diff --git a/packages/opencode/src/provider/transform.ts b/packages/opencode/src/provider/transform.ts
index 4f809a4b..50aa55cb 100644
--- a/packages/opencode/src/provider/transform.ts
+++ b/packages/opencode/src/provider/transform.ts
@@ -91,7 +91,7 @@ export namespace ProviderTransform {
     }
 
     if (modelID.includes("gpt-5") && !modelID.includes("gpt-5-chat")) {
-      result["reasoningEffort"] = "minimal"
+      result["reasoningEffort"] = "high"
       if (providerID !== "azure") {
         result["textVerbosity"] = "low"
       }
diff --git a/packages/opencode/src/session/system.ts b/packages/opencode/src/session/system.ts
index 41119c4f..b7555a64 100644
--- a/packages/opencode/src/session/system.ts
+++ b/packages/opencode/src/session/system.ts
@@ -14,7 +14,7 @@ import PROMPT_GEMINI from "./prompt/gemini.txt"
 import PROMPT_ANTHROPIC_SPOOF from "./prompt/anthropic_spoof.txt"
 import PROMPT_SUMMARIZE from "./prompt/summarize.txt"
 import PROMPT_TITLE from "./prompt/title.txt"
-import PROMPT_COPILOT_GPT_5 from "./prompt/copilot-gpt-5.txt"
+import PROMPT_CODEX from "./prompt/codex.txt"
 
 export namespace SystemPrompt {
   export function header(providerID: string) {
@@ -23,7 +23,7 @@ export namespace SystemPrompt {
   }
 
   export function provider(modelID: string) {
-    if (modelID.includes("gpt-5")) return [PROMPT_COPILOT_GPT_5]
+    if (modelID.includes("gpt-5")) return [PROMPT_CODEX]
     if (modelID.includes("gpt-") || modelID.includes("o1") || modelID.includes("o3")) return [PROMPT_BEAST]
     if (modelID.includes("gemini-")) return [PROMPT_GEMINI]
     if (modelID.includes("claude")) return [PROMPT_ANTHROPIC]