diff --git a/backend/app/db.py b/backend/app/db.py
index 290d0b0..c0afdcd 100644
--- a/backend/app/db.py
+++ b/backend/app/db.py
@@ -58,6 +58,9 @@ def init_db() -> None:
         scols = {r["name"] for r in conn.execute("PRAGMA table_info(sessions)")}
         if "summary" not in scols:
             conn.execute("ALTER TABLE sessions ADD COLUMN summary TEXT")
+        # Migration douce : projet (sous-dossier de travail) de la session.
+        if "project" not in scols:
+            conn.execute("ALTER TABLE sessions ADD COLUMN project TEXT")
         # Table clé/valeur pour la configuration de l'agent.
         conn.execute(
             "CREATE TABLE IF NOT EXISTS config (key TEXT PRIMARY KEY, value TEXT)"
@@ -69,19 +72,28 @@ def _now() -> float:
 
 
 # ── Sessions ─────────────────────────────────────────────────────────────
-def create_session(title: str, model: str | None) -> dict:
+def create_session(
+    title: str, model: str | None, project: str | None = None
+) -> dict:
     sid = uuid.uuid4().hex
     now = _now()
     with _LOCK, _connect() as conn:
         conn.execute(
-            "INSERT INTO sessions (id, title, model, created_at, updated_at)"
-            " VALUES (?, ?, ?, ?, ?)",
-            (sid, title, model, now, now),
+            "INSERT INTO sessions (id, title, model, project, created_at, updated_at)"
+            " VALUES (?, ?, ?, ?, ?, ?)",
+            (sid, title, model, project, now, now),
         )
-    return {"id": sid, "title": title, "model": model,
+    return {"id": sid, "title": title, "model": model, "project": project,
             "created_at": now, "updated_at": now, "message_count": 0}
 
 
+def set_session_project(sid: str, project: str | None) -> None:
+    with _LOCK, _connect() as conn:
+        conn.execute(
+            "UPDATE sessions SET project = ? WHERE id = ?", (project, sid)
+        )
+
+
 def list_sessions() -> list[dict]:
     with _LOCK, _connect() as conn:
         rows = conn.execute(
diff --git a/backend/app/routes/chat.py b/backend/app/routes/chat.py
index c5a834f..eb3064b 100644
--- a/backend/app/routes/chat.py
+++ b/backend/app/routes/chat.py
@@ -20,7 +20,7 @@ from fastapi import APIRouter, HTTPException
 from fastapi.responses import StreamingResponse
 from pydantic import BaseModel
 
-from .. import agent_config, coder, db, enhance, memory, rag, skills
+from .. import agent_config, coder, db, enhance, memory, rag, skills, tools
 from .. import router as msg_router
 from ..tools import check_html, _safe_path
 from ..agent import run_agent
@@ -87,7 +87,7 @@ def _session_code_context(history: list[dict]) -> tuple[str, list[str]]:
     et quelle demande d'origine. Les fichiers touchés servent aussi de cible
     par défaut pour Aider.
     """
-    root = os.path.abspath(settings.workspace_dir)
+    root = tools.active_root()
     files: list[str] = []
     for m in history:
         if m["role"] != "assistant":
@@ -121,8 +121,8 @@ def _session_code_context(history: list[dict]) -> tuple[str, list[str]]:
 
 
 def _workspace_listing(limit: int = 40) -> list[str]:
-    """Chemins relatifs des fichiers du workspace (aperçu compact)."""
-    root = os.path.abspath(settings.workspace_dir)
+    """Chemins relatifs des fichiers de la racine active (aperçu compact)."""
+    root = tools.active_root()
     out: list[str] = []
     for dirpath, dirnames, filenames in os.walk(root):
         dirnames[:] = [d for d in dirnames if not d.startswith(".")]
@@ -145,7 +145,7 @@ def _mentioned_files(text: str) -> list[str]:
     Transmis au moteur code pour qu'Aider travaille directement sur les bons
     fichiers au lieu de deviner via la repo map.
     """
-    root = os.path.abspath(settings.workspace_dir)
+    root = tools.active_root()
     out: list[str] = []
     for raw in _FILE_MENTION.findall(text):
         rel = raw.replace("\\", "/").lstrip("./")
diff --git a/backend/app/routes/sessions.py b/backend/app/routes/sessions.py
index ba3d1fe..99fa1f3 100644
--- a/backend/app/routes/sessions.py
+++ b/backend/app/routes/sessions.py
@@ -4,7 +4,7 @@ from __future__ import annotations
 from fastapi import APIRouter, HTTPException
 from pydantic import BaseModel
 
-from .. import db
+from .. import db, tools
 
 router = APIRouter(prefix="/api/sessions", tags=["sessions"])
 
@@ -12,10 +12,12 @@ router = APIRouter(prefix="/api/sessions", tags=["sessions"])
 class CreateSession(BaseModel):
     title: str = "Nouvelle session"
     model: str | None = None
+    project: str | None = None
 
 
-class RenameSession(BaseModel):
-    title: str
+class UpdateSession(BaseModel):
+    title: str | None = None
+    project: str | None = None  # "" = retour à la racine du workspace
 
 
 @router.get("")
@@ -25,7 +27,9 @@ async def get_sessions() -> dict:
 
 @router.post("")
 async def post_session(req: CreateSession) -> dict:
-    return db.create_session(req.title, req.model)
+    if req.project and not tools.PROJECT_NAME.match(req.project):
+        raise HTTPException(400, "nom de projet invalide")
+    return db.create_session(req.title, req.model, req.project or None)
 
 
 @router.get("/{sid}")
@@ -37,10 +41,16 @@ async def get_one(sid: str) -> dict:
 
 
 @router.patch("/{sid}")
-async def patch_session(sid: str, req: RenameSession) -> dict:
+async def patch_session(sid: str, req: UpdateSession) -> dict:
     if not db.get_session(sid):
         raise HTTPException(404, "session introuvable")
-    db.rename_session(sid, req.title)
+    if req.title is not None:
+        db.rename_session(sid, req.title)
+    if req.project is not None:
+        project = req.project or None
+        if project and not tools.PROJECT_NAME.match(project):
+            raise HTTPException(400, "nom de projet invalide")
+        db.set_session_project(sid, project)
     return {"ok": True}
 
 
diff --git a/backend/app/tools.py b/backend/app/tools.py
index 6dac043..5bc8c44 100644
--- a/backend/app/tools.py
+++ b/backend/app/tools.py
@@ -15,6 +15,7 @@ import os
 import re
 import shutil
 import subprocess
+from contextvars import ContextVar
 
 import httpx
 
@@ -25,8 +26,30 @@ class ToolError(Exception):
     """Erreur d'exécution d'un outil (message destiné au modèle)."""
 
 
+# Projet actif pour la requête en cours : re-racine tous les outils sur
+# workspace/<projet>. None = racine du workspace (comportement historique).
+_ACTIVE_PROJECT: ContextVar[str | None] = ContextVar("loki_project", default=None)
+
+PROJECT_NAME = re.compile(r"^[a-z0-9][a-z0-9_-]{0,40}$")
+
+
+def set_project(name: str | None) -> None:
+    """Fixe le projet actif de la requête (None = racine)."""
+    if name is not None and not PROJECT_NAME.match(name):
+        raise ToolError(f"nom de projet invalide : {name!r}")
+    _ACTIVE_PROJECT.set(name)
+
+
+def active_root() -> str:
+    """Racine effective (workspace ou projet), créée si nécessaire."""
+    return _workspace_root()
+
+
 def _workspace_root() -> str:
     root = os.path.abspath(settings.workspace_dir)
+    project = _ACTIVE_PROJECT.get()
+    if project:
+        root = os.path.join(root, project)
     os.makedirs(root, exist_ok=True)
     return root
 
diff --git a/backend/tests/test_projects.py b/backend/tests/test_projects.py
new file mode 100644
index 0000000..0a1965a
--- /dev/null
+++ b/backend/tests/test_projects.py
@@ -0,0 +1,62 @@
+import os
+import tempfile
+
+os.environ.setdefault("DATA_DIR", tempfile.mkdtemp())
+os.environ.setdefault("WORKSPACE_DIR", tempfile.mkdtemp())
+
+import pytest  # noqa: E402
+
+from app import db, tools  # noqa: E402
+from app.config import settings  # noqa: E402
+
+db.init_db()
+_ROOT = os.path.abspath(settings.workspace_dir)
+
+
+@pytest.fixture(autouse=True)
+def _reset_project():
+    tools.set_project(None)
+    yield
+    tools.set_project(None)
+
+
+def test_racine_par_defaut():
+    assert tools.active_root() == _ROOT
+
+
+def test_set_project_reracine():
+    tools.set_project("demo")
+    root = tools.active_root()
+    assert root == os.path.join(_ROOT, "demo")
+    assert os.path.isdir(root)  # créée à la volée
+
+
+def test_confinement_conserve():
+    tools.set_project("demo")
+    with pytest.raises(tools.ToolError):
+        tools._safe_path("../hors-projet")
+
+
+def test_nom_projet_invalide():
+    for bad in ("../x", "UPPER", "a b", "", "x" * 50):
+        with pytest.raises(tools.ToolError):
+            tools.set_project(bad)
+
+
+def test_session_porte_son_projet():
+    s = db.create_session("t", None, project="demo")
+    assert db.get_session(s["id"])["project"] == "demo"
+    db.set_session_project(s["id"], None)
+    assert db.get_session(s["id"])["project"] is None
+
+
+def test_aides_contexte_suivent_le_projet():
+    from app.routes.chat import _mentioned_files, _workspace_listing
+    tools.set_project("ctxdemo")
+    root = tools.active_root()
+    with open(os.path.join(root, "app.py"), "w", encoding="utf-8") as f:
+        f.write("x = 1")
+    assert "app.py" in _workspace_listing()
+    assert _mentioned_files("corrige app.py") == ["app.py"]
+    tools.set_project(None)
+    assert _mentioned_files("corrige app.py") == []  # absent de la racine