diff --git a/.changeset/fix-discriminated-union-schema.md b/.changeset/fix-discriminated-union-schema.md
new file mode 100644
index 000000000..06b0b3927
--- /dev/null
+++ b/.changeset/fix-discriminated-union-schema.md
@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/server": patch
+---
+
+fix: ensure tool inputSchema includes type: object for discriminated unions
diff --git a/packages/server/src/server/mcp.ts b/packages/server/src/server/mcp.ts
index 05136f5b6..67fbaafd5 100644
--- a/packages/server/src/server/mcp.ts
+++ b/packages/server/src/server/mcp.ts
@@ -147,7 +147,7 @@ export class McpServer {
                             title: tool.title,
                             description: tool.description,
                             inputSchema: tool.inputSchema
-                                ? (schemaToJson(tool.inputSchema, { io: 'input' }) as Tool['inputSchema'])
+                                ? ({ type: 'object' as const, ...schemaToJson(tool.inputSchema, { io: 'input' }) } as Tool['inputSchema'])
                                 : EMPTY_OBJECT_JSON_SCHEMA,
                             annotations: tool.annotations,
                             execution: tool.execution,