fix(codex): await image URL fetches before sending to upstream (closes #575)

Remote HTTP(S) image URLs are fetched and inlined as base64 data URIs
in a new prefetchImages() step run before super.execute(), so the body
sent to Codex contains resolved image bytes instead of URLs the backend
cannot access.

Scope is limited to the Codex executor — base executor and other
providers are untouched.

Co-authored-by: anuragg-saxenaa <anuragg.saxenaa@gmail.com>
Made-with: Cursor

tests/unit/codex-image-fetch.test.js

@@ -0,0 +1,145 @@
+/**
+ * Codex executor: verify remote image URLs are fetched and inlined as
+ * base64 data URIs BEFORE the request body reaches the upstream API.
+ *
+ * Covers bug #575:
+ *  - prefetchImages must await async image fetches
+ *  - execute() must run prefetchImages before super.execute so the body
+ *    sent to upstream contains base64 data, not remote URLs
+ */
+
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import { CodexExecutor } from "../../open-sse/executors/codex.js";
+import * as proxyFetchModule from "../../open-sse/utils/proxyFetch.js";
+
+const IMAGE_1MB_BYTES = 1024 * 1024;
+const REMOTE_URL = "https://example.com/big.jpg";
+const DATA_URI = "data:image/png;base64,iVBORw0KGgo=";
+
+function makeImageBuffer(sizeBytes) {
+  const buf = new Uint8Array(sizeBytes);
+  for (let i = 0; i < sizeBytes; i++) buf[i] = i & 0xff;
+  return buf.buffer;
+}
+
+function mockImageFetch(sizeBytes, mimeType = "image/jpeg") {
+  return {
+    ok: true,
+    headers: { get: (k) => (k === "Content-Type" ? mimeType : null) },
+    arrayBuffer: async () => makeImageBuffer(sizeBytes),
+  };
+}
+
+describe("CodexExecutor image handling", () => {
+  let originalFetch;
+
+  beforeEach(() => {
+    originalFetch = global.fetch;
+  });
+
+  afterEach(() => {
+    global.fetch = originalFetch;
+    vi.restoreAllMocks();
+  });
+
+  it("fetches 1MB remote image and inlines it as base64 data URI", async () => {
+    global.fetch = vi.fn(async () => mockImageFetch(IMAGE_1MB_BYTES));
+
+    const executor = new CodexExecutor();
+    const body = {
+      input: [
+        {
+          role: "user",
+          content: [
+            { type: "input_text", text: "describe this" },
+            { type: "image_url", image_url: { url: REMOTE_URL, detail: "high" } },
+          ],
+        },
+      ],
+    };
+
+    await executor.prefetchImages(body);
+
+    const imgBlock = body.input[0].content.find((c) => c.type === "input_image");
+    expect(imgBlock, "input_image block must be present after prefetch").toBeDefined();
+    expect(imgBlock.image_url.startsWith("data:image/jpeg;base64,")).toBe(true);
+    expect(imgBlock.detail).toBe("high");
+
+    const base64Payload = imgBlock.image_url.split(",")[1];
+    const decodedLen = Buffer.from(base64Payload, "base64").length;
+    expect(decodedLen).toBe(IMAGE_1MB_BYTES);
+    expect(global.fetch).toHaveBeenCalledTimes(1);
+  });
+
+  it("passes through existing data URIs without calling fetch", async () => {
+    global.fetch = vi.fn();
+
+    const executor = new CodexExecutor();
+    const body = {
+      input: [
+        {
+          role: "user",
+          content: [{ type: "image_url", image_url: { url: DATA_URI } }],
+        },
+      ],
+    };
+
+    await executor.prefetchImages(body);
+
+    const imgBlock = body.input[0].content.find((c) => c.type === "input_image");
+    expect(imgBlock.image_url).toBe(DATA_URI);
+    expect(global.fetch).not.toHaveBeenCalled();
+  });
+
+  it("falls back to original URL when remote fetch fails", async () => {
+    global.fetch = vi.fn(async () => { throw new Error("network down"); });
+
+    const executor = new CodexExecutor();
+    const body = {
+      input: [
+        {
+          role: "user",
+          content: [{ type: "image_url", image_url: { url: REMOTE_URL } }],
+        },
+      ],
+    };
+
+    await executor.prefetchImages(body);
+
+    const imgBlock = body.input[0].content.find((c) => c.type === "input_image");
+    expect(imgBlock.image_url).toBe(REMOTE_URL);
+  });
+
+  it("execute() prefetches images before sending to upstream", async () => {
+    global.fetch = vi.fn(async () => mockImageFetch(IMAGE_1MB_BYTES));
+
+    let capturedBodyString = null;
+    vi.spyOn(proxyFetchModule, "proxyAwareFetch").mockImplementation(async (url, init) => {
+      capturedBodyString = init.body;
+      return { ok: true, status: 200, headers: new Map() };
+    });
+
+    const executor = new CodexExecutor();
+    const body = {
+      input: [
+        {
+          role: "user",
+          content: [{ type: "image_url", image_url: { url: REMOTE_URL } }],
+        },
+      ],
+    };
+
+    await executor.execute({
+      model: "gpt-5.3-codex",
+      body,
+      stream: true,
+      credentials: { accessToken: "test" },
+    });
+
+    expect(capturedBodyString).toBeTypeOf("string");
+    expect(capturedBodyString).not.toBe("{}");
+    const parsed = JSON.parse(capturedBodyString);
+    const imgBlock = parsed.input[0].content.find((c) => c.type === "input_image");
+    expect(imgBlock.image_url.startsWith("data:image/jpeg;base64,")).toBe(true);
+  });
+});