diff --git a/core/comprehension_attempt/__init__.py b/core/comprehension_attempt/__init__.py
index 642224af..cf2ec183 100644
--- a/core/comprehension_attempt/__init__.py
+++ b/core/comprehension_attempt/__init__.py
@@ -8,5 +8,15 @@ from __future__ import annotations
 
 from core.comprehension_attempt.classify import classify_r1, classify_r2
 from core.comprehension_attempt.model import ComprehensionAttempt, Organ, Outcome
+from core.comprehension_attempt.router import RouteResult, RouteStatus, route_setup
 
-__all__ = ["ComprehensionAttempt", "Organ", "Outcome", "classify_r1", "classify_r2"]
+__all__ = [
+    "ComprehensionAttempt",
+    "Organ",
+    "Outcome",
+    "RouteResult",
+    "RouteStatus",
+    "classify_r1",
+    "classify_r2",
+    "route_setup",
+]
diff --git a/core/comprehension_attempt/router.py b/core/comprehension_attempt/router.py
new file mode 100644
index 00000000..4a7b78e8
--- /dev/null
+++ b/core/comprehension_attempt/router.py
@@ -0,0 +1,54 @@
+"""Deterministic multi-organ setup router (N3).
+
+Boring on purpose: attempt the R1 and R2 setup compilers, collect their typed attempts, and
+select a setup ONLY when exactly one organ produced an admissible one. No dynamic "best"
+scoring, no priority heuristics.
+
+```text
+exactly one setup_correct          -> routed (use it)
+zero setup_correct                 -> all_refused (classify downstream)
+>= 2 setup_correct, signatures agree-> routed (organs concur)
+>= 2 setup_correct, signatures differ-> ambiguous (refuse — never pick)
+```
+
+Cross-organ signatures are produced by different functions and never coincide, so in practice
+two admitting organs always resolve to ``ambiguous``. The router never solves and never emits
+``setup_wrong`` — that is an eval-only outcome; against gold the routed setup must match (the
+wrong=0 invariant, asserted by the router tests).
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Literal
+
+from core.comprehension_attempt.classify import classify_r1, classify_r2
+from core.comprehension_attempt.model import ComprehensionAttempt
+
+RouteStatus = Literal["routed", "all_refused", "ambiguous"]
+
+
+@dataclass(frozen=True, slots=True)
+class RouteResult:
+    """The outcome of routing one problem across the organs: every attempt, the selected setup
+    (or ``None``), and the routing status."""
+
+    attempts: tuple[ComprehensionAttempt, ...]
+    selected: ComprehensionAttempt | None
+    status: RouteStatus
+
+
+def route_setup(text: str, *, case_id: str | None = None) -> RouteResult:
+    """Route *text* to the single organ that admits an honest setup, or refuse."""
+    attempts = (classify_r1(text, case_id=case_id), classify_r2(text, case_id=case_id))
+    correct = tuple(a for a in attempts if a.is_setup_correct)
+    if len(correct) == 1:
+        return RouteResult(attempts, correct[0], "routed")
+    if not correct:
+        return RouteResult(attempts, None, "all_refused")
+    if len({a.setup_signature for a in correct}) == 1:
+        return RouteResult(attempts, correct[0], "routed")
+    return RouteResult(attempts, None, "ambiguous")
+
+
+__all__ = ["RouteResult", "RouteStatus", "route_setup"]
diff --git a/tests/test_setup_router.py b/tests/test_setup_router.py
new file mode 100644
index 00000000..1f8d57d6
--- /dev/null
+++ b/tests/test_setup_router.py
@@ -0,0 +1,52 @@
+"""Tests for the deterministic multi-organ setup router (N3).
+
+Pins that each problem routes to exactly the organ that owns it (R1 relational, R2 constraint),
+that unreadable problems refuse, that no gold problem is read by both organs (no ambiguity), and
+— the wrong=0 invariant — that every routed R2 setup matches the independent gold signature.
+"""
+
+from __future__ import annotations
+
+from core.comprehension_attempt import route_setup
+from core.comprehension_attempt.classify import _r2_signature
+from evals.constraint_oracle.runner import _load_r2_gold, gold_to_problem
+from evals.setup_oracle.runner import _load_r1_gold
+
+
+def test_r2_well_formed_routes_to_r2_and_matches_gold_signature() -> None:
+    for fx in _load_r2_gold():
+        result = route_setup(fx["text"], case_id=fx["id"])
+        if fx["expect"] in ("solved", "solver_refuses"):
+            assert result.status == "routed", f"{fx['id']}: {result.status}"
+            assert result.selected is not None and result.selected.organ == "r2_constraints"
+            # wrong=0: the routed setup is byte-equal to the independent gold setup.
+            assert result.selected.setup_signature == _r2_signature(gold_to_problem(fx)), fx["id"]
+        else:  # reader_refuses
+            assert result.status == "all_refused" and result.selected is None
+
+
+def test_r1_admitted_routes_to_r1_refused_all_refused() -> None:
+    routed = refused = 0
+    for fx in _load_r1_gold():
+        result = route_setup(fx["text"], case_id=fx["id"])
+        if result.status == "routed":
+            routed += 1
+            assert result.selected.organ == "r1_quantitative", fx["id"]
+        else:
+            refused += 1
+            assert result.status == "all_refused"
+    assert routed == 7 and refused == 3
+
+
+def test_no_gold_problem_is_ambiguous() -> None:
+    # Each organ is exclusive on the gold corpora — no text is admitted by both.
+    for fx in _load_r2_gold() + _load_r1_gold():
+        assert route_setup(fx["text"]).status != "ambiguous", fx.get("id")
+
+
+def test_router_never_selects_a_refusal() -> None:
+    for fx in _load_r2_gold() + _load_r1_gold():
+        result = route_setup(fx["text"])
+        if result.selected is not None:
+            assert result.selected.outcome == "setup_correct"
+        assert len(result.attempts) == 2  # always one R1 + one R2 attempt