feat: reassign on-call days and weeks to another person

Assigning over a day somebody else held did nothing but flash a 409 for
three seconds. The server holds one person per date and refused any that
was taken, all-or-nothing, so pressing W on a week where a single day was
already assigned placed none of the other six either. The only way
through was d on each day first — seven delete-and-confirm cycles to move
one week.

The clash is already on screen, so it is found before the request rather
than read back out of an error: the picker hands off to a confirmation
naming who loses the days and how many there are, and accepting sends the
whole selection with replace, which terdut-server v0.8.0 added. One
question to move a week, and nobody's shift moves without somebody being
asked. A day nobody holds still assigns with no prompt at all.

Reassigning somebody to a day they already hold raises no prompt, since
it takes nothing from anyone, but it does send replace: the server
rejects any date that exists, so without it a harmless no-op would fail.

internal/tui/model.go

@@ -54,6 +54,7 @@ const (
 	confirmResolveIncident
 	confirmDeleteScheduleEntry
 	confirmDeleteUser
+	confirmReassignSchedule
 )
 
 // pickerTarget says what the user picker is choosing a person for.
@@ -143,6 +144,18 @@ type scheduleDay struct {
 	entry *api.ScheduleEntry
 }
 
+// pendingAssign is an on-call assignment held back by the reassignment
+// confirmation, because some of its dates belong to somebody else.
+type pendingAssign struct {
+	userID   int64
+	username string
+	dates    []string
+	// taken are the dates currently held by other people, and holders the
+	// distinct names holding them — both only for wording the prompt.
+	taken   []string
+	holders []string
+}
+
 type Model struct {
 	client          *api.Client
 	serverURL       string
@@ -194,6 +207,7 @@ type Model struct {
 	confirmTarget      confirmTarget
 	pendingDeleteID    int64 // note event ID
 	pendingDeleteEntry *api.ScheduleEntry
+	pendingAssign      *pendingAssign
 
 	// Stats
 	topAlerts []api.TopAlert
@@ -866,9 +880,9 @@ func fetchScheduleCmd(client *api.Client, from, to time.Time) tea.Cmd {
 	}
 }
 
-func assignScheduleCmd(client *api.Client, userID int64, dates []string, from, to time.Time) tea.Cmd {
+func assignScheduleCmd(client *api.Client, userID int64, dates []string, replace bool, from, to time.Time) tea.Cmd {
 	return func() tea.Msg {
-		if _, err := client.AssignSchedule(userID, dates); err != nil {
+		if _, err := client.AssignSchedule(userID, dates, replace); err != nil {
 			return scheduleActionErrMsg{err}
 		}
 		entries, err := client.GetSchedule(from.Format("2006-01-02"), to.Format("2006-01-02"))

internal/tui/model_test.go

@@ -277,3 +277,95 @@ func TestBuildScheduleDays(t *testing.T) {
 		t.Error("expected unassigned days to have no entry")
 	}
 }
+
+// ── Schedule reassignment ─────────────────────────────────────────────────
+
+// scheduledWeek builds a model showing the week of 2026-07-27 with the given
+// entries already on the rota.
+func scheduledWeek(entries []api.ScheduleEntry) Model {
+	m := NewModel(nil, "http://test", time.Minute)
+	m.width, m.height = 120, 40
+	m.connected = true
+	m.activeSection = sectionSchedule
+	m.scheduleWindow = time.Date(2026, 7, 27, 0, 0, 0, 0, time.UTC)
+	m.scheduleEntries = entries
+	m.scheduleDays = buildScheduleDays(m.scheduleWindow, entries)
+	m.rebuildScheduleTable()
+	return m
+}
+
+func TestScheduleConflicts(t *testing.T) {
+	m := scheduledWeek([]api.ScheduleEntry{
+		{ID: 1, Date: "2026-07-27", UserID: 1, Username: "niklas"},
+		{ID: 2, Date: "2026-07-28", UserID: 3, Username: "sam"},
+		{ID: 3, Date: "2026-07-29", UserID: 2, Username: "alex"},
+	})
+	week := []string{"2026-07-27", "2026-07-28", "2026-07-29", "2026-07-30"}
+
+	// Assigning alex: the days niklas and sam hold are conflicts, the day alex
+	// already holds is not, and the free day is not.
+	taken, holders := m.scheduleConflicts(week, 2)
+	if len(taken) != 2 || taken[0] != "2026-07-27" || taken[1] != "2026-07-28" {
+		t.Errorf("expected the two other people's days, got %v", taken)
+	}
+	if len(holders) != 2 || holders[0] != "niklas" || holders[1] != "sam" {
+		t.Errorf("expected both holders named once, got %v", holders)
+	}
+}
+
+// Reassigning somebody to a day they already hold takes nothing from anyone, so
+// it must not raise a prompt — but it still needs replace, because the server
+// rejects any date that already exists.
+func TestScheduleConflicts_OwnDayIsNotAConflict(t *testing.T) {
+	m := scheduledWeek([]api.ScheduleEntry{
+		{ID: 1, Date: "2026-07-27", UserID: 2, Username: "alex"},
+	})
+	dates := []string{"2026-07-27"}
+
+	if taken, _ := m.scheduleConflicts(dates, 2); len(taken) != 0 {
+		t.Errorf("expected no conflict on the user's own day, got %v", taken)
+	}
+	if !m.scheduleOccupied(dates) {
+		t.Error("expected the day to still count as occupied, so replace is sent")
+	}
+}
+
+func TestScheduleOccupied_FreeDays(t *testing.T) {
+	m := scheduledWeek(nil)
+	if m.scheduleOccupied([]string{"2026-07-27", "2026-07-28"}) {
+		t.Error("expected an empty rota to need no replace")
+	}
+}
+
+func TestDayCount(t *testing.T) {
+	tests := []struct {
+		taken, total int
+		want         string
+	}{
+		{1, 1, "This day is"},
+		{7, 7, "All 7 days are"},
+		{3, 7, "3 of 7 days are"},
+	}
+	for _, tt := range tests {
+		if got := dayCount(tt.taken, tt.total); got != tt.want {
+			t.Errorf("dayCount(%d, %d) = %q, want %q", tt.taken, tt.total, got, tt.want)
+		}
+	}
+}
+
+func TestJoinNames(t *testing.T) {
+	tests := []struct {
+		names []string
+		want  string
+	}{
+		{nil, "somebody else"},
+		{[]string{"niklas"}, "niklas"},
+		{[]string{"niklas", "alex"}, "niklas and alex"},
+		{[]string{"niklas", "alex", "sam"}, "niklas, alex and sam"},
+	}
+	for _, tt := range tests {
+		if got := joinNames(tt.names); got != tt.want {
+			t.Errorf("joinNames(%v) = %q, want %q", tt.names, got, tt.want)
+		}
+	}
+}

internal/tui/update.go

@@ -822,6 +822,7 @@ func (m Model) handleConfirmKey(msg tea.KeyMsg) (Model, tea.Cmd) {
 		}
 		m.pendingDeleteID = 0
 		m.pendingDeleteEntry = nil
+		m.pendingAssign = nil
 		return m, nil
 	}
 
@@ -851,6 +852,17 @@ func (m Model) handleConfirmKey(msg tea.KeyMsg) (Model, tea.Cmd) {
 		m.mode = modeDashboard
 		m.usersLoading = true
 		return m, deleteUserCmd(m.client, userID)
+
+	case confirmReassignSchedule:
+		p := m.pendingAssign
+		m.mode = modeDashboard
+		m.pendingAssign = nil
+		if p == nil {
+			return m, nil
+		}
+		m.scheduleLoading = true
+		return m, assignScheduleCmd(m.client, p.userID, p.dates, true,
+			m.scheduleWindow, m.scheduleWindow.AddDate(0, 0, 6))
 	}
 
 	return m, nil
@@ -899,15 +911,84 @@ func (m Model) handleUserPickerKey(msg tea.KeyMsg) (Model, tea.Cmd) {
 		} else {
 			dates = []string{d.Format("2006-01-02")}
 		}
+
+		// The server refuses a date somebody else holds, so ask before taking
+		// it rather than letting the request come back 409. The answer is
+		// already on screen — no round trip is needed to work out who loses
+		// their shift.
+		taken, holders := m.scheduleConflicts(dates, user.ID)
+		if len(taken) > 0 {
+			m.pendingAssign = &pendingAssign{
+				userID:   user.ID,
+				username: user.Username,
+				dates:    dates,
+				taken:    taken,
+				holders:  holders,
+			}
+			m.confirmTarget = confirmReassignSchedule
+			m.mode = modeConfirm
+			return m, nil
+		}
+
 		m.mode = modeDashboard
 		m.scheduleLoading = true
-		return m, assignScheduleCmd(m.client, user.ID, dates,
+		// Nobody else loses anything, but the server rejects any date that
+		// already exists — including days this same person already holds, which
+		// is a no-op worth letting through silently.
+		return m, assignScheduleCmd(m.client, user.ID, dates, m.scheduleOccupied(dates),
 			m.scheduleWindow, m.scheduleWindow.AddDate(0, 0, 6))
 	}
 
 	return m, nil
 }
 
+// scheduleConflicts reports which of dates are already held by somebody other
+// than newUserID, and the distinct names holding them.
+//
+// Days the target already owns are not conflicts — reassigning somebody to
+// their own shift takes nothing from anyone, and prompting for it would be
+// noise. The server still needs replace for those, since it rejects any date
+// that exists.
+func (m Model) scheduleConflicts(dates []string, newUserID int64) (taken, holders []string) {
+	held := make(map[string]api.ScheduleEntry, len(m.scheduleDays))
+	for _, d := range m.scheduleDays {
+		if d.entry != nil {
+			held[d.entry.Date] = *d.entry
+		}
+	}
+	seen := make(map[string]bool)
+	for _, date := range dates {
+		e, ok := held[date]
+		if !ok || e.UserID == newUserID {
+			continue
+		}
+		taken = append(taken, date)
+		if !seen[e.Username] {
+			seen[e.Username] = true
+			holders = append(holders, e.Username)
+		}
+	}
+	return taken, holders
+}
+
+// scheduleOccupied reports whether any of dates already has an entry at all,
+// including one belonging to the incoming user. That is what decides whether
+// the request needs replace, as opposed to whether it needs confirming.
+func (m Model) scheduleOccupied(dates []string) bool {
+	held := make(map[string]bool, len(m.scheduleDays))
+	for _, d := range m.scheduleDays {
+		if d.entry != nil {
+			held[d.entry.Date] = true
+		}
+	}
+	for _, date := range dates {
+		if held[date] {
+			return true
+		}
+	}
+	return false
+}
+
 // ── User management ───────────────────────────────────────────────────────────
 
 func (m Model) handleUserCreateKey(msg tea.KeyMsg) (Model, tea.Cmd) {

internal/tui/update_test.go

@@ -309,6 +309,109 @@ func TestDeleteNote_ConfirmsThenActs(t *testing.T) {
 	}
 }
 
+// ── Schedule reassignment ─────────────────────────────────────────────────
+
+// pickingOnCall opens the user picker for the schedule day at dayIndex, which
+// is where a reassignment actually starts.
+func pickingOnCall(entries []api.ScheduleEntry, dayIndex int, week bool) Model {
+	m := scheduledWeek(entries)
+	m.users = []api.User{
+		{ID: 1, Username: "niklas", Email: "n@example.com"},
+		{ID: 2, Username: "alex", Email: "a@example.com"},
+	}
+	m.rebuildUserPickerTable()
+	m.scheduleTable.SetCursor(dayIndex)
+	m.pickerAssignWeek = week
+	m.pickerTarget = pickerSchedule
+	m.mode = modeUserPicker
+	m.userPickerTable.SetCursor(1) // alex
+	return m
+}
+
+// The bug: a day somebody already holds could not be handed to anybody else.
+// The server refuses it, so the TUI has to ask first and then say so.
+func TestSchedule_ReassigningATakenDayAsksFirst(t *testing.T) {
+	m := pickingOnCall([]api.ScheduleEntry{
+		{ID: 1, Date: "2026-07-27", UserID: 1, Username: "niklas"},
+	}, 0, false)
+
+	m, cmd := press(t, m, "enter")
+
+	if m.mode != modeConfirm || m.confirmTarget != confirmReassignSchedule {
+		t.Fatalf("expected a reassignment confirmation, got mode %v target %v",
+			m.mode, m.confirmTarget)
+	}
+	if cmd != nil {
+		t.Error("expected nothing sent to the server before confirming")
+	}
+	mustContain(t, m.confirmPrompt(), "This day is assigned to niklas", "Reassign to alex?")
+}
+
+func TestSchedule_ReassignConfirmedSends(t *testing.T) {
+	m := pickingOnCall([]api.ScheduleEntry{
+		{ID: 1, Date: "2026-07-27", UserID: 1, Username: "niklas"},
+	}, 0, false)
+	m, _ = press(t, m, "enter")
+
+	m, cmd := press(t, m, "y")
+	if cmd == nil {
+		t.Fatal("expected the confirmed reassignment to be sent")
+	}
+	if m.mode != modeDashboard {
+		t.Errorf("expected a return to the dashboard, got mode %v", m.mode)
+	}
+	if m.pendingAssign != nil {
+		t.Error("expected the pending assignment cleared")
+	}
+}
+
+// Declining must leave the rota alone — that is the whole point of the guard.
+func TestSchedule_ReassignDeclinedSendsNothing(t *testing.T) {
+	m := pickingOnCall([]api.ScheduleEntry{
+		{ID: 1, Date: "2026-07-27", UserID: 1, Username: "niklas"},
+	}, 0, false)
+	m, _ = press(t, m, "enter")
+
+	m, cmd := press(t, m, "n")
+	if cmd != nil {
+		t.Error("expected nothing sent when the reassignment is declined")
+	}
+	if m.pendingAssign != nil {
+		t.Error("expected the pending assignment discarded")
+	}
+}
+
+// A free day is the path that always worked, and must not grow a prompt.
+func TestSchedule_AssigningAFreeDayDoesNotAsk(t *testing.T) {
+	m := pickingOnCall(nil, 0, false)
+
+	m, cmd := press(t, m, "enter")
+	if m.mode != modeDashboard {
+		t.Errorf("expected no prompt for a free day, got mode %v", m.mode)
+	}
+	if cmd == nil {
+		t.Error("expected the assignment to be sent straight away")
+	}
+}
+
+// The week case is the one that was worst: a single taken day rejected all
+// seven. One prompt now covers the lot, and it says how much is being taken.
+func TestSchedule_ReassigningAPartlyTakenWeekAsksOnce(t *testing.T) {
+	m := pickingOnCall([]api.ScheduleEntry{
+		{ID: 1, Date: "2026-07-28", UserID: 1, Username: "niklas"},
+		{ID: 2, Date: "2026-07-30", UserID: 3, Username: "sam"},
+	}, 0, true)
+
+	m, _ = press(t, m, "enter")
+	if m.confirmTarget != confirmReassignSchedule {
+		t.Fatalf("expected one confirmation for the week, got target %v", m.confirmTarget)
+	}
+	if got := len(m.pendingAssign.dates); got != 7 {
+		t.Errorf("expected all 7 days in the assignment, got %d", got)
+	}
+	mustContain(t, m.confirmPrompt(), "2 of 7 days are assigned to niklas and sam")
+}
+
 // ── Ntfy topic ────────────────────────────────────────────────────────────
 
 // onUsers puts the model in the Users section with a loaded table.

internal/tui/view.go

@@ -178,10 +178,44 @@ func (m Model) confirmPrompt() string {
 		return "Delete schedule entry? [y/N]"
 	case confirmDeleteUser:
 		return fmt.Sprintf("Delete user %s (cascades all API keys)? [y/N]", m.selectedUser.Username)
+	case confirmReassignSchedule:
+		if p := m.pendingAssign; p != nil {
+			return fmt.Sprintf("%s assigned to %s. Reassign to %s? [y/N]",
+				dayCount(len(p.taken), len(p.dates)), joinNames(p.holders), p.username)
+		}
+		return "Reassign these days? [y/N]"
 	}
 	return "Are you sure? [y/N]"
 }
 
+// dayCount phrases how much of an assignment is being taken from somebody. A
+// single day says so plainly; a partial week says which part, because "3 of 7"
+// is the difference between taking a shift and taking somebody's whole week.
+func dayCount(taken, total int) string {
+	switch {
+	case total == 1:
+		return "This day is"
+	case taken == total:
+		return fmt.Sprintf("All %d days are", total)
+	default:
+		return fmt.Sprintf("%d of %d days are", taken, total)
+	}
+}
+
+// joinNames renders a list of people as prose.
+func joinNames(names []string) string {
+	switch len(names) {
+	case 0:
+		return "somebody else"
+	case 1:
+		return names[0]
+	case 2:
+		return names[0] + " and " + names[1]
+	default:
+		return strings.Join(names[:len(names)-1], ", ") + " and " + names[len(names)-1]
+	}
+}
+
 // ── Dashboard ──────────────────────────────────────────────────────────────
 
 func (m Model) renderDashboard() string {