diff --git a/.changeset/schema-object-type-for-unions.md b/.changeset/schema-object-type-for-unions.md
new file mode 100644
index 000000000..7749bb6c5
--- /dev/null
+++ b/.changeset/schema-object-type-for-unions.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/core': patch
+---
+
+Ensure `standardSchemaToJsonSchema` emits `type: "object"` at the root, fixing discriminated-union tool/prompt schemas that previously produced `{oneOf: [...]}` without the MCP-required top-level type. Also throws a clear error when given an explicitly non-object schema (e.g. `z.string()`). Fixes #1643.
diff --git a/packages/core/src/util/standardSchema.ts b/packages/core/src/util/standardSchema.ts
index d95572c0e..9817dc39a 100644
--- a/packages/core/src/util/standardSchema.ts
+++ b/packages/core/src/util/standardSchema.ts
@@ -138,8 +138,25 @@ export function isStandardSchemaWithJSON(schema: unknown): schema is StandardSch
 
 // JSON Schema conversion
 
+/**
+ * Converts a StandardSchema to JSON Schema for use as an MCP tool/prompt schema.
+ *
+ * MCP requires `type: "object"` at the root of tool inputSchema/outputSchema and
+ * prompt argument schemas. Zod's discriminated unions emit `{oneOf: [...]}` without
+ * a top-level `type`, so this function defaults `type` to `"object"` when absent.
+ *
+ * Throws if the schema has an explicit non-object `type` (e.g. `z.string()`),
+ * since that cannot satisfy the MCP spec.
+ */
 export function standardSchemaToJsonSchema(schema: StandardJSONSchemaV1, io: 'input' | 'output' = 'input'): Record<string, unknown> {
-    return schema['~standard'].jsonSchema[io]({ target: 'draft-2020-12' });
+    const result = schema['~standard'].jsonSchema[io]({ target: 'draft-2020-12' });
+    if (result.type !== undefined && result.type !== 'object') {
+        throw new Error(
+            `MCP tool and prompt schemas must describe objects (got type: ${JSON.stringify(result.type)}). ` +
+                `Wrap your schema in z.object({...}) or equivalent.`
+        );
+    }
+    return { type: 'object', ...result };
 }
 
 // Validation
diff --git a/packages/core/test/util/standardSchema.test.ts b/packages/core/test/util/standardSchema.test.ts
new file mode 100644
index 000000000..6c3de99d7
--- /dev/null
+++ b/packages/core/test/util/standardSchema.test.ts
@@ -0,0 +1,42 @@
+import * as z from 'zod/v4';
+
+import { standardSchemaToJsonSchema } from '../../src/util/standardSchema.js';
+
+describe('standardSchemaToJsonSchema', () => {
+    test('emits type:object for plain z.object schemas', () => {
+        const schema = z.object({ name: z.string(), age: z.number() });
+        const result = standardSchemaToJsonSchema(schema, 'input');
+
+        expect(result.type).toBe('object');
+        expect(result.properties).toBeDefined();
+    });
+
+    test('emits type:object for discriminated unions', () => {
+        const schema = z.discriminatedUnion('action', [
+            z.object({ action: z.literal('create'), name: z.string() }),
+            z.object({ action: z.literal('delete'), id: z.string() })
+        ]);
+        const result = standardSchemaToJsonSchema(schema, 'input');
+
+        expect(result.type).toBe('object');
+        // Zod emits oneOf for discriminated unions; the catchall on Tool.inputSchema
+        // accepts it, but the top-level type must be present per MCP spec.
+        expect(result.oneOf ?? result.anyOf).toBeDefined();
+    });
+
+    test('throws for schemas with explicit non-object type', () => {
+        expect(() => standardSchemaToJsonSchema(z.string(), 'input')).toThrow(/must describe objects/);
+        expect(() => standardSchemaToJsonSchema(z.array(z.string()), 'input')).toThrow(/must describe objects/);
+        expect(() => standardSchemaToJsonSchema(z.number(), 'input')).toThrow(/must describe objects/);
+    });
+
+    test('preserves existing type:object without modification', () => {
+        const schema = z.object({ x: z.string() });
+        const result = standardSchemaToJsonSchema(schema, 'input');
+
+        // Spread order means zod's own type:"object" wins; verify no double-wrap.
+        const keys = Object.keys(result);
+        expect(keys.filter(k => k === 'type')).toHaveLength(1);
+        expect(result.type).toBe('object');
+    });
+});