diff --git a/harness/.gitignore b/harness/.gitignore
index e38bdab7..af38b5f0 100644
--- a/harness/.gitignore
+++ b/harness/.gitignore
@@ -4,6 +4,10 @@ __pycache__/
 .venv/
 sessions/
 indexes/
+artifacts/
+# Per-customer active workspace (points at the customer repo via -c or symlink).
+# The committed templates live in workspaces/*.example.yaml; config/ holds the active one.
+config/tht.yaml
 *.egg-info/
 dist/
 build/
diff --git a/harness/tht/session/models.py b/harness/tht/session/models.py
index beb0c22e..89d8c154 100644
--- a/harness/tht/session/models.py
+++ b/harness/tht/session/models.py
@@ -1,14 +1,30 @@
 from datetime import datetime
-from typing import Literal
+from pathlib import Path
+from typing import Literal, Self
 
+import yaml
 from pydantic import BaseModel, Field, ConfigDict
 
 
-# Stub locale di _YamlModel (in the reference implementation vive in mschema/models.py).
-# Qui serve solo come base con populate_by_name; mschema/ sara' portato nel Task A9.
+# Stub locale di _YamlModel. In the reference implementation questa base vive in
+# mschema/models.py; qui la si replica perche' SessionManifest ha bisogno di
+# populate_by_name=True (db_schema usa l'alias "schema"), che la base mschema non
+# imposta. Identici to_yaml/from_yaml della controparte mschema.
 class _YamlModel(BaseModel):
     model_config = ConfigDict(populate_by_name=True)
 
+    def to_yaml(self, path: Path) -> None:
+        path.parent.mkdir(parents=True, exist_ok=True)
+        data = self.model_dump(by_alias=True, mode="json", exclude_defaults=False)
+        path.write_text(
+            yaml.safe_dump(data, sort_keys=False, allow_unicode=True, width=120)
+        )
+
+    @classmethod
+    def from_yaml(cls, path: Path) -> Self:
+        raw = yaml.safe_load(path.read_text())
+        return cls.model_validate(raw)
+
 
 class SessionManifest(_YamlModel):
     id: str