diff --git a/hermes_cli/config.py b/hermes_cli/config.py
index 8c8c79b639..366a66f77f 100644
--- a/hermes_cli/config.py
+++ b/hermes_cli/config.py
@@ -766,6 +766,7 @@ _FALLBACK_MODEL_COMMENT = """
 #
 # Supported providers:
 #   openrouter   (OPENROUTER_API_KEY)  — routes to any model
+#   openai-codex (OAuth — hermes login) — OpenAI Codex
 #   zai          (ZAI_API_KEY)         — Z.AI / GLM
 #   kimi-coding  (KIMI_API_KEY)        — Kimi / Moonshot
 #   minimax      (MINIMAX_API_KEY)     — MiniMax
diff --git a/run_agent.py b/run_agent.py
index 23f7ac719c..9a47f40026 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -2163,6 +2163,7 @@ class AIAgent:
     # Maps provider id → (default_base_url, [env_var_names])
     # Only includes providers that Hermes actually supports.
     # For anything else, use base_url + api_key_env in the config.
+    # Note: openai-codex is handled specially (OAuth, not env var).
     _FALLBACK_PROVIDERS = {
         "openrouter": (OPENROUTER_BASE_URL, ["OPENROUTER_API_KEY"]),
         "zai": ("https://api.z.ai/api/paas/v4", ["ZAI_API_KEY", "Z_AI_API_KEY"]),
@@ -2188,32 +2189,47 @@ class AIAgent:
         if not fb_provider or not fb_model:
             return False
 
-        # Resolve API key
-        fb_key = (fb.get("api_key") or "").strip()
-        if not fb_key:
-            key_env = (fb.get("api_key_env") or "").strip()
-            if key_env:
-                fb_key = os.getenv(key_env, "")
-            elif fb_provider in self._FALLBACK_PROVIDERS:
-                for env_var in self._FALLBACK_PROVIDERS[fb_provider][1]:
-                    fb_key = os.getenv(env_var, "")
-                    if fb_key:
-                        break
-        if not fb_key:
-            logging.warning(
-                "Fallback model configured but no API key found for provider '%s'",
-                fb_provider,
-            )
-            return False
+        # ── Resolve credentials ──────────────────────────────────────────
+        # OpenAI Codex uses OAuth (not a simple env var), so handle it
+        # separately from the standard env-var-based providers.
+        fb_api_mode = "chat_completions"
 
-        # Resolve base URL
-        fb_base_url = (fb.get("base_url") or "").strip()
-        if not fb_base_url and fb_provider in self._FALLBACK_PROVIDERS:
-            fb_base_url = self._FALLBACK_PROVIDERS[fb_provider][0]
-        if not fb_base_url:
-            fb_base_url = OPENROUTER_BASE_URL
+        if fb_provider == "openai-codex":
+            try:
+                from hermes_cli.auth import resolve_codex_runtime_credentials
+                creds = resolve_codex_runtime_credentials()
+                fb_key = creds["api_key"]
+                fb_base_url = creds["base_url"]
+                fb_api_mode = "codex_responses"
+            except Exception as e:
+                logging.warning("Fallback to openai-codex failed (no credentials): %s", e)
+                return False
+        else:
+            # Standard env-var resolution
+            fb_key = (fb.get("api_key") or "").strip()
+            if not fb_key:
+                key_env = (fb.get("api_key_env") or "").strip()
+                if key_env:
+                    fb_key = os.getenv(key_env, "")
+                elif fb_provider in self._FALLBACK_PROVIDERS:
+                    for env_var in self._FALLBACK_PROVIDERS[fb_provider][1]:
+                        fb_key = os.getenv(env_var, "")
+                        if fb_key:
+                            break
+            if not fb_key:
+                logging.warning(
+                    "Fallback model configured but no API key found for provider '%s'",
+                    fb_provider,
+                )
+                return False
 
-        # Build new client
+            fb_base_url = (fb.get("base_url") or "").strip()
+            if not fb_base_url and fb_provider in self._FALLBACK_PROVIDERS:
+                fb_base_url = self._FALLBACK_PROVIDERS[fb_provider][0]
+            if not fb_base_url:
+                fb_base_url = OPENROUTER_BASE_URL
+
+        # ── Build new client ──────────────────────────────────────────
         try:
             client_kwargs = {"api_key": fb_key, "base_url": fb_base_url}
             if "openrouter" in fb_base_url.lower():
@@ -2231,7 +2247,7 @@ class AIAgent:
             self.model = fb_model
             self.provider = fb_provider
             self.base_url = fb_base_url
-            self.api_mode = "chat_completions"
+            self.api_mode = fb_api_mode
             self._fallback_activated = True
 
             # Re-evaluate prompt caching for the new provider/model
diff --git a/tests/test_fallback_model.py b/tests/test_fallback_model.py
index 2603c16ccb..7c3717486b 100644
--- a/tests/test_fallback_model.py
+++ b/tests/test_fallback_model.py
@@ -218,6 +218,40 @@ class TestTryActivateFallback:
             call_kwargs = mock_openai.call_args[1]
             assert call_kwargs["api_key"] == "sk-alt-key"
 
+    def test_activates_codex_fallback(self):
+        """OpenAI Codex fallback should use OAuth credentials and codex_responses mode."""
+        agent = _make_agent(
+            fallback_model={"provider": "openai-codex", "model": "gpt-5.3-codex"},
+        )
+        mock_creds = {
+            "api_key": "codex-oauth-token",
+            "base_url": "https://chatgpt.com/backend-api/codex",
+        }
+        with (
+            patch("hermes_cli.auth.resolve_codex_runtime_credentials", return_value=mock_creds),
+            patch("run_agent.OpenAI") as mock_openai,
+        ):
+            result = agent._try_activate_fallback()
+            assert result is True
+            assert agent.model == "gpt-5.3-codex"
+            assert agent.provider == "openai-codex"
+            assert agent.api_mode == "codex_responses"
+            call_kwargs = mock_openai.call_args[1]
+            assert call_kwargs["api_key"] == "codex-oauth-token"
+            assert "chatgpt.com" in call_kwargs["base_url"]
+
+    def test_codex_fallback_fails_gracefully_without_credentials(self):
+        """Codex fallback should return False if no OAuth credentials available."""
+        agent = _make_agent(
+            fallback_model={"provider": "openai-codex", "model": "gpt-5.3-codex"},
+        )
+        with patch(
+            "hermes_cli.auth.resolve_codex_runtime_credentials",
+            side_effect=Exception("No Codex credentials"),
+        ):
+            assert agent._try_activate_fallback() is False
+            assert agent._fallback_activated is False
+
 
 # =============================================================================
 # Fallback config init