From 0c9b2bc75e6ae776600c115850e7a79d3eee6227 Mon Sep 17 00:00:00 2001
From: Dave-London <hello@os4us.org>
Date: Tue, 10 Feb 2026 07:52:39 +0200
Subject: [PATCH 1/2] fix: widen requestedSchema type to accept additional JSON
 Schema fields

The `requestedSchema` object in `ElicitRequestFormParamsSchema` was too
narrow, rejecting valid JSON Schema fields like `additionalProperties`
that Zod v4's `.toJSONSchema()` and other JSON Schema generators produce.

Add `.passthrough()` to the `requestedSchema` Zod object and include
the `$schema` field (already present in the spec types) so that standard
JSON Schema output is accepted without type errors.

Fixes #1362

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 packages/core/src/types/types.ts      | 13 ++++++++-----
 packages/core/test/spec.types.test.ts | 13 +++++++++++--
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/packages/core/src/types/types.ts b/packages/core/src/types/types.ts
index 2a79944b9..58104a891 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;
     },

From 9b4547d30357c636fe768c477c9f7db91afb0664 Mon Sep 17 00:00:00 2001
From: Dave-London <hello@os4us.org>
Date: Wed, 11 Feb 2026 06:15:08 +0200
Subject: [PATCH 2/2] chore: add changeset for requestedSchema type fix

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .changeset/widen-requested-schema-types.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/widen-requested-schema-types.md

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