diff --git a/open-sse/executors/codex.js b/open-sse/executors/codex.js
index e7033623..daf9e712 100644
--- a/open-sse/executors/codex.js
+++ b/open-sse/executors/codex.js
@@ -3,6 +3,7 @@ import { BaseExecutor } from "./base.js";
 import { CODEX_DEFAULT_INSTRUCTIONS } from "../config/codexInstructions.js";
 import { PROVIDERS } from "../config/providers.js";
 import { normalizeResponsesInput } from "../translator/helpers/responsesApiHelper.js";
+import { fetchImageAsBase64 } from "../translator/helpers/imageHelper.js";
 import { getConsistentMachineId } from "../../src/shared/utils/machineId.js";
 
 // In-memory map: hash(machineId + first assistant content) → { sessionId, lastUsed }
@@ -93,7 +94,36 @@ export class CodexExecutor extends BaseExecutor {
   }
 
   /**
-   * Transform request before sending - inject default instructions if missing
+   * Prefetch remote image URLs and inline them as base64 data URIs.
+   * Runs before execute() because Codex backend cannot fetch remote images.
+   * Mutates body.input in place.
+   */
+  async prefetchImages(body) {
+    if (!Array.isArray(body?.input)) return;
+    for (const item of body.input) {
+      if (!Array.isArray(item.content)) continue;
+      const pending = item.content.map(async (c) => {
+        if (c.type !== "image_url") return c;
+        const url = typeof c.image_url === "string" ? c.image_url : c.image_url?.url;
+        const detail = c.image_url?.detail || "auto";
+        if (!url) return c;
+        if (url.startsWith("data:")) return { type: "input_image", image_url: url, detail };
+        const fetched = await fetchImageAsBase64(url, { timeoutMs: 15000 });
+        return { type: "input_image", image_url: fetched?.url || url, detail };
+      });
+      item.content = await Promise.all(pending);
+    }
+  }
+
+  async execute(args) {
+    // Fetch remote images before the synchronous transform/execute pipeline
+    await this.prefetchImages(args.body);
+    return super.execute(args);
+  }
+
+  /**
+   * Transform request before sending - inject default instructions if missing.
+   * Image fetching is handled separately in prefetchImages() so this stays sync.
    */
   transformRequest(model, body, stream, credentials) {
     this._isCompact = !!body._compact;
@@ -109,21 +139,6 @@ export class CodexExecutor extends BaseExecutor {
       body.input = [{ type: "message", role: "user", content: [{ type: "input_text", text: "..." }] }];
     }
 
-    // Normalize image content: image_url → input_image (Responses API format)
-    if (Array.isArray(body.input)) {
-      for (const item of body.input) {
-        if (Array.isArray(item.content)) {
-          item.content = item.content.map(c => {
-            if (c.type === "image_url") {
-              const url = typeof c.image_url === "string" ? c.image_url : c.image_url?.url;
-              return { type: "input_image", image_url: url, detail: c.image_url?.detail || "auto" };
-            }
-            return c;
-          });
-        }
-      }
-    }
-
     // Ensure streaming is enabled (Codex API requires it)
     body.stream = true;
 
diff --git a/open-sse/translator/helpers/imageHelper.js b/open-sse/translator/helpers/imageHelper.js
new file mode 100644
index 00000000..1df04d8d
--- /dev/null
+++ b/open-sse/translator/helpers/imageHelper.js
@@ -0,0 +1,34 @@
+/**
+ * Fetch a remote image URL and return it as a base64 data URI.
+ * Used when upstream providers (Codex, etc.) require inline base64 images
+ * instead of remote URLs they cannot fetch.
+ * Returns null if fetch fails.
+ *
+ * @param {string} imageUrl - HTTP(S) URL of the image
+ * @param {object} options - { signal, timeoutMs }
+ * @returns {Promise<{url: string, mimeType: string}|null>}
+ */
+export async function fetchImageAsBase64(imageUrl, options = {}) {
+  const { signal, timeoutMs = 10000 } = options;
+  if (!imageUrl || (!imageUrl.startsWith("http://") && !imageUrl.startsWith("https://"))) {
+    return null;
+  }
+
+  const controller = new AbortController();
+  const timeout = signal ? null : setTimeout(() => controller.abort(), timeoutMs);
+  const fetchSignal = signal || controller.signal;
+
+  try {
+    const response = await fetch(imageUrl, { signal: fetchSignal });
+    if (!response.ok) return null;
+
+    const mimeType = response.headers.get("Content-Type") || "image/jpeg";
+    const arrayBuffer = await response.arrayBuffer();
+    const base64 = Buffer.from(arrayBuffer).toString("base64");
+    return { url: `data:${mimeType};base64,${base64}`, mimeType };
+  } catch {
+    return null;
+  } finally {
+    if (timeout) clearTimeout(timeout);
+  }
+}
diff --git a/tests/unit/codex-image-fetch.test.js b/tests/unit/codex-image-fetch.test.js
new file mode 100644
index 00000000..b2c6906a
--- /dev/null
+++ b/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);
+  });
+});