diff --git a/agent/model_metadata.py b/agent/model_metadata.py
index 41c8fe6796..636034f558 100644
--- a/agent/model_metadata.py
+++ b/agent/model_metadata.py
@@ -604,6 +604,14 @@ def grok_supports_reasoning_effort(model: str) -> bool:
     return any(name.startswith(prefix) for prefix in _GROK_EFFORT_CAPABLE_PREFIXES)
 
 
+def is_grok_46_family(model: str) -> bool:
+    """Return whether *model* is a Grok 4.6 family identifier."""
+    name = (model or "").strip().lower().replace("_", "-")
+    if "/" in name:
+        name = name.rsplit("/", 1)[-1]
+    return name == "grok-4.6" or name.startswith("grok-4.6-")
+
+
 _CONTEXT_LENGTH_KEYS = (
     "context_length",
     "context_window",
diff --git a/agent/transports/codex.py b/agent/transports/codex.py
index 9c361e3d58..2676dd1fc0 100644
--- a/agent/transports/codex.py
+++ b/agent/transports/codex.py
@@ -300,8 +300,13 @@ class ResponsesApiTransport(ProviderTransport):
             # Ultra is the Codex product tier; the Responses API wire value is max.
             _effort_clamp["ultra"] = "max"
         if params.get("is_xai_responses", False):
-            # xAI Responses tops out at high; keep generic stronger values usable.
-            _effort_clamp.update({"xhigh": "high", "max": "high", "ultra": "high"})
+            from agent.model_metadata import is_grok_46_family
+
+            # Grok 4.6 accepts xhigh as a wire value. Older Grok models top out
+            # at high, while max/ultra remain Hermes aliases for every xAI model.
+            if not is_grok_46_family(model):
+                _effort_clamp["xhigh"] = "high"
+            _effort_clamp.update({"max": "high", "ultra": "high"})
         if (params.get("provider") or "").strip().lower() == "actual":
             # Actual Computer relays to SGLang/vLLM backends that accept only
             # none/low/medium/high/max for reasoning effort — a forwarded
@@ -441,16 +446,18 @@ class ResponsesApiTransport(ProviderTransport):
             else:
                 kwargs.pop("prompt_cache_key", None)
 
-        # xAI Responses API rejects ``service_tier`` (HTTP 400 "Argument not
-        # supported: service_tier") — hit when ``/fast`` priority-processing
-        # mode lingers from a prior model in the same session, or when a
-        # user explicitly sets ``agent.service_tier`` in config.yaml.  The
-        # main-loop guard (``resolve_fast_mode_overrides`` only returns
-        # ``service_tier`` for OpenAI fast-eligible models) doesn't cover
-        # those leak paths, so strip defensively when targeting xAI.  See
-        # #28490 for the original report.
+        # Older xAI Responses models reject ``service_tier`` (HTTP 400
+        # "Argument not supported: service_tier"). Grok 4.6 accepts Priority
+        # Processing, but continue stripping stale or unsupported tier values
+        # on every other xAI path. See #28490 and #84799.
         if is_xai_responses:
-            kwargs.pop("service_tier", None)
+            from agent.model_metadata import is_grok_46_family
+
+            if not (
+                is_grok_46_family(model)
+                and kwargs.get("service_tier") == "priority"
+            ):
+                kwargs.pop("service_tier", None)
 
         # Forward per-request timeout to the SDK so OpenAI/Anthropic clients
         # honor it.  Without this, ``providers.<id>.request_timeout_seconds``
diff --git a/hermes_cli/models.py b/hermes_cli/models.py
index 838289d9fd..7b6c7d698f 100644
--- a/hermes_cli/models.py
+++ b/hermes_cli/models.py
@@ -2870,7 +2870,13 @@ def _strip_vendor_prefix(model_id: str) -> str:
 
 def model_supports_fast_mode(model_id: Optional[str]) -> bool:
     """Return whether Hermes should expose the /fast toggle for this model."""
-    return _is_anthropic_fast_model(model_id) or _is_openai_fast_model(model_id)
+    from agent.model_metadata import is_grok_46_family
+
+    return (
+        _is_anthropic_fast_model(model_id)
+        or _is_openai_fast_model(model_id)
+        or is_grok_46_family(str(model_id or ""))
+    )
 
 
 def _is_anthropic_fast_model(model_id: Optional[str]) -> bool:
diff --git a/tests/agent/transports/test_codex_transport.py b/tests/agent/transports/test_codex_transport.py
index 70c4ba0e04..5041a32da3 100644
--- a/tests/agent/transports/test_codex_transport.py
+++ b/tests/agent/transports/test_codex_transport.py
@@ -689,6 +689,47 @@ class TestCodexTransportTimeout:
 
 
 
+class TestCodexTransportXaiReasoningEffort:
+    @pytest.fixture
+    def transport(self):
+        from agent.transports.codex import ResponsesApiTransport
+        return ResponsesApiTransport()
+
+    def test_grok_46_preserves_xhigh(self, transport):
+        kw = transport.build_kwargs(
+            model="grok-4.6",
+            messages=[{"role": "user", "content": "hi"}],
+            tools=[],
+            is_xai_responses=True,
+            reasoning_config={"effort": "xhigh"},
+        )
+
+        assert kw["reasoning"]["effort"] == "xhigh"
+
+    @pytest.mark.parametrize("effort", ["max", "ultra"])
+    def test_grok_46_clamps_hermes_aliases_to_high(self, transport, effort):
+        kw = transport.build_kwargs(
+            model="x-ai/grok-4.6-latest",
+            messages=[{"role": "user", "content": "hi"}],
+            tools=[],
+            is_xai_responses=True,
+            reasoning_config={"effort": effort},
+        )
+
+        assert kw["reasoning"]["effort"] == "high"
+
+    def test_older_grok_clamps_xhigh_to_high(self, transport):
+        kw = transport.build_kwargs(
+            model="grok-4.5",
+            messages=[{"role": "user", "content": "hi"}],
+            tools=[],
+            is_xai_responses=True,
+            reasoning_config={"effort": "xhigh"},
+        )
+
+        assert kw["reasoning"]["effort"] == "high"
+
+
 class TestCodexTransportXaiServiceTierStrip:
     """xAI Responses API rejects ``service_tier`` (#28490).
 
@@ -721,6 +762,28 @@ class TestCodexTransportXaiServiceTierStrip:
             f"got {kw.get('service_tier')!r}"
         )
 
+    def test_grok_46_preserves_priority_service_tier(self, transport):
+        kw = transport.build_kwargs(
+            model="x-ai/grok-4.6-latest",
+            messages=[{"role": "user", "content": "hi"}],
+            tools=[],
+            is_xai_responses=True,
+            request_overrides={"service_tier": "priority"},
+        )
+
+        assert kw.get("service_tier") == "priority"
+
+    def test_grok_46_strips_non_priority_service_tier(self, transport):
+        kw = transport.build_kwargs(
+            model="grok-4.6",
+            messages=[{"role": "user", "content": "hi"}],
+            tools=[],
+            is_xai_responses=True,
+            request_overrides={"service_tier": "unsupported"},
+        )
+
+        assert "service_tier" not in kw
+
     def test_non_xai_codex_preserves_service_tier(self, transport):
         """The strip is xAI-only — native Codex DOES accept
         service_tier=priority (OpenAI Priority Processing).  Stripping
diff --git a/tests/cli/test_fast_command.py b/tests/cli/test_fast_command.py
index 5b7f74d74d..87a6b2689c 100644
--- a/tests/cli/test_fast_command.py
+++ b/tests/cli/test_fast_command.py
@@ -123,6 +123,17 @@ class TestPriorityProcessingModels(unittest.TestCase):
 
 
 
+    def test_grok_46_supports_priority_processing(self):
+        from hermes_cli.models import (
+            model_supports_fast_mode,
+            resolve_fast_mode_overrides,
+        )
+
+        assert model_supports_fast_mode("grok-4.6") is True
+        assert model_supports_fast_mode("x-ai/grok-4.6-latest") is True
+        assert model_supports_fast_mode("grok-4.5") is False
+        assert resolve_fast_mode_overrides("grok-4.6") == {"service_tier": "priority"}
+
     def test_resolve_overrides_returns_service_tier(self):
         from hermes_cli.models import resolve_fast_mode_overrides