diff --git a/backend/data/default_agent_types.json b/backend/data/default_agent_types.json
index bfc72bc..5eeddfc 100644
--- a/backend/data/default_agent_types.json
+++ b/backend/data/default_agent_types.json
@@ -9,7 +9,8 @@
     "fallback_models": ["claude-haiku-3-5-20241022"],
     "model_params": {
       "temperature": 0.7,
-      "max_tokens": 4096
+      "max_tokens": 4096,
+      "top_p": 0.95
     },
     "mcp_servers": ["gitea", "knowledge-base"],
     "tool_permissions": {
@@ -29,7 +30,8 @@
     "fallback_models": ["claude-haiku-3-5-20241022"],
     "model_params": {
       "temperature": 0.5,
-      "max_tokens": 8192
+      "max_tokens": 8192,
+      "top_p": 0.95
     },
     "mcp_servers": ["gitea", "knowledge-base"],
     "tool_permissions": {
@@ -49,7 +51,8 @@
     "fallback_models": ["claude-haiku-3-5-20241022"],
     "model_params": {
       "temperature": 0.6,
-      "max_tokens": 8192
+      "max_tokens": 8192,
+      "top_p": 0.95
     },
     "mcp_servers": ["gitea", "knowledge-base", "filesystem"],
     "tool_permissions": {
@@ -69,7 +72,8 @@
     "fallback_models": ["claude-haiku-3-5-20241022"],
     "model_params": {
       "temperature": 0.3,
-      "max_tokens": 16384
+      "max_tokens": 16384,
+      "top_p": 0.95
     },
     "mcp_servers": ["gitea", "knowledge-base", "filesystem"],
     "tool_permissions": {
@@ -89,7 +93,8 @@
     "fallback_models": ["claude-haiku-3-5-20241022"],
     "model_params": {
       "temperature": 0.4,
-      "max_tokens": 8192
+      "max_tokens": 8192,
+      "top_p": 0.95
     },
     "mcp_servers": ["gitea", "knowledge-base", "filesystem"],
     "tool_permissions": {
@@ -109,7 +114,8 @@
     "fallback_models": ["claude-haiku-3-5-20241022"],
     "model_params": {
       "temperature": 0.4,
-      "max_tokens": 8192
+      "max_tokens": 8192,
+      "top_p": 0.95
     },
     "mcp_servers": ["gitea", "knowledge-base", "filesystem"],
     "tool_permissions": {
diff --git a/frontend/src/components/agents/AgentTypeForm.tsx b/frontend/src/components/agents/AgentTypeForm.tsx
index 052d861..cc6ce0b 100644
--- a/frontend/src/components/agents/AgentTypeForm.tsx
+++ b/frontend/src/components/agents/AgentTypeForm.tsx
@@ -57,28 +57,37 @@ export function AgentTypeForm({
   const [activeTab, setActiveTab] = useState('basic');
   const [expertiseInput, setExpertiseInput] = useState('');
 
+  // Build initial values with proper defaults for missing fields
+  const getInitialValues = useCallback((): AgentTypeCreateFormValues => {
+    if (!agentType) return defaultAgentTypeValues;
+
+    // Ensure model_params has all required fields with proper defaults
+    const modelParams = agentType.model_params ?? {};
+    const safeModelParams = {
+      temperature: typeof modelParams.temperature === 'number' ? modelParams.temperature : 0.7,
+      max_tokens: typeof modelParams.max_tokens === 'number' ? modelParams.max_tokens : 8192,
+      top_p: typeof modelParams.top_p === 'number' ? modelParams.top_p : 0.95,
+    };
+
+    return {
+      name: agentType.name,
+      slug: agentType.slug,
+      description: agentType.description,
+      expertise: agentType.expertise ?? [],
+      personality_prompt: agentType.personality_prompt ?? '',
+      primary_model: agentType.primary_model ?? 'claude-opus-4-5-20251101',
+      fallback_models: agentType.fallback_models ?? [],
+      model_params: safeModelParams,
+      mcp_servers: agentType.mcp_servers ?? [],
+      tool_permissions: agentType.tool_permissions ?? {},
+      is_active: agentType.is_active ?? false,
+    };
+  }, [agentType]);
+
   // Always use create schema for validation - editing requires all fields too
   const form = useForm<AgentTypeCreateFormValues>({
     resolver: zodResolver(agentTypeCreateSchema),
-    defaultValues: agentType
-      ? {
-          name: agentType.name,
-          slug: agentType.slug,
-          description: agentType.description,
-          expertise: agentType.expertise,
-          personality_prompt: agentType.personality_prompt,
-          primary_model: agentType.primary_model,
-          fallback_models: agentType.fallback_models,
-          model_params: (agentType.model_params ?? {
-            temperature: 0.7,
-            max_tokens: 8192,
-            top_p: 0.95,
-          }) as AgentTypeCreateFormValues['model_params'],
-          mcp_servers: agentType.mcp_servers,
-          tool_permissions: agentType.tool_permissions,
-          is_active: agentType.is_active,
-        }
-      : defaultAgentTypeValues,
+    defaultValues: getInitialValues(),
   });
 
   const {
@@ -90,29 +99,58 @@ export function AgentTypeForm({
     formState: { errors },
   } = form;
 
+  // Helper to extract first error message from nested errors
+  const getFirstErrorMessage = useCallback(
+    (formErrors: FieldErrors<AgentTypeCreateFormValues>): { field: string; message: string } => {
+      for (const key of Object.keys(formErrors)) {
+        const error = formErrors[key as keyof typeof formErrors];
+        if (!error) continue;
+
+        // Handle nested errors (like model_params.temperature)
+        if (typeof error === 'object' && !('message' in error)) {
+          for (const nestedKey of Object.keys(error)) {
+            const nestedError = (error as Record<string, { message?: string }>)[nestedKey];
+            if (nestedError?.message) {
+              return { field: `${key}.${nestedKey}`, message: nestedError.message };
+            }
+          }
+        }
+
+        // Handle direct error
+        if ('message' in error && error.message) {
+          return { field: key, message: error.message as string };
+        }
+      }
+      return { field: 'unknown', message: 'Validation error' };
+    },
+    []
+  );
+
   // Handle form validation errors - show toast with first error
-  const handleFormError = useCallback((formErrors: FieldErrors<AgentTypeCreateFormValues>) => {
-    // Find the first error and show it
-    const firstErrorKey = Object.keys(formErrors)[0] as keyof AgentTypeCreateFormValues;
-    if (firstErrorKey) {
-      const error = formErrors[firstErrorKey];
-      const message = error && 'message' in error ? error.message : 'Validation error';
+  const handleFormError = useCallback(
+    (formErrors: FieldErrors<AgentTypeCreateFormValues>) => {
+      // Log for debugging
+      console.error('[AgentTypeForm] Validation errors:', formErrors);
+
+      const { field, message } = getFirstErrorMessage(formErrors);
       toast.error('Please fix form errors', {
-        description: `${String(firstErrorKey)}: ${message}`,
+        description: `${field}: ${message}`,
       });
 
       // Navigate to the tab containing the error
-      if (['name', 'slug', 'description', 'expertise', 'is_active'].includes(firstErrorKey)) {
+      const topLevelField = field.split('.')[0];
+      if (['name', 'slug', 'description', 'expertise', 'is_active'].includes(topLevelField)) {
         setActiveTab('basic');
-      } else if (['primary_model', 'fallback_models', 'model_params'].includes(firstErrorKey)) {
+      } else if (['primary_model', 'fallback_models', 'model_params'].includes(topLevelField)) {
         setActiveTab('model');
-      } else if (['mcp_servers', 'tool_permissions'].includes(firstErrorKey)) {
+      } else if (['mcp_servers', 'tool_permissions'].includes(topLevelField)) {
         setActiveTab('permissions');
-      } else if (firstErrorKey === 'personality_prompt') {
+      } else if (topLevelField === 'personality_prompt') {
         setActiveTab('personality');
       }
-    }
-  }, []);
+    },
+    [getFirstErrorMessage]
+  );
 
   const watchName = watch('name');
   /* istanbul ignore next -- defensive fallback, expertise always has default */
@@ -120,6 +158,14 @@ export function AgentTypeForm({
   /* istanbul ignore next -- defensive fallback, mcp_servers always has default */
   const watchMcpServers = watch('mcp_servers') || [];
 
+  // Reset form when agentType changes (e.g., switching to edit mode)
+  useEffect(() => {
+    if (agentType) {
+      const values = getInitialValues();
+      form.reset(values);
+    }
+  }, [agentType?.id, form, getInitialValues]);
+
   // Auto-generate slug from name for new agent types
   useEffect(() => {
     if (!isEditing && watchName) {