fix(server): preserve inputSchema for z.discriminatedUnion / z.union

[ankitvirdi4]

Summary

Fixes #1643. registerTool / registerPrompt silently advertised an empty schema in tools/list when given a z.discriminatedUnion(...) or a z.union([z.object(...), ...]). Tool calls still validated correctly via the fallback in validateToolInput, which masked the bug from end-to-end tests.

Repro on v1.x HEAD

import * as z from 'zod/v4';
import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';

const server = new McpServer({ name: 't', version: '1' });
server.registerTool('mutate', {
  inputSchema: z.discriminatedUnion('action', [
    z.object({ action: z.literal('create'), name: z.string() }),
    z.object({ action: z.literal('delete'), id: z.string() }),
  ]),
}, async args => ({ content: [{ type: 'text', text: JSON.stringify(args) }] }));

// tools/list returns: inputSchema: { type: 'object', properties: {} }
// branches dropped on the wire.

Root cause

normalizeObjectSchema() (in src/server/zod-compat.ts) returned undefined for any schema whose root was not z.object(...). The tools/list handler then substituted EMPTY_OBJECT_JSON_SCHEMA. The downstream toJsonSchemaCompat already handles unions correctly (zod-to-json-schema for v3, Mini's z.toJSONSchema for v4) but never got called.

Fix

Two surgical changes:

1. src/server/zod-compat.ts — normalizeObjectSchema() now also passes through:

   • v4 schemas with _zod.def.type === 'union' (covers both z.union and z.discriminatedUnion)
   • v3 schemas with _def.typeName of ZodDiscriminatedUnion or ZodUnion

   Schemas whose root is genuinely non-object (z.string(), z.array(...)) still return undefined, matching prior behavior.

2. src/server/mcp.ts — wraps the conversion result with a small ensureObjectRoot() helper that defaults type: "object" when the converter omits it. Discriminated unions and unions emit oneOf / anyOf without a root type, but the MCP spec requires tool input/output schemas to describe objects, so we default it on the way out. Plain z.object(...) already sets type: "object", so this is a no-op for the common case. Schemas with an explicit non-object root type are left alone (they fall out as user errors downstream, which is the right signal).

Tests

Three regression tests under "Tools with union and intersection schemas". The file uses describe.each(zodTestMatrix) so each runs once for v3 and once for v4 — six actual cases:

• discriminated-union inputSchema is exposed in tools/list with type: "object" + oneOf / anyOf
• regular union of objects, same
• discriminated-union outputSchema, same, plus end-to-end callTool round trip

Includes a changeset entry.

Test plan

• npx vitest run test/server/mcp.test.ts — 238 / 238 passing
• npx vitest run (full suite) — 1585 / 1585 passing
• npm run typecheck clean
• npm run lint clean
• Manual end-to-end with the SDK's in-memory transport: tools/list returns the proper schema with branches; valid input round-trips; malformed input returns proper validation error

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