diff --git a/packages/opencode/src/agent/agent.ts b/packages/opencode/src/agent/agent.ts
index e30d05e9350..fd07ebc85b7 100644
--- a/packages/opencode/src/agent/agent.ts
+++ b/packages/opencode/src/agent/agent.ts
@@ -322,11 +322,11 @@ export namespace Agent {
       }),
     } satisfies Parameters<typeof generateObject>[0]
 
+    // TODO: clean this up so provider specific logic doesnt bleed over
     if (defaultModel.providerID === "openai" && (await Auth.get(defaultModel.providerID))?.type === "oauth") {
       const result = streamObject({
         ...params,
         providerOptions: ProviderTransform.providerOptions(model, {
-          instructions: SystemPrompt.instructions(),
           store: false,
         }),
         onError: () => {},
diff --git a/packages/opencode/src/session/llm.ts b/packages/opencode/src/session/llm.ts
index daf70180e52..48a36741c9c 100644
--- a/packages/opencode/src/session/llm.ts
+++ b/packages/opencode/src/session/llm.ts
@@ -63,14 +63,14 @@ export namespace LLM {
       Provider.getProvider(input.model.providerID),
       Auth.get(input.model.providerID),
     ])
-    const isCodex = provider.id === "openai" && auth?.type === "oauth"
+    // TODO: move this to a proper hook
+    const isOpenaiOauth = provider.id === "openai" && auth?.type === "oauth"
 
-    const system = []
+    const system: string[] = []
     system.push(
       [
         // use agent prompt otherwise provider prompt
-        // For Codex sessions, skip SystemPrompt.provider() since it's sent via options.instructions
-        ...(input.agent.prompt ? [input.agent.prompt] : isCodex ? [] : SystemPrompt.provider(input.model)),
+        ...(input.agent.prompt ? [input.agent.prompt] : SystemPrompt.provider(input.model)),
         // any custom prompt passed into this call
         ...input.system,
         // any custom prompt from last user message
@@ -108,10 +108,22 @@ export namespace LLM {
       mergeDeep(input.agent.options),
       mergeDeep(variant),
     )
-    if (isCodex) {
-      options.instructions = SystemPrompt.instructions()
+    if (isOpenaiOauth) {
+      options.instructions = system.join("\n")
     }
 
+    const messages = isOpenaiOauth
+      ? input.messages
+      : [
+          ...system.map(
+            (x): ModelMessage => ({
+              role: "system",
+              content: x,
+            }),
+          ),
+          ...input.messages,
+        ]
+
     const params = await Plugin.trigger(
       "chat.params",
       {
@@ -146,7 +158,9 @@ export namespace LLM {
     )
 
     const maxOutputTokens =
-      isCodex || provider.id.includes("github-copilot") ? undefined : ProviderTransform.maxOutputTokens(input.model)
+      isOpenaiOauth || provider.id.includes("github-copilot")
+        ? undefined
+        : ProviderTransform.maxOutputTokens(input.model)
 
     const tools = await resolveTools(input)
 
@@ -217,15 +231,7 @@ export namespace LLM {
         ...headers,
       },
       maxRetries: input.retries ?? 0,
-      messages: [
-        ...system.map(
-          (x): ModelMessage => ({
-            role: "system",
-            content: x,
-          }),
-        ),
-        ...input.messages,
-      ],
+      messages,
       model: wrapLanguageModel({
         model: language,
         middleware: [
diff --git a/packages/opencode/src/session/prompt/codex_header.txt b/packages/opencode/src/session/prompt/codex.txt
similarity index 100%
rename from packages/opencode/src/session/prompt/codex_header.txt
rename to packages/opencode/src/session/prompt/codex.txt
diff --git a/packages/opencode/src/session/system.ts b/packages/opencode/src/session/system.ts
index 63230e6426c..80c89618e33 100644
--- a/packages/opencode/src/session/system.ts
+++ b/packages/opencode/src/session/system.ts
@@ -7,7 +7,7 @@ import PROMPT_DEFAULT from "./prompt/default.txt"
 import PROMPT_BEAST from "./prompt/beast.txt"
 import PROMPT_GEMINI from "./prompt/gemini.txt"
 
-import PROMPT_CODEX from "./prompt/codex_header.txt"
+import PROMPT_CODEX from "./prompt/codex.txt"
 import PROMPT_TRINITY from "./prompt/trinity.txt"
 import type { Provider } from "@/provider/provider"
 import type { Agent } from "@/agent/agent"
@@ -15,14 +15,10 @@ import { PermissionNext } from "@/permission"
 import { Skill } from "@/skill"
 
 export namespace SystemPrompt {
-  export function instructions() {
-    return PROMPT_CODEX.trim()
-  }
-
   export function provider(model: Provider.Model) {
-    if (model.api.id.includes("gpt-5")) return [PROMPT_CODEX]
-    if (model.api.id.includes("gpt-") || model.api.id.includes("o1") || model.api.id.includes("o3"))
+    if (model.api.id.includes("gpt-4") || model.api.id.includes("o1") || model.api.id.includes("o3"))
       return [PROMPT_BEAST]
+    if (model.api.id.includes("gpt")) return [PROMPT_CODEX]
     if (model.api.id.includes("gemini-")) return [PROMPT_GEMINI]
     if (model.api.id.includes("claude")) return [PROMPT_ANTHROPIC]
     if (model.api.id.toLowerCase().includes("trinity")) return [PROMPT_TRINITY]