fix: support harness codex model override

scripts/test_execute.py

@@ -1,5 +1,6 @@
 import importlib.util
 import json
+import os
 import subprocess
 import sys
 import tempfile
@@ -321,6 +322,47 @@ class ExecuteRunnerSafetyTests(unittest.TestCase):
         with patch.object(execute.shutil, "which", side_effect=fake_which):
             self.assertEqual(execute.StepExecutor._codex_command(), "C:/tools/codex.cmd")
 
+    def test_codex_exec_command_uses_model_env_override(self):
+        execute = load_execute()
+        with tempfile.TemporaryDirectory() as tmp:
+            root = Path(tmp)
+            write_phase(root)
+            executor = make_executor(execute, root)
+
+            with patch.object(executor, "_codex_command", return_value="codex.cmd"):
+                with patch.dict(os.environ, {"HARNESS_CODEX_MODEL": "gpt-5"}):
+                    self.assertEqual(
+                        executor._codex_exec_command(),
+                        [
+                            "codex.cmd",
+                            "exec",
+                            "-m",
+                            "gpt-5",
+                            "--dangerously-bypass-approvals-and-sandbox",
+                            "--json",
+                            "-",
+                        ],
+                    )
+
+    def test_configure_output_encoding_sets_utf8_replace(self):
+        execute = load_execute()
+
+        class Stream:
+            def __init__(self):
+                self.calls = []
+
+            def reconfigure(self, **kwargs):
+                self.calls.append(kwargs)
+
+        stdout = Stream()
+        stderr = Stream()
+        with patch.object(execute.sys, "stdout", stdout):
+            with patch.object(execute.sys, "stderr", stderr):
+                execute.configure_output_encoding()
+
+        self.assertEqual(stdout.calls, [{"encoding": "utf-8", "errors": "replace"}])
+        self.assertEqual(stderr.calls, [{"encoding": "utf-8", "errors": "replace"}])
+
 
 if __name__ == "__main__":
     unittest.main()