diff --git a/src/tracer/fit/pipeline.py b/src/tracer/fit/pipeline.py
index 4b0f056..f7e3a8b 100644
--- a/src/tracer/fit/pipeline.py
+++ b/src/tracer/fit/pipeline.py
@@ -233,7 +233,11 @@ def _calibrate_threshold(scores, preds, y_teacher, target_ta, alpha=0.1, min_acc
 
 def _predict(clf, X):
     probs = clf.predict_proba(X)
-    preds = probs.argmax(axis=1).astype(int)
+    idx = probs.argmax(axis=1)
+    # predict_proba columns follow clf.classes_. A stage trained on a subset of
+    # the label space (the RSB residual stage) must map them back to label ids.
+    classes = getattr(clf, "classes_", None)
+    preds = (np.asarray(classes)[idx] if classes is not None else idx).astype(int)
     return preds, probs
 
 
diff --git a/tests/test_stage_labels.py b/tests/test_stage_labels.py
new file mode 100644
index 0000000..4c2fa3d
--- /dev/null
+++ b/tests/test_stage_labels.py
@@ -0,0 +1,92 @@
+"""A stage's predictions must be teacher label ids, not predict_proba columns.
+
+The RSB second stage is trained only on the rows the first stage rejected, so
+its surrogate usually sees a subset of the label space (e.g. the easy class the
+first stage fully absorbs is missing). For such a model, column ``i`` of
+``predict_proba`` is ``clf.classes_[i]``, not label id ``i``.
+"""
+import json
+import tempfile
+from pathlib import Path
+
+import numpy as np
+from sklearn.linear_model import LogisticRegression
+
+from tracer.fit.pipeline import _predict, apply_stage
+
+LINEAR_ONLY = ("gbt", "rf", "et", "xgb", "dt", "mlp_1h", "mlp_2h")
+
+
+def test_predict_maps_proba_columns_to_class_labels():
+    rng = np.random.RandomState(0)
+    # Trained on label ids {1, 3} only (0 and 2 absent from this subset).
+    X = np.vstack([rng.randn(50, 2) - 4, rng.randn(50, 2) + 4])
+    y = np.r_[np.full(50, 1), np.full(50, 3)]
+    clf = LogisticRegression().fit(X, y)
+
+    preds, probs = _predict(clf, X)
+    assert probs.shape == (100, 2)
+    assert set(np.unique(preds)) == {1, 3}
+    assert (preds == y).mean() > 0.99
+
+    stage = {"clf": clf, "accept_all": True}
+    stage_preds, _, _ = apply_stage(stage, X)
+    assert set(np.unique(stage_preds)) == {1, 3}
+
+
+def _residual_traces(tmp, n=4000):
+    """Class 0 is trivially separable, so stage 1 accepts every class-0 row.
+
+    Classes 1-3 come in two groups. The 'easy' group carries a per-class
+    marker; the 'hard' group encodes the class along a direction that the easy
+    group uses the opposite way, which a global linear surrogate cannot
+    resolve. Those rows are rejected by stage 1 and are exactly what the RSB
+    residual stage should pick up, with only labels {1, 2, 3} in its pool.
+    """
+    rng = np.random.RandomState(0)
+    X, y = [], []
+    for _ in range(n):
+        c = rng.randint(0, 4)
+        x = rng.randn(6) * 0.5
+        if c == 0:
+            x[0] += 10
+        elif rng.rand() > 0.3:
+            x[c] += 4
+            x[5] -= (c - 2) * 8
+            x[4] -= 3
+        else:
+            x[4] += 3
+            x[5] += (c - 2) * 2
+        X.append(x)
+        y.append(c)
+    X = np.asarray(X, dtype=np.float32)
+    names = ["cls_0", "cls_1", "cls_2", "cls_3"]
+    path = Path(tmp) / "traces.jsonl"
+    with path.open("w") as f:
+        for i, c in enumerate(y):
+            f.write(json.dumps({"input": f"t{i}", "teacher": names[c]}) + "\n")
+    return path, X, np.asarray([names[c] for c in y])
+
+
+def test_rsb_residual_stage_routes_with_teacher_labels():
+    from tracer.api import fit, load_router
+    from tracer.config import FitConfig
+
+    with tempfile.TemporaryDirectory() as tmp:
+        path, X, teacher = _residual_traces(tmp)
+        out = Path(tmp) / ".tracer"
+        cfg = FitConfig(target_teacher_agreement=0.95, frontier_targets=(0.95,),
+                        skip_candidates=LINEAR_ONLY, verbose=False)
+        result = fit(path, artifact_dir=out, embeddings=X, config=cfg)
+
+        # The residual stage is certified, so RSB beats single-stage L2D.
+        assert result.manifest.selected_method == "rsb"
+        assert result.manifest.coverage_cal > 0.95
+
+        router = load_router(out)
+        batch = router.predict_batch(X)
+        stage2 = np.asarray(batch["stage_id"]) == 1
+        assert stage2.sum() > 100
+        labels = np.asarray(batch["labels"], dtype=object)[stage2]
+        # Stage-2 rows carry the class they belong to, not a shifted label.
+        assert (labels == teacher[stage2]).mean() >= 0.9