diff --git a/agent/transports/codex.py b/agent/transports/codex.py
index ef7ffbaffd24..2ed8699bfe5c 100644
--- a/agent/transports/codex.py
+++ b/agent/transports/codex.py
@@ -265,6 +265,9 @@ def build_kwargs(
         # the cache-scope routing headers below. Falls back to session_id when
         # there is no static content to hash.
         cache_key = _content_cache_key(instructions, response_tools) or session_id
+        # OpenAI rejects prompt_cache_key > 64 chars with HTTP 400.
+        if cache_key and len(cache_key) > 64:
+            cache_key = cache_key[:64]
         # xAI Responses takes prompt_cache_key in extra_body (set further
         # down); GitHub Models opts out of cache-key routing entirely.
         if not is_github_responses and not is_xai_responses and cache_key: