From 192d981d693b0ebb7899b558a1109ee7e29b2eac Mon Sep 17 00:00:00 2001
From: Felix Weinberger <fweinberger@anthropic.com>
Date: Thu, 27 Nov 2025 11:47:06 +0000
Subject: [PATCH] fix: prevent TS2589 type instantiation error in ToolCallback

Use [T] extends [U] pattern on the AnySchema check to prevent
distributive conditional types. This fixes the "Type instantiation
is excessively deep and possibly infinite" error that occurs when
using registerTool after upgrading to v1.23.0.

The issue was that AnySchema (z3.ZodTypeAny | z4.$ZodType) as a union
caused TypeScript to distribute the conditional type check, leading
to exponential type instantiation.

Fixes #1180
---
 src/server/mcp.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/server/mcp.ts b/src/server/mcp.ts
index b9b6d5596..b91d488dd 100644
--- a/src/server/mcp.ts
+++ b/src/server/mcp.ts
@@ -1088,7 +1088,7 @@ export class ResourceTemplate {
  */
 export type ToolCallback<Args extends undefined | ZodRawShapeCompat | AnySchema = undefined> = Args extends ZodRawShapeCompat
     ? (args: ShapeOutput<Args>, extra: RequestHandlerExtra<ServerRequest, ServerNotification>) => CallToolResult | Promise<CallToolResult>
-    : Args extends AnySchema
+    : [Args] extends [AnySchema]
       ? (
             args: SchemaOutput<Args>,
             extra: RequestHandlerExtra<ServerRequest, ServerNotification>