fix(core): accept MCP tools whose output schema is not a root object - #58224

Fixes #57643

Problem

If one tool in an MCP server's tools/list has an outputSchema with a top-level anyOf/oneOf and no "type": "object", the whole server fails to connect and none of its tools load:

MCP server failed: <server_name>
Invalid result for tools/list:
  path: ["tools", 3, "outputSchema", "type"]
  message: "Invalid input: expected \"object\""

The MCP client SDK validates legacy (pre-2026-07-28) tools/list results with a tool schema whose outputSchema must be type: "object". 2025-06-18 and 2025-11-25 did declare that restriction, but 2026-07-28 allows any JSON Schema 2020-12 here, and real servers (cqs, 10 of 31 tools) already use it. The SDK rejects the whole list over one tool's metadata, and SDK 2.3.1 behaves the same, so upgrading would not fix this.

Change

Our @modelcontextprotocol/client@2.0.0 patch now gives the legacy tool schema the same outputSchema shape as the SDK's own 2026-07-28 schema in the same file:

 const ToolSchema$1 = z.object({
   ...
   inputSchema: z.object({ type: z.literal("object"), ... }).catchall(z.unknown()),
-  outputSchema: z.object({
-    type: z.literal("object"),
-    properties: z.record(z.string(), JSONValueSchema$1).optional(),
-    required: z.array(z.string()).optional()
-  }).catchall(z.unknown()).optional(),
+  outputSchema: z.looseObject({ $schema: z.string().optional() }).optional(),

inputSchema keeps its type: "object" requirement. listTools() still caches the schemas, so callTool still validates structuredContent against them. Only the root-shape restriction on outputSchema is gone.

Out of scope

• How a structuredContent mismatch is handled (mcp: output schema mismatch hard-fails tool calls instead of degrading gracefully #56116). Calls still fail on a mismatch, as before.
• Upgrading the SDK.

Verification

• New test accepts legacy MCP tools whose output schema is not a root object, using a raw JSON-RPC stdio fixture. The server SDK can't be used here, because it rewrites non-object output schemas for legacy clients. With the patch, the tool lists with its anyOf schema intact and the call returns its structured result. Without it, the test fails with the error from the issue.
• Against a minimal raw stdio server with one type: "object" tool and one $defs + anyOf tool: unpatched 2.0.0 and 2.3.1 both fail with Invalid result for tools/list; patched 2.0.0 lists both tools and calls the anyOf one successfully.
• bun test test/mcp.test.ts test/mcp-oauth.test.ts test/mcp-instructions.test.ts test/mcp-import-boundary.test.ts in packages/core: 97 pass, 0 fail.
• bun run check from the repo root: 34/34 tasks pass.