diff --git a/src/server/features/ai-search/services/promptExplorer.ts b/src/server/features/ai-search/services/promptExplorer.ts
index 1328ce8..796262b 100644
--- a/src/server/features/ai-search/services/promptExplorer.ts
+++ b/src/server/features/ai-search/services/promptExplorer.ts
@@ -28,8 +28,13 @@ import {
 /** LLM responses are stable enough for a 7-day cache. */
 const PROMPT_RESPONSE_TTL_SECONDS = 7 * 24 * 60 * 60;
 
-/** Hard cap on response length to keep payloads sane. */
-const PROMPT_RESPONSE_MAX_TOKENS = 1024;
+/**
+ * Hard cap on response length. Set to the DataForSEO per-call maximum because
+ * reasoning models (gpt-5, gemini-2.5-pro) count hidden chain-of-thought
+ * tokens against this budget — at 1024 ChatGPT regularly burns the whole
+ * budget on reasoning and returns a near-empty visible message.
+ */
+const PROMPT_RESPONSE_MAX_TOKENS = 4096;
 
 type DataforseoClient = ReturnType<typeof createDataforseoClient>;
 
@@ -94,7 +99,7 @@ async function runModel(
     webSearch: args.input.webSearch,
     webSearchCountryCode: args.input.webSearchCountryCode ?? null,
     // Bumped when prompt/payload shape changes — busts stale cache entries.
-    systemPromptV: 4,
+    systemPromptV: 5,
   });
 
   const cached = promptExplorerModelResultSchema.safeParse(
diff --git a/src/server/lib/dataforseoLlm.ts b/src/server/lib/dataforseoLlm.ts
index b9c3d44..39ca7ac 100644
--- a/src/server/lib/dataforseoLlm.ts
+++ b/src/server/lib/dataforseoLlm.ts
@@ -324,15 +324,19 @@ export async function fetchLlmResponseRaw(
   input: LlmResponsesInput,
 ): Promise<DataforseoApiResponse<LlmResponseResult>> {
   const path = `/v3/ai_optimization/${input.modelSlug}/llm_responses/live`;
+  // DataForSEO's Gemini endpoint rejects `web_search_country_iso_code` with a
+  // 40501 "Invalid Field" error. The other three models accept it.
+  const supportsCountry = input.modelSlug !== "gemini";
   const payload = [
     {
       user_prompt: input.userPrompt,
       model_name: input.modelName,
       web_search: input.webSearch ?? true,
       max_output_tokens: clampLimit(input.maxOutputTokens ?? 1024, 256, 4096),
-      ...(input.webSearchCountryCode && {
-        web_search_country_iso_code: input.webSearchCountryCode,
-      }),
+      ...(supportsCountry &&
+        input.webSearchCountryCode && {
+          web_search_country_iso_code: input.webSearchCountryCode,
+        }),
     },
   ];