M8-R03: hand off channel binding during meter swap

app/api/routes/api/meters.py

@@ -61,7 +61,12 @@ from app.schemas.meter import (
     MeterPatchRequest,
     MeterResponse,
 )
-from app.services.meter_sources import ChannelNotFoundError, MeterSourceError, create_binding
+from app.services.meter_sources import (
+    ChannelNotFoundError,
+    MeterSourceError,
+    create_binding,
+    create_binding_for_meter_swap,
+)
 from app.services import timezone as _tz_mod
 from app.services.auth import AuthenticatedSession
 from app.services.energy_cost import recompute_range
@@ -231,6 +236,9 @@ def declare_energy_meter(
     started_at_utc = _localize_started_at(body.started_at)
 
     try:
+        old_meter = db.execute(
+            select(Meter).where(Meter.commodity == body.commodity, Meter.ended_at.is_(None))
+        ).scalar_one_or_none()
         new_meter = declare_meter(
             db,
             label=body.label,
@@ -246,28 +254,39 @@ def declare_energy_meter(
             ).scalar_one_or_none()
             if channel is None:
                 raise ChannelNotFoundError("Meter source channel was not found.")
+            if body.reason.value == "meter_swap":
+                create_binding_for_meter_swap(
+                    db,
+                    old_meter_id=old_meter.id if old_meter is not None else None,
+                    new_meter_id=new_meter.id,
+                    channel_id=channel.id,
+                    started_at=started_at_utc,
+                )
+            else:
                 create_binding(
                     db,
                     meter_id=new_meter.id,
                     channel_id=channel.id,
                     started_at=started_at_utc,
                 )
+
+        # Keep recompute in this transaction: a failure must not leave a new
+        # meter, its predecessor, or either binding at a half-applied boundary.
+        now = datetime.now(UTC)
+        if started_at_utc < now:
+            _trigger_recompute(db, started_at_utc, "POST /api/energy/meters")
+        db.commit()
     except (MeterOverlapError, MeterSourceError) as exc:
-        # declare_meter may already have closed the previous epoch.  Rolling back
-        # here makes Meter + binding declaration genuinely atomic.
         db.rollback()
         raise HTTPException(
             status_code=(status.HTTP_404_NOT_FOUND if isinstance(exc, ChannelNotFoundError)
                          else status.HTTP_422_UNPROCESSABLE_ENTITY),
             detail=str(exc),
         )
+    except Exception:
+        db.rollback()
+        raise
 
-    # Retroactive recompute: re-judge attribution from the new boundary onward.
-    now = datetime.now(UTC)
-    if started_at_utc < now:
-        _trigger_recompute(db, started_at_utc, "POST /api/energy/meters")
-
-    db.commit()
     db.refresh(new_meter)
 
     # Trigger HA discovery re-publish so the new active meter's energy-cost

app/services/meter_sources.py

@@ -315,6 +315,77 @@ def create_binding(
     return binding
 
 
+def create_binding_for_meter_swap(
+    session: Session,
+    *,
+    old_meter_id: int | None,
+    new_meter_id: int,
+    channel_id: int,
+    started_at: datetime,
+) -> MeterSourceBinding:
+    """Create a binding during a physical meter swap, handing off one channel if safe.
+
+    A channel is transferable only when exactly one of its bindings covered the
+    instant immediately before ``started_at`` and that binding belongs to the
+    meter which this declaration just closed.  All other occupied or ambiguous
+    cases retain the normal fail-closed overlap behaviour.
+
+    This function deliberately does not commit.  The caller must keep the meter
+    declaration, binding handoff, and any billing recompute in one transaction.
+    """
+    new_meter = _get_meter(session, new_meter_id)
+    channel = get_channel(session, channel_id)
+    expected_unit = COMMODITY_UNITS.get(new_meter.commodity)
+    if expected_unit is None or channel.unit != expected_unit:
+        raise BindingValidationError(
+            f"Meter commodity {new_meter.commodity!r} requires unit {expected_unit!r}, "
+            f"but channel has {channel.unit!r}."
+        )
+
+    boundary = _as_utc(started_at)
+    if _as_utc(new_meter.started_at) != boundary:
+        raise BindingValidationError(
+            "Meter-swap binding must start at the new meter's started_at boundary."
+        )
+    covering_bindings = [
+        binding
+        for binding in session.execute(
+            select(MeterSourceBinding).where(MeterSourceBinding.channel_id == channel_id)
+        ).scalars()
+        if _as_utc(binding.started_at) < boundary
+        and (binding.ended_at is None or _as_utc(binding.ended_at) >= boundary)
+    ]
+
+    if not covering_bindings:
+        return create_binding(
+            session,
+            meter_id=new_meter_id,
+            channel_id=channel_id,
+            started_at=started_at,
+        )
+
+    if old_meter_id is None or len(covering_bindings) != 1:
+        raise BindingOverlapError("Channel is occupied or has an ambiguous binding at meter swap.")
+
+    old_meter = _get_meter(session, old_meter_id)
+    old_binding = covering_bindings[0]
+    if (
+        old_meter.commodity != new_meter.commodity
+        or old_meter.ended_at is None
+        or _as_utc(old_meter.ended_at) != boundary
+        or old_binding.meter_id != old_meter.id
+    ):
+        raise BindingOverlapError("Channel is occupied by a binding that cannot be handed off.")
+
+    update_binding(session, old_binding.id, ended_at=started_at)
+    return create_binding(
+        session,
+        meter_id=new_meter_id,
+        channel_id=channel_id,
+        started_at=started_at,
+    )
+
+
 def update_binding(
     session: Session,
     binding_id: int,

frontend/src/energy/MeterManager.test.tsx

@@ -61,6 +61,7 @@ const ACTIVE_METER = {
   reason: 'initial',
   note: null,
   created_at: '2024-01-15T00:00:00Z',
+  bindings: [],
 }
 
 const CLOSED_METER = {
@@ -246,6 +247,202 @@ describe('MeterManager — declare new meter', () => {
     })
   })
 
+  it('offers a channel with closed history and one current old-meter binding for a safe swap', async () => {
+    const user = userEvent.setup()
+    mockGet.mockImplementation((path: string) => {
+      if (path === '/api/energy/meters') return Promise.resolve({ data: { items: [{
+        ...CLOSED_METER,
+        bindings: [{
+          uuid: 'binding-history', source_channel_uuid: 'handoff-channel', source_uuid: 'source-1',
+          started_at: '2023-06-01T00:00:00Z', ended_at: '2024-01-15T00:00:00Z',
+        }],
+      }, {
+        ...ACTIVE_METER,
+        bindings: [{
+          uuid: 'binding-current', source_channel_uuid: 'handoff-channel', source_uuid: 'source-1',
+          started_at: '2024-01-15T00:00:00Z', ended_at: null,
+        }],
+      }], total: 2 } })
+      if (path === '/api/energy/sources') return Promise.resolve({ data: { items: [{ uuid: 'source-1', name: 'DSMR' }] } })
+      if (path === '/api/energy/sources/{source_uuid}/channels') {
+        return Promise.resolve({ data: { items: [
+          { uuid: 'handoff-channel', label: 'Current total', unit: 'kWh', binding_count: 2, bound_meter_ids: [CLOSED_METER.id, ACTIVE_METER.id] },
+          { uuid: 'occupied-channel', label: 'Other total', unit: 'kWh', binding_count: 1, bound_meter_ids: [999] },
+        ] } })
+      }
+      return Promise.resolve({ data: { items: [] } })
+    })
+    mockPost.mockResolvedValue({ data: ACTIVE_METER })
+
+    renderWithProviders(<MeterManager />)
+    await user.click(await screen.findByTestId('meter-declare-button'))
+    await user.type(screen.getByTestId('meter-label'), 'Replacement meter')
+    await user.type(screen.getByTestId('meter-started-at'), '2026-01-01')
+    await user.click(screen.getByTestId('meter-reason'))
+    await user.click(await screen.findByText('Meter swap (same address)'))
+    await user.click(screen.getAllByLabelText('Bind source (optional)')[0])
+    await user.click(await screen.findByText('DSMR'))
+    await user.click((await screen.findAllByLabelText('Compatible source channel (optional)'))[0])
+
+    expect(await screen.findByText('Current total (kWh) — hand off from current meter')).toBeInTheDocument()
+    expect(screen.getByRole('option', { name: 'Other total (kWh)' })).toHaveAttribute('data-combobox-disabled')
+    await user.click(screen.getByText('Current total (kWh) — hand off from current meter'))
+    await user.click(screen.getByTestId('declare-meter-submit'))
+
+    await waitFor(() => expect(mockPost).toHaveBeenCalledWith(
+      '/api/energy/meters',
+      expect.objectContaining({ body: expect.objectContaining({ source_channel_uuid: 'handoff-channel' }) }),
+    ))
+  })
+
+  it('keeps a channel disabled when its current open binding is ambiguous', async () => {
+    const user = userEvent.setup()
+    const competingMeter = {
+      ...ACTIVE_METER,
+      id: 999,
+      label: 'Competing meter',
+      bindings: [{
+        uuid: 'binding-competing', source_channel_uuid: 'handoff-channel', source_uuid: 'source-1',
+        started_at: '2025-01-01T00:00:00Z', ended_at: null,
+      }],
+    }
+    mockGet.mockImplementation((path: string) => {
+      if (path === '/api/energy/meters') return Promise.resolve({ data: { items: [{
+        ...ACTIVE_METER,
+        bindings: [{
+          uuid: 'binding-current', source_channel_uuid: 'handoff-channel', source_uuid: 'source-1',
+          started_at: '2024-01-15T00:00:00Z', ended_at: null,
+        }],
+      }, competingMeter], total: 2 } })
+      if (path === '/api/energy/sources') return Promise.resolve({ data: { items: [{ uuid: 'source-1', name: 'DSMR' }] } })
+      if (path === '/api/energy/sources/{source_uuid}/channels') {
+        return Promise.resolve({ data: { items: [
+          { uuid: 'handoff-channel', label: 'Current total', unit: 'kWh', binding_count: 2, bound_meter_ids: [ACTIVE_METER.id, competingMeter.id] },
+        ] } })
+      }
+      return Promise.resolve({ data: { items: [] } })
+    })
+
+    renderWithProviders(<MeterManager />)
+    await user.click(await screen.findByTestId('meter-declare-button'))
+    await user.type(screen.getByTestId('meter-label'), 'Replacement meter')
+    await user.type(screen.getByTestId('meter-started-at'), '2026-01-01')
+    await user.click(screen.getByTestId('meter-reason'))
+    await user.click(await screen.findByText('Meter swap (same address)'))
+    await user.click(screen.getAllByLabelText('Bind source (optional)')[0])
+    await user.click(await screen.findByText('DSMR'))
+    await user.click((await screen.findAllByLabelText('Compatible source channel (optional)'))[0])
+
+    expect(await screen.findByRole('option', { name: 'Current total (kWh)' })).toHaveAttribute(
+      'data-combobox-disabled',
+    )
+  })
+
+  it('clears a selected handoff channel when the swap date becomes unsafe', async () => {
+    const user = userEvent.setup()
+    mockGet.mockImplementation((path: string) => {
+      if (path === '/api/energy/meters') return Promise.resolve({ data: { items: [{
+        ...ACTIVE_METER,
+        bindings: [{
+          uuid: 'binding-1', source_channel_uuid: 'handoff-channel', source_uuid: 'source-1',
+          started_at: '2024-01-15T00:00:00Z', ended_at: null,
+        }],
+      }], total: 1 } })
+      if (path === '/api/energy/sources') return Promise.resolve({ data: { items: [{ uuid: 'source-1', name: 'DSMR' }] } })
+      if (path === '/api/energy/sources/{source_uuid}/channels') {
+        return Promise.resolve({ data: { items: [
+          { uuid: 'handoff-channel', label: 'Current total', unit: 'kWh', binding_count: 1, bound_meter_ids: [ACTIVE_METER.id] },
+        ] } })
+      }
+      return Promise.resolve({ data: { items: [] } })
+    })
+    mockPost.mockResolvedValue({ data: ACTIVE_METER })
+
+    renderWithProviders(<MeterManager />)
+    await user.click(await screen.findByTestId('meter-declare-button'))
+    await user.type(screen.getByTestId('meter-label'), 'Replacement meter')
+    await user.type(screen.getByTestId('meter-started-at'), '2026-01-01')
+    await user.click(screen.getByTestId('meter-reason'))
+    await user.click(await screen.findByText('Meter swap (same address)'))
+    await user.click(screen.getAllByLabelText('Bind source (optional)')[0])
+    await user.click(await screen.findByText('DSMR'))
+    await user.click((await screen.findAllByLabelText('Compatible source channel (optional)'))[0])
+    await user.click(await screen.findByText('Current total (kWh) — hand off from current meter'))
+
+    const startedAt = screen.getByTestId('meter-started-at')
+    await user.clear(startedAt)
+    await user.type(startedAt, '2024-01-15')
+    expect(screen.getAllByLabelText('Compatible source channel (optional)')[0]).toHaveValue('')
+
+    await user.click(screen.getByTestId('declare-meter-submit'))
+
+    await waitFor(() => expect(mockPost).toHaveBeenCalledWith(
+      '/api/energy/meters',
+      expect.objectContaining({ body: expect.not.objectContaining({ source_channel_uuid: 'handoff-channel' }) }),
+    ))
+  })
+
+  it('treats a naive UTC binding timestamp as Amsterdam local time when validating a handoff', async () => {
+    vi.stubEnv('TZ', 'Europe/Amsterdam')
+    try {
+      const user = userEvent.setup()
+      mockGet.mockImplementation((path: string) => {
+        if (path === '/api/energy/meters') return Promise.resolve({ data: { items: [{
+          ...ACTIVE_METER,
+          bindings: [{
+            uuid: 'binding-1', source_channel_uuid: 'handoff-channel', source_uuid: 'source-1',
+            // SQLite commonly round-trips this UTC instant without a timezone suffix.
+            started_at: '2024-01-14T23:00:00', ended_at: null,
+          }],
+        }], total: 1 } })
+        if (path === '/api/energy/sources') return Promise.resolve({ data: { items: [{ uuid: 'source-1', name: 'DSMR' }] } })
+        if (path === '/api/energy/sources/{source_uuid}/channels') {
+          return Promise.resolve({ data: { items: [
+            { uuid: 'handoff-channel', label: 'Current total', unit: 'kWh', binding_count: 1, bound_meter_ids: [ACTIVE_METER.id] },
+          ] } })
+        }
+        return Promise.resolve({ data: { items: [] } })
+      })
+      mockPost.mockRejectedValueOnce(new Error('keep modal open after unsafe submission'))
+      mockPost.mockResolvedValueOnce({ data: ACTIVE_METER })
+
+      renderWithProviders(<MeterManager />)
+      await user.click(await screen.findByTestId('meter-declare-button'))
+      await user.type(screen.getByTestId('meter-label'), 'Replacement meter')
+      await user.type(screen.getByTestId('meter-started-at'), '2024-01-16')
+      await user.click(screen.getByTestId('meter-reason'))
+      await user.click(await screen.findByText('Meter swap (same address)'))
+      await user.click(screen.getAllByLabelText('Bind source (optional)')[0])
+      await user.click(await screen.findByText('DSMR'))
+      await user.click((await screen.findAllByLabelText('Compatible source channel (optional)'))[0])
+      await user.click(await screen.findByText('Current total (kWh) — hand off from current meter'))
+
+      const startedAt = screen.getByTestId('meter-started-at')
+      await user.clear(startedAt)
+      await user.type(startedAt, '2024-01-15')
+      expect(screen.getAllByLabelText('Compatible source channel (optional)')[0]).toHaveValue('')
+
+      await user.click(screen.getByTestId('declare-meter-submit'))
+      await waitFor(() => expect(mockPost).toHaveBeenLastCalledWith(
+        '/api/energy/meters',
+        expect.objectContaining({ body: expect.not.objectContaining({ source_channel_uuid: 'handoff-channel' }) }),
+      ))
+
+      await user.clear(startedAt)
+      await user.type(startedAt, '2024-01-16')
+      await user.click(screen.getAllByLabelText('Compatible source channel (optional)')[0])
+      await user.click(await screen.findByText('Current total (kWh) — hand off from current meter'))
+      await user.click(screen.getByTestId('declare-meter-submit'))
+
+      await waitFor(() => expect(mockPost).toHaveBeenLastCalledWith(
+        '/api/energy/meters',
+        expect.objectContaining({ body: expect.objectContaining({ source_channel_uuid: 'handoff-channel' }) }),
+      ))
+    } finally {
+      vi.unstubAllEnvs()
+    }
+  })
+
   it('displays error when POST fails with 422 (倒挂 / validation error)', async () => {
     const user = userEvent.setup()
     mockGet.mockResolvedValue({ data: { items: [ACTIVE_METER], total: 1 } })

frontend/src/energy/MeterManager.tsx

@@ -83,11 +83,12 @@ function toLocalDateInputString(d: Date): string {
 // ---------------------------------------------------------------------------
 
 interface DeclareMeterFormProps {
+  meters: MeterResponse[]
   onClose: () => void
   onSaved: () => void
 }
 
-function DeclareMeterForm({ onClose, onSaved }: DeclareMeterFormProps) {
+function DeclareMeterForm({ meters, onClose, onSaved }: DeclareMeterFormProps) {
   const [label, setLabel] = useState('')
   const [dateStr, setDateStr] = useState('')
   const [reason, setReason] = useState<string | null>(null)
@@ -100,8 +101,43 @@ function DeclareMeterForm({ onClose, onSaved }: DeclareMeterFormProps) {
   const channels = useSourceChannels(sourceUuid)
 
   const declareMutation = useDeclareMeter()
-  const compatible = (channel: { unit: string; binding_count: number; bound_meter_ids: number[] }) =>
+  const expectedUnit = ({ electricity: 'kWh', heating: 'GJ', hot_water: 'm³' } as Record<string, string>)[commodity ?? 'electricity']
-    ({ electricity: 'kWh', heating: 'GJ', hot_water: 'm³' } as Record<string, string>)[commodity ?? 'electricity'] === channel.unit && channel.binding_count === 0 && channel.bound_meter_ids.length === 0
+  const oldMeter = meters.find((meter) => meter.commodity === commodity && meter.ended_at === null)
+  const isChannelEligible = (uuid: string, startedAt: string) => {
+    const channel = channels.data?.items.find((item) => item.uuid === uuid)
+    if (!channel || channel.unit !== expectedUnit) return false
+
+    // Channel aggregates include closed binding history.  For a meter swap, only
+    // currently open bindings determine whether this channel can be handed off.
+    const openBindings = meters.flatMap((meter) =>
+      (meter.bindings ?? [])
+        .filter((binding) => binding.source_channel_uuid === channel.uuid && binding.ended_at === null)
+        .map((binding) => ({ meter, binding })),
+    )
+    const isUnbound = channel.binding_count === 0 &&
+      channel.bound_meter_ids.length === 0 && openBindings.length === 0
+    const oldBinding = openBindings[0]
+    const canHandoff = reason === 'meter_swap' && oldMeter !== undefined &&
+      openBindings.length === 1 && oldBinding !== undefined &&
+      oldBinding.meter.id === oldMeter.id && oldBinding.meter.commodity === commodity &&
+      startedAt > toLocalDateInputString(parseBackendTimestamp(oldBinding.binding.started_at))
+    return isUnbound || canHandoff
+  }
+  const channelOptions = channels.data?.items
+    .filter((channel) => channel.unit === expectedUnit)
+    .map((channel) => {
+      const canHandoff = !(
+        channel.binding_count === 0 && channel.bound_meter_ids.length === 0
+      ) && isChannelEligible(channel.uuid, dateStr)
+      return {
+        value: channel.uuid,
+        label: `${channel.label} (${channel.unit})${canHandoff ? ' — hand off from current meter' : ''}`,
+        disabled: !isChannelEligible(channel.uuid, dateStr),
+      }
+    }) ?? []
+  const selectedChannelUuid = channelUuid && isChannelEligible(channelUuid, dateStr)
+    ? channelUuid
+    : null
 
   async function handleSubmit(e: React.FormEvent) {
     e.preventDefault()
@@ -127,7 +163,7 @@ function DeclareMeterForm({ onClose, onSaved }: DeclareMeterFormProps) {
         reason: reason as MeterReason,
         note: note.trim() || undefined,
         commodity: commodity ?? 'electricity',
-        ...(channelUuid ? { source_channel_uuid: channelUuid } : {}),
+        ...(selectedChannelUuid ? { source_channel_uuid: selectedChannelUuid } : {}),
       })
       onSaved()
       onClose()
@@ -166,7 +202,11 @@ function DeclareMeterForm({ onClose, onSaved }: DeclareMeterFormProps) {
             type="date"
             required
             value={dateStr}
-            onChange={(e) => setDateStr(e.currentTarget.value)}
+            onChange={(e) => {
+              const nextDateStr = e.currentTarget.value
+              setDateStr(nextDateStr)
+              setChannelUuid((uuid) => uuid && !isChannelEligible(uuid, nextDateStr) ? null : uuid)
+            }}
             data-testid="meter-started-at"
           />
 
@@ -175,17 +215,17 @@ function DeclareMeterForm({ onClose, onSaved }: DeclareMeterFormProps) {
             required
             data={REASON_OPTIONS}
             value={reason}
-            onChange={setReason}
+            onChange={(value) => { setReason(value); setChannelUuid(null) }}
             data-testid="meter-reason"
           />
 
-          <Select label="Commodity" value={commodity} onChange={setCommodity} data={[
+          <Select label="Commodity" value={commodity} onChange={(value) => { setCommodity(value); setChannelUuid(null) }} data={[
             { value: 'electricity', label: 'Electricity' },
             { value: 'heating', label: 'Heating' },
             { value: 'hot_water', label: 'Hot water' },
           ]} />
           <Select label="Bind source (optional)" value={sourceUuid} onChange={(value) => { setSourceUuid(value); setChannelUuid(null) }} data={sources.data?.items.filter((source) => typeof source.uuid === 'string').map((source) => ({ value: source.uuid, label: source.name })) ?? []} />
-          {sourceUuid && <Select label="Compatible source channel (optional)" value={channelUuid} onChange={setChannelUuid} description="Only unbound channels with the required unit are eligible. Suggestions are informational only." data={channels.data?.items.filter(compatible).map((channel) => ({ value: channel.uuid, label: `${channel.label} (${channel.unit})${channel.suggested_commodity ? ` — suggestion: ${channel.suggested_commodity}` : ''}` })) ?? []} />}
+          {sourceUuid && <Select label="Compatible source channel (optional)" value={selectedChannelUuid} onChange={setChannelUuid} description={reason === 'meter_swap' ? 'An unbound channel, or the current same-commodity meter’s single binding, can be selected. Other occupied channels stay unavailable.' : 'Only unbound channels with the required unit are eligible.'} data={channelOptions} />}
 
           <Textarea
             label="Note (optional)"
@@ -534,6 +574,7 @@ export function MeterManager() {
       {/* Declare new meter */}
       {showDeclareForm && (
         <DeclareMeterForm
+          meters={meters}
           onClose={() => setShowDeclareForm(false)}
           onSaved={() => setShowDeclareForm(false)}
         />

tests/test_api_meters.py

@@ -31,7 +31,7 @@ Retroactive recompute integration
 
 from __future__ import annotations
 
-from datetime import UTC, datetime
+from datetime import UTC, datetime, timedelta
 from unittest.mock import patch
 
 import pytest
@@ -40,6 +40,7 @@ from sqlalchemy import create_engine, select
 from sqlalchemy.orm import Session
 
 from app.models.energy import Meter
+from app.models.meter_source import MeterSource, MeterSourceBinding, MeterSourceChannel
 
 # ---------------------------------------------------------------------------
 # Shared helpers
@@ -67,6 +68,43 @@ def _declare_payload(**overrides) -> dict:
     return base
 
 
+def _add_bound_channel(engine, *, meter_id: int, started_at: datetime) -> str:
+    """Persist one test-only DSMR channel binding and return its public UUID."""
+    with Session(engine) as session:
+        source = MeterSource(
+            name="Test DSMR",
+            kind="dsmr_mqtt",
+            enabled=True,
+            config={},
+            status="online",
+            created_at=started_at,
+            updated_at=started_at,
+        )
+        session.add(source)
+        session.flush()
+        channel = MeterSourceChannel(
+            source_id=source.id,
+            channel_key="electricity-total",
+            label="Electricity total",
+            unit="kWh",
+            created_at=started_at,
+            updated_at=started_at,
+        )
+        session.add(channel)
+        session.flush()
+        session.add(
+            MeterSourceBinding(
+                meter_id=meter_id,
+                channel_id=channel.id,
+                started_at=started_at,
+                created_at=started_at,
+                updated_at=started_at,
+            )
+        )
+        session.commit()
+        return channel.uuid
+
+
 # ---------------------------------------------------------------------------
 # Fixtures
 # ---------------------------------------------------------------------------
@@ -252,6 +290,159 @@ def test_declare_meter_swap_closes_previous(meters_client):
         assert ended_naive == t1
 
 
+def test_declare_meter_swap_hands_off_previous_meter_channel_atomically(meters_client):
+    client, engine = meters_client
+    _login(client)
+    t0 = datetime(2024, 6, 1, tzinfo=UTC)
+    boundary = datetime(2025, 3, 15, 12, tzinfo=UTC)
+
+    with patch("app.api.routes.api.meters.recompute_range", return_value=0):
+        old_response = client.post(
+            "/api/energy/meters",
+            json=_declare_payload(label="Old meter", started_at=t0.isoformat()),
+            headers={"X-CSRF-Token": _CSRF},
+        )
+        assert old_response.status_code == 201
+        old_id = old_response.json()["id"]
+        channel_uuid = _add_bound_channel(engine, meter_id=old_id, started_at=t0)
+        response = client.post(
+            "/api/energy/meters",
+            json=_declare_payload(
+                label="New meter",
+                started_at=boundary.isoformat(),
+                reason="meter_swap",
+                source_channel_uuid=channel_uuid,
+            ),
+            headers={"X-CSRF-Token": _CSRF},
+        )
+
+    assert response.status_code == 201
+    new_id = response.json()["id"]
+    with Session(engine) as session:
+        bindings = session.execute(
+            select(MeterSourceBinding).order_by(MeterSourceBinding.id)
+        ).scalars().all()
+        old_binding_ended_at = bindings[0].ended_at
+        if old_binding_ended_at is not None and old_binding_ended_at.tzinfo is None:
+            old_binding_ended_at = old_binding_ended_at.replace(tzinfo=UTC)
+        new_binding_started_at = bindings[1].started_at
+        if new_binding_started_at.tzinfo is None:
+            new_binding_started_at = new_binding_started_at.replace(tzinfo=UTC)
+        assert [(bindings[0].meter_id, old_binding_ended_at), (bindings[1].meter_id, bindings[1].ended_at)] == [
+            (old_id, boundary),
+            (new_id, None),
+        ]
+        assert new_binding_started_at == boundary
+
+
+def test_declare_meter_swap_rejects_other_meter_channel_and_rolls_back(meters_client):
+    client, engine = meters_client
+    _login(client)
+    t0 = datetime(2024, 6, 1, tzinfo=UTC)
+    boundary = datetime(2025, 3, 15, 12, tzinfo=UTC)
+
+    with patch("app.api.routes.api.meters.recompute_range", return_value=0):
+        old_response = client.post(
+            "/api/energy/meters",
+            json=_declare_payload(label="Old meter", started_at=t0.isoformat()),
+            headers={"X-CSRF-Token": _CSRF},
+        )
+        old_id = old_response.json()["id"]
+        other = Meter(
+            label="Other meter",
+            commodity="electricity",
+            started_at=t0,
+            ended_at=boundary + timedelta(days=1),
+            reason="initial",
+            created_at=t0,
+        )
+        with Session(engine) as session:
+            session.add(other)
+            session.commit()
+            other_id = other.id
+        channel_uuid = _add_bound_channel(engine, meter_id=other_id, started_at=t0)
+        response = client.post(
+            "/api/energy/meters",
+            json=_declare_payload(
+                label="Rejected meter",
+                started_at=boundary.isoformat(),
+                reason="meter_swap",
+                source_channel_uuid=channel_uuid,
+            ),
+            headers={"X-CSRF-Token": _CSRF},
+        )
+
+    assert response.status_code == 422
+    with Session(engine) as session:
+        assert session.execute(select(Meter).where(Meter.label == "Rejected meter")).scalar_one_or_none() is None
+        assert session.get(Meter, old_id).ended_at is None
+        binding = session.execute(select(MeterSourceBinding)).scalar_one()
+        assert binding.ended_at is None
+
+
+def test_declare_meter_non_swap_cannot_take_previous_meter_channel(meters_client):
+    client, engine = meters_client
+    _login(client)
+    t0 = datetime(2024, 6, 1, tzinfo=UTC)
+    boundary = datetime(2025, 3, 15, 12, tzinfo=UTC)
+    with patch("app.api.routes.api.meters.recompute_range", return_value=0):
+        old_response = client.post(
+            "/api/energy/meters",
+            json=_declare_payload(label="Old meter", started_at=t0.isoformat()),
+            headers={"X-CSRF-Token": _CSRF},
+        )
+        old_id = old_response.json()["id"]
+        channel_uuid = _add_bound_channel(engine, meter_id=old_id, started_at=t0)
+        response = client.post(
+            "/api/energy/meters",
+            json=_declare_payload(
+                label="Moved meter",
+                started_at=boundary.isoformat(),
+                reason="home_move",
+                source_channel_uuid=channel_uuid,
+            ),
+            headers={"X-CSRF-Token": _CSRF},
+        )
+
+    assert response.status_code == 422
+    with Session(engine) as session:
+        assert session.get(Meter, old_id).ended_at is None
+        assert session.execute(select(MeterSourceBinding)).scalar_one().ended_at is None
+
+
+def test_declare_meter_recompute_failure_rolls_back_handoff(meters_client):
+    client, engine = meters_client
+    _login(client)
+    t0 = datetime(2024, 6, 1, tzinfo=UTC)
+    boundary = datetime(2025, 3, 15, 12, tzinfo=UTC)
+    with patch("app.api.routes.api.meters.recompute_range", return_value=0):
+        old_response = client.post(
+            "/api/energy/meters",
+            json=_declare_payload(label="Old meter", started_at=t0.isoformat()),
+            headers={"X-CSRF-Token": _CSRF},
+        )
+    old_id = old_response.json()["id"]
+    channel_uuid = _add_bound_channel(engine, meter_id=old_id, started_at=t0)
+
+    with patch("app.api.routes.api.meters.recompute_range", side_effect=RuntimeError("recompute failed")):
+        with pytest.raises(RuntimeError, match="recompute failed"):
+            client.post(
+                "/api/energy/meters",
+                json=_declare_payload(
+                    label="Failed meter",
+                    started_at=boundary.isoformat(),
+                    reason="meter_swap",
+                    source_channel_uuid=channel_uuid,
+                ),
+                headers={"X-CSRF-Token": _CSRF},
+            )
+
+    with Session(engine) as session:
+        assert session.execute(select(Meter).where(Meter.label == "Failed meter")).scalar_one_or_none() is None
+        assert session.get(Meter, old_id).ended_at is None
+        assert session.execute(select(MeterSourceBinding)).scalar_one().ended_at is None
+
+
 def test_declare_meter_overlap_returns_422(meters_client):
     """Declaring a meter with started_at before active meter's started_at → 422."""
     client, _ = meters_client

tests/test_meter_source_services.py

@@ -23,6 +23,7 @@ from app.services.meter_sources import (
     SourceDeleteRestrictedError,
     close_binding,
     create_binding,
+    create_binding_for_meter_swap,
     create_source,
     delete_source,
     upsert_discovered_channel,
@@ -178,6 +179,131 @@ def test_binding_rejects_incompatible_unit_and_close_keeps_transaction_open(sess
     assert session.get(MeterSourceBinding, binding.id) is None
 
 
+def test_meter_swap_hands_off_only_the_previous_meter_binding(session):
+    start = datetime(2026, 8, 22, tzinfo=UTC)
+    boundary = start + timedelta(days=1)
+    old_meter = _meter(session, "heating", "old")
+    old_meter.started_at = start
+    old_meter.ended_at = boundary
+    new_meter = Meter(
+        label="new",
+        commodity="heating",
+        started_at=boundary,
+        reason="meter_swap",
+        created_at=boundary,
+    )
+    session.add(new_meter)
+    session.flush()
+    _, channel = _source_and_channel(session, "warmtelink_serial", "GJ")
+    old_binding = create_binding(
+        session, meter_id=old_meter.id, channel_id=channel.id, started_at=start
+    )
+    session.flush()
+
+    new_binding = create_binding_for_meter_swap(
+        session,
+        old_meter_id=old_meter.id,
+        new_meter_id=new_meter.id,
+        channel_id=channel.id,
+        started_at=boundary,
+    )
+    session.flush()
+
+    assert old_binding.ended_at == boundary
+    assert new_binding.started_at == boundary
+    assert new_binding.ended_at is None
+
+
+def test_meter_swap_rejects_channel_owned_by_a_different_meter(session):
+    start = datetime(2026, 8, 22, tzinfo=UTC)
+    boundary = start + timedelta(days=1)
+    old_meter = _meter(session, "heating", "old")
+    new_meter = _meter(session, "heating", "new")
+    other_meter = _meter(session, "heating", "other")
+    old_meter.started_at = start
+    old_meter.ended_at = boundary
+    new_meter.started_at = boundary
+    _, channel = _source_and_channel(session, "warmtelink_serial", "GJ")
+    create_binding(session, meter_id=other_meter.id, channel_id=channel.id, started_at=start)
+
+    with pytest.raises(BindingOverlapError, match="cannot be handed off"):
+        create_binding_for_meter_swap(
+            session,
+            old_meter_id=old_meter.id,
+            new_meter_id=new_meter.id,
+            channel_id=channel.id,
+            started_at=boundary,
+        )
+
+
+def test_meter_swap_rejects_ambiguous_channel_without_closing_any_binding(session):
+    start = datetime(2026, 8, 22, tzinfo=UTC)
+    boundary = start + timedelta(days=1)
+    old_meter = _meter(session, "heating", "old")
+    old_meter.started_at = start
+    old_meter.ended_at = boundary
+    new_meter = Meter(
+        label="new",
+        commodity="heating",
+        started_at=boundary,
+        reason="meter_swap",
+        created_at=boundary,
+    )
+    other_meter = _meter(session, "heating", "other")
+    session.add(new_meter)
+    session.flush()
+    _, channel = _source_and_channel(session, "warmtelink_serial", "GJ")
+    old_binding = create_binding(session, meter_id=old_meter.id, channel_id=channel.id, started_at=start)
+    session.add(
+        MeterSourceBinding(
+            meter_id=other_meter.id,
+            channel_id=channel.id,
+            started_at=start,
+            created_at=start,
+            updated_at=start,
+        )
+    )
+    session.flush()
+
+    with pytest.raises(BindingOverlapError, match="occupied or has an ambiguous binding"):
+        create_binding_for_meter_swap(
+            session,
+            old_meter_id=old_meter.id,
+            new_meter_id=new_meter.id,
+            channel_id=channel.id,
+            started_at=boundary,
+        )
+
+    assert old_binding.ended_at is None
+
+
+def test_meter_swap_rejects_incompatible_channel(session):
+    start = datetime(2026, 8, 22, tzinfo=UTC)
+    boundary = start + timedelta(days=1)
+    old_meter = _meter(session, "heating", "old")
+    old_meter.started_at = start
+    old_meter.ended_at = boundary
+    new_meter = Meter(
+        label="new",
+        commodity="heating",
+        started_at=boundary,
+        reason="meter_swap",
+        created_at=boundary,
+    )
+    session.add(new_meter)
+    session.flush()
+    _, channel = _source_and_channel(session, "dsmr_mqtt", "kWh")
+
+    with pytest.raises(BindingValidationError, match="requires unit"):
+        create_binding_for_meter_swap(
+            session,
+            old_meter_id=old_meter.id,
+            new_meter_id=new_meter.id,
+            channel_id=channel.id,
+            started_at=boundary,
+        )
+
+
 def test_source_delete_is_restricted_by_discovered_channel(session):
     source, _ = _source_and_channel(session, "dsmr_mqtt", "kWh")
     with pytest.raises(SourceDeleteRestrictedError):