From 3fd356f4338574d7f0b797860bbf8ca14a0819b0 Mon Sep 17 00:00:00 2001
From: kai-agent-free <kai@kdn.agency>
Date: Sun, 29 Mar 2026 08:42:31 +0000
Subject: [PATCH] fix: allow $schema and additionalProperties in elicitation
 requestedSchema

Zod's .toJSONSchema() adds $schema and additionalProperties fields
which are not harmful but cause TypeScript errors when passed directly
to elicitInput(). This adds both as optional fields to the schema
definition and TypeScript interface.

Fixes #1362
---
 .changeset/zod-elicit-compat.md       | 5 +++++
 packages/core/src/types/schemas.ts    | 4 +++-
 packages/core/src/types/spec.types.ts | 3 +++
 3 files changed, 11 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/zod-elicit-compat.md

diff --git a/.changeset/zod-elicit-compat.md b/.changeset/zod-elicit-compat.md
new file mode 100644
index 000000000..5274b825f
--- /dev/null
+++ b/.changeset/zod-elicit-compat.md
@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/core": patch
+---
+
+Allow `$schema` and `additionalProperties` in elicitation `requestedSchema` for Zod `.toJSONSchema()` compatibility
diff --git a/packages/core/src/types/schemas.ts b/packages/core/src/types/schemas.ts
index 309b6ade2..dba423361 100644
--- a/packages/core/src/types/schemas.ts
+++ b/packages/core/src/types/schemas.ts
@@ -1862,9 +1862,11 @@ export const ElicitRequestFormParamsSchema = TaskAugmentedRequestParamsSchema.ex
      * Only top-level properties are allowed, without nesting.
      */
     requestedSchema: z.object({
+        $schema: z.string().optional(),
         type: z.literal('object'),
         properties: z.record(z.string(), PrimitiveSchemaDefinitionSchema),
-        required: z.array(z.string()).optional()
+        required: z.array(z.string()).optional(),
+        additionalProperties: z.boolean().optional()
     })
 });
 
diff --git a/packages/core/src/types/spec.types.ts b/packages/core/src/types/spec.types.ts
index ba849c546..6e35eaaea 100644
--- a/packages/core/src/types/spec.types.ts
+++ b/packages/core/src/types/spec.types.ts
@@ -1740,6 +1740,7 @@ export interface Tool extends BaseMetadata, Icons {
         type: 'object';
         properties?: { [key: string]: JSONValue };
         required?: string[];
+        additionalProperties?: boolean;
     };
 
     /**
@@ -1759,6 +1760,7 @@ export interface Tool extends BaseMetadata, Icons {
         type: 'object';
         properties?: { [key: string]: JSONValue };
         required?: string[];
+        additionalProperties?: boolean;
     };
 
     /**
@@ -2801,6 +2803,7 @@ export interface ElicitRequestFormParams extends TaskAugmentedRequestParams {
             [key: string]: PrimitiveSchemaDefinition;
         };
         required?: string[];
+        additionalProperties?: boolean;
     };
 }