fix(headroom): translate openai-responses input through OpenAI for compression

Codex (openai-responses) body.input holds Responses items, not OpenAI
messages. Translate input -> OpenAI -> compress -> back to input so the
Responses contract is preserved. Fixes #1998

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/unit/headroom-responses-format.test.js

@@ -0,0 +1,50 @@
+// #1998 — Headroom compression treated a Codex (openai-responses) body.input
+// array as OpenAI messages: it sent Responses items to /v1/compress and then
+// assigned the returned OpenAI messages back to body.input, violating the
+// Responses format contract. body.input must stay Responses-shaped.
+import { describe, it, expect, vi, afterEach } from "vitest";
+import { compressWithHeadroom } from "../../open-sse/rtk/headroom.js";
+
+describe("compressWithHeadroom openai-responses format (#1998)", () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("keeps body.input in Responses format after compressing an openai-responses request", async () => {
+    // Headroom always returns compressed OpenAI-style messages.
+    global.fetch = vi.fn(async () => ({
+      ok: true,
+      json: async () => ({
+        messages: [{ role: "user", content: "compressed text" }],
+        tokens_before: 100,
+        tokens_after: 90,
+        tokens_saved: 10,
+      }),
+    }));
+
+    const body = {
+      input: [
+        {
+          type: "message",
+          role: "user",
+          content: [{ type: "input_text", text: "a long original message ".repeat(20) }],
+        },
+      ],
+    };
+
+    const data = await compressWithHeadroom(body, {
+      enabled: true,
+      url: "http://headroom.test",
+      model: "gpt-5",
+      format: "openai-responses",
+    });
+
+    expect(data).not.toBeNull();
+    // body.input must remain Responses items (type:"message" + content array),
+    // NOT the raw OpenAI messages ({ role, content: "<string>" }) the bug produced.
+    expect(Array.isArray(body.input)).toBe(true);
+    expect(body.input[0]).toMatchObject({ type: "message", role: "user" });
+    expect(Array.isArray(body.input[0].content)).toBe(true);
+    expect(typeof body.input[0].content).not.toBe("string");
+  });
+});