diff --git a/.changeset/widen-requested-schema-types.md b/.changeset/widen-requested-schema-types.md
new file mode 100644
index 000000000..294adb04b
--- /dev/null
+++ b/.changeset/widen-requested-schema-types.md
@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/core": patch
+---
+
+fix: widen requestedSchema type to accept additional JSON Schema fields
diff --git a/packages/core/src/types/types.ts b/packages/core/src/types/types.ts
index 070ff0279..72b5ea2fc 100644
--- a/packages/core/src/types/types.ts
+++ b/packages/core/src/types/types.ts
@@ -1993,11 +1993,14 @@ 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({
+            $schema: z.string().optional(),
+            type: z.literal('object'),
+            properties: z.record(z.string(), PrimitiveSchemaDefinitionSchema),
+            required: z.array(z.string()).optional()
+        })
+        .passthrough()
 });
 
 /**
diff --git a/packages/core/test/spec.types.test.ts b/packages/core/test/spec.types.test.ts
index 8ec29b096..c73b42a7a 100644
--- a/packages/core/test/spec.types.test.ts
+++ b/packages/core/test/spec.types.test.ts
@@ -53,6 +53,12 @@ type MakeUnknownsNotOptional<T> =
                       }
           : T;
 
+// Targeted fix: in spec, requestedSchema needs index signature to match SDK's .passthrough()
+// which allows additional JSON Schema fields like `additionalProperties` from Zod's .toJSONSchema()
+type FixSpecRequestedSchema<T> = T extends { requestedSchema: infer R }
+    ? Omit<T, 'requestedSchema'> & { requestedSchema: R & { [x: string]: unknown } }
+    : T;
+
 // Targeted fix: in spec, treat ClientCapabilities.elicitation?: object as Record<string, unknown>
 type FixSpecClientCapabilities<T> = T extends { elicitation?: object }
     ? Omit<T, 'elicitation'> & { elicitation?: Record<string, unknown> }
@@ -160,11 +166,14 @@ const sdkTypeChecks = {
         sdk = spec;
         spec = sdk;
     },
-    ElicitRequestParams: (sdk: SDKTypes.ElicitRequestParams, spec: SpecTypes.ElicitRequestParams) => {
+    ElicitRequestParams: (
+        sdk: SDKTypes.ElicitRequestParams,
+        spec: FixSpecRequestedSchema<SpecTypes.ElicitRequestFormParams> | SpecTypes.ElicitRequestURLParams
+    ) => {
         sdk = spec;
         spec = sdk;
     },
-    ElicitRequestFormParams: (sdk: SDKTypes.ElicitRequestFormParams, spec: SpecTypes.ElicitRequestFormParams) => {
+    ElicitRequestFormParams: (sdk: SDKTypes.ElicitRequestFormParams, spec: FixSpecRequestedSchema<SpecTypes.ElicitRequestFormParams>) => {
         sdk = spec;
         spec = sdk;
     },