outputSchema always emitted as JSON Schema draft-07, breaking clients that only accept 2020-12

Package: @modelcontextprotocol/sdk 1.30.0
Affects: any server built with McpServer that declares an outputSchema (e.g. @modelcontextprotocol/server-filesystem 2026.7.10), consumed by a client with a strict output-schema validator.

Bug

In src/server/zod-json-schema-compat.ts (compiled to dist/{cjs,esm}/server/zod-json-schema-compat.js), mapMiniTarget() falls back to 'draft-7' whenever no explicit target is passed:

function mapMiniTarget(t) {
    if (!t) return 'draft-7';   // <-- always hit in practice
    ...
}

McpServer's tool-listing handler (src/server/mcp.ts) never passes a target when calling toJsonSchemaCompat() for either inputSchema or outputSchema:

toolDefinition.outputSchema = toJsonSchemaCompat(obj, {
    strictUnions: true,
    pipeStrategy: 'output'
});

So every registered tool's outputSchema (Zod v4 branch, via zod/v4-mini's toJSONSchema) is emitted with "$schema": "http://json-schema.org/draft-07/schema#", with no way for the server author to opt into 2020-12.

Impact

A client that validates outputSchema strictly against JSON Schema 2020-12 rejects every tool from every SDK-based server that declares an outputSchema, e.g. (observed in Claude Desktop 1.37937.1):

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

This breaks @modelcontextprotocol/server-filesystem entirely — every tool call fails client-side before even reaching the server.

Suggested fix

Default mapMiniTarget's fallback to 'draft-2020-12' instead of 'draft-7' (2020-12 is a superset-compatible, more current dialect and z4mini.toJSONSchema already supports it via the existing branch), or have McpServer pass target: 'draft-2020-12' explicitly.

Confirmed locally that swapping the fallback produces "$schema": "https://json-schema.org/draft/2020-12/schema" and resolves the client-side rejection with no other code changes needed.

Repro

1. Any MCP server built with McpServer + outputSchema (e.g. server-filesystem@2026.7.10).
2. A client validating outputSchema strictly against 2020-12 (e.g. Claude Desktop 1.37937.1+).
3. Every tool call fails with the "unsupported dialect" error above.

Start in src/server/zod-json-schema-compat.ts by reading mapMiniTarget(), then inspect the tool-listing handler in src/server/mcp.ts where toJsonSchemaCompat() is called. Reproduce a tool with an outputSchema and inspect the emitted $schema value; done means the listing emits the JSON Schema 2020-12 dialect and no longer triggers the strict client rejection described in the issue.