diff --git a/host.py b/host.py
index d58a2f4..0d8b7f3 100644
--- a/host.py
+++ b/host.py
@@ -50,6 +50,7 @@ import os
 import queue
 import re
 import shlex
+import shutil
 import subprocess
 import sys
 import threading
@@ -201,6 +202,22 @@ def default_codex_bin():
     paired = os.path.join(HERE, "bin", "codex")
     if os.path.exists(paired):
         return paired
+    # part 5（RETEST4 新发现 1）：单元内既无 CODEX_BIN 又无
+    # HERE/bin/codex 时，旧代码返回裸 "codex"，run_health_check
+    # 的 os.path.exists("codex") 以相对路径判恒为 False，
+    # health 的 ok 恒 False（单元内 which codex 明明是
+    # /usr/local/bin/codex）。回落到 shutil.which("codex")：
+    # PATH 里找得到即返回绝对路径，health 与 Server 都直接
+    # 受益；找不到时仍返回裸 "codex"，行为与旧代码一致。
+    # 不选 (a) CODEX_BIN 进 FORWARD_ENV_ALLOWLIST：要执行器
+    # 与 chv-cell 两边接线、还得在客机环境里定一个单元内路径，
+    # 改动面大，且只修执行器驱动的调用；不选 (b) 执行器显式
+    # 加 --codex-bin：把客机镜像路径硬编码进执行器，且直接
+    # 跑 host.py 的 health 同样修不到。单点修 host.py 改动
+    # 最小、覆盖全部调用方。
+    found = shutil.which("codex")
+    if found:
+        return found
     return "codex"
 
 
diff --git a/tests/test_host.py b/tests/test_host.py
index 3e4e16c..fa6e279 100644
--- a/tests/test_host.py
+++ b/tests/test_host.py
@@ -6,6 +6,7 @@ import sys
 import tempfile
 import unittest
 from pathlib import Path
+from unittest import mock
 
 
 REPO_ROOT = Path(__file__).resolve().parents[1]
@@ -558,6 +559,98 @@ class HealthRestoreRecoverTest(unittest.TestCase):
             self.assertTrue(report["checks"]["server_ok"])
 
 
+class DefaultCodexBinTest(unittest.TestCase):
+    """part 5 修复 1（RETEST4 新发现 1）：default_codex_bin()
+    回落到 shutil.which("codex")。
+
+    模拟单元内环境：PATH 含 codex 所在目录、cwd 不含 codex、
+    无 CODEX_BIN、无 HERE/bin/codex。旧代码此时返回裸 "codex"，
+    run_health_check 的 os.path.exists("codex") 恒 False，
+    health 的 ok 恒 False。"""
+
+    class FakeServer:
+        def initialize(self):
+            return {}
+
+        def list_threads(self):
+            return {"data": []}
+
+        def close(self):
+            pass
+
+    def _fake_bin_dir(self, tmp):
+        bindir = os.path.join(tmp, "bin")
+        os.makedirs(bindir)
+        codex = os.path.join(bindir, "codex")
+        with open(codex, "w") as fh:
+            fh.write("#!/bin/sh\nexit 0\n")
+        os.chmod(codex, 0o755)
+        return bindir, codex
+
+    def test_which_fallback_returns_absolute_path(self):
+        # 仓库根下没有配对的 bin/codex（修的是无配对的情形）。
+        self.assertFalse(
+            os.path.exists(os.path.join(host.HERE, "bin", "codex")))
+        with tempfile.TemporaryDirectory() as tmp:
+            bindir, codex = self._fake_bin_dir(tmp)
+            workdir = os.path.join(tmp, "work")
+            os.makedirs(workdir)
+            old_cwd = os.getcwd()
+            try:
+                os.chdir(workdir)
+                env = {"PATH": bindir}
+                with mock.patch.dict(os.environ, env, clear=True):
+                    self.assertEqual(host.default_codex_bin(), codex)
+            finally:
+                os.chdir(old_cwd)
+
+    def test_codex_bin_env_wins_over_path(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            bindir, _codex = self._fake_bin_dir(tmp)
+            env = {"PATH": bindir, "CODEX_BIN": "/custom/codex"}
+            with mock.patch.dict(os.environ, env, clear=True):
+                self.assertEqual(host.default_codex_bin(),
+                                 "/custom/codex")
+
+    def test_no_codex_anywhere_returns_bare_name(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            empty = os.path.join(tmp, "empty")
+            os.makedirs(empty)
+            with mock.patch.dict(os.environ, {"PATH": empty},
+                                 clear=True):
+                self.assertEqual(host.default_codex_bin(), "codex")
+
+    def test_health_ok_with_which_resolved_bin(self):
+        # 验收口径：不加任何前缀的单元内 health 应 rc=0 且 ok=true。
+        # 这里钉 host 侧：codex_bin_exists 为真且 report ok 为真。
+        with tempfile.TemporaryDirectory() as tmp:
+            bindir, _codex = self._fake_bin_dir(tmp)
+            workdir = os.path.join(tmp, "work")
+            os.makedirs(workdir)
+            path = os.path.join(tmp, "host-state.json")
+            with open(path, "w") as f:
+                json.dump({"tasks": {}, "automations": [],
+                           "version": 1}, f)
+            with open(path + ".bak", "w") as f:
+                json.dump({"tasks": {}, "automations": [],
+                           "version": 1}, f)
+            old_cwd = os.getcwd()
+            try:
+                os.chdir(workdir)
+                with mock.patch.dict(os.environ, {"PATH": bindir},
+                                     clear=True):
+                    codex_bin = host.default_codex_bin()
+                    self.assertTrue(os.path.isabs(codex_bin))
+                    report = host.run_health_check(
+                        path, {}, codex_bin, tmp, "deny",
+                        server_factory=lambda: self.FakeServer())
+            finally:
+                os.chdir(old_cwd)
+            self.assertTrue(report["checks"]["codex_bin_exists"],
+                            report)
+            self.assertTrue(report["ok"], report)
+
+
 class GithubPRTest(unittest.TestCase):
     class FakeGh:
         def __init__(self, checks_output="", pr_url=""):