fix: widen elicitInput requestedSchema type to accept Zod's toJSONSchema output

Zod v4's `.toJSONSchema()` produces standard JSON Schema output that includes
fields like `$schema` and `additionalProperties`. The `requestedSchema` type
in `ElicitRequestFormParams` was too narrow to accept these extra fields,
forcing users to cast through `unknown`.

This adds `.passthrough()` to the Zod runtime schema and an index signature
to the spec type, allowing any additional JSON Schema fields through while
keeping the required fields (`type`, `properties`) strictly typed.

Fixes #1362

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: travisbreaks

.changeset/widen-elicit-requested-schema.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/core': patch
+---
+
+Widen `requestedSchema` type in `ElicitRequestFormParams` to accept additional JSON Schema fields (e.g., `$schema`, `additionalProperties`) that tools like Zod's `.toJSONSchema()` produce. This removes the need for users to cast through `unknown` when passing Zod-generated schemas to `elicitInput()`.

packages/core/src/types/spec.types.ts

@@ -2795,6 +2795,7 @@ export interface ElicitRequestFormParams extends TaskAugmentedRequestParams {
      * Only top-level properties are allowed, without nesting.
      */
     requestedSchema: {
+        [key: string]: unknown;
         $schema?: string;
         type: 'object';
         properties: {

packages/core/src/types/types.ts

@@ -2024,11 +2024,13 @@ export const ElicitRequestFormParamsSchema = TaskAugmentedRequestParamsSchema.ex
      * A restricted subset of JSON Schema.
      * Only top-level properties are allowed, without nesting.
      */
-    requestedSchema: z.object({
-        type: z.literal('object'),
-        properties: z.record(z.string(), PrimitiveSchemaDefinitionSchema),
-        required: z.array(z.string()).optional()
-    })
+    requestedSchema: z
+        .object({
+            type: z.literal('object'),
+            properties: z.record(z.string(), PrimitiveSchemaDefinitionSchema),
+            required: z.array(z.string()).optional()
+        })
+        .passthrough()
 });
 
 /**