feat(providers): support Codex fast mode

docs/configuration.md

@@ -676,6 +676,25 @@ Then run:
 nanobot agent -m "Hello!"
 ```
 
+To opt in to Codex Fast mode, merge this provider setting into `config.json`:
+
+```json
+{
+  "providers": {
+    "openaiCodex": {
+      "extraBody": {
+        "service_tier": "priority"
+      }
+    }
+  }
+}
+```
+
+`priority` is the Responses API request value used by Codex Fast mode. The setting only works
+for models and accounts that support Fast mode; remove `service_tier` to return to standard
+processing. Fast mode consumes Codex credits at a higher rate. See the
+[OpenAI Codex rate card](https://help.openai.com/en/articles/20001106) for current details.
+
 For proxy, remote/headless login, model-name, or config-key errors, see [`troubleshooting.md`](./troubleshooting.md#provider-and-model-problems).
 
 </details>

nanobot/providers/factory.py

@@ -88,6 +88,7 @@ def _make_provider_core(
         provider = OpenAICodexProvider(
             default_model=model,
             proxy=getattr(p, "proxy", None) if p else None,
+            extra_body=p.extra_body if p else None,
         )
     elif backend == "azure_openai":
         from nanobot.providers.azure_openai_provider import AzureOpenAIProvider

nanobot/providers/openai_codex_provider.py

@@ -37,10 +37,12 @@ class OpenAICodexProvider(LLMProvider):
         self,
         default_model: str = "openai-codex/gpt-5.6-sol",
         proxy: str | None = None,
+        extra_body: dict[str, Any] | None = None,
     ):
         super().__init__(api_key=None, api_base=None)
         self.default_model = default_model
         self.proxy = proxy or None
+        self._extra_body = dict(extra_body or {})
 
     async def _call_codex(
         self,
@@ -74,6 +76,9 @@ class OpenAICodexProvider(LLMProvider):
             body["reasoning"] = reasoning_options
         if tools:
             body["tools"] = convert_tools(tools)
+        if self._extra_body:
+            # Apply explicit provider overrides last, matching other provider backends.
+            body.update(self._extra_body)
 
         stage = "oauth_token"
         try:

tests/providers/test_openai_codex_provider.py

@@ -9,6 +9,8 @@ import pytest
 from loguru import logger
 
 import nanobot.providers.base as provider_base
+from nanobot.config.schema import Config
+from nanobot.providers.factory import make_provider
 from nanobot.providers.openai_codex_provider import (
     OpenAICodexProvider,
     _build_reasoning_options,
@@ -220,6 +222,38 @@ async def test_codex_prompt_cache_key_uses_stable_conversation_prefix(monkeypatc
 
     assert bodies[0]["prompt_cache_key"] == bodies[1]["prompt_cache_key"]
     assert bodies[0]["prompt_cache_key"] != bodies[2]["prompt_cache_key"]
+    assert all("service_tier" not in body for body in bodies)
+
+
+@pytest.mark.asyncio
+async def test_codex_provider_applies_extra_body_from_config(monkeypatch) -> None:
+    bodies: list[dict[str, Any]] = []
+    _mock_codex_token(monkeypatch)
+
+    async def fake_request(_url, _headers, body, **_kwargs):
+        bodies.append(body)
+        return "ok", [], "stop", {}, None
+
+    monkeypatch.setattr("nanobot.providers.openai_codex_provider._request_codex", fake_request)
+    config = Config.model_validate({
+        "agents": {
+            "defaults": {
+                "model": "openai-codex/gpt-5.6-sol",
+                "provider": "openai_codex",
+            },
+        },
+        "providers": {
+            "openaiCodex": {
+                "extraBody": {"service_tier": "priority"},
+            },
+        },
+    })
+
+    provider = make_provider(config)
+    response = await provider.chat([{"role": "user", "content": "hello"}])
+
+    assert response.content == "ok"
+    assert bodies[0]["service_tier"] == "priority"
 
 
 @pytest.mark.asyncio