fix(translator): normalize developer role to system for OpenAI-format providers

`filterToOpenAIFormat` rewrites the OpenAI Responses-style `developer` role
to `system` before downstream dispatch so OpenAI-compatible providers that
reject the newer role keep working on passthrough routes.

Co-authored-by: Tran Hoang Nguyen <tranhoangnguyen03@gmail.com>
Inspired-by: https://github.com/decolua/9router/pull/1011

open-sse/translator/helpers/openaiHelper.ts

@@ -35,6 +35,11 @@ export function filterToOpenAIFormat(body) {
   if (!body.messages || !Array.isArray(body.messages)) return body;
 
   body.messages = body.messages.map((msg) => {
+    // Normalize OpenAI Responses-style `developer` role to `system` — many
+    // OpenAI-compatible providers reject `developer` (ported from
+    // decolua/9router#1011).
+    if (msg.role === "developer") msg = { ...msg, role: "system" };
+
     // Keep tool messages as-is (OpenAI format)
     if (msg.role === "tool") return msg;
 

tests/unit/openai-developer-role-normalize.test.ts

@@ -0,0 +1,38 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+import { filterToOpenAIFormat } from "../../open-sse/translator/helpers/openaiHelper.ts";
+
+// Ported from upstream decolua/9router#1011 — many OpenAI-compatible providers
+// reject the `developer` role (introduced by OpenAI Responses API). Normalize to
+// `system` so passthrough requests keep working downstream.
+test("filterToOpenAIFormat normalizes developer role to system", () => {
+  const body = {
+    messages: [
+      { role: "developer", content: "You are helpful" },
+      { role: "user", content: "Hi" },
+    ],
+  };
+
+  const out = filterToOpenAIFormat(body);
+
+  assert.equal(out.messages[0].role, "system");
+  assert.equal(out.messages[0].content, "You are helpful");
+  assert.equal(out.messages[1].role, "user");
+});
+
+test("filterToOpenAIFormat normalizes developer role with array content", () => {
+  const body = {
+    messages: [
+      {
+        role: "developer",
+        content: [{ type: "text", text: "Be concise" }],
+      },
+    ],
+  };
+
+  const out = filterToOpenAIFormat(body);
+
+  assert.equal(out.messages[0].role, "system");
+  assert.deepEqual(out.messages[0].content, [{ type: "text", text: "Be concise" }]);
+});