fix(codex): avoid duplicate client instructions

src/providers/proxies/codex-proxy.ts

@@ -88,15 +88,21 @@ export const transformCodexBodyJson = (
     ...nextBody
   } = bodyJson;
 
-  // OpenCode injects instructions internally in its Codex/OAuth path:
-  // https://github.com/anomalyco/opencode/blob/d848c9b6a32f408e8b9bf6448b83af05629454d0/packages/opencode/src/session/llm.ts#L110-L112
-  // Non-Codex clients won't, so we fall back to OpenCode's default instructions for Codex.
-  // https://github.com/anomalyco/opencode/blob/d848c9b6a32f408e8b9bf6448b83af05629454d0/packages/opencode/src/session/prompt/codex_header.txt
+  const incomingInstructions = trimString(nextBody.instructions);
+  const input = Array.isArray(nextBody.input) ? nextBody.input : [];
+  const firstInput = input[0];
+  const promotedInstructions =
+    !incomingInstructions &&
+    isObjectRecord(firstInput) &&
+    (firstInput.role === "system" || firstInput.role === "developer")
+      ? trimString(firstInput.content)
+      : "";
   const instructions =
-    trimString(nextBody.instructions) || CODEX_DEFAULT_INSTRUCTIONS;
+    incomingInstructions || promotedInstructions || CODEX_DEFAULT_INSTRUCTIONS;
 
   return {
     ...nextBody,
+    ...(promotedInstructions ? { input: input.slice(1) } : {}),
     instructions,
     store: false,
     ...(sessionId ? { prompt_cache_key: sessionId } : {}),

tests/providers/proxy-contract.test.ts

@@ -338,6 +338,39 @@ describe("proxy contract: codex", () => {
     );
   });
 
+  test("promotes OpenCode developer input instead of adding fallback instructions", () => {
+    const headers = new Headers();
+    const bodyJson = {
+      model: "gpt-5.6-sol",
+      input: [
+        {
+          role: "developer",
+          content: "Current OpenCode GPT instructions",
+        },
+        {
+          role: "user",
+          content: [{ type: "input_text", text: "hello" }],
+        },
+      ],
+    };
+
+    const result = prepareCodexProxyRequest({
+      headers,
+      accessToken: "codex-access",
+      accountId: "acct-fallback",
+      metadata: null,
+      bodyText: JSON.stringify(bodyJson),
+      bodyJson,
+    });
+
+    expect(JSON.parse(result.bodyText)).toEqual({
+      model: "gpt-5.6-sol",
+      input: [bodyJson.input[1]],
+      instructions: "Current OpenCode GPT instructions",
+      store: false,
+    });
+  });
+
   test("uses derived session headers and prompt cache key for codex requests", () => {
     const headers = new Headers({
       session_id: "raw-session-underscore",