feat(R03): unify model routing and centralise the provider catalogue

EPIC R03 (Team Duy) — Model Providers & Routing. All six tasks done.

R03-T02 — Provider catalogue
  domain/models/provider_descriptor.py     ProviderDescriptor (frozen), WireProtocol, AuthKind
  infrastructure/providers/provider_registry.py
                                           thread-safe registry: id/alias lookup, dynamic
                                           lookup by model id, adapter selection by protocol
  providers/factory.py                     drops its own _REGISTRY table and delegates to the
                                           registry, still raising ProviderError for callers

R03-T03 — RoutingApplicationService (pure Python, 4 modes)
  application/model_routing/routing_models.py
                                           RoutingMode (off/auto/manual/fallback),
                                           RoutingRequest (immutable snapshot), RouteEvaluation,
                                           RoutingOutcome
  application/model_routing/routing_application_service.py
                                           the single decision flow, reached through two narrow
                                           ports plus a caller-supplied confirm callback, so no
                                           Qt import is needed
  application/model_routing/core_routing_adapter.py
                                           binds the ports to core/routing and AppContext

  Fallback is a new resilience mode: keep the selected model while it can serve the turn,
  re-route only when it cannot. Wired end to end through config.py, state.py,
  ui/routing_toggle.py and i18n.py (EN/JA/VI).

R03-T04 / T05 — Remove the duplicated routing flow
  ui/chat_panel.py (#L638), ui/co4e_tab.py, ui/folder_tab.py each drop ~35 lines of copied
  logic and call the shared service; the widgets now only build a RoutingRequest, host the
  Manual-mode modal and render the outcome.

R03-T06 — Token usage as an event
  infrastructure/telemetry/usage_sink.py   UsageEvent + UsageEventSink protocol, with tracker,
                                           in-memory and composite sinks
  providers/openai_compat.py, providers/anthropic.py
                                           publish a UsageEvent instead of writing to the
                                           usage tracker themselves
  core/usage_tracker.py                    adds current_context() so a sink can borrow and
                                           restore a thread's attribution

R03-T01 — Contract tests
  tests/contracts/test_providers.py parametrises over every provider in the registry: chat()
  signature, canonical assistant message, normalised tool calls, response closed, tool schema
  translation, ProviderError, list_models/test_connection, one UsageEvent per turn.

Test infrastructure fix (required to verify any of the above): tests/conftest.py used to put
the repository's PARENT directory on sys.path, so `import cowork_local.*` resolved against
whichever sibling folder happened to carry that name — on a dev machine, an unrelated older
checkout. The suite reported green while exercising different code. The conftest now binds
this checkout to the cowork_local name in sys.modules.

Verification
  pytest tests/                    236 passed in ~1.8s (102 before this change)
  scripts/check_imports.py         PASS, 0 forbidden imports in domain/ and application/
  new production files             largest is 288 lines, all under the 400 LOC ceiling
  new tests                        134 (50 contract, 70 unit, 14 integration), all offline

scripts/run_quality_gate.py does not exist yet (R10-T02), so DoD item 7 was covered by
check_imports.py plus the full suite.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

application/model_routing/__init__.py

@@ -1 +1,54 @@
-"""Application model routing package: model route decisions and multi-provider balancing."""
+"""Application model routing package: model route decisions and multi-provider balancing.
+
+Public surface (R03-T03 — the single routing entry point every chat surface uses):
+
+* :class:`RoutingApplicationService` — decides one turn's provider/model.
+* :class:`RoutingRequest` / :class:`RoutingOutcome` — the immutable DTOs in and out.
+* :class:`RoutingMode` — Off / Auto / Manual / Fallback.
+* :func:`build_routing_application_service` — wires the service to a live
+  ``AppContext`` (engine + per-workspace mode + confirm timeout).
+
+Typical call site (see ``ui/chat_panel.py::_apply_routing``)::
+
+    service = build_routing_application_service(self.ctx)
+    outcome = service.resolve(
+        RoutingRequest(surface="cowork", prompt=text,
+                       current_provider=provider, current_model=model),
+        confirm=lambda decision, timeout: confirm_switch(self, decision, timeout),
+    )
+
+Only ``core_routing_adapter`` touches ``core/routing``; the service and the DTOs
+stay pure Python so the whole rule set is testable without Qt or the engine.
+"""
+
+from .core_routing_adapter import (
+    AppContextModeResolver,
+    CoreRoutingEngine,
+    build_routing_application_service,
+)
+from .routing_application_service import (
+    ConfirmationCallback,
+    ModeResolver,
+    RoutingApplicationService,
+    RoutingDecisionPort,
+)
+from .routing_models import (
+    RouteEvaluation,
+    RoutingMode,
+    RoutingOutcome,
+    RoutingRequest,
+)
+
+__all__ = [
+    "AppContextModeResolver",
+    "ConfirmationCallback",
+    "CoreRoutingEngine",
+    "ModeResolver",
+    "RouteEvaluation",
+    "RoutingApplicationService",
+    "RoutingDecisionPort",
+    "RoutingMode",
+    "RoutingOutcome",
+    "RoutingRequest",
+    "build_routing_application_service",
+]

application/model_routing/core_routing_adapter.py

@@ -0,0 +1,169 @@
+"""Adapters that plug the existing routing engine into the application service.
+
+:mod:`routing_application_service` is written against two narrow ports so it can
+be unit-tested with plain fakes. This module supplies the real implementations —
+the assessment/scoring engine in ``core/routing`` and the per-workspace mode
+lookup on ``AppContext`` — and is therefore the ONLY file in
+``application/model_routing/`` that knows those concrete types exist.
+
+All engine imports are deferred into method bodies. Importing the routing stack
+pulls in Pydantic models and the on-disk assessment store, and the UI must be
+able to import this module during startup without paying that cost (the same
+lazy-wiring reason ``state.py::AppContext.routing`` gives).
+"""
+
+from __future__ import annotations
+
+import logging
+from typing import Any, Optional
+
+from .routing_application_service import RoutingApplicationService
+from .routing_models import RouteEvaluation, RoutingMode, RoutingRequest
+
+logger = logging.getLogger("cowork_local.application.model_routing")
+
+
+class CoreRoutingEngine:
+    """:class:`RoutingDecisionPort` backed by ``core/routing/service.py``.
+
+    Translates in both directions: application DTOs in, and the engine's
+    ``RouteResult``/``SwitchDecision``/``TaskType`` flattened back out into a
+    :class:`RouteEvaluation`, so no ``core.routing`` type ever escapes into the
+    application service or the UI call sites.
+    """
+
+    def __init__(self, routing_service: Any) -> None:
+        self._routing_service = routing_service
+
+    def evaluate(self, request: RoutingRequest, mode: RoutingMode) -> RouteEvaluation:
+        """Rank candidates for this turn and report the engine's verdict."""
+        from ...core.routing.models import TaskType, candidate_key
+
+        result = self._routing_service.route(
+            request.surface,
+            request.prompt,
+            request.current_provider,
+            request.current_model,
+            # The engine only knows off/auto/manual; FALLBACK was already mapped
+            # to AUTO upstream so the value handed over here is always valid.
+            mode_override=mode.value,
+            required_capabilities=list(request.required_capabilities) or None,
+            task_type=self._parse_task_type(request.task_type, TaskType),
+        )
+
+        decision = result.decision
+        target = result.target()  # (provider, model_id) or None
+        current_key = (
+            candidate_key(request.current_provider, request.current_model)
+            if request.current_model
+            else ""
+        )
+        return RouteEvaluation(
+            task_type=self._task_type_value(result.task_type),
+            should_switch=bool(result.should_switch),
+            target_provider=target[0] if target else None,
+            target_model=target[1] if target else None,
+            score_gain=float(getattr(decision, "score_gain", 0.0) or 0.0),
+            reason=str(getattr(decision, "reason", "") or ""),
+            current_is_usable=self._current_is_usable(result, current_key),
+            decision=decision,
+        )
+
+    # -- translation helpers --------------------------------------------- #
+    @staticmethod
+    def _parse_task_type(raw: Optional[str], task_type_enum) -> Optional[Any]:
+        """Coerce a task-type string to the engine's enum.
+
+        ``None`` (the common case) means "let the engine classify the prompt".
+        An unrecognised string is also downgraded to ``None`` rather than
+        raising, so a stale value in a saved workspace cannot break a turn.
+        """
+        if raw is None:
+            return None
+        if isinstance(raw, task_type_enum):
+            return raw
+        try:
+            return task_type_enum(str(raw).strip().lower())
+        except ValueError:
+            logger.warning("routing: unknown task type %r — classifying from the prompt", raw)
+            return None
+
+    @staticmethod
+    def _task_type_value(task_type: Any) -> str:
+        """The plain string form of the engine's task type enum."""
+        return str(getattr(task_type, "value", task_type) or "")
+
+    @staticmethod
+    def _current_is_usable(result: Any, current_key: str) -> bool:
+        """Whether the currently selected model can still serve this task.
+
+        This is the signal FALLBACK mode acts on. A model is usable when the
+        ranking scored it above zero; ``rank_models`` already drops candidates
+        that are unavailable, lack a probe for this task type, or failed their
+        last probe, so "absent from the ranking" is precisely "cannot serve it".
+
+        With no ranking (routing off, or the engine's internal error path) or no
+        current model, we answer True: absence of evidence must not trigger a
+        surprise switch in a mode whose whole promise is not to surprise.
+        """
+        ranking = getattr(result, "ranking", None)
+        if ranking is None or not current_key:
+            return True
+        try:
+            return float(ranking.score_of(current_key)) > 0.0
+        except Exception:  # noqa: BLE001 — defensive: never fail a turn on telemetry-ish data
+            logger.debug("routing: could not score current model %r", current_key, exc_info=True)
+            return True
+
+
+class AppContextModeResolver:
+    """:class:`ModeResolver` backed by the active workspace's settings.
+
+    Reads through ``AppContext.project_routing_mode``, which already layers the
+    workspace override on top of the global default — so per-workspace routing
+    modes keep working unchanged now that the mode lookup moved out of the
+    widgets.
+    """
+
+    def __init__(self, ctx: Any) -> None:
+        self._ctx = ctx
+
+    def mode_for(self, surface: str) -> RoutingMode:
+        """Effective mode for ``surface`` in the active workspace."""
+        return RoutingMode.parse(self._ctx.project_routing_mode(surface))
+
+
+def build_routing_application_service(ctx: Any) -> RoutingApplicationService:
+    """The shared :class:`RoutingApplicationService` for this app context.
+
+    Cached on the context (like ``AppContext.routing()`` caches the engine) so
+    every surface talks to the same instance and a future stateful addition —
+    per-surface cool-down, switch history — is shared rather than duplicated per
+    widget. Falls back to a fresh instance if the context refuses attribute
+    assignment, which keeps tests using lightweight stand-ins working.
+    """
+    cached = getattr(ctx, "_routing_app_service", None)
+    if cached is not None:
+        return cached
+
+    service = RoutingApplicationService(
+        CoreRoutingEngine(ctx.routing()),
+        AppContextModeResolver(ctx),
+        # Read at call time: the user can change the confirm timeout in Settings
+        # between two turns and the next Manual dialog should honour it.
+        confirm_timeout_sec=lambda: float(
+            (ctx.config.routing or {}).get("confirm_timeout_sec", 60) or 60
+        ),
+    )
+    try:
+        ctx._routing_app_service = service
+    except Exception:  # noqa: BLE001 — read-only/slotted stand-ins stay supported
+        logger.debug("routing: could not cache the application service on the context", exc_info=True)
+    return service
+
+
+__all__ = [
+    "AppContextModeResolver",
+    "CoreRoutingEngine",
+    "build_routing_application_service",
+]

application/model_routing/routing_application_service.py

@@ -0,0 +1,236 @@
+"""The one place that decides how a turn is routed (R03-T03).
+
+Before this service, ``ui/chat_panel.py#L638``, ``ui/co4e_tab.py`` and
+``ui/folder_tab.py`` each carried their own copy of the same eight-step dance:
+clear last turn's override → read the surface's mode → bail on "off" → call the
+routing engine → check ``should_switch`` → resolve the target → show the Manual
+confirm dialog → publish the override and a status line. Three copies meant
+three chances to drift, and none of them could be tested without a Qt widget.
+
+The dance now lives here, once, in pure Python:
+
+* the routing engine is reached through :class:`RoutingDecisionPort`;
+* the surface's Off/Auto/Manual/Fallback mode through :class:`ModeResolver`;
+* the Manual-mode confirmation through a ``confirm`` callback supplied per call,
+  so the Qt dialog stays in the presentation layer where it belongs.
+
+Every failure path degrades to "keep the current model": a routing problem must
+never be the reason a user cannot send a message.
+"""
+
+from __future__ import annotations
+
+import logging
+from typing import Any, Callable, Optional, Protocol, runtime_checkable
+
+from .routing_models import (
+    RouteEvaluation,
+    RoutingMode,
+    RoutingOutcome,
+    RoutingRequest,
+)
+
+logger = logging.getLogger("cowork_local.application.model_routing")
+
+# Asks the user to approve a Manual-mode switch. Receives the underlying
+# decision object (for rendering) plus the timeout in seconds; returns True to
+# approve. Supplied by the caller so this module never imports a UI toolkit.
+ConfirmationCallback = Callable[[Any, float], bool]
+
+
+@runtime_checkable
+class RoutingDecisionPort(Protocol):
+    """The routing engine, as this service needs it.
+
+    Narrowed to a single method on purpose: the concrete engine
+    (``core/routing/service.py::RoutingService``) exposes assessment,
+    persistence and scheduling too, none of which a turn-time decision needs.
+    """
+
+    def evaluate(self, request: RoutingRequest, mode: RoutingMode) -> RouteEvaluation:
+        """Rank candidates for ``request`` and report whether to switch."""
+
+
+@runtime_checkable
+class ModeResolver(Protocol):
+    """Resolves the effective routing mode for a surface.
+
+    In the app this reads the active workspace's per-surface override with the
+    global default behind it (``AppContext.project_routing_mode``); in tests it
+    is a two-line stub.
+    """
+
+    def mode_for(self, surface: str) -> RoutingMode:
+        """Effective mode for ``surface``."""
+
+
+class RoutingApplicationService:
+    """Turn-time routing decisions for every chat surface."""
+
+    # Matches DEFAULT_CONFIG["routing"]["confirm_timeout_sec"]; used only when
+    # no timeout provider is wired, so a bare service is still usable in tests.
+    DEFAULT_CONFIRM_TIMEOUT_SEC = 60.0
+
+    def __init__(
+        self,
+        decision_port: RoutingDecisionPort,
+        mode_resolver: Optional[ModeResolver] = None,
+        *,
+        confirm_timeout_sec: Optional[Callable[[], float]] = None,
+    ) -> None:
+        self._decision_port = decision_port
+        self._mode_resolver = mode_resolver
+        # A callable rather than a number: the timeout lives in mutable config
+        # the user can change in Settings between two turns.
+        self._confirm_timeout_sec = confirm_timeout_sec
+
+    # -- public API ------------------------------------------------------ #
+    def resolve(
+        self,
+        request: RoutingRequest,
+        confirm: Optional[ConfirmationCallback] = None,
+    ) -> RoutingOutcome:
+        """Decide this turn's provider/model.
+
+        Returns a :class:`RoutingOutcome`; ``provider``/``model`` are ``None``
+        whenever the surface should keep its own selection. Never raises — an
+        unexpected failure is logged and reported as "keep current", because a
+        broken assessment store must not block chatting.
+        """
+        mode = request.mode or self._resolve_mode(request.surface)
+        try:
+            return self._resolve_unguarded(request, mode, confirm)
+        except Exception:  # noqa: BLE001 — routing must never break a turn
+            logger.exception("routing.resolve failed — keeping the current model")
+            return RoutingOutcome.keep_current(mode, reason="routing error — keeping current model")
+
+    def confirm_timeout(self) -> float:
+        """Seconds to wait for a Manual-mode confirmation.
+
+        Falls back to the built-in default when the provider is missing or
+        returns something unusable, so a corrupted config value cannot produce a
+        zero-second dialog that instantly declines every switch.
+        """
+        if self._confirm_timeout_sec is None:
+            return self.DEFAULT_CONFIRM_TIMEOUT_SEC
+        try:
+            value = float(self._confirm_timeout_sec())
+        except (TypeError, ValueError):
+            return self.DEFAULT_CONFIRM_TIMEOUT_SEC
+        return value if value > 0 else self.DEFAULT_CONFIRM_TIMEOUT_SEC
+
+    # -- internals ------------------------------------------------------- #
+    def _resolve_mode(self, surface: str) -> RoutingMode:
+        """The surface's configured mode, defaulting to OFF when unresolvable —
+        routing stays opt-in, so "we don't know" must mean "don't switch"."""
+        if self._mode_resolver is None:
+            return RoutingMode.OFF
+        try:
+            return RoutingMode.parse(self._mode_resolver.mode_for(surface))
+        except Exception:  # noqa: BLE001 — a config read must not break a turn
+            logger.exception("routing: could not resolve mode for surface %r", surface)
+            return RoutingMode.OFF
+
+    def _resolve_unguarded(
+        self,
+        request: RoutingRequest,
+        mode: RoutingMode,
+        confirm: Optional[ConfirmationCallback],
+    ) -> RoutingOutcome:
+        """The decision flow proper; :meth:`resolve` owns the safety net."""
+        # 1. Routing disabled, or nothing to classify -> keep the selection.
+        if mode is RoutingMode.OFF:
+            return RoutingOutcome.keep_current(mode, reason="routing off")
+        if not request.has_prompt:
+            return RoutingOutcome.keep_current(mode, reason="empty prompt — nothing to route")
+
+        # 2. Ask the engine. FALLBACK is evaluated with AUTO's ranking because
+        #    it needs the same candidate list; only the accept/reject rule below
+        #    differs, so the engine stays unaware of the extra mode.
+        engine_mode = RoutingMode.AUTO if mode is RoutingMode.FALLBACK else mode
+        evaluation = self._decision_port.evaluate(request, engine_mode)
+
+        # 3. Apply the mode's own accept rule to the engine's verdict.
+        if mode is RoutingMode.FALLBACK:
+            accepted, reason = self._fallback_verdict(evaluation)
+        else:
+            accepted, reason = evaluation.should_switch, evaluation.reason
+
+        if not accepted or not evaluation.has_target:
+            return RoutingOutcome.keep_current(
+                mode,
+                reason=reason or evaluation.reason,
+                task_type=evaluation.task_type,
+                decision=evaluation.decision,
+            )
+
+        # 4. Manual mode asks first; a decline or a timeout keeps the current
+        #    model (and is reported as such, so the surface can tell the two
+        #    cases apart from "nothing better was found").
+        if mode is RoutingMode.MANUAL and not self._approved(evaluation, confirm):
+            return RoutingOutcome.keep_current(
+                mode,
+                reason="switch declined by user or confirmation timed out",
+                task_type=evaluation.task_type,
+                declined=True,
+                decision=evaluation.decision,
+            )
+
+        # 5. Publish the override for THIS turn only. The provider falls back to
+        #    the request's current provider when the engine named a model but no
+        #    provider (same-provider switch).
+        return RoutingOutcome(
+            mode=mode,
+            switched=True,
+            provider=evaluation.target_provider or request.current_provider,
+            model=evaluation.target_model or "",
+            task_type=evaluation.task_type,
+            score_gain=evaluation.score_gain,
+            reason=reason or evaluation.reason,
+            decision=evaluation.decision,
+        )
+
+    @staticmethod
+    def _fallback_verdict(evaluation: RouteEvaluation) -> tuple:
+        """FALLBACK's accept rule: switch ONLY to rescue an unusable selection.
+
+        The user's pinned model wins as long as it can serve the turn, even when
+        a higher-scoring candidate exists — that is the whole point of the mode.
+        A switch happens only when the current model is not a usable candidate
+        (never assessed, marked unavailable, or its last probe failed) and the
+        engine has something to move to.
+        """
+        if evaluation.current_is_usable:
+            return False, "fallback mode — current model is healthy, keeping it"
+        if not evaluation.has_target:
+            return False, "fallback mode — current model unusable and no replacement available"
+        return True, "fallback mode — current model unavailable, switching to the best alternative"
+
+    def _approved(
+        self,
+        evaluation: RouteEvaluation,
+        confirm: Optional[ConfirmationCallback],
+    ) -> bool:
+        """Run the Manual-mode confirmation callback.
+
+        No callback means no way to ask, and silently switching in Manual mode
+        would violate the mode's contract — so a missing callback is treated as
+        "not approved". A callback that raises is treated the same way, since a
+        broken dialog must not auto-approve a model change.
+        """
+        if confirm is None:
+            logger.warning("routing: manual mode without a confirmation callback — keeping current model")
+            return False
+        try:
+            return bool(confirm(evaluation.decision, self.confirm_timeout()))
+        except Exception:  # noqa: BLE001
+            logger.exception("routing: confirmation callback failed — keeping current model")
+            return False
+
+
+__all__ = [
+    "ConfirmationCallback",
+    "ModeResolver",
+    "RoutingApplicationService",
+    "RoutingDecisionPort",
+]

application/model_routing/routing_models.py

@@ -0,0 +1,158 @@
+"""Pure-Python DTOs exchanged with :mod:`routing_application_service`.
+
+These types are the vocabulary the chat surfaces (Cowork chat, Co4E, AI-Edit)
+now speak instead of each re-deriving routing state from raw config lookups and
+``core/routing`` internals.
+
+Layer rules (``docs/architecture/ADR-001-layered-architecture.md``): application
+code is 100% pure Python. Nothing here imports PySide6, and nothing here imports
+``core.routing`` either — the concrete routing engine is reached only through
+the adapter in :mod:`core_routing_adapter`, which keeps this module trivially
+testable with plain fakes.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from enum import Enum
+from typing import Any, Optional, Tuple
+
+
+class RoutingMode(str, Enum):
+    """The four routing behaviours a surface can be in (R03-T03).
+
+    ``OFF``/``AUTO``/``MANUAL`` map 1:1 onto the existing per-surface toggle and
+    onto ``core/routing/models.py::SwitchMode``. ``FALLBACK`` is new and
+    deliberately NOT an optimisation mode: it keeps whatever model the user
+    chose and only re-routes when that model cannot serve the turn, which is the
+    behaviour a resilience-minded workspace wants (never surprise me, but never
+    leave me stuck either).
+    """
+
+    OFF = "off"
+    AUTO = "auto"
+    MANUAL = "manual"
+    FALLBACK = "fallback"
+
+    @classmethod
+    def parse(cls, raw: Any, default: "RoutingMode" = None) -> "RoutingMode":
+        """Best-effort coercion from config/UI strings.
+
+        Routing must never break a turn, so an unrecognised value degrades to
+        ``default`` (``OFF`` unless told otherwise) instead of raising — the same
+        defensive posture ``config.routing_mode_for`` already takes.
+        """
+        fallback = default if default is not None else cls.OFF
+        if isinstance(raw, cls):
+            return raw
+        try:
+            return cls(str(raw or "").strip().lower())
+        except ValueError:
+            return fallback
+
+
+@dataclass(frozen=True)
+class RoutingRequest:
+    """Everything needed to decide how ONE turn should be routed.
+
+    Frozen: the request is captured from live UI state (the selected model, the
+    typed prompt) and then handed to code that may run on a worker thread. An
+    immutable snapshot means the user changing the model picker mid-turn cannot
+    retroactively alter the decision that was already made — the same rationale
+    behind R04's ``ConversationExecutionRequest``.
+    """
+
+    surface: str                       # "cowork" | "co4e" | "ai_edit" | ...
+    prompt: str                        # the user's text; drives task classification
+    current_provider: str              # provider the surface would use as-is
+    current_model: str = ""            # model the surface would use ("" = provider default)
+    mode: Optional[RoutingMode] = None  # explicit override; None -> resolve per surface
+    # Pre-classified task type ("coding", "qa", ...). AI-Edit always knows its
+    # turns are coding work, so it pins this and skips prompt classification.
+    task_type: Optional[str] = None
+    required_capabilities: Tuple[str, ...] = ()   # e.g. ("vision",)
+
+    @property
+    def has_prompt(self) -> bool:
+        """Whether there is anything to classify. An empty prompt cannot be
+        routed meaningfully, so every surface short-circuits on it."""
+        return bool((self.prompt or "").strip())
+
+
+@dataclass(frozen=True)
+class RouteEvaluation:
+    """A routing engine's verdict, normalised away from ``core/routing`` types.
+
+    The adapter flattens ``RouteResult``/``SwitchDecision`` into these plain
+    fields so the application service never touches Pydantic models or enums
+    owned by another layer. ``decision`` still carries the original object
+    because the Manual-mode confirm dialog renders its ``reason``.
+    """
+
+    task_type: str
+    should_switch: bool
+    target_provider: Optional[str] = None
+    target_model: Optional[str] = None
+    score_gain: float = 0.0
+    reason: str = ""
+    # False when the currently selected model is not a usable candidate for this
+    # task (unranked, unavailable, or failed its last probe) — the single signal
+    # FALLBACK mode acts on.
+    current_is_usable: bool = True
+    decision: Any = None               # original SwitchDecision, for the UI dialog
+
+    @property
+    def has_target(self) -> bool:
+        """A switch is only actionable when the engine named a model to move to."""
+        return bool(self.target_model or self.target_provider)
+
+
+@dataclass(frozen=True)
+class RoutingOutcome:
+    """What the calling surface should actually do for this turn.
+
+    A surface needs exactly three things from routing — "which provider/model do
+    I build?", "do I tell the user?" and "was I told to stand down?" — so those
+    are the fields here, and nothing else. ``provider``/``model`` are ``None``
+    when the surface should keep its own selection untouched.
+    """
+
+    mode: RoutingMode
+    switched: bool = False
+    provider: Optional[str] = None
+    model: Optional[str] = None
+    task_type: str = ""
+    score_gain: float = 0.0
+    reason: str = ""
+    # True when Manual mode proposed a switch and the user declined or the
+    # confirmation timed out. Distinct from "no switch proposed" so a surface
+    # can tell "routing had nothing to offer" from "the user said no".
+    declined: bool = False
+    decision: Any = field(default=None, repr=False)
+
+    @property
+    def should_notify(self) -> bool:
+        """Whether the surface should post the "switched model" status bubble.
+        Only an executed switch is worth interrupting the transcript for."""
+        return self.switched
+
+    @classmethod
+    def keep_current(
+        cls,
+        mode: RoutingMode,
+        *,
+        reason: str = "",
+        task_type: str = "",
+        declined: bool = False,
+        decision: Any = None,
+    ) -> "RoutingOutcome":
+        """The no-change outcome — the single constructor for every path that
+        leaves the surface's own model selection in place (routing off, empty
+        prompt, no better candidate, user declined, internal error)."""
+        return cls(
+            mode=mode, switched=False, provider=None, model=None,
+            task_type=task_type, reason=reason, declined=declined, decision=decision,
+        )
+
+
+__all__ = ["RoutingMode", "RoutingRequest", "RouteEvaluation", "RoutingOutcome"]