diff --git a/agent/transports/chat_completions.py b/agent/transports/chat_completions.py
index fa36301bd81..96997afca43 100644
--- a/agent/transports/chat_completions.py
+++ b/agent/transports/chat_completions.py
@@ -113,9 +113,8 @@ class ChatCompletionsTransport(ProviderTransport):
         self, messages: list[dict[str, Any]], **kwargs
     ) -> list[dict[str, Any]]:
         """Messages are already in OpenAI format — strip internal fields
-        that strict chat-completions providers reject with HTTP 400/422.
-
-        Strips:
+        that strict chat-completions providers reject with HTTP 400/422
+        (or, in the case of some OpenAI-compatible gateways, 5xx):
 
         - Codex Responses API fields: ``codex_reasoning_items`` /
           ``codex_message_items`` on the message, ``call_id`` /
@@ -127,6 +126,16 @@ class ChatCompletionsTransport(ProviderTransport):
           ``Extra inputs are not permitted, field: 'messages[N].tool_name'``.
           Permissive providers (OpenRouter, MiniMax) silently ignore the
           field, which masked the bug for months.
+        - Hermes-internal scaffolding markers — any top-level message key
+          starting with ``_`` (e.g. ``_empty_recovery_synthetic``,
+          ``_empty_terminal_sentinel``, ``_thinking_prefill``). These are
+          bookkeeping flags the agent loop attaches to messages so the
+          persistence layer can later strip its own scaffolding; they must
+          never reach the wire. Permissive providers (real OpenAI,
+          Anthropic) silently drop unknown message keys, but strict
+          gateways (e.g. opencode-go, codex.nekos.me) reject with
+          ``Extra inputs are not permitted, field: 'messages[N]._empty_recovery_synthetic'``,
+          which then poisons every subsequent request in the session.
         """
         needs_sanitize = False
         for msg in messages:
@@ -139,6 +148,9 @@ class ChatCompletionsTransport(ProviderTransport):
             ):
                 needs_sanitize = True
                 break
+            if any(isinstance(k, str) and k.startswith("_") for k in msg):
+                needs_sanitize = True
+                break
             tool_calls = msg.get("tool_calls")
             if isinstance(tool_calls, list):
                 for tc in tool_calls:
@@ -160,6 +172,11 @@ class ChatCompletionsTransport(ProviderTransport):
             msg.pop("codex_reasoning_items", None)
             msg.pop("codex_message_items", None)
             msg.pop("tool_name", None)
+            # 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.
+            for key in [k for k in msg if isinstance(k, str) and k.startswith("_")]:
+                msg.pop(key, None)
             tool_calls = msg.get("tool_calls")
             if isinstance(tool_calls, list):
                 for tc in tool_calls:
diff --git a/tests/agent/transports/test_chat_completions.py b/tests/agent/transports/test_chat_completions.py
index 2e7b9da2f8d..9f3a205f8a8 100644
--- a/tests/agent/transports/test_chat_completions.py
+++ b/tests/agent/transports/test_chat_completions.py
@@ -66,6 +66,38 @@ class TestChatCompletionsBasic:
         # Original list untouched (deepcopy-on-demand)
         assert msgs[2]["tool_name"] == "execute_code"
 
+    def test_convert_messages_strips_internal_scaffolding_markers(self, transport):
+        """Hermes-internal ``_``-prefixed markers must never reach the wire.
+
+        The empty-response recovery path appends synthetic messages tagged
+        with ``_empty_recovery_synthetic``; permissive providers ignore the
+        unknown key, but strict gateways (opencode-go, codex.nekos.me)
+        reject the request, poisoning every later turn in the session.
+        """
+        msgs = [
+            {"role": "user", "content": "run the task"},
+            {"role": "assistant", "content": "(empty)", "_empty_recovery_synthetic": True},
+            {"role": "user", "content": "continue", "_empty_recovery_synthetic": True},
+            {"role": "assistant", "content": "done", "_thinking_prefill": True,
+             "_empty_terminal_sentinel": True},
+        ]
+        result = transport.convert_messages(msgs)
+        for m in result:
+            assert not any(k.startswith("_") for k in m), m
+        # Visible content preserved
+        assert result[1]["content"] == "(empty)"
+        assert result[2]["content"] == "continue"
+        # Original list untouched (deepcopy-on-demand)
+        assert msgs[1]["_empty_recovery_synthetic"] is True
+
+    def test_convert_messages_clean_list_is_identity(self, transport):
+        """A list with no internal/codex keys is returned as-is (no copy)."""
+        msgs = [
+            {"role": "user", "content": "hi"},
+            {"role": "assistant", "content": "hello"},
+        ]
+        assert transport.convert_messages(msgs) is msgs
+
 
 class TestChatCompletionsBuildKwargs: