diff --git a/harness/tests/test_set_schema_linking.py b/harness/tests/test_set_schema_linking.py
new file mode 100644
index 00000000..ba8f0091
--- /dev/null
+++ b/harness/tests/test_set_schema_linking.py
@@ -0,0 +1,35 @@
+"""L1: store.set_schema_linking — validate against SchemaLinking, then write."""
+import json
+
+import pytest
+from pydantic import ValidationError
+
+from tht.config import DatabaseConfig
+from tht.session.models import SchemaLinking
+from tht.session.store import create_session, set_schema_linking
+
+
+def _db():
+    return DatabaseConfig(database="testdb", user="u", password="p", **{"schema": "public"})  # noqa: S106
+
+
+def test_writes_and_revalidates(tmp_path):
+    m = create_session("q", _db(), tmp_path)
+    data = {
+        "question": "q riscritta",
+        "candidates": [{"kind": "table", "name": "fact_x", "decision": "promoted"}],
+        "joins": [{"from": "a.k", "to": "b.k"}],
+    }
+    path = set_schema_linking(m.id, data, tmp_path)
+    assert path.exists()
+    reloaded = json.loads(path.read_text())
+    assert reloaded["candidates"][0]["name"] == "fact_x"
+    assert reloaded["joins"][0]["from"] == "a.k"  # 'from' alias round-trips
+    SchemaLinking.model_validate(reloaded)  # re-validates clean
+
+
+def test_rejects_invalid_and_writes_nothing(tmp_path):
+    m = create_session("q", _db(), tmp_path)
+    with pytest.raises(ValidationError):
+        set_schema_linking(m.id, {"question": "q", "bogus": 1}, tmp_path)  # extra=forbid
+    assert not (tmp_path / m.id / "schema_linking.json").exists()
diff --git a/harness/tht/session/store.py b/harness/tht/session/store.py
index c94958dd..859f1de3 100644
--- a/harness/tht/session/store.py
+++ b/harness/tht/session/store.py
@@ -1,3 +1,4 @@
+import json
 import os
 import shutil
 from datetime import UTC, datetime
@@ -158,6 +159,28 @@ def set_question(
     return path
 
 
+def set_schema_linking(
+    session_id: str,
+    data: dict,
+    sessions_root: Path,
+) -> Path:
+    """Valida e scrive schema_linking.json (Fase 4) in modo deterministico.
+
+    Valida `data` contro il modello SchemaLinking (ValidationError se invalido) PRIMA
+    di scrivere, così un artefatto malformato non tocca mai il disco. Ritorna il path.
+    """
+    from tht.session.models import SchemaLinking
+
+    load_session(session_id, sessions_root)
+    model = SchemaLinking.model_validate(data)
+    path = sessions_root / session_id / "schema_linking.json"
+    path.write_text(
+        json.dumps(model.model_dump(by_alias=True), indent=2, ensure_ascii=False)
+    )
+    touch_manifest(session_id, sessions_root)
+    return path
+
+
 def load_session(session_id: str, sessions_root: Path) -> SessionManifest:
     path = sessions_root / session_id / MANIFEST
     if not path.exists():