diff --git a/client/src/components/DynamicJsonForm.tsx b/client/src/components/DynamicJsonForm.tsx
index 96b03b3a..685401ab 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 740cc5ed..26f988e9 100644
--- a/client/src/components/ToolsTab.tsx
+++ b/client/src/components/ToolsTab.tsx
@@ -18,6 +18,7 @@ import {
   generateDefaultValue,
   isPropertyRequired,
   normalizeUnionType,
+  resolveRef,
 } from "@/utils/schemaUtils";
 import {
   CompatibilityCallToolResult,
@@ -90,14 +91,21 @@ const ToolsTab = ({
   useEffect(() => {
     const params = Object.entries(
       selectedTool?.inputSchema.properties ?? [],
-    ).map(([key, value]) => [
-      key,
-      generateDefaultValue(
+    ).map(([key, value]) => {
+      // First resolve any $ref references
+      const resolvedValue = resolveRef(
         value as JsonSchemaType,
-        key,
         selectedTool?.inputSchema as JsonSchemaType,
-      ),
-    ]);
+      );
+      return [
+        key,
+        generateDefaultValue(
+          resolvedValue,
+          key,
+          selectedTool?.inputSchema as JsonSchemaType,
+        ),
+      ];
+    });
     setParams(Object.fromEntries(params));
 
     // Reset validation errors when switching tools
@@ -154,7 +162,12 @@ const ToolsTab = ({
                 </p>
                 {Object.entries(selectedTool.inputSchema.properties ?? []).map(
                   ([key, value]) => {
-                    const prop = normalizeUnionType(value as JsonSchemaType);
+                    // First resolve any $ref references
+                    const resolvedValue = resolveRef(
+                      value as JsonSchemaType,
+                      selectedTool.inputSchema as JsonSchemaType,
+                    );
+                    const prop = normalizeUnionType(resolvedValue);
                     const inputSchema =
                       selectedTool.inputSchema as JsonSchemaType;
                     const required = isPropertyRequired(key, inputSchema);
@@ -181,16 +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 3a9336f1..334dafc2 100644
--- a/client/src/utils/jsonUtils.ts
+++ b/client/src/utils/jsonUtils.ts
@@ -48,6 +48,7 @@ export type JsonSchemaType = {
   const?: JsonValue;
   oneOf?: (JsonSchemaType | JsonSchemaConst)[];
   anyOf?: (JsonSchemaType | JsonSchemaConst)[];
+  $ref?: string;
 };
 
 export type JsonObject = { [key: string]: JsonValue };
@@ -84,8 +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 73137805..36587021 100644
--- a/client/src/utils/schemaUtils.ts
+++ b/client/src/utils/schemaUtils.ts
@@ -145,6 +145,50 @@ export function isPropertyRequired(
   return schema.required?.includes(propertyName) ?? false;
 }
 
+/**
+ * Resolves $ref references in JSON schema
+ * @param schema The schema that may contain $ref
+ * @param rootSchema The root schema to resolve references against
+ * @returns The resolved schema without $ref
+ */
+export function resolveRef(
+  schema: JsonSchemaType,
+  rootSchema: JsonSchemaType,
+): JsonSchemaType {
+  if (!("$ref" in schema) || !schema.$ref) {
+    return schema;
+  }
+
+  const ref = schema.$ref;
+
+  // Handle simple #/properties/name references
+  if (ref.startsWith("#/")) {
+    const path = ref.substring(2).split("/");
+    let current: unknown = rootSchema;
+
+    for (const segment of path) {
+      if (
+        current &&
+        typeof current === "object" &&
+        current !== null &&
+        segment in current
+      ) {
+        current = (current as Record<string, unknown>)[segment];
+      } else {
+        // If reference cannot be resolved, return the original schema
+        console.warn(`Could not resolve $ref: ${ref}`);
+        return schema;
+      }
+    }
+
+    return current as JsonSchemaType;
+  }
+
+  // For other types of references, return the original schema
+  console.warn(`Unsupported $ref format: ${ref}`);
+  return schema;
+}
+
 /**
  * Normalizes union types (like string|null from FastMCP) to simple types for form rendering
  * @param schema The JSON schema to normalize
@@ -191,6 +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) &&