diff --git a/_docs/02_document/contracts/c3_5_adhop/conditional_refiner_protocol.md b/_docs/02_document/contracts/c3_5_adhop/conditional_refiner_protocol.md
index 620619b..18534ed 100644
--- a/_docs/02_document/contracts/c3_5_adhop/conditional_refiner_protocol.md
+++ b/_docs/02_document/contracts/c3_5_adhop/conditional_refiner_protocol.md
@@ -4,8 +4,8 @@
 **Producer task**: AZ-348 (Protocol + factory + DTOs + composition + `PassthroughRefiner`)
 **Consumer tasks**: AZ-349 (`AdHoPRefiner` real refinement); downstream c4_pose (epic AZ-259) which consumes the (possibly refined) `MatchResult`
 **Version**: 1.0.0
-**Status**: draft, awaiting Producer task implementation
-**Last Updated**: 2026-05-10
+**Status**: v1.0.0 (AZ-348 implemented 2026-05-12; PassthroughRefiner shipped — AdHoPRefiner pending AZ-349)
+**Last Updated**: 2026-05-12
 **Module-layout home**: `src/gps_denied_onboard/components/c3_5_adhop/interface.py` (Protocol), `src/gps_denied_onboard/components/c3_5_adhop/__init__.py` (re-exports), `src/gps_denied_onboard/runtime_root/refiner_factory.py` (factory)
 
 > **Public API symbol naming.** The component's public interface symbol is named `ConditionalRefiner` in `description.md` § 2 and `AdHoPRefinementStrategy` in `module-layout.md` § c3_5_adhop. Both refer to the SAME Protocol; the canonical class name in code is `ConditionalRefiner` — it is the role description-first name and matches the method `refine_if_needed`. The producer task ALSO updates `module-layout.md` to align (`AdHoPRefinementStrategy` → `ConditionalRefiner`) so the two documents agree.
diff --git a/_docs/02_document/module-layout.md b/_docs/02_document/module-layout.md
index 727ad6a..13da49c 100644
--- a/_docs/02_document/module-layout.md
+++ b/_docs/02_document/module-layout.md
@@ -89,11 +89,15 @@ Bootstrap reference: `_docs/02_tasks/todo/AZ-263_initial_structure.md`. Architec
 - **Epic**: AZ-258 (E-C3.5 AdHoP Refinement)
 - **Directory**: `src/gps_denied_onboard/components/c3_5_adhop/`
 - **Public API**:
-  - `__init__.py` (re-exports `AdHoPRefinementStrategy`)
-  - `interface.py` (`AdHoPRefinementStrategy` Protocol)
-- **Internal**: `default_refiner.py`
-- **Owns**: `src/gps_denied_onboard/components/c3_5_adhop/**`, `tests/unit/c3_5_adhop/**`
-- **Imports from**: `_types`, `helpers.ransac_filter`, `helpers.se3_utils`, `config`, `logging`, `fdr_client`
+  - `__init__.py` (re-exports `ConditionalRefiner`, `C3_5RefinerConfig`)
+  - `interface.py` (`ConditionalRefiner` Protocol)
+  - `config.py` (`C3_5RefinerConfig`)
+  - `errors.py` (`RefinerError`, `RefinerBackboneError`, `RefinerConfigError` — held internal to the component; consumers reach them only via tests)
+- **Internal**:
+  - `passthrough_refiner.py` (reference baseline; AZ-348)
+  - `adhop_refiner.py` (production-default; AZ-349 pending)
+- **Owns**: `src/gps_denied_onboard/components/c3_5_adhop/**`, `tests/unit/c3_5_adhop/**`, `src/gps_denied_onboard/runtime_root/refiner_factory.py`
+- **Imports from**: `_types`, `helpers.ransac_filter` (R14: SHARED with C3 and C4 — owned by helper, NOT by C3.5), `helpers.se3_utils`, `components.c7_inference`, `config`, `logging`, `fdr_client`
 - **Consumed by**: `c4_pose`, `runtime_root`
 
 ### Component: c4_pose
diff --git a/_docs/02_tasks/todo/AZ-348_c3_5_refiner_protocol.md b/_docs/02_tasks/done/AZ-348_c3_5_refiner_protocol.md
similarity index 100%
rename from _docs/02_tasks/todo/AZ-348_c3_5_refiner_protocol.md
rename to _docs/02_tasks/done/AZ-348_c3_5_refiner_protocol.md
diff --git a/_docs/_autodev_state.md b/_docs/_autodev_state.md
index ff7fef6..5f5e82e 100644
--- a/_docs/_autodev_state.md
+++ b/_docs/_autodev_state.md
@@ -8,7 +8,7 @@ status: in_progress
 sub_step:
   phase: 2
   name: detect-progress
-  detail: "batch 24 in flight (per-task commits): AZ-336 done, AZ-342 done, AZ-344 done; next AZ-348"
+  detail: "batch 24 complete (AZ-336, AZ-342, AZ-344, AZ-348 per-task commits); ready to plan batch 25"
 retry_count: 0
 cycle: 1
 tracker: jira
diff --git a/src/gps_denied_onboard/_types/matcher.py b/src/gps_denied_onboard/_types/matcher.py
index ce00dd0..3c7af25 100644
--- a/src/gps_denied_onboard/_types/matcher.py
+++ b/src/gps_denied_onboard/_types/matcher.py
@@ -52,8 +52,12 @@ class CandidateMatchSet:
 class MatchResult:
     """Cross-domain match outcome for one frame.
 
-    Consumed by C3.5 :class:`ConditionalRefiner` (AZ-348). The
-    ``per_candidate`` tuple is sorted descending by
+    Consumed by C3.5 :class:`ConditionalRefiner` (AZ-348) — which
+    may either pass the result through unchanged or emit a new
+    instance via :func:`dataclasses.replace` with the refinement
+    fields set.
+
+    The ``per_candidate`` tuple is sorted descending by
     ``inlier_count`` with ties broken ascending by
     ``per_candidate_residual_px`` (INV-3) so
     ``best_candidate_idx == 0`` by construction.
@@ -62,6 +66,15 @@ class MatchResult:
     residual (mirrors ``per_candidate[0].per_candidate_residual_px``)
     surfaced separately so consumers do not have to know the
     ranking encoding.
+
+    The two refinement fields are populated by C3.5; they default
+    to the passthrough values so a C3 producer that never goes
+    through C3.5 still yields a valid downstream-readable
+    :class:`MatchResult` (AZ-348 AC-2). ``refinement_label`` is
+    one of ``"adhop"`` or ``"passthrough"``;
+    ``refinement_added_latency_ms`` covers exactly the work done
+    inside :meth:`ConditionalRefiner.refine_if_needed` and is
+    ``0.0`` on pure passthrough.
     """
 
     frame_id: int
@@ -72,6 +85,8 @@ class MatchResult:
     matcher_label: str
     candidates_input: int
     candidates_dropped: int
+    refinement_label: str = "passthrough"
+    refinement_added_latency_ms: float = 0.0
 
 
 @dataclass(frozen=True, slots=True)
diff --git a/src/gps_denied_onboard/components/c3_5_adhop/__init__.py b/src/gps_denied_onboard/components/c3_5_adhop/__init__.py
index 2e04160..2ee4953 100644
--- a/src/gps_denied_onboard/components/c3_5_adhop/__init__.py
+++ b/src/gps_denied_onboard/components/c3_5_adhop/__init__.py
@@ -1,5 +1,26 @@
-"""C3.5 AdHoP Refinement component — Public API."""
+"""C3.5 AdHoP / conditional refiner — Public API (AZ-348).
 
-from gps_denied_onboard.components.c3_5_adhop.interface import AdHoPRefinementStrategy
+Per ``conditional_refiner_protocol.md`` v1.0.0 the public surface
+consists of:
 
-__all__ = ["AdHoPRefinementStrategy"]
+- :class:`ConditionalRefiner` Protocol (two methods).
+- :class:`C3_5RefinerConfig` config block (registered on import).
+
+The error family (:class:`RefinerError`,
+:class:`RefinerBackboneError`, :class:`RefinerConfigError`) and
+both concrete strategies (:class:`PassthroughRefiner`,
+:class:`AdHoPRefiner`) are intentionally NOT in ``__all__`` per
+task spec AC-8: consumers see only the Protocol; concrete
+strategies are reached via the runtime-root factory.
+"""
+
+from gps_denied_onboard.components.c3_5_adhop.config import C3_5RefinerConfig
+from gps_denied_onboard.components.c3_5_adhop.interface import ConditionalRefiner
+from gps_denied_onboard.config.schema import register_component_block
+
+register_component_block("c3_5_adhop", C3_5RefinerConfig)
+
+__all__ = [
+    "C3_5RefinerConfig",
+    "ConditionalRefiner",
+]
diff --git a/src/gps_denied_onboard/components/c3_5_adhop/config.py b/src/gps_denied_onboard/components/c3_5_adhop/config.py
new file mode 100644
index 0000000..dfe0df6
--- /dev/null
+++ b/src/gps_denied_onboard/components/c3_5_adhop/config.py
@@ -0,0 +1,64 @@
+"""C3.5 ``ConditionalRefiner`` config block (AZ-348).
+
+Registered into ``config.components['c3_5_adhop']`` by the
+package ``__init__.py``. The composition-root factory
+:func:`gps_denied_onboard.runtime_root.refiner_factory.build_refiner_strategy`
+reads this block to select the strategy and configure thresholds.
+
+``strategy`` selects one of the two concrete refiners
+(``adhop`` — production-default; ``passthrough`` — baseline /
+smoke / IT-12 comparison). Both modules are linked
+unconditionally: there is NO ``BUILD_REFINER_*`` flag (NOT ADR-002
+territory). Runtime selection only.
+
+``residual_threshold_px`` is the conditional-gate threshold: a
+:class:`MatchResult` whose ``reprojection_residual_px <=
+threshold`` is passed through unchanged; ``>`` invokes the
+strategy's refinement procedure. Default 2.5 px (the AC-NEW-5 /
+R10 tunable from operator tooling).
+
+``invocation_rate_warn_threshold`` is the rolling-60 s
+invocation-rate ceiling above which a WARN log fires
+(C3.5-IT-03 / NFT-PERF-01). Must be in ``(0, 1)``; default 0.25.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Final
+
+from gps_denied_onboard.config.schema import ConfigError
+
+__all__ = [
+    "C3_5RefinerConfig",
+    "KNOWN_STRATEGIES",
+]
+
+
+KNOWN_STRATEGIES: Final[frozenset[str]] = frozenset({"adhop", "passthrough"})
+
+
+@dataclass(frozen=True)
+class C3_5RefinerConfig:
+    """Per-component config for C3.5 conditional refiner."""
+
+    strategy: str = "adhop"
+    residual_threshold_px: float = 2.5
+    invocation_rate_warn_threshold: float = 0.25
+
+    def __post_init__(self) -> None:
+        if self.strategy not in KNOWN_STRATEGIES:
+            raise ConfigError(
+                f"C3_5RefinerConfig.strategy={self.strategy!r} not in "
+                f"{sorted(KNOWN_STRATEGIES)}"
+            )
+        if self.residual_threshold_px <= 0.0:
+            raise ConfigError(
+                "C3_5RefinerConfig.residual_threshold_px must be > 0; "
+                f"got {self.residual_threshold_px}"
+            )
+        if not (0.0 < self.invocation_rate_warn_threshold < 1.0):
+            raise ConfigError(
+                "C3_5RefinerConfig.invocation_rate_warn_threshold must be in "
+                f"(0, 1); got {self.invocation_rate_warn_threshold}"
+            )
diff --git a/src/gps_denied_onboard/components/c3_5_adhop/errors.py b/src/gps_denied_onboard/components/c3_5_adhop/errors.py
new file mode 100644
index 0000000..69f66ba
--- /dev/null
+++ b/src/gps_denied_onboard/components/c3_5_adhop/errors.py
@@ -0,0 +1,44 @@
+"""C3.5 ``ConditionalRefiner`` error taxonomy (AZ-348).
+
+The family is intentionally small: per-candidate failures are
+handled inside C3 (drop-and-continue); at C3.5 the only failure
+mode is the AdHoP backbone (TensorRT exception, OOM, NaN, shape
+mismatch) and it is contained within the strategy via
+passthrough fall-through (contract Invariant 4) — never re-raised
+out of :meth:`refine_if_needed`.
+
+:class:`RefinerConfigError` is the composition-root rejection at
+startup; never raised per-frame.
+"""
+
+from __future__ import annotations
+
+__all__ = [
+    "RefinerBackboneError",
+    "RefinerConfigError",
+    "RefinerError",
+]
+
+
+class RefinerError(Exception):
+    """Base class for all C3.5 refinement-strategy errors."""
+
+
+class RefinerBackboneError(RefinerError):
+    """AdHoP backbone forward-pass failed.
+
+    TensorRT exception, OOM, NaN, shape mismatch. Caught inside
+    :meth:`ConditionalRefiner.refine_if_needed`, converted to
+    passthrough fall-through (Invariant 4), logged at ERROR with
+    one FDR record. NEVER re-raised out of the strategy.
+    """
+
+
+class RefinerConfigError(RefinerError):
+    """Composition-root rejected the refiner config.
+
+    Unknown strategy label OR invalid threshold (``<= 0``). Raised
+    at startup ONLY — never per-frame. The composition root logs
+    ERROR (``kind="c3_5.refiner.strategy_unknown"`` or
+    ``kind="c3_5.refiner.invalid_threshold"``) before raising.
+    """
diff --git a/src/gps_denied_onboard/components/c3_5_adhop/interface.py b/src/gps_denied_onboard/components/c3_5_adhop/interface.py
index f028222..f00e54b 100644
--- a/src/gps_denied_onboard/components/c3_5_adhop/interface.py
+++ b/src/gps_denied_onboard/components/c3_5_adhop/interface.py
@@ -1,16 +1,123 @@
-"""C3.5 `AdHoPRefinementStrategy` Protocol.
+"""C3.5 ``ConditionalRefiner`` Protocol (AZ-348).
 
-Concrete impl: AdHoP refiner. See `_docs/02_document/components/05_c3_5_adhop/`.
+PEP 544 ``typing.Protocol`` with ``runtime_checkable=True``; a
+two-method surface: :meth:`refine_if_needed` and
+:meth:`was_invoked`.
+
+Concrete impls — :class:`PassthroughRefiner` (this task) and
+:class:`AdHoPRefiner` (AZ-349) — live in sibling modules.
+Both are linked into the production binary unconditionally per
+ADR-001; runtime selection is via ``config.refiner.strategy``
+(NO ``BUILD_REFINER_*`` flag — NOT ADR-002 territory because
+both strategies are tiny and the AdHoP TRT engine is shared C7
+infrastructure).
+
+The contract at
+``_docs/02_document/contracts/c3_5_adhop/conditional_refiner_protocol.md``
+v1.0.0 is the authoritative shape; this module mirrors it 1:1.
 """
 
 from __future__ import annotations
 
-from typing import Protocol
+from typing import TYPE_CHECKING, Protocol, runtime_checkable
 
-from gps_denied_onboard._types.matcher import MatchResult
+if TYPE_CHECKING:
+    from gps_denied_onboard._types.matcher import MatchResult
+    from gps_denied_onboard._types.nav import NavCameraFrame
+
+__all__ = ["ConditionalRefiner"]
 
 
-class AdHoPRefinementStrategy(Protocol):
-    """Conditional refinement of a `MatchResult` (geometric verification + outlier purge)."""
+@runtime_checkable
+class ConditionalRefiner(Protocol):
+    """Conditional refinement strategy between C3 (matcher) and C4 (pose).
 
-    def refine(self, match: MatchResult) -> MatchResult: ...
+    Stateless per-frame; the only persistent state is the
+    constructor-injected backbone runtime handle (when the strategy
+    uses one) and the last-invocation flag.
+
+    Invariants (see ``conditional_refiner_protocol.md`` v1.0.0):
+
+    - **INV-1 single-threaded** — each instance is bound to one
+      ingest thread; same thread as C3 (shared C-frame ingest
+      path).
+    - **INV-2 stateless per-frame** — except for the
+      ``was_invoked`` flag, no implicit dependency on prior frames;
+      reordering :meth:`refine_if_needed` calls (tests only) MUST
+      yield identical output ``MatchResult`` content.
+    - **INV-3 conditional gate is a pure comparison** —
+      ``mr.reprojection_residual_px <= threshold`` → passthrough;
+      ``>`` → invoke. No tolerance, no smoothing, no hysteresis.
+    - **INV-4 passthrough fall-through on backbone error** —
+      :class:`RefinerBackboneError` raised inside the invoked path
+      is caught by the strategy and converted to passthrough output
+      with ``refinement_label = "passthrough"``; the error is
+      logged at ERROR + emitted to FDR; NEVER re-raised out of
+      :meth:`refine_if_needed`.
+    - **INV-5 bit-identical correspondences on passthrough** —
+      when ``refinement_label == "passthrough"``, every
+      ``inlier_correspondences`` ndarray in the output equals the
+      input ndarray bit-for-bit (``np.array_equal`` AND same
+      ``dtype``).
+    - **INV-6 ``refinement_label`` is `"adhop"` OR
+      `"passthrough"`** — exactly one of those two values; matches
+      the strategy's selected variant. Readers check
+      :meth:`was_invoked` to discriminate "AdHoP ran" from
+      "AdHoP-fell-through-to-passthrough".
+    - **INV-7 ``refinement_added_latency_ms`` is strategy-internal
+      added latency** — always ``>= 0``; near-zero on passthrough;
+      up to ~90 ms on AdHoP invoke per C3.5-PT-01.
+    - **INV-8 ``was_invoked()`` semantics** — set to ``True`` iff
+      the strategy entered the refinement procedure (post-gate,
+      regardless of whether AdHoP succeeded or fell through). On
+      pure passthrough strategy + every gate-decided-passthrough
+      call: ``False``.
+    - **INV-9 threshold validation** — the strategy MUST reject
+      ``residual_threshold_px <= 0`` (raise :class:`ValueError`);
+      the composition root validates the config-loaded threshold
+      at startup so this in-method check is defensive.
+    """
+
+    def refine_if_needed(
+        self,
+        frame: "NavCameraFrame",
+        mr: "MatchResult",
+        residual_threshold_px: float,
+    ) -> "MatchResult":
+        """Either pass ``mr`` through unchanged or run refinement.
+
+        If ``mr.reprojection_residual_px <= residual_threshold_px``
+        (the steady-state path), return ``mr`` unchanged AND set
+        :meth:`was_invoked` to ``False``. Otherwise run the
+        strategy's refinement procedure and return an enriched
+        :class:`MatchResult` (typically via
+        :func:`dataclasses.replace`) with ``refinement_label``
+        set, AND set :meth:`was_invoked` to ``True``.
+
+        On :class:`RefinerBackboneError` (AdHoP backbone failure
+        during the invoked path), the refiner MUST fall through to
+        passthrough — return ``mr`` unchanged with
+        ``refinement_label = "passthrough"`` AND :meth:`was_invoked`
+        ``True`` (the attempt counts towards the invocation rate
+        even on failure). The error is logged at ERROR + emitted
+        to FDR; downstream pose estimation gets a usable
+        :class:`MatchResult` and decides whether to trigger F6.
+
+        Determinism: same inputs MUST produce the same output. No
+        probabilistic gating; no time-based gating.
+
+        Raises:
+            ValueError: ``residual_threshold_px <= 0`` (defensive;
+                composition root should have caught this already).
+        """
+        ...
+
+    def was_invoked(self) -> bool:
+        """Return ``True`` iff the last :meth:`refine_if_needed`
+        call entered the refinement procedure.
+
+        Set at the start of every :meth:`refine_if_needed` call.
+        Used by FDR per-frame provenance and by
+        NFT-PERF-01 / C3.5-IT-03 invocation-rate accounting.
+        """
+        ...
diff --git a/src/gps_denied_onboard/components/c3_5_adhop/passthrough_refiner.py b/src/gps_denied_onboard/components/c3_5_adhop/passthrough_refiner.py
new file mode 100644
index 0000000..4046d07
--- /dev/null
+++ b/src/gps_denied_onboard/components/c3_5_adhop/passthrough_refiner.py
@@ -0,0 +1,84 @@
+"""``PassthroughRefiner`` — no-op :class:`ConditionalRefiner` (AZ-348).
+
+The reference baseline implementation. Returns the input
+:class:`MatchResult` unchanged (same object reference;
+``inlier_correspondences`` ndarrays are bit-identical and share
+references per contract INV-5). Always sets :meth:`was_invoked`
+to ``False`` per INV-8.
+
+Both helpers (``ransac_filter``, ``inference_runtime``) are held
+by reference for parity with :class:`AdHoPRefiner` (AZ-349) but
+neither is invoked. ``inference_runtime`` is typed as ``object``
+because the C3 matcher → C3.5 refiner → C4 pose layering forbids
+L3-to-L3 imports (architecture test ``test_az270_compose_root``);
+the composition-root factory at
+:mod:`gps_denied_onboard.runtime_root.refiner_factory`
+narrows the type at construction time.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from gps_denied_onboard._types.matcher import MatchResult
+    from gps_denied_onboard._types.nav import NavCameraFrame
+    from gps_denied_onboard.config.schema import Config
+    from gps_denied_onboard.helpers.ransac_filter import RansacFilter
+
+__all__ = ["PassthroughRefiner", "create"]
+
+
+class PassthroughRefiner:
+    """Reference passthrough strategy.
+
+    See module docstring. Stateless except for the ``_was_invoked``
+    flag (always ``False`` per INV-8); concurrent calls are unsafe
+    (single-thread invariant covers).
+    """
+
+    def __init__(
+        self,
+        *,
+        ransac_filter: "RansacFilter",
+        inference_runtime: object,
+    ) -> None:
+        # Held for parity with AdHoPRefiner; neither is invoked.
+        self._ransac_filter = ransac_filter
+        self._inference_runtime = inference_runtime
+        self._was_invoked: bool = False
+
+    def refine_if_needed(
+        self,
+        frame: "NavCameraFrame",
+        mr: "MatchResult",
+        residual_threshold_px: float,
+    ) -> "MatchResult":
+        if residual_threshold_px <= 0.0:
+            raise ValueError(
+                "residual_threshold_px must be > 0; "
+                f"got {residual_threshold_px}"
+            )
+        self._was_invoked = False
+        # `MatchResult` defaults `refinement_label="passthrough"`
+        # and `refinement_added_latency_ms=0.0` already; the input
+        # is bit-identical per contract INV-5 so we return it by
+        # reference rather than recreating via `dataclasses.replace`.
+        return mr
+
+    def was_invoked(self) -> bool:
+        return self._was_invoked
+
+
+def create(
+    config: "Config",
+    *,
+    ransac_filter: "RansacFilter",
+    inference_runtime: object,
+) -> PassthroughRefiner:
+    """Module-level factory entry point consumed by the
+    composition-root :func:`build_refiner_strategy`."""
+    return PassthroughRefiner(
+        ransac_filter=ransac_filter,
+        inference_runtime=inference_runtime,
+    )
diff --git a/src/gps_denied_onboard/runtime_root/refiner_factory.py b/src/gps_denied_onboard/runtime_root/refiner_factory.py
new file mode 100644
index 0000000..fd63cbc
--- /dev/null
+++ b/src/gps_denied_onboard/runtime_root/refiner_factory.py
@@ -0,0 +1,142 @@
+"""C3.5 refiner strategy composition-root factory (AZ-348).
+
+:func:`build_refiner_strategy` selects exactly one strategy by
+``config.components['c3_5_adhop'].strategy``. Both concrete
+strategies are linked into the production binary
+**unconditionally** (NO ``BUILD_REFINER_*`` flag — this is NOT
+ADR-002 territory). Runtime selection only per ADR-001.
+
+Strategy resolution table — mirrors the contract's
+``conditional_refiner_protocol.md`` v1.0.0 § Composition-root
+factory table verbatim:
+
+* ``"adhop"`` → ``gps_denied_onboard.components.c3_5_adhop.adhop_refiner.AdHoPRefiner`` (AZ-349; placeholder today).
+* ``"passthrough"`` → ``gps_denied_onboard.components.c3_5_adhop.passthrough_refiner.PassthroughRefiner``.
+
+The shared :class:`RansacFilter` and C7 :class:`InferenceRuntime`
+handles are constructor-injected — the factory does NOT own their
+lifecycles. The runtime root constructs ONE
+:class:`RansacFilter` instance and identity-shares it across C3,
+C3.5, and C4 (per ``ransac_filter.md`` v1.0.0).
+"""
+
+from __future__ import annotations
+
+import logging
+from typing import TYPE_CHECKING
+
+from gps_denied_onboard.components.c3_5_adhop.errors import RefinerConfigError
+
+if TYPE_CHECKING:
+    from gps_denied_onboard.components.c3_5_adhop import (
+        C3_5RefinerConfig,
+        ConditionalRefiner,
+    )
+    from gps_denied_onboard.components.c7_inference import InferenceRuntime
+    from gps_denied_onboard.config.schema import Config
+    from gps_denied_onboard.helpers.ransac_filter import RansacFilter
+
+__all__ = ["build_refiner_strategy"]
+
+
+_LOG = logging.getLogger("gps_denied_onboard.c3_5_adhop")
+
+
+# Strategy resolution table — mirrors the contract verbatim. ANY
+# mutation of this dict MUST be mirrored in the contract.
+_STRATEGY_TO_MODULE: dict[str, tuple[str, str]] = {
+    "adhop": (
+        "gps_denied_onboard.components.c3_5_adhop.adhop_refiner",
+        "AdHoPRefiner",
+    ),
+    "passthrough": (
+        "gps_denied_onboard.components.c3_5_adhop.passthrough_refiner",
+        "PassthroughRefiner",
+    ),
+}
+
+
+def _c3_5_config(config: "Config") -> "C3_5RefinerConfig":
+    """Pull the registered C3.5 config block.
+
+    ``c3_5_adhop.__init__`` registers it on import; a missing
+    registration is a developer error and surfaces as ``KeyError``
+    rather than a silent fallback.
+    """
+    return config.components["c3_5_adhop"]
+
+
+def build_refiner_strategy(
+    config: "Config",
+    *,
+    ransac_filter: "RansacFilter",
+    inference_runtime: "InferenceRuntime",
+) -> "ConditionalRefiner":
+    """Construct the :class:`ConditionalRefiner` impl selected by config.
+
+    1. Reads ``config.components['c3_5_adhop'].{strategy, residual_threshold_px}``.
+    2. Validates ``residual_threshold_px > 0`` — defensive
+       redundancy on top of the config-load-time check
+       (:class:`C3_5RefinerConfig.__post_init__`); raises
+       :class:`RefinerConfigError` on failure.
+    3. Imports the concrete strategy module via the resolution
+       table (NOT lazy — both strategies are linked
+       unconditionally).
+    4. Constructs the strategy via its module-level
+       ``create(config, ransac_filter, inference_runtime)``
+       factory function.
+    5. Emits ONE INFO log ``kind="c3_5.refiner.strategy_loaded"``
+       with ``{strategy, residual_threshold_px}``.
+
+    Raises:
+        RefinerConfigError: unknown strategy label OR invalid
+            threshold (``<= 0``).
+    """
+    block = _c3_5_config(config)
+    strategy = block.strategy
+    module_info = _STRATEGY_TO_MODULE.get(strategy)
+    if module_info is None:
+        _LOG.error(
+            "c3_5.refiner.strategy_unknown",
+            extra={"strategy": strategy},
+        )
+        raise RefinerConfigError(f"Unknown refiner strategy: {strategy}")
+    if block.residual_threshold_px <= 0.0:
+        # Config-load validation should have rejected this already;
+        # defensive in case a caller constructed Config bypassing
+        # __post_init__ (e.g., via dataclasses.replace on a partial
+        # block).
+        _LOG.error(
+            "c3_5.refiner.invalid_threshold",
+            extra={
+                "strategy": strategy,
+                "residual_threshold_px": block.residual_threshold_px,
+            },
+        )
+        raise RefinerConfigError(
+            "residual_threshold_px must be > 0; "
+            f"got {block.residual_threshold_px}"
+        )
+    module_name, class_name = module_info
+    module = __import__(module_name, fromlist=[class_name])
+    create_fn = getattr(module, "create", None)
+    if create_fn is None:
+        strategy_cls = getattr(module, class_name)
+        instance = strategy_cls(
+            ransac_filter=ransac_filter,
+            inference_runtime=inference_runtime,
+        )
+    else:
+        instance = create_fn(
+            config,
+            ransac_filter=ransac_filter,
+            inference_runtime=inference_runtime,
+        )
+    _LOG.info(
+        "c3_5.refiner.strategy_loaded",
+        extra={
+            "strategy": strategy,
+            "residual_threshold_px": block.residual_threshold_px,
+        },
+    )
+    return instance
diff --git a/tests/unit/c3_5_adhop/test_protocol_conformance.py b/tests/unit/c3_5_adhop/test_protocol_conformance.py
new file mode 100644
index 0000000..a53a250
--- /dev/null
+++ b/tests/unit/c3_5_adhop/test_protocol_conformance.py
@@ -0,0 +1,490 @@
+"""AZ-348 — C3.5 ConditionalRefiner Protocol + DTO + error + factory conformance.
+
+Covers AC-1..AC-14 + NFRs. AC-9 (single-thread binding) is
+deferred per the task spec's Risk-4 escape clause — the generic
+``compose_root`` thread-binding registry lives with AZ-270 and
+the broader runtime-root composition. Each factory owns its own
+thread binding today; this protocol task does not add a new
+binding registry.
+
+AC-7 (strategy resolution table) is asserted up to the lookup
+point for ``"adhop"``: the AdHoP module is the AZ-349 placeholder
+that does not exist yet, so the assertion on that path stops at
+``ModuleNotFoundError`` — explicitly documented as the "import +
+class lookup not __init__" rule in the task spec.
+"""
+
+from __future__ import annotations
+
+import dataclasses
+import logging
+import sys
+import time
+
+import numpy as np
+import pytest
+
+from gps_denied_onboard._types.matcher import (
+    CandidateMatchSet,
+    MatchResult,
+)
+from gps_denied_onboard.components.c3_5_adhop import (
+    C3_5RefinerConfig,
+    ConditionalRefiner,
+)
+from gps_denied_onboard.components.c3_5_adhop.config import KNOWN_STRATEGIES
+from gps_denied_onboard.components.c3_5_adhop.errors import (
+    RefinerBackboneError,
+    RefinerConfigError,
+    RefinerError,
+)
+from gps_denied_onboard.components.c3_5_adhop.passthrough_refiner import (
+    PassthroughRefiner,
+    create,
+)
+from gps_denied_onboard.config.schema import Config, ConfigError
+from gps_denied_onboard.runtime_root.refiner_factory import build_refiner_strategy
+
+
+# ----------------------------------------------------------------------
+# Fakes.
+
+
+class _FakeRansacFilter:
+    def filter(self, *args, **kwargs):
+        raise NotImplementedError
+
+
+class _FakeInferenceRuntime:
+    def deserialize_engine(self, *args, **kwargs):
+        raise NotImplementedError
+
+    def thermal_state(self):
+        raise NotImplementedError
+
+
+class _FakeFrame:
+    frame_id = 7
+
+
+class _PartialRefinerMissingWasInvoked:
+    def refine_if_needed(self, frame, mr, residual_threshold_px):
+        raise NotImplementedError
+
+
+class _PartialRefinerMissingRefine:
+    def was_invoked(self):
+        raise NotImplementedError
+
+
+def _config_with_strategy(
+    strategy: str = "passthrough",
+    *,
+    threshold: float = 2.5,
+) -> Config:
+    return Config.with_blocks(
+        c3_5_adhop=C3_5RefinerConfig(
+            strategy=strategy,
+            residual_threshold_px=threshold,
+        )
+    )
+
+
+def _make_candidate(*, inliers: int = 40, residual: float = 1.0) -> CandidateMatchSet:
+    return CandidateMatchSet(
+        tile_id=(18, 49.9, 36.3),
+        inlier_count=inliers,
+        inlier_correspondences=np.ones((inliers, 4), dtype=np.float32) * 0.5,
+        ransac_outlier_count=3,
+        per_candidate_residual_px=residual,
+    )
+
+
+def _make_result(
+    *,
+    reprojection_residual: float = 1.0,
+    refinement_label: str = "passthrough",
+    refinement_latency_ms: float = 0.0,
+) -> MatchResult:
+    candidate = _make_candidate()
+    return MatchResult(
+        frame_id=7,
+        per_candidate=(candidate,),
+        best_candidate_idx=0,
+        reprojection_residual_px=reprojection_residual,
+        matched_at=1_000_000_000,
+        matcher_label="disk_lightglue",
+        candidates_input=3,
+        candidates_dropped=2,
+        refinement_label=refinement_label,
+        refinement_added_latency_ms=refinement_latency_ms,
+    )
+
+
+# ----------------------------------------------------------------------
+# AC-1: Protocol conformance.
+
+
+def test_ac1_passthrough_refiner_conformance() -> None:
+    refiner = PassthroughRefiner(
+        ransac_filter=_FakeRansacFilter(),
+        inference_runtime=_FakeInferenceRuntime(),
+    )
+    assert isinstance(refiner, ConditionalRefiner)
+
+
+@pytest.mark.parametrize(
+    "partial_cls",
+    [_PartialRefinerMissingWasInvoked, _PartialRefinerMissingRefine],
+)
+def test_ac1_partial_refiners_fail_conformance(partial_cls) -> None:
+    assert not isinstance(partial_cls(), ConditionalRefiner)
+
+
+# ----------------------------------------------------------------------
+# AC-2: MatchResult backward-compatible defaults.
+
+
+def test_ac2_match_result_construction_without_new_fields() -> None:
+    candidate = _make_candidate()
+    mr = MatchResult(
+        frame_id=7,
+        per_candidate=(candidate,),
+        best_candidate_idx=0,
+        reprojection_residual_px=candidate.per_candidate_residual_px,
+        matched_at=1_000_000_000,
+        matcher_label="disk_lightglue",
+        candidates_input=3,
+        candidates_dropped=2,
+    )
+    assert mr.refinement_label == "passthrough"
+    assert mr.refinement_added_latency_ms == 0.0
+
+
+# ----------------------------------------------------------------------
+# AC-3: MatchResult immutability + slots preserved.
+
+
+def test_ac3_match_result_slots_include_new_fields() -> None:
+    assert "refinement_label" in MatchResult.__slots__
+    assert "refinement_added_latency_ms" in MatchResult.__slots__
+
+
+def test_ac3_match_result_remains_frozen() -> None:
+    mr = _make_result()
+    with pytest.raises(dataclasses.FrozenInstanceError):
+        mr.refinement_label = "adhop"
+    with pytest.raises(dataclasses.FrozenInstanceError):
+        mr.refinement_added_latency_ms = 12.5
+
+
+# ----------------------------------------------------------------------
+# AC-4: Factory rejects unknown strategy. (Validated at config-load.)
+
+
+@pytest.mark.parametrize(
+    "bad_label",
+    ["ADHOP", "passthrough_v2", "garbage", "", "default"],
+)
+def test_ac4_unknown_strategy_rejected_at_config_load(bad_label: str) -> None:
+    with pytest.raises(ConfigError) as exc_info:
+        C3_5RefinerConfig(strategy=bad_label)
+    msg = str(exc_info.value)
+    for valid in KNOWN_STRATEGIES:
+        assert valid in msg
+
+
+def test_ac4_factory_emits_error_log_on_unknown_strategy(caplog) -> None:
+    # Build a Config that bypasses __post_init__ validation by
+    # constructing the block via dataclasses.replace on an already-valid
+    # block — this is what catches the defensive factory path.
+    valid_block = C3_5RefinerConfig(strategy="passthrough")
+    object.__setattr__(valid_block, "strategy", "garbage")
+    config = Config.with_blocks(c3_5_adhop=valid_block)
+    with caplog.at_level(logging.ERROR, logger="gps_denied_onboard.c3_5_adhop"):
+        with pytest.raises(RefinerConfigError) as exc_info:
+            build_refiner_strategy(
+                config,
+                ransac_filter=_FakeRansacFilter(),
+                inference_runtime=_FakeInferenceRuntime(),
+            )
+    assert "Unknown refiner strategy: garbage" in str(exc_info.value)
+    assert any(
+        r.message == "c3_5.refiner.strategy_unknown" for r in caplog.records
+    )
+
+
+# ----------------------------------------------------------------------
+# AC-5: Factory rejects invalid threshold.
+
+
+@pytest.mark.parametrize("bad_threshold", [0.0, -0.1, -10.0])
+def test_ac5_invalid_threshold_rejected_at_config_load(bad_threshold: float) -> None:
+    with pytest.raises(ConfigError):
+        C3_5RefinerConfig(residual_threshold_px=bad_threshold)
+
+
+def test_ac5_factory_emits_error_log_on_invalid_threshold(caplog) -> None:
+    valid_block = C3_5RefinerConfig(strategy="passthrough")
+    object.__setattr__(valid_block, "residual_threshold_px", 0.0)
+    config = Config.with_blocks(c3_5_adhop=valid_block)
+    with caplog.at_level(logging.ERROR, logger="gps_denied_onboard.c3_5_adhop"):
+        with pytest.raises(RefinerConfigError):
+            build_refiner_strategy(
+                config,
+                ransac_filter=_FakeRansacFilter(),
+                inference_runtime=_FakeInferenceRuntime(),
+            )
+    assert any(
+        r.message == "c3_5.refiner.invalid_threshold" for r in caplog.records
+    )
+
+
+# ----------------------------------------------------------------------
+# AC-6: Successful factory load emits INFO log.
+
+
+def test_ac6_factory_emits_info_log_on_success(caplog) -> None:
+    config = _config_with_strategy("passthrough", threshold=2.5)
+    with caplog.at_level(logging.INFO, logger="gps_denied_onboard.c3_5_adhop"):
+        instance = build_refiner_strategy(
+            config,
+            ransac_filter=_FakeRansacFilter(),
+            inference_runtime=_FakeInferenceRuntime(),
+        )
+    assert isinstance(instance, ConditionalRefiner)
+    records = [
+        r for r in caplog.records if r.message == "c3_5.refiner.strategy_loaded"
+    ]
+    assert len(records) == 1
+    record = records[0]
+    assert getattr(record, "strategy", None) == "passthrough"
+    assert getattr(record, "residual_threshold_px", None) == 2.5
+
+
+# ----------------------------------------------------------------------
+# AC-7: Strategy resolution table.
+
+
+def test_ac7_passthrough_resolution() -> None:
+    config = _config_with_strategy("passthrough")
+    instance = build_refiner_strategy(
+        config,
+        ransac_filter=_FakeRansacFilter(),
+        inference_runtime=_FakeInferenceRuntime(),
+    )
+    assert isinstance(instance, PassthroughRefiner)
+    assert isinstance(instance, ConditionalRefiner)
+    assert (
+        "gps_denied_onboard.components.c3_5_adhop.passthrough_refiner"
+        in sys.modules
+    )
+
+
+def test_ac7_adhop_resolution_stops_at_module_lookup() -> None:
+    """Task spec: the full-success path for "adhop" belongs to the
+    AdHoP task (AZ-349); this assertion verifies the factory reaches
+    the import step but the module does not exist yet.
+    """
+    config = _config_with_strategy("adhop")
+    with pytest.raises(ModuleNotFoundError):
+        build_refiner_strategy(
+            config,
+            ransac_filter=_FakeRansacFilter(),
+            inference_runtime=_FakeInferenceRuntime(),
+        )
+
+
+# ----------------------------------------------------------------------
+# AC-8: Public API.
+
+
+def test_ac8_public_api_re_exports() -> None:
+    from gps_denied_onboard.components import c3_5_adhop
+
+    assert "ConditionalRefiner" in c3_5_adhop.__all__
+    assert "C3_5RefinerConfig" in c3_5_adhop.__all__
+
+
+def test_ac8_internals_not_in_public_api() -> None:
+    from gps_denied_onboard.components import c3_5_adhop
+
+    for internal in (
+        "PassthroughRefiner",
+        "AdHoPRefiner",
+        "RefinerError",
+        "RefinerBackboneError",
+        "RefinerConfigError",
+    ):
+        assert internal not in c3_5_adhop.__all__, (
+            f"internal name leaked into public API: {internal}"
+        )
+
+
+# ----------------------------------------------------------------------
+# AC-9: deferred.
+
+
+def test_ac9_single_thread_binding_deferred() -> None:
+    """AC-9 (single-thread binding) is deferred per task spec
+    Risk 4: the generic ``compose_root`` thread-binding registry
+    lives with AZ-270 and the broader runtime-root composition.
+    Each factory owns its own thread binding today; this protocol
+    task does not add a new binding registry."""
+
+
+# ----------------------------------------------------------------------
+# AC-10: PassthroughRefiner byte-identical correspondences.
+
+
+def test_ac10_passthrough_returns_same_match_result_reference() -> None:
+    refiner = create(
+        _config_with_strategy(),
+        ransac_filter=_FakeRansacFilter(),
+        inference_runtime=_FakeInferenceRuntime(),
+    )
+    mr = _make_result()
+    out = refiner.refine_if_needed(_FakeFrame(), mr, residual_threshold_px=2.5)
+    assert out is mr
+    assert out.refinement_label == "passthrough"
+    assert out.refinement_added_latency_ms == 0.0
+
+
+def test_ac10_passthrough_correspondences_bit_identical() -> None:
+    refiner = PassthroughRefiner(
+        ransac_filter=_FakeRansacFilter(),
+        inference_runtime=_FakeInferenceRuntime(),
+    )
+    mr = _make_result()
+    out = refiner.refine_if_needed(_FakeFrame(), mr, residual_threshold_px=2.5)
+    for in_cand, out_cand in zip(mr.per_candidate, out.per_candidate, strict=True):
+        assert np.array_equal(
+            out_cand.inlier_correspondences, in_cand.inlier_correspondences
+        )
+        assert out_cand.inlier_correspondences.dtype == in_cand.inlier_correspondences.dtype
+        # Same object reference — INV-5 forbids silent copies on the
+        # passthrough path.
+        assert out_cand.inlier_correspondences is in_cand.inlier_correspondences
+
+
+# ----------------------------------------------------------------------
+# AC-11: was_invoked always False on PassthroughRefiner.
+
+
+def test_ac11_was_invoked_always_false_on_passthrough() -> None:
+    refiner = PassthroughRefiner(
+        ransac_filter=_FakeRansacFilter(),
+        inference_runtime=_FakeInferenceRuntime(),
+    )
+    assert refiner.was_invoked() is False
+    mr = _make_result()
+    for _ in range(10):
+        refiner.refine_if_needed(_FakeFrame(), mr, residual_threshold_px=2.5)
+        assert refiner.was_invoked() is False
+
+
+# ----------------------------------------------------------------------
+# AC-12: Threshold validation in refine_if_needed.
+
+
+@pytest.mark.parametrize("bad_threshold", [0.0, -0.1, -50.0])
+def test_ac12_threshold_validation_in_method(bad_threshold: float) -> None:
+    refiner = PassthroughRefiner(
+        ransac_filter=_FakeRansacFilter(),
+        inference_runtime=_FakeInferenceRuntime(),
+    )
+    mr = _make_result()
+    with pytest.raises(ValueError):
+        refiner.refine_if_needed(
+            _FakeFrame(), mr, residual_threshold_px=bad_threshold
+        )
+
+
+# ----------------------------------------------------------------------
+# AC-13: Error hierarchy.
+
+
+@pytest.mark.parametrize(
+    "exc_factory",
+    [RefinerBackboneError, RefinerConfigError],
+)
+def test_ac13_all_refiner_errors_caught_as_family(exc_factory) -> None:
+    with pytest.raises(RefinerError):
+        raise exc_factory("boom")
+
+
+# ----------------------------------------------------------------------
+# AC-14: module-layout.md symbol rename.
+
+
+def test_ac14_module_layout_references_conditional_refiner() -> None:
+    from pathlib import Path
+
+    doc = Path("_docs/02_document/module-layout.md").read_text()
+    assert "ConditionalRefiner" in doc, (
+        "module-layout.md must reference the canonical Public API symbol"
+    )
+    # Old symbol name must NOT be present (per AC-14).
+    assert "AdHoPRefinementStrategy" not in doc, (
+        "module-layout.md still references the legacy AdHoPRefinementStrategy "
+        "symbol; rename per AZ-348 AC-14"
+    )
+
+
+# ----------------------------------------------------------------------
+# NFRs.
+
+
+def test_nfr_perf_factory_under_20ms_p99(caplog) -> None:
+    config = _config_with_strategy("passthrough")
+    ransac_filter = _FakeRansacFilter()
+    inference_runtime = _FakeInferenceRuntime()
+    durations_ms: list[float] = []
+    for _ in range(100):
+        t0 = time.perf_counter()
+        build_refiner_strategy(
+            config,
+            ransac_filter=ransac_filter,
+            inference_runtime=inference_runtime,
+        )
+        durations_ms.append((time.perf_counter() - t0) * 1000.0)
+    durations_ms.sort()
+    p99 = durations_ms[int(0.99 * len(durations_ms))]
+    assert p99 <= 20.0, f"factory p99={p99:.2f} ms exceeded 20 ms budget"
+
+
+def test_nfr_perf_passthrough_under_0_5ms_p99() -> None:
+    refiner = PassthroughRefiner(
+        ransac_filter=_FakeRansacFilter(),
+        inference_runtime=_FakeInferenceRuntime(),
+    )
+    mr = _make_result()
+    frame = _FakeFrame()
+    durations_us: list[float] = []
+    for _ in range(10_000):
+        t0 = time.perf_counter()
+        refiner.refine_if_needed(frame, mr, residual_threshold_px=2.5)
+        durations_us.append((time.perf_counter() - t0) * 1_000_000.0)
+    durations_us.sort()
+    p99_us = durations_us[int(0.99 * len(durations_us))]
+    assert p99_us <= 500.0, f"refine p99={p99_us:.2f} us exceeded 500 us budget"
+
+
+# ----------------------------------------------------------------------
+# Surface coverage — config defaults.
+
+
+def test_c3_5_config_defaults() -> None:
+    cfg = C3_5RefinerConfig()
+    assert cfg.strategy == "adhop"
+    assert cfg.residual_threshold_px == 2.5
+    assert cfg.invocation_rate_warn_threshold == 0.25
+
+
+def test_c3_5_config_invocation_rate_validation() -> None:
+    with pytest.raises(ConfigError):
+        C3_5RefinerConfig(invocation_rate_warn_threshold=0.0)
+    with pytest.raises(ConfigError):
+        C3_5RefinerConfig(invocation_rate_warn_threshold=1.0)
+    with pytest.raises(ConfigError):
+        C3_5RefinerConfig(invocation_rate_warn_threshold=-0.5)
diff --git a/tests/unit/c3_5_adhop/test_smoke.py b/tests/unit/c3_5_adhop/test_smoke.py
deleted file mode 100644
index ad551ad..0000000
--- a/tests/unit/c3_5_adhop/test_smoke.py
+++ /dev/null
@@ -1,8 +0,0 @@
-"""C3.5 AdHoP smoke test — AC-9."""
-
-
-def test_interface_importable() -> None:
-    # Assert
-    from gps_denied_onboard.components.c3_5_adhop import AdHoPRefinementStrategy
-
-    assert AdHoPRefinementStrategy is not None