diff --git a/packages/core/src/model/nodes/ChatNodeBase.ts b/packages/core/src/model/nodes/ChatNodeBase.ts
index 06df5157c..873abee7a 100644
--- a/packages/core/src/model/nodes/ChatNodeBase.ts
+++ b/packages/core/src/model/nodes/ChatNodeBase.ts
@@ -1119,6 +1119,12 @@ export const ChatNodeBase = {
 
           if (isReasoningModel) {
             options.max_completion_tokens = maxTokens;
+            // Reasoning models (o1, o3, o4, gpt-5) don't support sampling parameters
+            options.top_p = undefined;
+            options.frequency_penalty = undefined;
+            options.presence_penalty = undefined;
+            options.stop = undefined;
+            options.n = undefined;
           } else {
             options.temperature = useTopP ? undefined : temperature; // Not supported in o1-preview
             options.max_tokens = maxTokens;
diff --git a/packages/core/src/utils/openai.ts b/packages/core/src/utils/openai.ts
index add544a0e..915234b15 100644
--- a/packages/core/src/utils/openai.ts
+++ b/packages/core/src/utils/openai.ts
@@ -39,7 +39,7 @@ export const openaiModels = {
   'gpt-5-mini': {
     maxTokens: 400000,
     cost: {
-      prompt: 0.25 - 6,
+      prompt: 0.25e-6,
       completion: 2e-6,
     },
     displayName: 'GPT-5 mini',