diff --git a/nanobot/providers/openai_codex_provider.py b/nanobot/providers/openai_codex_provider.py
index 945cae9ba..0d37b5ece 100644
--- a/nanobot/providers/openai_codex_provider.py
+++ b/nanobot/providers/openai_codex_provider.py
@@ -56,7 +56,7 @@ class OpenAICodexProvider(LLMProvider):
             "input": input_items,
             "text": {"verbosity": "medium"},
             "include": ["reasoning.encrypted_content"],
-            "prompt_cache_key": _prompt_cache_key(messages),
+            "prompt_cache_key": _prompt_cache_key(messages[:2]),
             "tool_choice": tool_choice or "auto",
             "parallel_tool_calls": True,
         }
diff --git a/tests/providers/test_openai_codex_provider.py b/tests/providers/test_openai_codex_provider.py
new file mode 100644
index 000000000..ef9a91a79
--- /dev/null
+++ b/tests/providers/test_openai_codex_provider.py
@@ -0,0 +1,50 @@
+from __future__ import annotations
+
+from types import SimpleNamespace
+
+import pytest
+
+from nanobot.providers.openai_codex_provider import OpenAICodexProvider
+
+
+@pytest.mark.asyncio
+async def test_codex_prompt_cache_key_uses_stable_conversation_prefix(monkeypatch) -> None:
+    bodies: list[dict] = []
+
+    monkeypatch.setattr(
+        "nanobot.providers.openai_codex_provider.get_codex_token",
+        lambda: SimpleNamespace(account_id="acct", access="token"),
+    )
+
+    async def fake_request(url, headers, body, verify, on_content_delta=None):
+        bodies.append(body)
+        return "ok", [], "stop"
+
+    monkeypatch.setattr("nanobot.providers.openai_codex_provider._request_codex", fake_request)
+
+    provider = OpenAICodexProvider()
+    await provider.chat(
+        [
+            {"role": "system", "content": "You are nanobot."},
+            {"role": "user", "content": "first request"},
+            {"role": "assistant", "content": "first answer"},
+        ],
+    )
+    await provider.chat(
+        [
+            {"role": "system", "content": "You are nanobot."},
+            {"role": "user", "content": "first request"},
+            {"role": "assistant", "content": "first answer"},
+            {"role": "user", "content": "follow up"},
+        ],
+    )
+    await provider.chat(
+        [
+            {"role": "system", "content": "You are nanobot."},
+            {"role": "user", "content": "different request"},
+            {"role": "assistant", "content": "first answer"},
+        ],
+    )
+
+    assert bodies[0]["prompt_cache_key"] == bodies[1]["prompt_cache_key"]
+    assert bodies[0]["prompt_cache_key"] != bodies[2]["prompt_cache_key"]