server-filesystem: tools/list schemas declare unsupported $schema draft-07 dialect, rejected by strict 2020-12 validators

[tomekep]

Summary

@modelcontextprotocol/server-filesystem (tested on the current latest, 2026.8.31, published 2026-09-17) emits tools/list responses where every tool's schema declares:

"$schema": "http://json-schema.org/draft-07/schema#"

Clients that validate tools/list strictly against JSON Schema 2020-12 reject every tool outright, e.g.:

Tool '<name>' has an invalid outputSchema/inputSchema: JSON Schema declares an unsupported dialect ("$schema": "http://json-schema.org/draft-07/schema#"). The default validator supports JSON Schema 2020-12 only

This is the same class of problem noted as a "separately noted, different, unrelated bug" in #4772 for server-memory, but here it reproduces for server-filesystem as well.

Steps to reproduce

Spawn the server directly and send a raw tools/list request over stdio:

printf '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}\n{"jsonrpc":"2.0","method":"notifications/initialized"}\n{"jsonrpc":"2.0","id":2,"method":"tools/list"}\n' \
  | node node_modules/@modelcontextprotocol/server-filesystem/dist/index.js /some/allowed/dir

Every tool's inputSchema includes "$schema":"http://json-schema.org/draft-07/schema#".

What I checked before filing

• Confirmed this reproduces on a fresh install of the latest published version (2026.8.31), not an old cached one — npm ls -g @modelcontextprotocol/server-filesystem shows 2026.8.31, installed minutes before reproducing.
• The package depends on zod-to-json-schema@3.25.2, whose zodToJsonSchema() sets combined.$schema = "http://json-schema.org/draft-07/schema#" whenever refs.target === "jsonSchema7" (its own default target) — dist/cjs/zodToJsonSchema.js and dist/esm/zodToJsonSchema.js, same line in both.
• However, patching that exact line in both files (changing the literal to a 2020-12 URI) and re-spawning the server (fresh process, no compile cache active, confirmed via node -e "console.log(process.env.NODE_COMPILE_CACHE)" → undefined) did not change the live tools/list output — it still returns draft-07. I verified with grep that the patched files on disk have zero occurrences of draft-07 after the edit, yet the live response is unchanged.
• I could not find any other occurrence of the literal string draft-07 anywhere else under server-filesystem's own node_modules/@modelcontextprotocol/sdk, and dist/index.js itself contains no zod-to-json-schema import and no toJSONSchema call.
• One more lead I did not have time to chase down: the package also pulls in zod v4, whose native toJSONSchema() (node_modules/zod/v4/core/json-schema-generator.js) also normalizes its default target to "draft-07" (see the module's own JSDoc example and the normalizedTarget = "draft-07" fallback). It's possible the actual schema generation path used by the current build goes through Zod v4's native generator instead of (or in addition to) zod-to-json-schema, which would explain why patching the latter had no effect. I did not confirm this — flagging it as the most promising next place to look.

Impact

Any MCP client with a strict JSON Schema 2020-12 validator on tools/list (e.g. Claude Code / Claude Desktop's "Cowork and Code sessions" context, similar to what's described in #4772) cannot use server-filesystem at all — every tool call fails at the schema-validation step before it's ever invoked.

Environment

• @modelcontextprotocol/server-filesystem@2026.8.31
• zod-to-json-schema@3.25.2 (as installed transitively)
• Node.js v24.15.0, Windows 11 ARM64