From 3d4a82e16cbdb469a74424cbbe670f148b89a6cc Mon Sep 17 00:00:00 2001
From: jyotir-aditya <adityaroy2169@gmail.com>
Date: Sun, 25 Oct 2026 01:22:45 +0530
Subject: [PATCH] mcp: reject null titled enum entries in elicitation schemas

validateTitledEnumEntry dereferenced entry.Const without checking
entry for nil. A JSON null inside oneOf (single-select) or anyOf
(multi-select) unmarshals to a nil *jsonschema.Schema, so the client
panicked while validating an elicitation/create request.

ServerSession.Elicit runs the same validateElicitSchema on the schema
it sent once the client accepts, so a server passing a nil entry could
panic there too.

Return an invalid-params error instead, as for other invalid schemas,
and add oneOf and anyOf regression cases to
TestElicitationSchemaValidation.
---
 mcp/client.go   |  3 +++
 mcp/mcp_test.go | 28 ++++++++++++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/mcp/client.go b/mcp/client.go
index 3b24b2e9d..38c9d6afd 100644
--- a/mcp/client.go
+++ b/mcp/client.go
@@ -1119,6 +1119,9 @@ func validateElicitArrayProperty(propName string, propSchema *jsonschema.Schema)
 }
 
 func validateTitledEnumEntry(entry *jsonschema.Schema) error {
+	if entry == nil {
+		return fmt.Errorf("titled enum entries must be objects")
+	}
 	if entry.Const == nil {
 		return fmt.Errorf("const is required for titled enum entries")
 	}
diff --git a/mcp/mcp_test.go b/mcp/mcp_test.go
index c53e6d571..b15cd7fb0 100644
--- a/mcp/mcp_test.go
+++ b/mcp/mcp_test.go
@@ -1650,6 +1650,34 @@ func TestElicitationSchemaValidation(t *testing.T) {
 			},
 			expectedError: "elicit schema property \"priority\" oneOf has invalid entry: const is required for titled enum entries",
 		},
+		{
+			name: "titled enum with null oneOf entry",
+			schema: &jsonschema.Schema{
+				Type: "object",
+				Properties: map[string]*jsonschema.Schema{
+					"priority": {
+						Type:  "string",
+						OneOf: []*jsonschema.Schema{nil},
+					},
+				},
+			},
+			expectedError: "elicit schema property \"priority\" oneOf has invalid entry: titled enum entries must be objects",
+		},
+		{
+			name: "multi select titled enum with null anyOf entry",
+			schema: &jsonschema.Schema{
+				Type: "object",
+				Properties: map[string]*jsonschema.Schema{
+					"priorities": {
+						Type: "array",
+						Items: &jsonschema.Schema{
+							AnyOf: []*jsonschema.Schema{nil},
+						},
+					},
+				},
+			},
+			expectedError: "elicit schema property \"priorities\" items has invalid entry: titled enum entries must be objects",
+		},
 		{
 			name: "untyped property",
 			schema: &jsonschema.Schema{