diff --git a/client/src/components/DynamicJsonForm.tsx b/client/src/components/DynamicJsonForm.tsx
index 84c51117..e2d7d7ed 100644
--- a/client/src/components/DynamicJsonForm.tsx
+++ b/client/src/components/DynamicJsonForm.tsx
@@ -106,8 +106,17 @@ const DynamicJsonForm = forwardRef<DynamicJsonFormRef, DynamicJsonFormProps>(
             const parsed = JSON.parse(jsonString);
             onChange(parsed);
             setJsonError(undefined);
-          } catch {
-            // Don't set error during normal typing
+          } catch (err) {
+            // For invalid JSON, set error and reset to default if it's clearly malformed
+            const errorMessage =
+              err instanceof Error ? err.message : "Invalid JSON";
+            setJsonError(errorMessage);
+
+            // Reset to default for clearly invalid JSON (not just incomplete typing)
+            const trimmed = jsonString.trim();
+            if (trimmed.length > 5 && !trimmed.match(/^[\s\[{]/)) {
+              onChange(generateDefaultValue(schema));
+            }
           }
         }, 300);
       },
@@ -164,7 +173,12 @@ const DynamicJsonForm = forwardRef<DynamicJsonFormRef, DynamicJsonFormProps>(
       try {
         const jsonStr = rawJsonValue.trim();
         if (!jsonStr) return { isValid: true, error: null };
-        JSON.parse(jsonStr);
+        const parsed = JSON.parse(jsonStr);
+        // Clear any pending debounced update and immediately update parent
+        if (timeoutRef.current) {
+          clearTimeout(timeoutRef.current);
+        }
+        onChange(parsed);
         setJsonError(undefined);
         return { isValid: true, error: null };
       } catch (err) {
diff --git a/client/src/components/ToolsTab.tsx b/client/src/components/ToolsTab.tsx
index c73fcd09..31979f0d 100644
--- a/client/src/components/ToolsTab.tsx
+++ b/client/src/components/ToolsTab.tsx
@@ -66,10 +66,20 @@ const ToolsTab = ({
   const [isToolRunning, setIsToolRunning] = useState(false);
   const [isOutputSchemaExpanded, setIsOutputSchemaExpanded] = useState(false);
   const [isMetaExpanded, setIsMetaExpanded] = useState(false);
+  const [hasValidationErrors, setHasValidationErrors] = useState(false);
   const formRefs = useRef<Record<string, DynamicJsonFormRef | null>>({});
   const { toast } = useToast();
   const { copied, setCopied } = useCopy();
 
+  // Function to check if any form has validation errors
+  const checkValidationErrors = () => {
+    const errors = Object.values(formRefs.current).some(
+      (ref) => ref && !ref.validateJson().isValid,
+    );
+    setHasValidationErrors(errors);
+    return errors;
+  };
+
   useEffect(() => {
     const params = Object.entries(
       selectedTool?.inputSchema.properties ?? [],
@@ -82,6 +92,12 @@ const ToolsTab = ({
       ),
     ]);
     setParams(Object.fromEntries(params));
+
+    // Reset validation errors when switching tools
+    setHasValidationErrors(false);
+
+    // Clear form refs for the previous tool
+    formRefs.current = {};
   }, [selectedTool]);
 
   return (
@@ -199,6 +215,8 @@ const ToolsTab = ({
                                   ...params,
                                   [key]: newValue,
                                 });
+                                // Check validation after a short delay to allow form to update
+                                setTimeout(checkValidationErrors, 100);
                               }}
                             />
                           </div>
@@ -235,6 +253,8 @@ const ToolsTab = ({
                                   ...params,
                                   [key]: newValue,
                                 });
+                                // Check validation after a short delay to allow form to update
+                                setTimeout(checkValidationErrors, 100);
                               }}
                             />
                           </div>
@@ -317,10 +337,7 @@ const ToolsTab = ({
                   <Button
                     onClick={async () => {
                       // Validate JSON inputs before calling tool
-                      const hasValidationErrors = Object.values(
-                        formRefs.current,
-                      ).some((ref) => ref && !ref.validateJson().isValid);
-                      if (hasValidationErrors) return;
+                      if (checkValidationErrors()) return;
 
                       try {
                         setIsToolRunning(true);
@@ -329,7 +346,7 @@ const ToolsTab = ({
                         setIsToolRunning(false);
                       }
                     }}
-                    disabled={isToolRunning}
+                    disabled={isToolRunning || hasValidationErrors}
                   >
                     {isToolRunning ? (
                       <>