Summary

This PR adds regression test coverage only for a directory_tree structured-content validation bug that appears to have already been fixed upstream.

There are no runtime behavior changes in this PR.

Context

The -32602 error reported in:

• In the filesystem, the outputSchema type of the directory_tree tool is incorrect #3106
• Filesystem MCP: move_file tool returns array instead of string (Error -32602) #3093

occurs when a tool declares outputSchema: { content: string } but returns structuredContent.content as an array.

From git history, this appears to have been fixed in:

• e6933ca — "Fix: Changed structuredContent output to match outputSchema (Fix: Changed structuredContent output to match outputSchema #3099)" (2025-12-11)

Note: I opened this PR after seeing the open bug reports and reproducing the issue in an older build, but I did not initially confirm that main already contained the fix. I’m keeping this PR scoped to test coverage in case it’s still useful as a regression guard.

What this PR does

• Adds src/filesystem/__tests__/directory-tree.mcp-sdk.test.ts
  • Uses @modelcontextprotocol/sdk + StdioClientTransport to call directory_tree end-to-end.
  • Asserts that structuredContent.content is a string and JSON-parseable.
• Makes structured-content.test.ts slightly more robust on macOS by resolving the temp directory with fs.realpath() so the allowlist path matches /private/... temp paths.

This is intended to help prevent reintroducing the same outputSchema/structuredContent mismatch.

Minimal reproduction snippet (for older builds)

I was able to reproduce the -32602 validation error by running the filesystem server over stdio and calling directory_tree through the MCP SDK client.

Maintainers can run the following snippet directly (Node 20+):

import { Client } from '@modelcontextprotocol/sdk/client/index.js';
import { StdioClientTransport } from '@modelcontextprotocol/sdk/client/stdio.js';
import fs from 'node:fs/promises';
import os from 'node:os';
import path from 'node:path';

const tmp = await fs.mkdtemp(path.join(os.tmpdir(), 'mcp-fs-tree-'));
const root = await fs.realpath(tmp);
await fs.writeFile(path.join(root, 'a.txt'), 'hello');

const transport = new StdioClientTransport({
  command: 'npx',
  args: ['-y', '@modelcontextprotocol/server-filesystem', root],
});

const client = new Client({ name: 'debug-client', version: '1.0.0' }, { capabilities: {} });
await client.connect(transport);

const result = await client.callTool({
  name: 'directory_tree',
  arguments: { path: root },
});

console.log(result.structuredContent);
await client.close();

On buggy builds, this call throws McpError: -32602 ... Expected string, received array at path: ["content"].

Release context

I have not traced the exact release tag for e6933ca, but based on timestamps it looks like the fix (2025-12-11) should be included in npm releases published after that date (for example @modelcontextprotocol/server-filesystem@2025.12.18 and later).

Happy to update this PR with the exact release tag if you'd like me to track it down precisely.

Test plan

• npm test --workspace @modelcontextprotocol/server-filesystem

Follow-up suggestion (optional)

If you'd like, I can follow up with a small CI-level "functional" test job that runs a tiny MCP SDK client against the built dist/index.js binaries for select servers (filesystem, memory, etc.) to assert that schema validation passes for a few representative tool calls.

I didn't include CI changes in this PR since I wasn't sure how you'd want to scope cross-package functional tests.

Authorship note

This contribution (bug identification, repro, test updates, and PR drafting) was authored with OpenAI Codex.