent concurrency cap on foreground sub-agents (#6290) (#6300)

The concurrency cap (QWEN_CODE_MAX_BACKGROUND_AGENTS) only checked
backgrounded agents, letting foreground agents like Explorer bypass
it entirely. Count all running agents toward the cap and apply the
preflight check regardless of background/foreground flavor.

Co-authored-by: Claude Opus 4 <noreply@anthropic.com>

packages/core/src/agents/background-tasks.test.ts

@@ -511,16 +511,24 @@ describe('BackgroundTaskRegistry', () => {
       expect(registry.get('bg-1')?.prompt).toBe('resumed continuation');
     });
 
-    it('does not count foreground, paused, or terminal entries toward the cap', () => {
+    it('counts foreground agents toward the cap but not paused or terminal entries', () => {
       registry = new BackgroundTaskRegistry({
         maxConcurrentBackgroundAgents: 1,
       });
 
+      // A foreground agent occupies a slot.
       registry.register(
         makeRegistration('fg-1', {
           isBackgrounded: false,
         }),
       );
+
+      expect(() => registry.register(makeRegistration('bg-1'))).toThrow(
+        'maximum concurrent background agents (1) reached',
+      );
+
+      // Paused entries do not occupy a slot.
+      registry.unregisterForeground('fg-1');
       registry.register(
         makeRegistration('paused-1', {
           status: 'paused',
@@ -535,7 +543,6 @@ describe('BackgroundTaskRegistry', () => {
       registry.complete('bg-1', 'done');
       registry.register(makeRegistration('bg-2'));
 
-      expect(registry.get('fg-1')).toBeDefined();
       expect(registry.get('paused-1')).toBeDefined();
       expect(registry.get('bg-2')?.status).toBe('running');
     });

packages/core/src/agents/background-tasks.ts

@@ -428,10 +428,9 @@ export class BackgroundTaskRegistry {
     registration: AgentTaskRegistration,
     options: BackgroundTaskRegisterOptions = {},
   ): AgentTask {
-    if (registration.isBackgrounded && registration.status === 'running') {
+    if (registration.status === 'running') {
       const existing = this.agents.get(registration.agentId);
-      const isReplacingRunning =
-        existing?.isBackgrounded === true && existing.status === 'running';
+      const isReplacingRunning = existing?.status === 'running';
       if (!isReplacingRunning) {
         this.assertCanStartBackgroundAgent();
       }
@@ -852,7 +851,7 @@ export class BackgroundTaskRegistry {
 
   private getRunningBackgroundCount(): number {
     return Array.from(this.agents.values()).filter(
-      (entry) => entry.isBackgrounded && entry.status === 'running',
+      (entry) => entry.status === 'running',
     ).length;
   }
 

packages/core/src/tools/agent/agent.ts

@@ -2135,26 +2135,22 @@ class AgentToolInvocation extends BaseToolInvocation<AgentParams, ToolResult> {
       // This is not redundant with registry.register() below — that call
       // remains the authoritative race guard, but by then the launch path
       // has already run hooks and created a child agent.
-      if (shouldRunInBackground) {
-        try {
-          this.config
-            .getBackgroundTaskRegistry()
-            .assertCanStartBackgroundAgent();
-        } catch (error) {
-          const errorMessage =
-            error instanceof Error ? error.message : String(error);
-          this.updateDisplay(
-            {
-              status: 'failed',
-              terminateReason: errorMessage,
-            },
-            updateOutput,
-          );
-          return {
-            llmContent: errorMessage,
-            returnDisplay: this.currentDisplay!,
-          };
-        }
+      try {
+        this.config.getBackgroundTaskRegistry().assertCanStartBackgroundAgent();
+      } catch (error) {
+        const errorMessage =
+          error instanceof Error ? error.message : String(error);
+        this.updateDisplay(
+          {
+            status: 'failed',
+            terminateReason: errorMessage,
+          },
+          updateOutput,
+        );
+        return {
+          llmContent: errorMessage,
+          returnDisplay: this.currentDisplay!,
+        };
       }
 
       // ── Optional worktree isolation (Phase 1: provision) ──────────