fix(vp): close #1780 race — bump slicer-MQTT wait to 5s + retroactive stamp

Round 2 (166e9f9e) fixed the stash-key mismatch, but @mkoreen's
      2026-06-23 bundle showed BS's MQTT project_file arrived 85 ms past the
      2.0 s wait timeout (FTP done 00:42:02.509, "No slicer options cached"
      00:42:04.509, MQTT 00:42:04.594). Queue item was committed with
      settings defaults; nozzle_mapping never made it onto the wire.

      Three pieces:

      1. _SLICER_OPTIONS_WAIT_TIMEOUT module constant, 2.0 -> 5.0 s. Covers
         wireless / loaded-Pi jitter; one-time +3 s cost only for legacy
         slicers that never send MQTT.

      2. _RECENT_QUEUE_ITEM_TTL fallback: on_print_command retroactively
         UPDATEs slicer-driven fields on a recently-committed queue item
         when the event wait already gave up. Tracked via
         _recent_queue_items dict (30 s TTL, evicted on every queue-add).
         Gated on status='pending' so we never race the dispatcher.
         Multi-plate covered via WHERE id IN (...).

      3. Post-commit last-chance pop. Audit caught a race in (2): MQTT could
         arrive during any await inside _add_to_print_queue (wait_for,
         archive_print, db.flush, db.commit), and on_print_command would
         stash data with no event consumer AND no _recent_queue_items entry
         yet. After populating _recent_queue_items, _add_to_print_queue now
         pops _slicer_print_options[file_path.name] one last time and
         routes any hit through _restamp inline.

backend/tests/integration/test_inventory_bulk.py

@@ -0,0 +1,212 @@
+"""Bulk inventory endpoint coverage for the batch-edit feature (#1795).
+
+Endpoints under test:
+- POST /api/v1/inventory/spools/bulk-update
+- POST /api/v1/inventory/spools/bulk-delete
+- POST /api/v1/inventory/spools/bulk-archive
+- POST /api/v1/inventory/spools/bulk-restore
+
+The Spoolman-mode equivalents live in test_spoolman_inventory_api.py.
+"""
+
+from datetime import datetime, timezone
+
+import pytest
+from httpx import AsyncClient
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from backend.app.models.spool import Spool
+
+
+@pytest.fixture
+async def spool_factory(db_session: AsyncSession):
+    async def _create(**kwargs):
+        defaults = {
+            "material": "PLA",
+            "subtype": "Basic",
+            "brand": "Bambu",
+            "color_name": "Red",
+            "rgba": "FF0000FF",
+            "label_weight": 1000,
+            "core_weight": 250,
+            "weight_used": 0,
+            "weight_used_baseline": 0,
+            "weight_locked": False,
+        }
+        defaults.update(kwargs)
+        spool = Spool(**defaults)
+        db_session.add(spool)
+        await db_session.commit()
+        await db_session.refresh(spool)
+        return spool
+
+    return _create
+
+
+class TestBulkUpdate:
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_applies_patch_to_all_listed_spools(self, async_client: AsyncClient, spool_factory, db_session):
+        a = await spool_factory(brand="Bambu", note=None)
+        b = await spool_factory(brand="Bambu", note=None)
+        c = await spool_factory(brand="Bambu", note=None)
+
+        resp = await async_client.post(
+            "/api/v1/inventory/spools/bulk-update",
+            json={"ids": [a.id, b.id, c.id], "update": {"brand": "Sunlu", "note": "From bulk edit"}},
+        )
+
+        assert resp.status_code == 200
+        body = resp.json()
+        assert body["updated"] == 3
+        assert body["not_found"] == []
+
+        for spool in (a, b, c):
+            await db_session.refresh(spool)
+            assert spool.brand == "Sunlu"
+            assert spool.note == "From bulk edit"
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_reports_unknown_ids_in_not_found(self, async_client: AsyncClient, spool_factory, db_session):
+        real = await spool_factory(brand="Bambu")
+        resp = await async_client.post(
+            "/api/v1/inventory/spools/bulk-update",
+            json={"ids": [real.id, 999_999], "update": {"brand": "Sunlu"}},
+        )
+
+        assert resp.status_code == 200
+        body = resp.json()
+        assert body["updated"] == 1
+        assert body["not_found"] == [999_999]
+
+        await db_session.refresh(real)
+        assert real.brand == "Sunlu"
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_empty_update_rejected(self, async_client: AsyncClient, spool_factory):
+        a = await spool_factory()
+        resp = await async_client.post(
+            "/api/v1/inventory/spools/bulk-update",
+            json={"ids": [a.id], "update": {}},
+        )
+        assert resp.status_code == 400
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_setting_weight_used_auto_locks(self, async_client: AsyncClient, spool_factory, db_session):
+        a = await spool_factory(weight_locked=False, weight_used=0.0)
+        resp = await async_client.post(
+            "/api/v1/inventory/spools/bulk-update",
+            json={"ids": [a.id], "update": {"weight_used": 250.5}},
+        )
+        assert resp.status_code == 200
+        await db_session.refresh(a)
+        assert a.weight_used == 250.5
+        assert a.weight_locked is True
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_empty_ids_list_rejected(self, async_client: AsyncClient):
+        resp = await async_client.post(
+            "/api/v1/inventory/spools/bulk-update",
+            json={"ids": [], "update": {"brand": "X"}},
+        )
+        assert resp.status_code == 422
+
+
+class TestBulkDelete:
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_deletes_listed_spools(self, async_client: AsyncClient, spool_factory, db_session):
+        a = await spool_factory()
+        b = await spool_factory()
+        kept = await spool_factory()
+
+        resp = await async_client.post(
+            "/api/v1/inventory/spools/bulk-delete",
+            json={"ids": [a.id, b.id]},
+        )
+
+        assert resp.status_code == 200
+        body = resp.json()
+        assert body["deleted"] == 2
+        assert body["not_found"] == []
+
+        remaining = (await db_session.execute(select(Spool.id))).scalars().all()
+        assert kept.id in remaining
+        assert a.id not in remaining
+        assert b.id not in remaining
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_reports_unknown_ids(self, async_client: AsyncClient, spool_factory):
+        a = await spool_factory()
+        resp = await async_client.post(
+            "/api/v1/inventory/spools/bulk-delete",
+            json={"ids": [a.id, 999_999]},
+        )
+        assert resp.status_code == 200
+        body = resp.json()
+        assert body["deleted"] == 1
+        assert body["not_found"] == [999_999]
+
+
+class TestBulkArchiveRestore:
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_bulk_archive_sets_archived_at(self, async_client: AsyncClient, spool_factory, db_session):
+        a = await spool_factory()
+        b = await spool_factory()
+
+        resp = await async_client.post(
+            "/api/v1/inventory/spools/bulk-archive",
+            json={"ids": [a.id, b.id]},
+        )
+
+        assert resp.status_code == 200
+        body = resp.json()
+        assert body["archived"] == 2
+        assert body["already_archived"] == []
+        assert body["not_found"] == []
+
+        for s in (a, b):
+            await db_session.refresh(s)
+            assert s.archived_at is not None
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_bulk_archive_skips_already_archived(self, async_client: AsyncClient, spool_factory, db_session):
+        active = await spool_factory()
+        already = await spool_factory(archived_at=datetime.now(timezone.utc))
+
+        resp = await async_client.post(
+            "/api/v1/inventory/spools/bulk-archive",
+            json={"ids": [active.id, already.id]},
+        )
+
+        assert resp.status_code == 200
+        body = resp.json()
+        assert body["archived"] == 1
+        assert body["already_archived"] == [already.id]
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_bulk_restore_clears_archived_at(self, async_client: AsyncClient, spool_factory, db_session):
+        archived = await spool_factory(archived_at=datetime.now(timezone.utc))
+        active = await spool_factory(archived_at=None)
+
+        resp = await async_client.post(
+            "/api/v1/inventory/spools/bulk-restore",
+            json={"ids": [archived.id, active.id]},
+        )
+
+        assert resp.status_code == 200
+        body = resp.json()
+        assert body["restored"] == 1
+        assert body["already_active"] == [active.id]
+
+        await db_session.refresh(archived)
+        assert archived.archived_at is None

backend/tests/integration/test_spoolman_inventory_bulk.py

@@ -0,0 +1,200 @@
+"""Bulk Spoolman inventory endpoint coverage for the batch-edit feature (#1795).
+
+Endpoints under test:
+- POST /api/v1/spoolman/inventory/spools/bulk-update
+- POST /api/v1/spoolman/inventory/spools/bulk-delete
+- POST /api/v1/spoolman/inventory/spools/bulk-archive
+- POST /api/v1/spoolman/inventory/spools/bulk-restore
+"""
+
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+from fastapi import HTTPException
+from httpx import AsyncClient
+
+SAMPLE_SPOOLMAN_SPOOL = {
+    "id": 42,
+    "filament": {
+        "id": 7,
+        "name": "PLA Basic",
+        "material": "PLA",
+        "color_hex": "FF0000",
+        "weight": 1000,
+        "vendor": {"id": 3, "name": "Bambu Lab"},
+    },
+    "remaining_weight": 750.0,
+    "used_weight": 250.0,
+    "location": "Printer1 - AMS A1",
+    "comment": "test note",
+    "first_used": "2024-01-01T00:00:00+00:00",
+    "last_used": "2024-02-01T00:00:00+00:00",
+    "registered": "2024-01-01T00:00:00+00:00",
+    "archived": False,
+    "price": None,
+    "extra": {},
+}
+
+
+@pytest.fixture
+async def spoolman_settings(db_session):
+    from backend.app.models.settings import Settings
+
+    db_session.add(Settings(key="spoolman_enabled", value="true"))
+    db_session.add(Settings(key="spoolman_url", value="http://localhost:7912"))
+    await db_session.commit()
+
+
+@pytest.fixture
+def mock_spoolman_client():
+    mock = MagicMock()
+    mock.base_url = "http://localhost:7912"
+    mock.health_check = AsyncMock(return_value=True)
+    mock.get_spool = AsyncMock(return_value=SAMPLE_SPOOLMAN_SPOOL)
+    mock.delete_spool = AsyncMock(return_value=True)
+    mock.set_spool_archived = AsyncMock(
+        side_effect=lambda spool_id, archived: {**SAMPLE_SPOOLMAN_SPOOL, "archived": archived}
+    )
+    mock.update_spool_full = AsyncMock(return_value=SAMPLE_SPOOLMAN_SPOOL)
+    mock.merge_spool_extra = AsyncMock(return_value=SAMPLE_SPOOLMAN_SPOOL)
+    mock.is_filament_shared = AsyncMock(return_value=False)
+    mock.patch_filament = AsyncMock(return_value={"id": 7})
+    mock.find_or_create_filament = AsyncMock(return_value=7)
+    mock.find_or_create_vendor = AsyncMock(return_value=3)
+    mock.ensure_extra_field = AsyncMock(return_value=True)
+    mock.get_distinct_locations = AsyncMock(return_value=[])
+
+    class _Lock:
+        async def __aenter__(self):
+            return self
+
+        async def __aexit__(self, *args):
+            return False
+
+    mock.extra_lock = lambda spool_id: _Lock()
+
+    with (
+        patch(
+            "backend.app.api.routes.spoolman_inventory.get_spoolman_client",
+            AsyncMock(return_value=mock),
+        ),
+        patch(
+            "backend.app.api.routes.spoolman_inventory.init_spoolman_client",
+            AsyncMock(return_value=mock),
+        ),
+    ):
+        yield mock
+
+
+class TestSpoolmanBulkUpdate:
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_calls_per_spool_update_for_each_id(
+        self, async_client: AsyncClient, spoolman_settings, mock_spoolman_client
+    ):
+        resp = await async_client.post(
+            "/api/v1/spoolman/inventory/spools/bulk-update",
+            json={"ids": [42, 43, 44], "update": {"note": "From bulk edit"}},
+        )
+
+        assert resp.status_code == 200
+        body = resp.json()
+        assert body["updated"] == 3
+        assert body["errors"] == []
+        # update_spool route loops through each, which calls update_spool_full once per ID
+        assert mock_spoolman_client.update_spool_full.await_count == 3
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_collects_per_spool_errors_without_aborting_batch(
+        self, async_client: AsyncClient, spoolman_settings, mock_spoolman_client
+    ):
+        # First two succeed, third raises
+        mock_spoolman_client.update_spool_full.side_effect = [
+            SAMPLE_SPOOLMAN_SPOOL,
+            SAMPLE_SPOOLMAN_SPOOL,
+            HTTPException(status_code=404, detail="Spool 999 not found"),
+        ]
+
+        resp = await async_client.post(
+            "/api/v1/spoolman/inventory/spools/bulk-update",
+            json={"ids": [42, 43, 999], "update": {"note": "Batched"}},
+        )
+
+        assert resp.status_code == 200
+        body = resp.json()
+        assert body["updated"] == 2
+        assert len(body["errors"]) == 1
+        assert body["errors"][0]["id"] == 999
+        assert body["errors"][0]["status"] == 404
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_empty_update_rejected(self, async_client: AsyncClient, spoolman_settings, mock_spoolman_client):
+        resp = await async_client.post(
+            "/api/v1/spoolman/inventory/spools/bulk-update",
+            json={"ids": [42], "update": {}},
+        )
+        assert resp.status_code == 400
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_empty_ids_rejected(self, async_client: AsyncClient, spoolman_settings, mock_spoolman_client):
+        resp = await async_client.post(
+            "/api/v1/spoolman/inventory/spools/bulk-update",
+            json={"ids": [], "update": {"note": "X"}},
+        )
+        assert resp.status_code == 422
+
+
+class TestSpoolmanBulkDelete:
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_deletes_listed_spools(self, async_client: AsyncClient, spoolman_settings, mock_spoolman_client):
+        resp = await async_client.post(
+            "/api/v1/spoolman/inventory/spools/bulk-delete",
+            json={"ids": [42, 43, 44]},
+        )
+
+        assert resp.status_code == 200
+        body = resp.json()
+        assert body["deleted"] == 3
+        assert body["errors"] == []
+        assert mock_spoolman_client.delete_spool.await_count == 3
+
+
+class TestSpoolmanBulkArchiveRestore:
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_bulk_archive_calls_per_spool(
+        self, async_client: AsyncClient, spoolman_settings, mock_spoolman_client
+    ):
+        resp = await async_client.post(
+            "/api/v1/spoolman/inventory/spools/bulk-archive",
+            json={"ids": [42, 43]},
+        )
+
+        assert resp.status_code == 200
+        body = resp.json()
+        assert body["archived"] == 2
+        # set_spool_archived(spool_id, archived=True) called for each id
+        assert mock_spoolman_client.set_spool_archived.await_count == 2
+        for call in mock_spoolman_client.set_spool_archived.call_args_list:
+            assert call.kwargs.get("archived") is True
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_bulk_restore_calls_per_spool(
+        self, async_client: AsyncClient, spoolman_settings, mock_spoolman_client
+    ):
+        resp = await async_client.post(
+            "/api/v1/spoolman/inventory/spools/bulk-restore",
+            json={"ids": [42, 43]},
+        )
+
+        assert resp.status_code == 200
+        body = resp.json()
+        assert body["restored"] == 2
+        assert mock_spoolman_client.set_spool_archived.await_count == 2
+        for call in mock_spoolman_client.set_spool_archived.call_args_list:
+            assert call.kwargs.get("archived") is False

backend/tests/unit/services/test_virtual_printer.py

@@ -1785,6 +1785,272 @@ class TestVirtualPrinterInstance:
         for item in added_items:
             assert _json.loads(item.nozzle_mapping) == [16, 0]
 
+    @pytest.mark.asyncio
+    async def test_on_print_command_late_mqtt_retroactively_stamps_queue_item(self, tmp_path):
+        """#1780 round 3: Bambu Studio's MQTT project_file can arrive AFTER
+        `_add_to_print_queue` already gave up waiting (observed at 2.085 s
+        on H2C wireless setups). The queue item was committed with settings
+        defaults; the slicer's nozzle_mapping + workflow flags must be
+        patched onto it when MQTT lands, otherwise the H2C firmware falls
+        back to auto-pick.
+        """
+        import json as _json
+
+        from backend.app.services.virtual_printer.manager import VirtualPrinterInstance
+
+        added_items: list = []
+        mock_db = AsyncMock()
+        mock_db.add = MagicMock(
+            side_effect=lambda item: (added_items.append(item), setattr(item, "id", 100 + len(added_items)))[0]
+        )
+
+        async def _flush():
+            # added_items[-1].id was set by `add`; nothing else to do.
+            return None
+
+        mock_db.flush = AsyncMock(side_effect=_flush)
+        mock_db.commit = AsyncMock()
+
+        # First execute() call (the position-max SELECT inside _add_to_print_queue)
+        # returns None; second (the eligible-pending SELECT in
+        # _restamp_recent_queue_item) returns the committed queue id; third
+        # (the UPDATE) is fire-and-forget.
+        position_max_result = MagicMock()
+        position_max_result.scalar = MagicMock(return_value=None)
+        select_pending_result = MagicMock()
+        select_pending_result.all = MagicMock(return_value=[(101,)])
+        update_result = MagicMock()
+        mock_db.execute = AsyncMock(side_effect=[position_max_result, select_pending_result, update_result])
+
+        mock_session_factory = MagicMock()
+        mock_session_ctx = AsyncMock()
+        mock_session_ctx.__aenter__ = AsyncMock(return_value=mock_db)
+        mock_session_ctx.__aexit__ = AsyncMock(return_value=False)
+        mock_session_factory.return_value = mock_session_ctx
+
+        inst = VirtualPrinterInstance(
+            vp_id=99,
+            name="LateMQTT",
+            mode="queue",
+            model="O1C2",
+            access_code="12345678",
+            serial_suffix="391800099",
+            base_dir=tmp_path,
+            session_factory=mock_session_factory,
+        )
+        # MQTT server presence enables the wait_for path; we don't actually
+        # use any methods on it.
+        inst._mqtt = MagicMock()
+
+        file_path = tmp_path / "test.3mf"
+        file_path.write_bytes(b"fake3mf")
+
+        mock_archive = MagicMock()
+        mock_archive.id = 1
+        mock_archive.print_name = "test"
+
+        # 1. _add_to_print_queue runs WITHOUT a prior on_print_command —
+        #    the wait_for times out (settings-default fallback) and the
+        #    queue item is committed.
+        with (
+            patch(
+                "backend.app.api.routes.settings.get_setting",
+                new_callable=AsyncMock,
+                return_value=None,
+            ),
+            patch(
+                "backend.app.services.archive.ArchiveService.archive_print",
+                new_callable=AsyncMock,
+                return_value=mock_archive,
+            ),
+            # Shorten the wait so the test isn't slow.
+            patch(
+                "backend.app.services.virtual_printer.manager._SLICER_OPTIONS_WAIT_TIMEOUT",
+                0.05,
+            ),
+        ):
+            await inst._add_to_print_queue(file_path, "192.168.1.100")
+
+        assert len(added_items) == 1
+        assert added_items[0].nozzle_mapping is None  # MQTT was never received
+        assert file_path.name in inst._recent_queue_items
+
+        # 2. MQTT project_file arrives AFTER the wait expired — must
+        #    retroactively patch the queue item.
+        await inst.on_print_command(
+            file_path.name,
+            {
+                "command": "project_file",
+                "file": file_path.name,
+                "nozzle_mapping": [16, -1, -1, 1],
+                "timelapse": True,
+                "bed_leveling": False,
+            },
+        )
+
+        # The UPDATE call is the third execute. Inspect its values.
+        update_call = mock_db.execute.await_args_list[2]
+        update_stmt = update_call.args[0]
+        compiled = update_stmt.compile(compile_kwargs={"literal_binds": False})
+        params = dict(compiled.params)
+        assert _json.loads(params["nozzle_mapping"]) == [16, -1, -1, 1]
+        assert params["timelapse"] is True
+        assert params["bed_levelling"] is False  # MQTT bed_leveling → column bed_levelling
+        # Recent-queue tracking dict is cleared after the patch.
+        assert file_path.name not in inst._recent_queue_items
+
+    @pytest.mark.asyncio
+    async def test_add_to_print_queue_catches_mqtt_stashed_post_wait_timeout(self, tmp_path):
+        """The actual race-window scenario: wait_for times out, then MQTT
+        arrives and stashes options AFTER the wait but BEFORE the
+        post-commit re-check. The post-commit pop must catch it.
+        """
+        import json as _json
+
+        from backend.app.services.virtual_printer.manager import VirtualPrinterInstance
+
+        added_items: list = []
+        mock_db = AsyncMock()
+        mock_db.add = MagicMock(
+            side_effect=lambda item: (added_items.append(item), setattr(item, "id", 300 + len(added_items)))[0]
+        )
+        mock_db.flush = AsyncMock()
+        mock_db.commit = AsyncMock()
+
+        position_max_result = MagicMock()
+        position_max_result.scalar = MagicMock(return_value=None)
+        select_pending_result = MagicMock()
+        select_pending_result.all = MagicMock(return_value=[(301,)])
+        update_result = MagicMock()
+        mock_db.execute = AsyncMock(side_effect=[position_max_result, select_pending_result, update_result])
+
+        mock_session_factory = MagicMock()
+        mock_session_ctx = AsyncMock()
+        mock_session_ctx.__aenter__ = AsyncMock(return_value=mock_db)
+        mock_session_ctx.__aexit__ = AsyncMock(return_value=False)
+        mock_session_factory.return_value = mock_session_ctx
+
+        inst = VirtualPrinterInstance(
+            vp_id=96,
+            name="RaceCommitYield",
+            mode="queue",
+            model="O1C2",
+            access_code="12345678",
+            serial_suffix="391800096",
+            base_dir=tmp_path,
+            session_factory=mock_session_factory,
+        )
+        inst._mqtt = MagicMock()
+
+        file_path = tmp_path / "test.3mf"
+        file_path.write_bytes(b"fake3mf")
+
+        # Stash MQTT data on the FIRST commit (simulating MQTT arrival
+        # during _add_to_print_queue's commit yield); _restamp also calls
+        # db.commit later, so we one-shot the side effect.
+        commit_calls = {"n": 0}
+
+        async def _delayed_stash(*_args, **_kwargs):
+            commit_calls["n"] += 1
+            if commit_calls["n"] == 1:
+                inst._slicer_print_options[file_path.name] = {
+                    "command": "project_file",
+                    "file": file_path.name,
+                    "nozzle_mapping": [0, 16, -1, -1],
+                    "timelapse": False,
+                }
+            return None
+
+        mock_db.commit = AsyncMock(side_effect=_delayed_stash)
+
+        mock_archive = MagicMock()
+        mock_archive.id = 1
+        mock_archive.print_name = "test"
+
+        with (
+            patch(
+                "backend.app.api.routes.settings.get_setting",
+                new_callable=AsyncMock,
+                return_value=None,
+            ),
+            patch(
+                "backend.app.services.archive.ArchiveService.archive_print",
+                new_callable=AsyncMock,
+                return_value=mock_archive,
+            ),
+            patch(
+                "backend.app.services.virtual_printer.manager._SLICER_OPTIONS_WAIT_TIMEOUT",
+                0.05,
+            ),
+        ):
+            await inst._add_to_print_queue(file_path, "192.168.1.100")
+
+        # Queue item INSERTed with defaults (wait timed out, no slicer_opts).
+        assert len(added_items) == 1
+        # But the post-commit pop caught the late stash and applied the
+        # slicer nozzle_mapping via _restamp's UPDATE.
+        update_call = mock_db.execute.await_args_list[2]
+        update_stmt = update_call.args[0]
+        compiled = update_stmt.compile(compile_kwargs={"literal_binds": False})
+        params = dict(compiled.params)
+        assert _json.loads(params["nozzle_mapping"]) == [0, 16, -1, -1]
+        assert params["timelapse"] is False
+        # _recent_queue_items entry was consumed by the post-commit
+        # _restamp call.
+        assert file_path.name not in inst._recent_queue_items
+        # And the stash is empty.
+        assert file_path.name not in inst._slicer_print_options
+
+    @pytest.mark.asyncio
+    async def test_on_print_command_late_mqtt_skips_already_dispatched_item(self, tmp_path):
+        """Once the scheduler has picked the queue item up (status != pending),
+        the retroactive patch is a no-op — racing the dispatcher would be
+        unsafe.
+        """
+        from backend.app.services.virtual_printer.manager import VirtualPrinterInstance
+
+        mock_db = AsyncMock()
+        # The eligible-pending SELECT returns nothing — item is no longer pending.
+        empty_result = MagicMock()
+        empty_result.all = MagicMock(return_value=[])
+        mock_db.execute = AsyncMock(return_value=empty_result)
+        mock_db.commit = AsyncMock()
+
+        mock_session_factory = MagicMock()
+        mock_session_ctx = AsyncMock()
+        mock_session_ctx.__aenter__ = AsyncMock(return_value=mock_db)
+        mock_session_ctx.__aexit__ = AsyncMock(return_value=False)
+        mock_session_factory.return_value = mock_session_ctx
+
+        inst = VirtualPrinterInstance(
+            vp_id=98,
+            name="LateMQTTDispatched",
+            mode="queue",
+            model="O1C2",
+            access_code="12345678",
+            serial_suffix="391800098",
+            base_dir=tmp_path,
+            session_factory=mock_session_factory,
+        )
+        inst._mqtt = MagicMock()
+        # Pre-seed the recent-queue dict — pretend _add_to_print_queue just
+        # committed item id 42.
+        inst._recent_queue_items["test.3mf"] = ([42], 1_000_000.0)
+        # Drive _restamp via on_print_command on the late-MQTT path.
+        with patch("backend.app.services.virtual_printer.manager.time.monotonic", return_value=1_000_001.0):
+            await inst.on_print_command(
+                "test.3mf",
+                {
+                    "command": "project_file",
+                    "file": "test.3mf",
+                    "nozzle_mapping": [16, -1],
+                },
+            )
+        # No UPDATE was issued — only the eligibility SELECT ran.
+        assert mock_db.execute.await_count == 1
+        mock_db.commit.assert_not_awaited()
+        assert "test.3mf" not in inst._recent_queue_items
+
 
 class TestVirtualPrinterManager:
     """Tests for VirtualPrinterManager orchestrator."""