diff --git a/src/harness/codex-harness.ts b/src/harness/codex-harness.ts
index e40eaf8f..cf174f05 100644
--- a/src/harness/codex-harness.ts
+++ b/src/harness/codex-harness.ts
@@ -199,6 +199,11 @@ export function codexChildEnv(source: NodeJS.ProcessEnv, jail: string): NodeJS.P
 export function prepareCodexHome(source: NodeJS.ProcessEnv, jail: string): string {
   const target = join(jail, "codex-home");
   mkdirSync(target, { recursive: true });
+  if (source.OPENAI_BASE_URL) {
+    writeFileSync(join(target, "config.toml"), `openai_base_url = ${JSON.stringify(source.OPENAI_BASE_URL)}\n`, {
+      mode: 0o600,
+    });
+  }
   if (source.OPENAI_API_KEY) {
     writeFileSync(
       join(target, "auth.json"),
diff --git a/test/codex-harness.test.ts b/test/codex-harness.test.ts
index 168d858c..e47b975c 100644
--- a/test/codex-harness.test.ts
+++ b/test/codex-harness.test.ts
@@ -304,6 +304,13 @@ test("Codex materializes API-key auth into its isolated home, and never an ambie
   assert.equal(existsSync(join(prepareCodexHome({ HOME: homedir() }, bare), "auth.json")), false);
 });
 
+test("Codex materializes an OpenAI-compatible endpoint into its isolated home", (t) => {
+  const jail = mkdtempSync(join(tmpdir(), "qm-codex-endpoint-test-"));
+  t.after(() => rmSync(jail, { recursive: true, force: true }));
+  const home = prepareCodexHome({ OPENAI_BASE_URL: "https://proxy.example.com/v1" }, jail);
+  assert.equal(readFileSync(join(home, "config.toml"), "utf8"), 'openai_base_url = "https://proxy.example.com/v1"\n');
+});
+
 test("Codex children cannot use parent surface, control, or terminal tools", () => {
   assert.equal(codexChildToolAllowed("history"), true);
   assert.equal(codexChildToolAllowed("execute"), true);