fix: ensure tool inputSchema includes type: object for discriminated unions

[MayCXC]

Fixes #1643

Problem

z.toJSONSchema() on a z.discriminatedUnion() produces { oneOf: [...] } without a top-level type field. The MCP protocol requires inputSchema.type === "object" (spec.types.ts), so clients that validate the tools/list response reject these schemas. The as Tool['inputSchema'] cast hides the mismatch at compile time.

Fix

// Before
inputSchema: tool.inputSchema
    ? (schemaToJson(tool.inputSchema, { io: 'input' }) as Tool['inputSchema'])
    : EMPTY_OBJECT_JSON_SCHEMA,

// After
inputSchema: tool.inputSchema
    ? ({ type: 'object' as const, ...schemaToJson(tool.inputSchema, { io: 'input' }) } as Tool['inputSchema'])
    : EMPTY_OBJECT_JSON_SCHEMA,

Spreading type: 'object' before the schema output is a no-op for z.object() (already has type) and adds the required field for unions, intersections, etc. This is semantically correct because discriminated unions are always unions of object types (each variant shares a discriminator key).

The result is valid JSON Schema that satisfies the protocol:

{
  "type": "object",
  "oneOf": [
    { "type": "object", "properties": { "type": { "const": "page" }, ... }, "required": ["type", "search"] },
    { "type": "object", "properties": { "type": { "const": "quiz" }, ... }, "required": ["type", "search"] }
  ]
}

Why not other approaches

• Auto-flatten unions into a merged object: complex, lossy (loses "exactly one of these shapes" semantics)
• Throw an error: doesn't solve the problem
• Change schemaToJson: too MCP-specific for a general utility
• Protocol spec change: unnecessary since type: "object" + oneOf is already valid JSON Schema

[changeset-bot]

🦋 Changeset detected

Latest commit: c624fc7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@modelcontextprotocol/server	Patch
@modelcontextprotocol/express	Patch
@modelcontextprotocol/hono	Patch
@modelcontextprotocol/node	Patch

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commit: 2ff1317