fix: widen requestedSchema type to accept standard JSON Schema output

.changeset/fix-elicit-json-schema-compat.md

@@ -0,0 +1,9 @@
+---
+"@modelcontextprotocol/sdk": patch
+---
+
+Widen `requestedSchema` type in `elicitInput()` to accept standard JSON Schema output from generators like Zod's `.toJSONSchema()`.
+
+Added `additionalProperties` as an explicit optional field and an index signature `[key: string]: unknown` to allow extra JSON Schema fields (e.g., `$schema`, `additionalProperties`) that schema generators commonly produce. Also added `.passthrough()` to the Zod validation schema so these extra fields are preserved at runtime.
+
+Fixes modelcontextprotocol/typescript-sdk#1362

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

@@ -1740,6 +1740,8 @@ export interface Tool extends BaseMetadata, Icons {
         type: 'object';
         properties?: { [key: string]: JSONValue };
         required?: string[];
+        additionalProperties?: boolean | { [key: string]: unknown };
+        [key: string]: unknown;
     };
 
     /**
@@ -1759,6 +1761,8 @@ export interface Tool extends BaseMetadata, Icons {
         type: 'object';
         properties?: { [key: string]: JSONValue };
         required?: string[];
+        additionalProperties?: boolean | { [key: string]: unknown };
+        [key: string]: unknown;
     };
 
     /**
@@ -2801,6 +2805,8 @@ export interface ElicitRequestFormParams extends TaskAugmentedRequestParams {
             [key: string]: PrimitiveSchemaDefinition;
         };
         required?: string[];
+        additionalProperties?: boolean | { [key: string]: unknown };
+        [key: string]: unknown;
     };
 }
 

packages/core/src/types/types.ts

@@ -2024,11 +2024,14 @@ 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(),
+            additionalProperties: z.union([z.boolean(), z.record(z.string(), z.unknown())]).optional()
+        })
+        .passthrough()
 });
 
 /**