Reuse dead VM names in sandbox auto-naming

src/mcqemu/tools/sandbox.py

@@ -25,11 +25,19 @@ async def _agent_responding(record) -> bool:
         return False
 
 
-def _auto_name(taken: list[str]) -> str:
-    if "sandbox" not in taken:
+def _auto_name(registry) -> str:
+    """First 'sandbox[-N]' name that is unregistered or belongs to a dead VM
+    (launch_vm reclaims dead names, so reusing them is safe and keeps names
+    predictable)."""
+
+    def usable(candidate: str) -> bool:
+        record = registry.get(candidate)
+        return record is None or not registry.is_process_alive(record)
+
+    if usable("sandbox"):
         return "sandbox"
     i = 2
-    while f"sandbox-{i}" in taken:
+    while not usable(f"sandbox-{i}"):
         i += 1
     return f"sandbox-{i}"
 
@@ -57,7 +65,7 @@ async def sandbox_vm(
         raise ToolError(f"Bad base image: {e}") from e
 
     if name is None:
-        name = _auto_name(state.registry.names())
+        name = _auto_name(state.registry)
     elif not valid_vm_name(name):
         raise ToolError(
             f"Invalid VM name {name!r}: use letters, digits, '.', '_', '-' (max 48 chars)."

tests/test_sandbox.py

@@ -99,13 +99,24 @@ async def test_sandbox_vm_creates_overlay_and_waits(dirs, base_image, fake_launc
     assert data["overlay"] == str(overlay)
 
 
-async def test_sandbox_auto_names_avoid_collisions(dirs, base_image, fake_launch, agent_up):
+async def test_sandbox_auto_names_avoid_live_collisions(
+    dirs, base_image, fake_launch, agent_up, all_pids_alive
+):
     write_registry(dirs, seeded_record(dirs, "sandbox"))
     async with Client(mcp) as client:
         data = result_data(await client.call_tool("sandbox_vm", {"base_image": str(base_image)}))
     assert data["name"] == "sandbox-2"
 
 
+async def test_sandbox_auto_name_reuses_dead(
+    dirs, base_image, fake_launch, agent_up, all_pids_dead
+):
+    write_registry(dirs, seeded_record(dirs, "sandbox"))
+    async with Client(mcp) as client:
+        data = result_data(await client.call_tool("sandbox_vm", {"base_image": str(base_image)}))
+    assert data["name"] == "sandbox"
+
+
 async def test_sandbox_agent_timeout_reports_hint(dirs, base_image, fake_launch, monkeypatch):
     async def never(record):
         return False