diff --git a/src/providers/__init__.py b/src/providers/__init__.py
index fce92636..d8f769f7 100644
--- a/src/providers/__init__.py
+++ b/src/providers/__init__.py
@@ -360,7 +360,42 @@ class is what carries the wire contract.
     from .anthropic_provider import AnthropicProvider
     from .minimax_provider import MinimaxProvider
 
-    return isinstance(provider, (AnthropicProvider, MinimaxProvider))
+    return isinstance(unwrap_provider(provider), (AnthropicProvider, MinimaxProvider))
+
+
+def unwrap_provider(provider: Any) -> Any:
+    """Follow a delegating wrapper down to the provider that owns the wire.
+
+    ``FusionProvider`` wraps a base provider to substitute image blocks; its
+    MRO is ``(FusionProvider, object)``, so it is not an instance of anything
+    the wire tests check. An ``isinstance`` test therefore reports
+    "OpenAI-compatible" for EVERY fusion model — including one whose base is
+    Anthropic or Minimax.
+
+    That is not cosmetic. Both things :func:`is_anthropic_wire` decides are
+    hard failures when decided wrongly: reasoning effort would go out as a
+    top-level ``reasoning_effort`` field, which the Anthropic API rejects with
+    ``400 invalid_request_error — reasoning_effort: Extra inputs are not
+    permitted``, and the system prompt would be prepended as a
+    ``{"role": "system"}`` message instead of passed as the ``system`` kwarg.
+    A fusion over ``anthropic:claude-opus-5`` is expressible today
+    (``fusion=anthropic:claude-opus-5+openai:gpt-5.6-luna``) and would fail on
+    its first request.
+
+    Bounded rather than a ``while True``: a cycle in ``inner`` would otherwise
+    hang the wire check. Depth 8 is far beyond any real nesting (one wrapper
+    today) and any excess simply falls back to the outermost object, which is
+    the pre-existing behaviour.
+    """
+    seen: set[int] = set()
+    current = provider
+    for _ in range(8):
+        inner = getattr(current, "inner", None)
+        if inner is None or inner is current or id(inner) in seen:
+            return current
+        seen.add(id(current))
+        current = inner
+    return current
 
 
 #: The provider ids whose classes :func:`is_anthropic_wire` matches. Kept
@@ -468,6 +503,7 @@ def resolve_api_key(
     "get_provider_class",
     "get_provider_info",
     "is_anthropic_wire",
+    "unwrap_provider",
     "canonical_provider_name",
     "provider_env_vars",
     "provider_has_credentials",
diff --git a/src/providers/base.py b/src/providers/base.py
index 7c8ab95d..bbf7c282 100644
--- a/src/providers/base.py
+++ b/src/providers/base.py
@@ -62,6 +62,44 @@ class BaseProvider(ABC):
     #: OpenRouter is intentionally NOT covered.
     is_deepseek: bool = False
 
+    #: Reasoning-effort levels this provider's API actually accepts, or
+    #: ``None`` for "the whole clawcodex ladder passes through untouched"
+    #: (the default, and correct for OpenAI/OpenRouter, which take all five).
+    #: See :meth:`normalize_reasoning_effort`.
+    supported_reasoning_efforts: tuple[str, ...] | None = None
+
+    #: Maps a clawcodex level this provider does NOT accept onto the nearest
+    #: one it does. Only consulted for levels absent from
+    #: ``supported_reasoning_efforts``.
+    reasoning_effort_aliases: dict[str, str] = {}
+
+    def normalize_reasoning_effort(self, effort: str | None) -> str | None:
+        """Translate a clawcodex effort level into this provider's vocabulary.
+
+        clawcodex exposes ``low | medium | high | xhigh | max``, but that
+        ladder is Anthropic's and not every API shares it. Sending a level a
+        provider does not know is not obviously harmful — nobody 400s on it —
+        which is exactly why it needs handling: the provider ignores the field
+        and silently applies its own default, so the user gets a level they
+        did not ask for and no diagnostic saying so.
+
+        The damaging direction is DOWNWARD. A user who selects ``xhigh``
+        against a provider whose ladder tops out differently is asking for
+        more than ``high``; if the value is dropped they get the default,
+        which is typically ``high`` — strictly less than requested, on the
+        setting people reach for precisely when a task is hard.
+
+        Default is identity: providers that accept the full ladder are
+        unaffected, and a provider that has not declared a vocabulary keeps
+        the pass-through behaviour it had before this hook existed.
+        """
+        if effort is None:
+            return None
+        supported = self.supported_reasoning_efforts
+        if not supported or effort in supported:
+            return effort
+        return self.reasoning_effort_aliases.get(effort, effort)
+
     def __init__(
         self, api_key: str, base_url: Optional[str] = None, model: Optional[str] = None
     ):
diff --git a/src/providers/deepseek_provider.py b/src/providers/deepseek_provider.py
index eca51bdd..a4f9783b 100644
--- a/src/providers/deepseek_provider.py
+++ b/src/providers/deepseek_provider.py
@@ -30,6 +30,25 @@ class DeepSeekProvider(OpenAICompatibleProvider):
     #: cache (see ``query._split_system_prompt_blocks``).
     is_deepseek = True
 
+    #: DeepSeek's OpenAI-format thinking vocabulary is ``low | high | max``
+    #: (api-docs.deepseek.com/guides/thinking_mode). Thinking is ON by
+    #: default at ``high``.
+    #:
+    #: The API does not VALIDATE this field — probed 2026-08-03 against
+    #: ``deepseek-v4-flash``, every one of ``low / medium / high / xhigh /
+    #: max / minimal`` returned 200, and so did a value the docs never list.
+    #: So an unsupported level is not an error, it is silently discarded and
+    #: the default (``high``) applies. Without the mapping below, ``xhigh``
+    #: — chosen precisely when a task is hard — quietly delivered LESS
+    #: reasoning than ``max``, which DeepSeek does support.
+    supported_reasoning_efforts = ("low", "high", "max")
+
+    #: ``medium`` has no DeepSeek equivalent and already behaved as ``high``
+    #: (unknown → default), so this makes the existing behaviour explicit
+    #: rather than changing it. ``xhigh`` is the real fix: it means "above
+    #: high", and ``max`` is the only DeepSeek level that is.
+    reasoning_effort_aliases = {"medium": "high", "xhigh": "max", "minimal": "low"}
+
     def __init__(
         self, api_key: str, base_url: Optional[str] = None, model: Optional[str] = None
     ):
diff --git a/src/query/query.py b/src/query/query.py
index ab0c90e5..39f0f443 100644
--- a/src/query/query.py
+++ b/src/query/query.py
@@ -1217,6 +1217,37 @@ async def _call_model_sync(
             getattr(provider, "model", None) or call_kwargs.get("model"),
             clamp_xhigh=False,
         )
+        # Translate onto the provider's own vocabulary before it hits the
+        # wire. The ladder above is Anthropic's; a provider that does not
+        # share it silently DISCARDS the unknown level and applies its own
+        # default, so the request goes out looking fine and the user gets a
+        # level they did not choose. Identity for providers that take the
+        # full ladder (OpenAI, OpenRouter) — see
+        # ``BaseProvider.normalize_reasoning_effort``.
+        #
+        # No explicit unwrap here, deliberately: ``FusionProvider.__getattr__``
+        # already delegates unknown attributes to its base, so this lookup
+        # lands on the BASE provider's method and its vocabulary. (Contrast
+        # ``is_anthropic_wire``, which cannot delegate — an ``isinstance``
+        # test sees the wrapper's class and needs ``unwrap_provider``.) The
+        # delegation is pinned by a test rather than left incidental.
+        # The result is VALIDATED before use, not trusted. This is a duck-typed
+        # ``getattr`` on whatever object the caller passed, and providers are
+        # not all real ``BaseProvider`` subclasses — mocks, gateway shims and
+        # third-party wrappers all reach here. A ``MagicMock`` in particular
+        # answers every attribute with a callable returning another Mock, so an
+        # unguarded assignment writes a ``<MagicMock ...>`` repr into the
+        # request body. Anything that is not a plain string on the known ladder
+        # is discarded in favour of the level we already resolved.
+        if resolved_effort is not None:
+            _normalize = getattr(provider, "normalize_reasoning_effort", None)
+            if callable(_normalize):
+                try:
+                    _mapped = _normalize(resolved_effort)
+                except Exception:  # noqa: BLE001 — never fail a request on this
+                    _mapped = None
+                if isinstance(_mapped, str) and _mapped in VALID_THINKING_EFFORT_LEVELS:
+                    resolved_effort = _mapped
         if resolved_effort is not None:
             extra_body = dict(call_kwargs.get("extra_body") or {})
             extra_body.setdefault("reasoning_effort", resolved_effort)
diff --git a/tests/test_deepseek_effort_vocabulary.py b/tests/test_deepseek_effort_vocabulary.py
new file mode 100644
index 00000000..839505d0
--- /dev/null
+++ b/tests/test_deepseek_effort_vocabulary.py
@@ -0,0 +1,230 @@
+"""Reasoning effort must reach the wire in the PROVIDER's own vocabulary.
+
+clawcodex exposes ``low | medium | high | xhigh | max`` — Anthropic's ladder.
+DeepSeek's OpenAI-format thinking mode accepts only ``low | high | max``
+(api-docs.deepseek.com/guides/thinking_mode), and it does NOT validate the
+field: probed 2026-08-03 against ``deepseek-v4-flash``, every one of
+low/medium/high/xhigh/max/minimal returned 200. An unsupported level is
+therefore silently discarded and the provider's default (``high``) applies —
+the request looks fine and the user gets a level they did not ask for.
+
+The damaging direction is downward: ``xhigh`` means "more than high", and
+dropping it delivers ``high`` when ``max`` was available — on the setting
+people reach for precisely when a task is hard.
+
+Also covers the wrapper case, because that is how the fusion models are
+built: a ``FusionProvider`` must consult its BASE provider's vocabulary and
+its base's wire family, not the wrapper's own (empty) defaults.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import types
+
+import pytest
+
+from src.providers import is_anthropic_wire, unwrap_provider
+from src.providers.anthropic_provider import AnthropicProvider
+from src.providers.deepseek_provider import DeepSeekProvider
+from src.providers.fusion_models import FusionModel, ModelRef
+from src.providers.fusion_provider import FusionProvider
+from src.providers.openrouter_provider import OpenRouterProvider
+
+
+def _fusion(base):
+    return FusionProvider(
+        base,
+        FusionModel(
+            name="deepseek-v4-flash-luna",
+            base=ModelRef("deepseek", "deepseek-v4-flash"),
+            vision=ModelRef("openai", "gpt-5.6-luna"),
+        ),
+    )
+
+
+def _deepseek():
+    return DeepSeekProvider(api_key="k", model="deepseek-v4-flash")
+
+
+# --- vocabulary translation ------------------------------------------------
+
+
+@pytest.mark.parametrize(
+    "requested,on_the_wire",
+    [
+        ("low", "low"),
+        ("high", "high"),
+        ("max", "max"),
+        # No DeepSeek equivalent. Already behaved as `high` (unknown ->
+        # default), so this makes the existing behaviour explicit.
+        ("medium", "high"),
+        # The real fix: "above high" must not silently become "high".
+        ("xhigh", "max"),
+    ],
+)
+def test_deepseek_translates_onto_its_own_ladder(requested, on_the_wire):
+    assert _deepseek().normalize_reasoning_effort(requested) == on_the_wire
+
+
+@pytest.mark.parametrize("effort", ["low", "medium", "high", "xhigh", "max"])
+def test_providers_taking_the_full_ladder_are_untouched(effort):
+    """The default must be identity. OpenRouter accepts all five, and
+    rewriting them there would be inventing a restriction that does not
+    exist — the failure this fix is meant to prevent, inverted."""
+    p = OpenRouterProvider(api_key="k", model="openai/gpt-5.6-luna")
+    assert p.normalize_reasoning_effort(effort) == effort
+
+
+def test_a_provider_with_no_declared_vocabulary_passes_through():
+    """Pre-existing behaviour for anything that has not opted in."""
+
+    class _Bare:
+        supported_reasoning_efforts = None
+        reasoning_effort_aliases: dict[str, str] = {}
+        normalize_reasoning_effort = (
+            DeepSeekProvider.normalize_reasoning_effort  # unbound, shared impl
+        )
+
+    assert _Bare().normalize_reasoning_effort("xhigh") == "xhigh"
+    assert _Bare().normalize_reasoning_effort(None) is None
+
+
+# --- wrapper transparency --------------------------------------------------
+
+
+def test_unwrap_reaches_the_base_provider():
+    base = _deepseek()
+    assert unwrap_provider(_fusion(base)) is base
+    assert unwrap_provider(base) is base
+
+
+def test_wire_family_follows_the_fusion_BASE():
+    """`is_anthropic_wire` used an isinstance test, and FusionProvider's MRO
+    is (FusionProvider, object) — so EVERY fusion model reported
+    OpenAI-compatible, including one whose base is Anthropic. That sends a
+    top-level `reasoning_effort` to the Anthropic API, which rejects it with
+    `400 ... Extra inputs are not permitted`, and prepends the system prompt
+    as a message instead of passing the `system` kwarg.
+    """
+    assert is_anthropic_wire(_fusion(_deepseek())) is False
+    anthropic_base = AnthropicProvider(api_key="k", model="claude-opus-5")
+    assert is_anthropic_wire(_fusion(anthropic_base)) is True
+    assert is_anthropic_wire(anthropic_base) is True
+
+
+def test_fusion_delegates_the_vocabulary_lookup_to_its_base():
+    """The wire boundary looks `normalize_reasoning_effort` up on whatever
+    provider it was handed, with no explicit unwrap — that is only correct
+    because `FusionProvider.__getattr__` delegates to the base. Pinned here
+    so the delegation is a tested property rather than an incidental one: if
+    the wrapper ever grows its own attribute (or the `__getattr__` guard list
+    changes), a fusion model would silently start sending the untranslated
+    clawcodex level.
+    """
+    fused = _fusion(_deepseek())
+    assert fused.normalize_reasoning_effort("xhigh") == "max"
+    assert fused.supported_reasoning_efforts == ("low", "high", "max")
+
+
+def test_unwrap_terminates_on_a_self_referential_wrapper():
+    """A cycle must not hang the wire check."""
+
+    class _Loop:
+        pass
+
+    a = _Loop()
+    a.inner = a
+    assert unwrap_provider(a) is a
+
+    b, c = _Loop(), _Loop()
+    b.inner, c.inner = c, b
+    assert unwrap_provider(b) in (b, c)  # terminates; which end is arbitrary
+
+
+# --- the whole chain, at the wire boundary ---------------------------------
+
+
+def _wire_kwargs(provider, effort):
+    """Drive the real `_call_model_sync` and capture what it would send."""
+    import importlib
+    import sys
+
+    importlib.import_module("src.query.query")
+    Q = sys.modules["src.query.query"]
+
+    captured: dict = {}
+
+    def _fake_stream(*args, **kwargs):
+        captured.update(kwargs)
+        return types.SimpleNamespace(
+            content="ok", model="m", usage={}, finish_reason="stop",
+            tool_uses=None, reasoning_content=None, raw_content_blocks=None,
+        )
+
+    target = unwrap_provider(provider)
+    target.chat_stream_response = _fake_stream  # type: ignore[method-assign]
+    try:
+        asyncio.run(
+            Q._call_model_sync(
+                provider=provider,
+                messages=[{"role": "user", "content": "hi"}],
+                system_prompt="s",
+                tools=[],
+                thinking_effort=effort,
+            )
+        )
+    except Exception:  # noqa: BLE001 — only the captured kwargs matter
+        pass
+    return captured
+
+
+@pytest.mark.parametrize(
+    "requested,on_the_wire",
+    [("low", "low"), ("medium", "high"), ("high", "high"),
+     ("xhigh", "max"), ("max", "max")],
+)
+def test_effort_reaches_the_deepseek_wire_translated(requested, on_the_wire):
+    kwargs = _wire_kwargs(_deepseek(), requested)
+    assert (kwargs.get("extra_body") or {}).get("reasoning_effort") == on_the_wire
+
+
+@pytest.mark.parametrize(
+    "requested,on_the_wire",
+    [("medium", "high"), ("xhigh", "max"), ("max", "max")],
+)
+def test_a_fusion_model_uses_its_BASE_providers_vocabulary(requested, on_the_wire):
+    """The fusion wrapper declares no vocabulary of its own; without
+    unwrapping, every fusion model would send the raw clawcodex level."""
+    kwargs = _wire_kwargs(_fusion(_deepseek()), requested)
+    assert (kwargs.get("extra_body") or {}).get("reasoning_effort") == on_the_wire
+
+
+def test_a_bogus_normalize_hook_cannot_corrupt_the_wire():
+    """The wire boundary duck-types this hook via `getattr`, and not every
+    provider-shaped object is a real BaseProvider — mocks, gateway shims and
+    third-party wrappers all reach it. A MagicMock answers EVERY attribute
+    with a callable returning another Mock, which without validation writes a
+    `<MagicMock ...>` repr into the request body as the effort level.
+    """
+    from unittest.mock import MagicMock
+
+    provider = MagicMock()
+    provider.model = "some-model"
+    # A bare MagicMock also answers `.inner` with a Mock, which would send
+    # `unwrap_provider` walking a chain that never ends in a real object.
+    # Pin it so this test exercises the normalize guard, not the unwrap loop.
+    provider.inner = None
+    kwargs = _wire_kwargs(provider, "xhigh")
+    assert (kwargs.get("extra_body") or {}).get("reasoning_effort") == "xhigh"
+
+
+def test_a_raising_normalize_hook_falls_back_instead_of_failing_the_turn():
+    base = _deepseek()
+
+    def _boom(_effort):
+        raise RuntimeError("provider bug")
+
+    base.normalize_reasoning_effort = _boom  # type: ignore[method-assign]
+    kwargs = _wire_kwargs(base, "max")
+    assert (kwargs.get("extra_body") or {}).get("reasoning_effort") == "max"