diff --git a/client/src/components/DynamicJsonForm.tsx b/client/src/components/DynamicJsonForm.tsx
index 96b03b3a4..685401ab9 100644
--- a/client/src/components/DynamicJsonForm.tsx
+++ b/client/src/components/DynamicJsonForm.tsx
@@ -113,8 +113,8 @@ const DynamicJsonForm = forwardRef<DynamicJsonFormRef, DynamicJsonFormProps>(
             setJsonError(errorMessage);
 
             // Reset to default for clearly invalid JSON (not just incomplete typing)
-            const trimmed = jsonString.trim();
-            if (trimmed.length > 5 && !trimmed.match(/^[\s[{]/)) {
+            const trimmed = jsonString?.trim();
+            if (trimmed && trimmed.length > 5 && !trimmed.match(/^[\s[{]/)) {
               onChange(generateDefaultValue(schema));
             }
           }
@@ -155,7 +155,7 @@ const DynamicJsonForm = forwardRef<DynamicJsonFormRef, DynamicJsonFormProps>(
 
     const formatJson = () => {
       try {
-        const jsonStr = rawJsonValue.trim();
+        const jsonStr = rawJsonValue?.trim();
         if (!jsonStr) {
           return;
         }
@@ -171,7 +171,7 @@ const DynamicJsonForm = forwardRef<DynamicJsonFormRef, DynamicJsonFormProps>(
     const validateJson = () => {
       if (!isJsonMode) return { isValid: true, error: null };
       try {
-        const jsonStr = rawJsonValue.trim();
+        const jsonStr = rawJsonValue?.trim();
         if (!jsonStr) return { isValid: true, error: null };
         const parsed = JSON.parse(jsonStr);
         // Clear any pending debounced update and immediately update parent
diff --git a/client/src/components/ToolsTab.tsx b/client/src/components/ToolsTab.tsx
index b8c163a2f..26f988e9b 100644
--- a/client/src/components/ToolsTab.tsx
+++ b/client/src/components/ToolsTab.tsx
@@ -194,16 +194,18 @@ const ToolsTab = ({
                                     ...params,
                                     [key]: checked
                                       ? null
-                                      : prop.default !== null
-                                        ? prop.default
-                                        : prop.type === "boolean"
-                                          ? false
-                                          : prop.type === "string"
-                                            ? ""
-                                            : prop.type === "number" ||
-                                                prop.type === "integer"
-                                              ? undefined
-                                              : undefined,
+                                      : prop.type === "array"
+                                        ? undefined
+                                        : prop.default !== null
+                                          ? prop.default
+                                          : prop.type === "boolean"
+                                            ? false
+                                            : prop.type === "string"
+                                              ? ""
+                                              : prop.type === "number" ||
+                                                  prop.type === "integer"
+                                                ? undefined
+                                                : undefined,
                                   })
                                 }
                               />
diff --git a/client/src/utils/jsonUtils.ts b/client/src/utils/jsonUtils.ts
index f83789668..334dafc24 100644
--- a/client/src/utils/jsonUtils.ts
+++ b/client/src/utils/jsonUtils.ts
@@ -85,8 +85,9 @@ export function tryParseJson(str: string): {
   success: boolean;
   data: JsonValue;
 } {
-  const trimmed = str.trim();
+  const trimmed = str?.trim();
   if (
+    trimmed &&
     !(trimmed.startsWith("{") && trimmed.endsWith("}")) &&
     !(trimmed.startsWith("[") && trimmed.endsWith("]"))
   ) {
diff --git a/client/src/utils/schemaUtils.ts b/client/src/utils/schemaUtils.ts
index 5beb25e2f..365870216 100644
--- a/client/src/utils/schemaUtils.ts
+++ b/client/src/utils/schemaUtils.ts
@@ -235,6 +235,16 @@ export function normalizeUnionType(schema: JsonSchemaType): JsonSchemaType {
     return { ...schema, type: "integer", anyOf: undefined, nullable: true };
   }
 
+  // Handle anyOf with exactly array and null (FastMCP pattern)
+  if (
+    schema.anyOf &&
+    schema.anyOf.length === 2 &&
+    schema.anyOf.some((t) => (t as JsonSchemaType).type === "array") &&
+    schema.anyOf.some((t) => (t as JsonSchemaType).type === "null")
+  ) {
+    return { ...schema, type: "array", anyOf: undefined, nullable: true };
+  }
+
   // Handle array type with exactly string and null
   if (
     Array.isArray(schema.type) &&