diff --git a/packages/ui/src/components/sections/agents/AgentsPage.tsx b/packages/ui/src/components/sections/agents/AgentsPage.tsx
index 6db27c18..01c30f4b 100644
--- a/packages/ui/src/components/sections/agents/AgentsPage.tsx
+++ b/packages/ui/src/components/sections/agents/AgentsPage.tsx
@@ -624,7 +624,7 @@ export const AgentsPage: React.FC = () => {
       const permissionConfig = buildPermissionConfigWithGlobal(globalPermission, permissionRules);
       const config: AgentConfig = {
         name: agentName,
-        description: description.trim() || undefined,
+        ...(description.trim() ? { description: description.trim() } : {}),
         mode,
         model: trimmedModel === '' ? null : trimmedModel,
         variant: trimmedVariant === '' ? null : trimmedVariant || undefined,
@@ -632,7 +632,7 @@ export const AgentsPage: React.FC = () => {
         top_p: topP ?? null,
         prompt: trimmedPrompt || (isNewAgent ? undefined : null),
         permission: permissionConfig,
-        scope: isNewAgent ? draftScope : undefined,
+        ...(isNewAgent && draftScope ? { scope: draftScope } : {}),
       };
 
       let result: AgentMutationResult;
diff --git a/packages/vscode/src/opencodeConfig.ts b/packages/vscode/src/opencodeConfig.ts
index 8478e1ab..79a7455d 100644
--- a/packages/vscode/src/opencodeConfig.ts
+++ b/packages/vscode/src/opencodeConfig.ts
@@ -1666,6 +1666,9 @@ export const updateAgent = (agentName: string, updates: Record<string, unknown>,
   const creatingNewMd = isBuiltinOverride;
 
   for (const [field, value] of Object.entries(updates || {})) {
+    // Skip undefined values — they would overwrite existing frontmatter fields with nothing
+    if (value === undefined) continue;
+
     if (field === 'prompt') {
       if (value === null) {
         if (mdExists || creatingNewMd) {
diff --git a/packages/web/server/lib/opencode/agents.js b/packages/web/server/lib/opencode/agents.js
index d2acff89..9dd85052 100644
--- a/packages/web/server/lib/opencode/agents.js
+++ b/packages/web/server/lib/opencode/agents.js
@@ -451,6 +451,9 @@ function updateAgent(agentName, updates, workingDirectory) {
   const creatingNewMd = isBuiltinOverride;
 
   for (const [field, value] of Object.entries(updates)) {
+    // Skip undefined values — they would overwrite existing frontmatter fields with nothing
+    if (value === undefined) continue;
+
     if (field === 'prompt') {
       if (value === null) {
         if (mdExists || creatingNewMd) {