From 6c078e7e3d8af3006739a39b2f8a49b30e3d5d98 Mon Sep 17 00:00:00 2001
From: kai-agent-free <kai@kdn.agency>
Date: Mon, 16 Mar 2026 17:39:43 +0000
Subject: [PATCH] fix: widen requestedSchema type to accept standard JSON
 Schema output

Zod's .toJSONSchema() and other JSON Schema generators produce extra
fields like $schema and additionalProperties that were rejected by
the narrow requestedSchema type in elicitInput().

Changes:
- Add additionalProperties as explicit optional field in both the
  TypeScript interface (spec.types.ts) and Zod schema (types.ts)
- Add index signature [key: string]: unknown to the interface to
  allow any additional JSON Schema fields
- Add .passthrough() to Zod schema so extra fields survive validation

Fixes modelcontextprotocol/typescript-sdk#1362
---
 .changeset/fix-elicit-json-schema-compat.md |  9 +++++++++
 packages/core/src/types/spec.types.ts       |  6 ++++++
 packages/core/src/types/types.ts            | 13 ++++++++-----
 3 files changed, 23 insertions(+), 5 deletions(-)
 create mode 100644 .changeset/fix-elicit-json-schema-compat.md

diff --git a/.changeset/fix-elicit-json-schema-compat.md b/.changeset/fix-elicit-json-schema-compat.md
new file mode 100644
index 000000000..649b42655
--- /dev/null
+++ b/.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
diff --git a/packages/core/src/types/spec.types.ts b/packages/core/src/types/spec.types.ts
index f36434bef..8251f1a57 100644
--- a/packages/core/src/types/spec.types.ts
+++ b/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;
     };
 }
 
diff --git a/packages/core/src/types/types.ts b/packages/core/src/types/types.ts
index 6ac79777b..468229af7 100644
--- a/packages/core/src/types/types.ts
+++ b/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()
 });
 
 /**