diff --git a/switchyard/lib/profiles/tier_target_builders.py b/switchyard/lib/profiles/tier_target_builders.py
index 4b87c56ac..3ad5768c1 100644
--- a/switchyard/lib/profiles/tier_target_builders.py
+++ b/switchyard/lib/profiles/tier_target_builders.py
@@ -20,6 +20,7 @@
     build_native_backend,
     resolve_llm_target,
 )
+from switchyard.lib.processors.reasoning_hint import model_accepts_reasoning_hint
 from switchyard.lib.profiles.deterministic_routing_config import (
     DEFAULT_DETERMINISTIC_TIER_TIMEOUT_S,
 )
@@ -27,10 +28,18 @@
 
 
 def apply_deepseek_overrides(target: LlmTarget) -> LlmTarget:
-    """Apply benchmark-specific DeepSeek extras without clobbering callers."""
+    """Apply benchmark-specific DeepSeek extras without clobbering callers.
+
+    The thinking-off default is a vLLM-side hint (``chat_template_kwargs``);
+    serving stacks on the :func:`model_accepts_reasoning_hint` deny list
+    reject the field outright (HTTP 400 ``Extra inputs are not permitted``),
+    so the default is gated on the same model-id check the LLM classifier
+    already uses for its own calls. The batch-priority header default is
+    provider-neutral and stays unconditional.
+    """
     default_body = (
         {"chat_template_kwargs": {"enable_thinking": False}}
-        if "deepseek-v4" in target.model
+        if "deepseek-v4" in target.model and model_accepts_reasoning_hint(target.model)
         else None
     )
     default_headers = (
diff --git a/tests/test_deterministic_routing_profile.py b/tests/test_deterministic_routing_profile.py
index 331b3bb21..c3cbd55e7 100644
--- a/tests/test_deterministic_routing_profile.py
+++ b/tests/test_deterministic_routing_profile.py
@@ -391,6 +391,21 @@ def test_caller_supplied_empty_body_wins(self) -> None:
         out = apply_deepseek_overrides(target)
         assert out.extra_body == {}
 
+    def test_hint_incompatible_model_id_skips_thinking_off(self) -> None:
+        """Deny-listed serving stacks reject the vLLM hint with HTTP 400,
+        so the thinking-off default must not be injected for their ids.
+        The provider-neutral batch-priority header still applies."""
+        target = LlmTarget(
+            id="weak",
+            model="bedrock/deepseek-ai/deepseek-v4-flash",
+            format=BackendFormat.OPENAI,
+            api_key="k",
+            base_url="https://e/v1",
+        )
+        out = apply_deepseek_overrides(target)
+        assert not out.extra_body
+        assert out.extra_headers == {"X-Inference-Priority": "batch"}
+
 
 class TestTierTimeoutDefaults:
     """Deterministic tiers get a bounded timeout unless callers set one."""