diff --git a/README.md b/README.md
index 91264da..2681874 100644
--- a/README.md
+++ b/README.md
@@ -118,6 +118,7 @@ curl http://127.0.0.1:8000/v1/chat/completions \
 - `gpt-5.2`
 - `gpt-5-codex`
 - `gpt-5.2-codex`
+- `gpt-5.3-codex`
 - `gpt-5.1-codex`
 - `gpt-5.1-codex-max`
 - `gpt-5.1-codex-mini`
@@ -129,7 +130,7 @@ curl http://127.0.0.1:8000/v1/chat/completions \
 
 - `--reasoning-effort` (choice of minimal,low,medium,high,xhigh)<br>
 GPT-5 has a configurable amount of "effort" it can put into thinking, which may cause it to take more time for a response to return, but may overall give a smarter answer. Applying this parameter after `serve` forces the server to use this reasoning effort by default, unless overrided by the API request with a different effort set. The default reasoning effort without setting this parameter is `medium`.<br>
-    The `gpt-5.1` family (including codex) supports `low`, `medium`, and `high` while `gpt-5.1-codex-max` adds `xhigh`. The `gpt-5.2` family (including codex) supports `low`, `medium`, `high`, and `xhigh`. 
+    The `gpt-5.1` family (including codex) supports `low`, `medium`, and `high` while `gpt-5.1-codex-max` adds `xhigh`. The `gpt-5.2` and `gpt-5.3` families (including codex) support `low`, `medium`, `high`, and `xhigh`. 
 
 ### Thinking summaries
 
@@ -173,4 +174,3 @@ When the model returns a thinking summary, the model will send back thinking tag
 
 [![Star History Chart](https://api.star-history.com/svg?repos=RayBytes/ChatMock&type=Timeline)](https://www.star-history.com/#RayBytes/ChatMock&Timeline)
 
-
diff --git a/chatmock/reasoning.py b/chatmock/reasoning.py
index 5b04ac2..a6df9c9 100644
--- a/chatmock/reasoning.py
+++ b/chatmock/reasoning.py
@@ -11,6 +11,8 @@ def allowed_efforts_for_model(model: str | None) -> Set[str]:
     if not base:
         return DEFAULT_REASONING_EFFORTS
     normalized = base.split(":", 1)[0]
+    if normalized.startswith("gpt-5.3"):
+        return {"low", "medium", "high", "xhigh"}
     if normalized.startswith("gpt-5.2"):
         return {"low", "medium", "high", "xhigh"}
     if normalized.startswith("gpt-5.1-codex-max"):
diff --git a/chatmock/routes_ollama.py b/chatmock/routes_ollama.py
index 413adff..6974f24 100644
--- a/chatmock/routes_ollama.py
+++ b/chatmock/routes_ollama.py
@@ -71,7 +71,7 @@ def ollama_version() -> Response:
 
 def _instructions_for_model(model: str) -> str:
     base = current_app.config.get("BASE_INSTRUCTIONS", BASE_INSTRUCTIONS)
-    if model.startswith("gpt-5-codex") or model.startswith("gpt-5.1-codex") or model.startswith("gpt-5.2-codex"):
+    if "codex" in (model or "").lower():
         codex = current_app.config.get("GPT5_CODEX_INSTRUCTIONS") or GPT5_CODEX_INSTRUCTIONS
         if isinstance(codex, str) and codex.strip():
             return codex
@@ -97,6 +97,7 @@ def ollama_tags() -> Response:
         "gpt-5",
         "gpt-5.1",
         "gpt-5.2",
+        "gpt-5.3-codex",
         "gpt-5-codex",
         "gpt-5.2-codex",
         "gpt-5.1-codex",
@@ -125,6 +126,10 @@ def ollama_tags() -> Response:
                 "gpt-5.2-codex-high",
                 "gpt-5.2-codex-medium",
                 "gpt-5.2-codex-low",
+                "gpt-5.3-codex-xhigh",
+                "gpt-5.3-codex-high",
+                "gpt-5.3-codex-medium",
+                "gpt-5.3-codex-low",
                 "gpt-5.1-codex-high",
                 "gpt-5.1-codex-medium",
                 "gpt-5.1-codex-low",
diff --git a/chatmock/routes_openai.py b/chatmock/routes_openai.py
index c7a2c94..219a15f 100644
--- a/chatmock/routes_openai.py
+++ b/chatmock/routes_openai.py
@@ -59,7 +59,7 @@ def _wrap_stream_logging(label: str, iterator, enabled: bool):
 
 def _instructions_for_model(model: str) -> str:
     base = current_app.config.get("BASE_INSTRUCTIONS", BASE_INSTRUCTIONS)
-    if model.startswith("gpt-5-codex") or model.startswith("gpt-5.1-codex") or model.startswith("gpt-5.2-codex"):
+    if "codex" in (model or "").lower():
         codex = current_app.config.get("GPT5_CODEX_INSTRUCTIONS") or GPT5_CODEX_INSTRUCTIONS
         if isinstance(codex, str) and codex.strip():
             return codex
@@ -535,6 +535,7 @@ def list_models() -> Response:
         ("gpt-5", ["high", "medium", "low", "minimal"]),
         ("gpt-5.1", ["high", "medium", "low"]),
         ("gpt-5.2", ["xhigh", "high", "medium", "low"]),
+        ("gpt-5.3-codex", ["xhigh", "high", "medium", "low"]),
         ("gpt-5-codex", ["high", "medium", "low"]),
         ("gpt-5.2-codex", ["xhigh", "high", "medium", "low"]),
         ("gpt-5.1-codex", ["high", "medium", "low"]),
diff --git a/chatmock/upstream.py b/chatmock/upstream.py
index 4803954..6dc4156 100644
--- a/chatmock/upstream.py
+++ b/chatmock/upstream.py
@@ -45,6 +45,9 @@ def normalize_model_name(name: str | None, debug_model: str | None = None) -> st
         "gpt5.2": "gpt-5.2",
         "gpt-5.2": "gpt-5.2",
         "gpt-5.2-latest": "gpt-5.2",
+        "gpt5.3-codex": "gpt-5.3-codex",
+        "gpt-5.3-codex": "gpt-5.3-codex",
+        "gpt-5.3-codex-latest": "gpt-5.3-codex",
         "gpt5.2-codex": "gpt-5.2-codex",
         "gpt-5.2-codex": "gpt-5.2-codex",
         "gpt-5.2-codex-latest": "gpt-5.2-codex",