diff --git a/packages/cli/src/acp-integration/session/Session.test.ts b/packages/cli/src/acp-integration/session/Session.test.ts
index 67213c446..8e7a5cd6c 100644
--- a/packages/cli/src/acp-integration/session/Session.test.ts
+++ b/packages/cli/src/acp-integration/session/Session.test.ts
@@ -1069,45 +1069,53 @@ describe('Session', () => {
 
     describe('tool call concurrency', () => {
       it('runs multiple Agent tool calls concurrently (issue #2516)', async () => {
-        // Build a controllable Agent invocation whose execute() blocks until
-        // the test resolves it. If the ACP loop runs Agent calls sequentially
-        // the second `execute` would never be invoked while the first is
-        // still pending — the concurrent implementation starts both before
-        // either resolves.
-        type Deferred = {
-          promise: Promise<core.ToolResult>;
-          resolve: (v: core.ToolResult) => void;
+        // Each Agent call has two controllable async boundaries:
+        //   - `called`  — resolves *when* the test code reaches `execute()`
+        //   - `result`  — the promise `execute()` returns, resolved by the
+        //                 test after observing both `called` signals.
+        //
+        // Under the old sequential for-loop, call-b's `execute()` would
+        // only run after call-a's `execute()` promise resolved — so the
+        // `await Promise.all([called-a, called-b])` below deadlocks and
+        // the test hits vitest's default per-test timeout. Under the
+        // concurrent implementation both `called` signals fire before
+        // either `result` is resolved.
+        type Deferred<T> = {
+          promise: Promise<T>;
+          resolve: (v: T) => void;
         };
-        const makeDeferred = (): Deferred => {
-          let resolve!: (v: core.ToolResult) => void;
-          const promise = new Promise<core.ToolResult>((r) => {
+        const makeDeferred = <T>(): Deferred<T> => {
+          let resolve!: (v: T) => void;
+          const promise = new Promise<T>((r) => {
             resolve = r;
           });
           return { promise, resolve };
         };
 
-        const defs: Record<string, Deferred> = {
-          'call-a': makeDeferred(),
-          'call-b': makeDeferred(),
+        const called: Record<string, Deferred<void>> = {
+          'call-a': makeDeferred<void>(),
+          'call-b': makeDeferred<void>(),
+        };
+        const result: Record<string, Deferred<core.ToolResult>> = {
+          'call-a': makeDeferred<core.ToolResult>(),
+          'call-b': makeDeferred<core.ToolResult>(),
         };
-        const executeSpies: Record<string, ReturnType<typeof vi.fn>> = {};
-        const invocationsBuilt: string[] = [];
 
         const agentTool = {
           name: core.ToolNames.AGENT,
           kind: core.Kind.Think,
           build: vi.fn().mockImplementation((args: Record<string, unknown>) => {
             const id = args['_test_id'] as string;
-            invocationsBuilt.push(id);
-            const execute = vi.fn().mockImplementation(() => defs[id].promise);
-            executeSpies[id] = execute;
             return {
               params: args,
               eventEmitter: undefined,
               getDefaultPermission: vi.fn().mockResolvedValue('allow'),
               getDescription: vi.fn().mockReturnValue(`agent ${id}`),
               toolLocations: vi.fn().mockReturnValue([]),
-              execute,
+              execute: vi.fn().mockImplementation(() => {
+                called[id].resolve();
+                return result[id].promise;
+              }),
             };
           }),
         };
@@ -1153,21 +1161,14 @@ describe('Session', () => {
           prompt: [{ type: 'text', text: 'spawn two agents' }],
         });
 
-        // Yield to the microtask queue repeatedly so the session has a chance
-        // to fan out both Agent calls before we resolve anything. 10 ticks is
-        // generous for the `build → permission-eval → execute` path.
-        for (let i = 0; i < 10; i++) await Promise.resolve();
-
-        // Proof of concurrency: both execute() were invoked before either
-        // resolved. Under the old sequential for-loop this assertion fails
-        // because call-b's execute would only run after call-a resolved.
-        expect(executeSpies['call-a']).toHaveBeenCalledTimes(1);
-        expect(executeSpies['call-b']).toHaveBeenCalledTimes(1);
+        // Wait until both `execute()` bodies have been entered. Sequential
+        // behaviour deadlocks here → vitest times out the test → failure.
+        await Promise.all([called['call-a'].promise, called['call-b'].promise]);
 
         // Resolve out of order to also verify that final part ordering
         // follows the original functionCalls order, not resolution order.
-        defs['call-b'].resolve({ llmContent: 'B-done', returnDisplay: 'B' });
-        defs['call-a'].resolve({ llmContent: 'A-done', returnDisplay: 'A' });
+        result['call-b'].resolve({ llmContent: 'B-done', returnDisplay: 'B' });
+        result['call-a'].resolve({ llmContent: 'A-done', returnDisplay: 'A' });
 
         await promptPromise;
 
diff --git a/packages/cli/src/acp-integration/session/Session.ts b/packages/cli/src/acp-integration/session/Session.ts
index ceb9a741e..1a0fd66b2 100644
--- a/packages/cli/src/acp-integration/session/Session.ts
+++ b/packages/cli/src/acp-integration/session/Session.ts
@@ -1105,12 +1105,41 @@ export class Session implements SessionContext {
       }
     }
 
+    // Bounded-concurrency runner: matches core's `runConcurrently`
+    // behaviour (`coreToolScheduler.ts:1506`), capped by
+    // `QWEN_CODE_MAX_TOOL_CONCURRENCY` (default 10). Results are returned
+    // in input order regardless of resolution order.
+    const runBounded = async (calls: FunctionCall[]): Promise<Part[][]> => {
+      const parsed = parseInt(
+        process.env['QWEN_CODE_MAX_TOOL_CONCURRENCY'] || '',
+        10,
+      );
+      const maxConcurrency =
+        Number.isFinite(parsed) && parsed >= 1 ? parsed : 10;
+      const results: Part[][] = new Array(calls.length);
+      const executing = new Set<Promise<void>>();
+      for (let i = 0; i < calls.length; i++) {
+        const idx = i;
+        const p = this.runTool(abortSignal, promptId, calls[idx])
+          .then((r) => {
+            results[idx] = r;
+          })
+          .finally(() => {
+            executing.delete(p);
+          });
+        executing.add(p);
+        if (executing.size >= maxConcurrency) {
+          await Promise.race(executing);
+        }
+      }
+      await Promise.all(executing);
+      return results;
+    };
+
     const parts: Part[] = [];
     for (const batch of batches) {
       if (batch.concurrent && batch.calls.length > 1) {
-        const results = await Promise.all(
-          batch.calls.map((fc) => this.runTool(abortSignal, promptId, fc)),
-        );
+        const results = await runBounded(batch.calls);
         for (const r of results) parts.push(...r);
       } else {
         for (const fc of batch.calls) {