diff --git a/agent/transports/chat_completions.py b/agent/transports/chat_completions.py
index c0b2a13d250..e7a7a0a133e 100644
--- a/agent/transports/chat_completions.py
+++ b/agent/transports/chat_completions.py
@@ -172,6 +172,7 @@ class ChatCompletionsTransport(ProviderTransport):
                 "codex_reasoning_items" in msg
                 or "codex_message_items" in msg
                 or "tool_name" in msg
+                or "timestamp" in msg  # #47868 — strict providers reject this
             ):
                 needs_sanitize = True
                 break
@@ -201,6 +202,7 @@ class ChatCompletionsTransport(ProviderTransport):
             msg.pop("codex_reasoning_items", None)
             msg.pop("codex_message_items", None)
             msg.pop("tool_name", None)
+            msg.pop("timestamp", None)  # #47868 — leak into strict providers
             # Drop all Hermes-internal scaffolding markers (``_``-prefixed).
             # OpenAI's message schema has no ``_``-prefixed fields, so this
             # is safe and future-proofs against new markers being added.