diff --git a/backend/app/services/copilot_service.py b/backend/app/services/copilot_service.py
index 175735ee..76327ff4 100644
--- a/backend/app/services/copilot_service.py
+++ b/backend/app/services/copilot_service.py
@@ -8,7 +8,7 @@ from datetime import datetime, timezone, timedelta
 from typing import Optional, Any
 from uuid import UUID
 
-from sqlalchemy import select
+from sqlalchemy import select, or_
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import selectinload
 
@@ -103,13 +103,23 @@ async def start_conversation(
 
     Returns (conversation, greeting_message).
     """
-    # Load tree
+    # Load tree — must be accessible to this account.
+    # Allows own account's trees, default trees, and public trees.
+    # Raises ValueError (caught by endpoint as 404) if not found or not accessible.
     result = await db.execute(
-        select(Tree).options(selectinload(Tree.tags)).where(Tree.id == tree_id)
+        select(Tree).options(selectinload(Tree.tags)).where(
+            Tree.id == tree_id,
+            or_(
+                Tree.account_id == account_id,
+                Tree.author_id == user_id,
+                Tree.is_default == True,
+                Tree.is_public == True,
+            ),
+        )
     )
     tree = result.scalar_one_or_none()
     if not tree:
-        raise ValueError(f"Tree {tree_id} not found")
+        raise ValueError(f"Tree {tree_id} not found or not accessible")
 
     conversation = CopilotConversation(
         user_id=user_id,
diff --git a/backend/tests/test_tenant_isolation_p0.py b/backend/tests/test_tenant_isolation_p0.py
new file mode 100644
index 00000000..04018f30
--- /dev/null
+++ b/backend/tests/test_tenant_isolation_p0.py
@@ -0,0 +1,107 @@
+"""Cross-tenant isolation tests for Phase 0 gap fixes."""
+import pytest
+from httpx import AsyncClient
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy import select
+import uuid
+
+from app.models.account import Account
+from app.models.user import User
+from app.models.tree import Tree
+from app.core.security import get_password_hash
+
+
+# ── Helpers ──────────────────────────────────────────────────────────────────
+
+async def _create_account_and_user(db: AsyncSession, suffix: str) -> tuple[Account, User, str]:
+    """Create an account + owner user. Returns (account, user, plain_password)."""
+    account = Account(
+        name=f"Test Corp {suffix}",
+        display_code=uuid.uuid4().hex[:8],
+    )
+    db.add(account)
+    await db.flush()
+
+    password = "TestPass123!"
+    user = User(
+        email=f"user-{suffix}-{uuid.uuid4().hex[:6]}@example.com",
+        name=f"User {suffix}",
+        password_hash=get_password_hash(password),
+        is_active=True,
+        account_id=account.id,
+        account_role="owner",
+    )
+    db.add(user)
+    await db.flush()
+    return account, user, password
+
+
+async def _login(client: AsyncClient, email: str, password: str) -> dict:
+    """Return auth headers for a user."""
+    resp = await client.post(
+        "/api/v1/auth/login/json",
+        json={"email": email, "password": password},
+    )
+    assert resp.status_code == 200, resp.text
+    return {"Authorization": f"Bearer {resp.json()['access_token']}"}
+
+
+async def _create_private_tree(db: AsyncSession, account: Account, user: User) -> Tree:
+    """Create a private tree owned by account."""
+    tree = Tree(
+        name=f"Private Tree {uuid.uuid4().hex[:6]}",
+        account_id=account.id,
+        author_id=user.id,
+        visibility="private",
+        tree_type="troubleshooting",
+        tree_structure={"id": "root", "type": "start", "children": []},
+        is_active=True,
+        status="published",
+    )
+    db.add(tree)
+    await db.flush()
+    return tree
+
+
+# ── Task 1: Copilot bypass ────────────────────────────────────────────────────
+
+@pytest.mark.asyncio
+async def test_copilot_cannot_start_conversation_with_other_account_tree(
+    client: AsyncClient, test_db: AsyncSession
+):
+    """Account A cannot start a copilot conversation using Account B's private tree UUID."""
+    acct_a, user_a, pass_a = await _create_account_and_user(test_db, "a")
+    acct_b, user_b, pass_b = await _create_account_and_user(test_db, "b")
+    tree_b = await _create_private_tree(test_db, acct_b, user_b)
+    await test_db.commit()
+
+    headers_a = await _login(client, user_a.email, pass_a)
+
+    resp = await client.post(
+        "/api/v1/copilot/conversations",
+        json={"tree_id": str(tree_b.id), "session_id": None, "current_node_id": None},
+        headers=headers_a,
+    )
+    # Must be 404 (not 200, not 403 — never confirm existence)
+    assert resp.status_code == 404, f"Expected 404, got {resp.status_code}: {resp.text}"
+
+
+@pytest.mark.asyncio
+async def test_copilot_service_rejects_cross_tenant_tree(test_db: AsyncSession):
+    """copilot_service.start_conversation raises ValueError for cross-tenant tree."""
+    from app.services import copilot_service
+
+    acct_a, user_a, pass_a = await _create_account_and_user(test_db, "svc-a")
+    acct_b, user_b, pass_b = await _create_account_and_user(test_db, "svc-b")
+    tree_b = await _create_private_tree(test_db, acct_b, user_b)
+    await test_db.commit()
+
+    with pytest.raises(ValueError, match="not found"):
+        await copilot_service.start_conversation(
+            user_id=user_a.id,
+            account_id=user_a.account_id,
+            tree_id=tree_b.id,
+            session_id=None,
+            current_node_id=None,
+            db=test_db,
+        )