diff --git a/src/filesystem/__tests__/directory-tree.mcp-sdk.test.ts b/src/filesystem/__tests__/directory-tree.mcp-sdk.test.ts
new file mode 100644
index 0000000000..0cbe48c65c
--- /dev/null
+++ b/src/filesystem/__tests__/directory-tree.mcp-sdk.test.ts
@@ -0,0 +1,57 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import * as fs from 'fs/promises';
+import * as path from 'path';
+import * as os from 'os';
+import { Client } from '@modelcontextprotocol/sdk/client/index.js';
+import { StdioClientTransport } from '@modelcontextprotocol/sdk/client/stdio.js';
+
+describe('directory_tree MCP SDK regression', () => {
+  let client: Client;
+  let transport: StdioClientTransport;
+  let testDir: string;
+
+  beforeEach(async () => {
+    const tmp = await fs.mkdtemp(path.join(os.tmpdir(), 'mcp-fs-tree-'));
+    testDir = await fs.realpath(tmp);
+
+    await fs.writeFile(path.join(testDir, 'root.txt'), 'root');
+    await fs.mkdir(path.join(testDir, 'nested'));
+    await fs.writeFile(path.join(testDir, 'nested', 'child.txt'), 'child');
+
+    const serverPath = path.resolve(__dirname, '../dist/index.js');
+    transport = new StdioClientTransport({
+      command: 'node',
+      args: [serverPath, testDir],
+    });
+
+    client = new Client(
+      { name: 'directory-tree-regression-test', version: '1.0.0' },
+      { capabilities: {} },
+    );
+
+    await client.connect(transport);
+  });
+
+  afterEach(async () => {
+    await client?.close();
+    await fs.rm(testDir, { recursive: true, force: true });
+  });
+
+  it('returns structuredContent.content as a string (not an array) when called via MCP SDK', async () => {
+    const result = await client.callTool({
+      name: 'directory_tree',
+      arguments: { path: testDir },
+    });
+
+    // Regression test for issues where structuredContent was returned as an array,
+    // which causes MCP SDK validation to throw -32602 (invalid structured content).
+    const structured = result.structuredContent as { content: unknown };
+    expect(structured).toBeDefined();
+    expect(typeof structured.content).toBe('string');
+    expect(Array.isArray(structured.content)).toBe(false);
+
+    const parsed = JSON.parse(structured.content as string);
+    expect(Array.isArray(parsed)).toBe(true);
+    expect(parsed.length).toBeGreaterThan(0);
+  });
+});
diff --git a/src/filesystem/__tests__/structured-content.test.ts b/src/filesystem/__tests__/structured-content.test.ts
index 4b8f92b0a3..52a3186d67 100644
--- a/src/filesystem/__tests__/structured-content.test.ts
+++ b/src/filesystem/__tests__/structured-content.test.ts
@@ -21,7 +21,9 @@ describe('structuredContent schema compliance', () => {
 
   beforeEach(async () => {
     // Create a temp directory for testing
-    testDir = await fs.mkdtemp(path.join(os.tmpdir(), 'mcp-fs-test-'));
+    const tmpDir = await fs.mkdtemp(path.join(os.tmpdir(), 'mcp-fs-test-'));
+    // macOS temp dirs may be symlinked (/var -> /private/var); resolve for server allowlist checks.
+    testDir = await fs.realpath(tmpDir);
 
     // Create test files
     await fs.writeFile(path.join(testDir, 'test.txt'), 'test content');