diff --git a/gateway/run.py b/gateway/run.py
index f6e86b29fd9a..15042bd4c394 100644
--- a/gateway/run.py
+++ b/gateway/run.py
@@ -2667,6 +2667,8 @@ def _normalize_empty_agent_response(
             )
         return response
     if api_calls > 0:
+        if _is_gateway_hidden_reasoning_incomplete_turn(agent_result):
+            return ""
         if agent_result.get("partial"):
             err = agent_result.get("error", "processing incomplete")
             return f"⚠️ Processing stopped: {str(err)[:200]}. Try again."
@@ -2694,6 +2696,20 @@ def _normalize_empty_agent_response(
     return response
 
 
+def _is_gateway_hidden_reasoning_incomplete_turn(agent_result: dict) -> bool:
+    """Detect retry-exhausted turns with hidden reasoning but no visible answer."""
+    if not isinstance(agent_result, dict):
+        return False
+    if agent_result.get("failed") or agent_result.get("interrupted"):
+        return False
+    if agent_result.get("final_response"):
+        return False
+    if not agent_result.get("partial"):
+        return False
+    error_text = str(agent_result.get("error", "") or "").lower()
+    return "remained incomplete after" in error_text
+
+
 def _should_clear_resume_pending_after_turn(agent_result: dict) -> bool:
     """Return True only when a gateway turn really completed successfully.
 
@@ -12039,6 +12055,9 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
             # forgets what was just asked.  Persist the user turn so the
             # conversation is preserved. (#7100)
             agent_failed_early = bool(agent_result.get("failed"))
+            hidden_reasoning_incomplete = _is_gateway_hidden_reasoning_incomplete_turn(
+                agent_result
+            )
             _err_str_for_classify = str(agent_result.get("error", "")).lower()
             # Use specific multi-word phrases (not bare "exceed" or "token")
             # to avoid false positives on transient errors like "rate limit
@@ -12067,6 +12086,13 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
                     "message so conversation context is preserved on retry.",
                     session_entry.session_id,
                 )
+            elif hidden_reasoning_incomplete:
+                logger.warning(
+                    "Suppressing hidden-reasoning-only incomplete gateway turn "
+                    "for session %s: %s",
+                    session_entry.session_id,
+                    agent_result.get("error", "processing incomplete"),
+                )
 
             # When compression is exhausted, the session is permanently too
             # large to process.  Auto-reset it so the next message starts
@@ -12150,11 +12176,14 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
             # entries that were stripped before the agent saw them.
             if is_context_overflow_failure:
                 pass  # handled above — skip all transcript writes
-            elif agent_failed_early:
+            elif agent_failed_early or hidden_reasoning_incomplete:
                 # Transient failure (429/timeout/5xx): persist only the user
                 # message so the next message can load a transcript that
                 # reflects what was said.  Skip the assistant error text since
-                # it's a gateway-generated hint, not model output. (#7100)
+                # it's a gateway-generated hint, not model output. Hidden-
+                # reasoning-only incomplete turns follow the same persistence
+                # rule so peer-agent channels don't ingest them as completed
+                # assistant turns. (#7100, #51628)
                 _user_entry = {
                     "role": "user",
                     "content": (
diff --git a/tests/gateway/test_incomplete_gateway_turns.py b/tests/gateway/test_incomplete_gateway_turns.py
new file mode 100644
index 000000000000..1ac356711c36
--- /dev/null
+++ b/tests/gateway/test_incomplete_gateway_turns.py
@@ -0,0 +1,159 @@
+"""Regression tests for hidden-reasoning-only incomplete gateway turns."""
+
+import asyncio
+from datetime import datetime
+from types import SimpleNamespace
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+import gateway.run as gateway_run
+from gateway.config import GatewayConfig, Platform, PlatformConfig
+from gateway.platforms.base import BasePlatformAdapter, MessageEvent, ProcessingOutcome, SendResult
+from gateway.session import SessionEntry, SessionSource, build_session_key
+
+
+class CaptureSlackAdapter(BasePlatformAdapter):
+    def __init__(self):
+        super().__init__(PlatformConfig(enabled=True, token="fake-token"), Platform.SLACK)
+        self.sent = []
+        self.processing_hooks = []
+
+    async def connect(self) -> bool:
+        return True
+
+    async def disconnect(self) -> None:
+        return None
+
+    async def send(self, chat_id, content, reply_to=None, metadata=None) -> SendResult:
+        self.sent.append(
+            {
+                "chat_id": chat_id,
+                "content": content,
+                "reply_to": reply_to,
+                "metadata": metadata,
+            }
+        )
+        return SendResult(success=True, message_id="slack-1")
+
+    async def send_typing(self, chat_id: str, metadata=None) -> None:
+        return None
+
+    async def get_chat_info(self, chat_id: str):
+        return {"id": chat_id}
+
+    async def on_processing_start(self, event: MessageEvent) -> None:
+        self.processing_hooks.append(("start", event.message_id))
+
+    async def on_processing_complete(self, event: MessageEvent, outcome: ProcessingOutcome) -> None:
+        self.processing_hooks.append(("complete", event.message_id, outcome))
+
+
+def _make_incomplete_result() -> dict:
+    return {
+        "final_response": None,
+        "messages": [
+            {"role": "user", "content": "hello"},
+            {"role": "assistant", "content": ""},
+        ],
+        "tools": [],
+        "history_offset": 0,
+        "api_calls": 3,
+        "partial": True,
+        "completed": False,
+        "interrupted": False,
+        "error": "Codex response remained incomplete after 3 continuation attempts",
+        "last_prompt_tokens": 0,
+    }
+
+
+def _make_runner(adapter: CaptureSlackAdapter) -> gateway_run.GatewayRunner:
+    runner = object.__new__(gateway_run.GatewayRunner)
+    runner.config = GatewayConfig(
+        platforms={Platform.SLACK: PlatformConfig(enabled=True, token="fake-token")}
+    )
+    runner.adapters = {Platform.SLACK: adapter}
+    runner._voice_mode = {}
+    runner.hooks = SimpleNamespace(emit=AsyncMock(), loaded_hooks=False)
+    runner.session_store = MagicMock()
+    runner.session_store.get_or_create_session.return_value = SessionEntry(
+        session_key="agent:main:slack:channel:C123:171717",
+        session_id="sess-1",
+        created_at=datetime.now(),
+        updated_at=datetime.now(),
+        platform=Platform.SLACK,
+        chat_type="channel",
+    )
+    runner.session_store.load_transcript.return_value = []
+    runner.session_store.has_any_sessions.return_value = True
+    runner.session_store.rewrite_transcript = MagicMock()
+    runner.session_store.append_to_transcript = MagicMock()
+    runner.session_store.update_session = MagicMock()
+    runner._running_agents = {}
+    runner._pending_messages = {}
+    runner._pending_approvals = {}
+    runner._session_db = None
+    runner._is_user_authorized = lambda _source: True
+    runner._set_session_env = lambda _context: None
+    runner._run_agent = AsyncMock(return_value=_make_incomplete_result())
+    return runner
+
+
+def _make_event() -> MessageEvent:
+    return MessageEvent(
+        text="hello",
+        source=SessionSource(
+            platform=Platform.SLACK,
+            chat_id="C123",
+            chat_type="channel",
+            thread_id="171717",
+            user_id="U123",
+        ),
+        message_id="m-1",
+    )
+
+
+def test_incomplete_codex_warning_is_not_surfaced_as_chat_text():
+    agent_result = _make_incomplete_result()
+
+    response = gateway_run._normalize_empty_agent_response(
+        agent_result,
+        agent_result.get("final_response") or "",
+        history_len=4,
+    )
+
+    assert response == ""
+
+
+@pytest.mark.asyncio
+async def test_incomplete_codex_turn_stays_out_of_slack_transcript(monkeypatch, tmp_path):
+    adapter = CaptureSlackAdapter()
+    runner = _make_runner(adapter)
+
+    monkeypatch.setattr(gateway_run, "_hermes_home", tmp_path)
+    monkeypatch.setattr(gateway_run, "_resolve_runtime_agent_kwargs", lambda: {"api_key": "fake"})
+    monkeypatch.setattr(
+        "agent.model_metadata.get_model_context_length",
+        lambda *_args, **_kwargs: 100,
+    )
+    monkeypatch.setenv("SLACK_HOME_CHANNEL", "C123")
+
+    adapter.set_message_handler(runner._handle_message)
+    adapter._keep_typing = lambda *_args, **_kwargs: asyncio.Event().wait()
+
+    event = _make_event()
+    await adapter._process_message_background(event, build_session_key(event.source))
+
+    assert adapter.sent == []
+    assert runner.session_store.update_session.called
+
+    transcript_roles = [
+        call.args[1]["role"]
+        for call in runner.session_store.append_to_transcript.call_args_list
+    ]
+    assert transcript_roles == ["session_meta", "user"]
+    assert runner.session_store.append_to_transcript.call_args_list[1].args[1]["content"] == "hello"
+    assert adapter.processing_hooks == [
+        ("start", "m-1"),
+        ("complete", "m-1", ProcessingOutcome.SUCCESS),
+    ]
diff --git a/website/docs/user-guide/messaging/slack.md b/website/docs/user-guide/messaging/slack.md
index 42acf9f3edb7..299b5fa0ad89 100644
--- a/website/docs/user-guide/messaging/slack.md
+++ b/website/docs/user-guide/messaging/slack.md
@@ -217,6 +217,12 @@ hermes gateway install      # Install as a user service
 sudo hermes gateway install --system   # Linux only: boot-time system service
 ```
 
+:::tip Codex reasoning-effort safety
+For Codex-backed Slack peer-agent channels, prefer `agent.reasoning_effort: high` or lower. `xhigh`
+can spend the entire turn in hidden reasoning and never produce visible assistant text; Hermes now
+suppresses those incomplete-turn warnings from the thread and keeps the diagnostics in gateway logs.
+:::
+
 ---
 
 ## Step 9: Invite the Bot to Channels