diff --git a/backend/packages/harness/deerflow/models/credential_loader.py b/backend/packages/harness/deerflow/models/credential_loader.py
index 05f839ed1..5a623222d 100644
--- a/backend/packages/harness/deerflow/models/credential_loader.py
+++ b/backend/packages/harness/deerflow/models/credential_loader.py
@@ -241,6 +241,9 @@ def load_codex_cli_credential() -> CodexCliCredential | None:
 
     access_token = data.get("access_token") or data.get("token") or tokens.get("access_token", "")
     account_id = data.get("account_id") or tokens.get("account_id", "")
+    if not isinstance(account_id, str):
+        logger.debug("Codex CLI credentials file has a non-string account_id; using no account")
+        account_id = ""
     if not access_token:
         logger.debug("Codex CLI credentials file exists but no token found")
         return None
diff --git a/backend/tests/test_codex_provider.py b/backend/tests/test_codex_provider.py
index 6735c7132..2744b407d 100644
--- a/backend/tests/test_codex_provider.py
+++ b/backend/tests/test_codex_provider.py
@@ -361,3 +361,20 @@ def test_parse_tool_call_arguments_non_dict_json():
     parsed, err = model._parse_tool_call_arguments({"arguments": '["list", "not", "dict"]', "name": "t", "call_id": "c"})
     assert parsed is None
     assert err is not None
+
+
+# ---------------------------------------------------------------------------
+# Credential loading
+# ---------------------------------------------------------------------------
+
+
+def test_model_post_init_accepts_null_account_id(tmp_path, monkeypatch):
+    auth_path = tmp_path / "auth.json"
+    auth_path.write_text(json.dumps({"tokens": {"access_token": "tok-test", "account_id": None}}))
+    monkeypatch.setenv("CODEX_AUTH_PATH", str(auth_path))
+
+    from deerflow.models.openai_codex_provider import CodexChatModel
+
+    model = CodexChatModel(model="gpt-5.4", reasoning_effort="medium")
+
+    assert model._account_id == ""
diff --git a/backend/tests/test_credential_loader.py b/backend/tests/test_credential_loader.py
index 2df804f68..e51b177ad 100644
--- a/backend/tests/test_credential_loader.py
+++ b/backend/tests/test_credential_loader.py
@@ -406,3 +406,45 @@ def test_codex_chat_model_reports_missing_credential_for_non_object_auth_file(tm
 
     with pytest.raises(ValueError, match="Codex CLI credential not found"):
         CodexChatModel(model="gpt-5.4")
+
+
+def test_load_codex_cli_credential_defaults_null_account_id(tmp_path, monkeypatch):
+    auth_path = tmp_path / "auth.json"
+    auth_path.write_text(
+        json.dumps(
+            {
+                "tokens": {
+                    "access_token": "codex-access-token",
+                    "account_id": None,
+                }
+            }
+        )
+    )
+    monkeypatch.setenv("CODEX_AUTH_PATH", str(auth_path))
+
+    cred = load_codex_cli_credential()
+
+    assert cred is not None
+    assert cred.access_token == "codex-access-token"
+    assert cred.account_id == ""
+
+
+def test_load_codex_cli_credential_ignores_non_string_account_id(tmp_path, monkeypatch):
+    auth_path = tmp_path / "auth.json"
+    auth_path.write_text(
+        json.dumps(
+            {
+                "tokens": {
+                    "access_token": "codex-access-token",
+                    "account_id": 12345,
+                }
+            }
+        )
+    )
+    monkeypatch.setenv("CODEX_AUTH_PATH", str(auth_path))
+
+    cred = load_codex_cli_credential()
+
+    assert cred is not None
+    assert cred.access_token == "codex-access-token"
+    assert cred.account_id == ""