diff --git a/backend/app/services/l1_session_service.py b/backend/app/services/l1_session_service.py
index b6a309a7..b0eb107e 100644
--- a/backend/app/services/l1_session_service.py
+++ b/backend/app/services/l1_session_service.py
@@ -98,6 +98,28 @@ async def start_adhoc_session(
     return session
 
 
+async def start_ai_build_session(
+    db: AsyncSession,
+    *,
+    account_id: UUID,
+    user: User,
+    ticket_id: str,
+    ticket_kind: str,
+) -> L1WalkSession:
+    """Start an AI-built tree session (nodes generated on demand via next-node)."""
+    session = L1WalkSession(
+        account_id=account_id,
+        created_by_user_id=user.id,
+        acting_as=_resolve_acting_as(user),
+        ticket_id=ticket_id,
+        ticket_kind=ticket_kind,
+        session_kind="ai_build",
+    )
+    db.add(session)
+    await db.flush()
+    return session
+
+
 async def record_step(
     db: AsyncSession,
     *,
diff --git a/backend/tests/test_l1_session_service.py b/backend/tests/test_l1_session_service.py
index b558b598..072060f4 100644
--- a/backend/tests/test_l1_session_service.py
+++ b/backend/tests/test_l1_session_service.py
@@ -778,6 +778,24 @@ async def test_escalate_without_walk_reason_is_optional(test_db: AsyncSession):
     assert session.escalation_reason_category == "no_kb_content"
 
 
+# ---------------------------------------------------------------------------
+# T7: start_ai_build_session
+# ---------------------------------------------------------------------------
+
+@pytest.mark.asyncio
+async def test_start_ai_build_session(test_db: AsyncSession):
+    from app.services import l1_session_service as svc
+    account = await _make_account(test_db)
+    l1_user = await _make_user(test_db, account_id=account.id)
+    s = await svc.start_ai_build_session(
+        test_db, account_id=account.id, user=l1_user,
+        ticket_id="t-ai", ticket_kind="internal",
+    )
+    assert s.session_kind == "ai_build"
+    assert s.flow_id is None and s.flow_proposal_id is None
+    assert s.status == "active"
+
+
 # ---------------------------------------------------------------------------
 # T14 audit log tests (spec §5.6.1)
 # ---------------------------------------------------------------------------