diff --git a/open-sse/translator/request/openai-responses.js b/open-sse/translator/request/openai-responses.js
index 98c516cd..23603a71 100644
--- a/open-sse/translator/request/openai-responses.js
+++ b/open-sse/translator/request/openai-responses.js
@@ -221,13 +221,14 @@ export function openaiToOpenAIResponsesRequest(model, body, stream, credentials)
   const messages = body.messages || [];
 
   for (const msg of messages) {
-    if (msg.role === ROLE.SYSTEM) {
-      // Use first system message as instructions
+    if (msg.role === ROLE.SYSTEM || msg.role === ROLE.DEVELOPER) {
+      // Use the first instruction-bearing message as instructions.
+      // OpenAI recommends role="developer" for GPT-5/Codex as the system-level prompt.
       if (!hasSystemMessage) {
         result.instructions = typeof msg.content === "string" ? msg.content : "";
         hasSystemMessage = true;
       }
-      continue; // Skip system messages in input
+      continue; // Skip instruction messages in input
     }
 
     // Convert user/assistant messages to input items
diff --git a/tests/translator/bugs-codexCli-responses.test.js b/tests/translator/bugs-codexCli-responses.test.js
index 6f947833..c3401341 100644
--- a/tests/translator/bugs-codexCli-responses.test.js
+++ b/tests/translator/bugs-codexCli-responses.test.js
@@ -46,6 +46,20 @@ describe("Codex CLI Responses → OpenAI", () => {
 });
 
 describe("OpenAI → Codex Responses (reverse)", () => {
+  it("maps developer messages to Responses API instructions", () => {
+    const out = O2R({
+      messages: [
+        { role: "developer", content: "Follow the project rules." },
+        { role: "user", content: "Hello" },
+      ],
+    });
+
+    expect(out.instructions).toBe("Follow the project rules.");
+    expect(out.input).toEqual([
+      { type: "message", role: "user", content: [{ type: "input_text", text: "Hello" }] },
+    ]);
+  });
+
   // openai-responses.js:13 — clampCallId NOT applied on Responses→Chat; but here Chat→Responses must clamp
   it("call_id longer than 64 chars is clamped", () => {
     const longId = "call_" + "x".repeat(80);