diff --git a/drivers/place/staff_api.cr b/drivers/place/staff_api.cr
index d080b8f497c..5f2cf29f19d 100644
--- a/drivers/place/staff_api.cr
+++ b/drivers/place/staff_api.cr
@@ -993,6 +993,13 @@ class Place::StaffAPI < PlaceOS::Driver
     JSON.parse(response.body)
   end
 
+  def event_guests(event_id : String, system_id : String)
+    logger.debug { "getting guests for event #{event_id} in system #{system_id}" }
+    response = get("/api/staff/v1/events/#{event_id}/guests?system_id=#{system_id}", headers: authentication)
+    raise "issue getting guests for event #{event_id}: #{response.status_code}" unless response.success?
+    JSON.parse(response.body)
+  end
+
   # lists asset IDs based on the parameters provided
   #
   # booking_type is required unless event_id or ical_uid is present
diff --git a/drivers/place/visitor_mailer.cr b/drivers/place/visitor_mailer.cr
index 409ce05e719..601c7c5a0a9 100644
--- a/drivers/place/visitor_mailer.cr
+++ b/drivers/place/visitor_mailer.cr
@@ -83,6 +83,9 @@ class Place::VisitorMailer < PlaceOS::Driver
     # Booking details have changed — notify all visitors if relevant fields changed
     monitor("staff/booking/changed") { |_subscription, payload| booking_changed_event(payload.gsub(/[^[:print:]]/, "")) }
 
+    # Calendar event details have changed — notify visitors / previous host
+    monitor("staff/event/changed") { |_subscription, payload| event_changed_event(payload.gsub(/[^[:print:]]/, "")) }
+
     on_update
   end
 
@@ -553,8 +556,10 @@ class Place::VisitorMailer < PlaceOS::Driver
     logger.debug { "received booking changed payload: #{payload}" }
     details = BookingChanged.from_json payload
 
-    # only respond to full changes, not metadata-only updates
-    return unless details.action == "changed"
+    # Only process actions that can carry visitor-relevant changes.
+    # Using an allowlist ensures new action types (e.g. "approved", "rejected",
+    # "checked_in") are ignored by default and don't trigger spurious emails.
+    return unless details.action.in?("changed", "metadata_changed")
 
     # ensure the event is for this building
     if zones = details.zones
@@ -574,6 +579,9 @@ class Place::VisitorMailer < PlaceOS::Driver
     if prev_start = details.previous_booking_start
       fields_changed = true if prev_start != details.booking_start
     end
+    if prev_end = details.previous_booking_end
+      fields_changed = true if prev_end != details.booking_end
+    end
 
     # Location changed: zones identify the building/room the visitor should attend
     if prev_zones = details.previous_zones
@@ -607,6 +615,134 @@ class Place::VisitorMailer < PlaceOS::Driver
     end
 
     guests = staff_api.booking_guests(details.id).get.as_a
+    send_booking_changed_emails(
+      guests,
+      details.user_email,
+      details.booking_start,
+      details.title,
+      details.previous_booking_start,
+      previous_building_name,
+      previous_room_name,
+    )
+  rescue error
+    logger.error { error.inspect_with_backtrace }
+    self[:error_count] = @error_count += 1
+    self[:last_error] = {
+      error: error.message,
+      time:  Time.local.to_s,
+      user:  payload,
+    }
+  end
+
+  protected def event_changed_event(payload)
+    logger.debug { "received event changed payload: #{payload}" }
+    details = EventChanged.from_json payload
+
+    # only respond to updates, not creates or cancellations
+    return unless details.action == "update"
+
+    # ensure the event is for this building
+    if zones = details.zones
+      check = [building_zone.id] + @parent_zone_ids
+
+      if (check & zones).empty?
+        logger.debug { "ignoring event_changed as does not match any zones: #{check}" }
+        return
+      end
+    end
+
+    # --- Host change notification
+    if prev_host = details.previous_host_email
+      if prev_host.downcase != details.host.downcase
+        send_original_host_email(
+          @notify_original_host_template,
+          prev_host,
+          details.host,
+          details.title,
+          details.event_start,
+        )
+      end
+    end
+
+    # --- Date / time / location change notification
+    fields_changed = false
+
+    # Date or time changed
+    if prev_start = details.previous_event_start
+      fields_changed = true if prev_start != details.event_start
+    end
+    if prev_end = details.previous_event_end
+      fields_changed = true if prev_end != details.event_end
+    end
+
+    # Location changed (system_id represents the room)
+    if prev_sys = details.previous_system_id
+      fields_changed = true if prev_sys != details.system_id
+    end
+
+    return unless fields_changed
+
+    # Resolve previous location names from previous_system_id when room changed.
+    # Default previous_room_name to "unknown" when we know there was a different
+    # previous system — if the lookup succeeds it will be overwritten with the
+    # real name; if it fails (rescue) the "unknown" default is preserved and the
+    # recipient can see that the original location could not be determined.
+    previous_building_name = building_zone.display_name.presence || building_zone.name
+    previous_room_name = @booking_space_name
+
+    if (prev_sys_id = details.previous_system_id) && prev_sys_id != details.system_id
+      previous_room_name = "unknown"
+      begin
+        prev_sys = get_room_details(prev_sys_id)
+        previous_room_name = prev_sys.display_name.presence || prev_sys.name
+        if prev_zones = prev_sys.zones
+          prev_zones.each do |zone_id|
+            begin
+              zone = fetch_zone(zone_id)
+              if zone.tags.includes?(@invite_zone_tag)
+                previous_building_name = zone.display_name.presence || zone.name
+                break
+              end
+            rescue error
+              logger.warn(exception: error) { "error looking up previous zone #{zone_id}" }
+            end
+          end
+        end
+      rescue error
+        logger.warn(exception: error) { "error looking up previous system #{prev_sys_id}" }
+      end
+    end
+
+    guests = staff_api.event_guests(details.event_id, details.system_id).get.as_a
+    send_booking_changed_emails(
+      guests,
+      details.host,
+      details.event_start,
+      details.title,
+      details.previous_event_start,
+      previous_building_name,
+      previous_room_name,
+    )
+  rescue error
+    logger.error { error.inspect_with_backtrace }
+    self[:error_count] = @error_count += 1
+    self[:last_error] = {
+      error: error.message,
+      time:  Time.local.to_s,
+      user:  payload,
+    }
+  end
+
+  # Sends booking-changed notification emails to each visitor in the guest list.
+  private def send_booking_changed_emails(
+    guests : Array(JSON::Any),
+    host_email : String,
+    event_start : Int64,
+    event_title : String?,
+    previous_start : Int64?,
+    previous_building_name : String,
+    previous_room_name : String,
+  )
     guests.each do |guest|
       visitor_email = guest["email"].as_s
       visitor_name = guest["name"].as_s?
@@ -614,10 +750,10 @@ class Place::VisitorMailer < PlaceOS::Driver
       # don't email staff members
       next if !@host_domain_filter.empty? && visitor_email.split('@', 2)[1].downcase.in?(@host_domain_filter)
 
-      local_start_time = Time.unix(details.booking_start).in(@time_zone)
+      local_start_time = Time.unix(event_start).in(@time_zone)
 
-      previous_date = details.previous_booking_start.try { |timestamp| Time.unix(timestamp).in(@time_zone).to_s(@date_format) }
-      previous_time = details.previous_booking_start.try { |timestamp| Time.unix(timestamp).in(@time_zone).to_s(@time_format) }
+      previous_date = previous_start.try { |timestamp| Time.unix(timestamp).in(@time_zone).to_s(@date_format) }
+      previous_time = previous_start.try { |timestamp| Time.unix(timestamp).in(@time_zone).to_s(@time_format) }
 
       mailer.send_template(
         visitor_email,
@@ -625,11 +761,11 @@ class Place::VisitorMailer < PlaceOS::Driver
         {
           visitor_email:          visitor_email,
           visitor_name:           visitor_name,
-          host_name:              get_host_name(details.user_email),
-          host_email:             details.user_email,
+          host_name:              get_host_name(host_email),
+          host_email:             host_email,
           room_name:              @booking_space_name,
           building_name:          building_zone.display_name.presence || building_zone.name,
-          event_title:            details.title,
+          event_title:            event_title,
           event_start:            local_start_time.to_s(@time_format),
           event_date:             local_start_time.to_s(@date_format),
           event_time:             local_start_time.to_s(@time_format),
@@ -642,14 +778,6 @@ class Place::VisitorMailer < PlaceOS::Driver
     rescue error
       logger.warn(exception: error) { "failed to send booking_changed email to #{visitor_email}" }
     end
-  rescue error
-    logger.error { error.inspect_with_backtrace }
-    self[:error_count] = @error_count += 1
-    self[:last_error] = {
-      error: error.message,
-      time:  Time.local.to_s,
-      user:  payload,
-    }
   end
 
   @[Security(Level::Support)]
@@ -830,6 +958,7 @@ class Place::VisitorMailer < PlaceOS::Driver
     property name : String
     property display_name : String?
     property map_id : String?
+    property zones : Array(String)?
   end
 
   protected def get_room_details(system_id : String, retries = 0)
diff --git a/drivers/place/visitor_mailer_spec.cr b/drivers/place/visitor_mailer_spec.cr
index 84f5c3db754..257da8c3cf1 100644
--- a/drivers/place/visitor_mailer_spec.cr
+++ b/drivers/place/visitor_mailer_spec.cr
@@ -130,6 +130,30 @@ class StaffAPIMock < DriverSpecs::MockDriver
       },
     ]
   end
+
+  def event_guests(event_id : String, system_id : String)
+    [
+      {
+        email:          "visitor@external.com",
+        name:           "Visitor One",
+        checked_in:     false,
+        visit_expected: true,
+      },
+    ]
+  end
+
+  def get_system(id : String, complete : Bool = false)
+    case id
+    when "sys-room1"
+      {id: "sys-room1", name: "Room 1", display_name: "Conference Room 1", map_id: nil, zones: ["zone-building", "zone-room"]}
+    when "sys-old-room"
+      {id: "sys-old-room", name: "Room 202", display_name: "Old Conference Room 202", map_id: nil, zones: ["zone-old-building", "zone-old-room"]}
+    when "sys-error"
+      raise "system not found: #{id}"
+    else
+      {id: id, name: "Unknown Room", display_name: nil, map_id: nil, zones: [] of String}
+    end
+  end
 end
 
 DriverSpecs.mock_driver "Place::VisitorMailer" do
@@ -346,4 +370,508 @@ DriverSpecs.mock_driver "Place::VisitorMailer" do
   sleep 0.5
 
   system(:Mailer)[:send_count].should eq 3
+
+  # ------------------------------------------------------------------
+  # Test 6b: booking_changed with "metadata_changed" action but time
+  #          window actually shrunk (e.g. 9am–5pm → 10am–4pm).
+  #          The driver should still send the notification because the
+  #          previous values differ from the current values.
+  # ------------------------------------------------------------------
+
+  changed_payload_shrunk = {
+    action:                 "metadata_changed",
+    id:                     106_i64,
+    booking_type:           "desk",
+    booking_start:          now + 7200,
+    booking_end:            now + 10800,
+    timezone:               "GMT",
+    resource_id:            "desk-1",
+    resource_ids:           ["desk-1"],
+    user_email:             "host@example.com",
+    title:                  "Shrunk Window Meeting",
+    zones:                  ["zone-building", "zone-room"],
+    previous_booking_start: now + 3600,
+    previous_booking_end:   now + 14400,
+  }.to_json
+
+  publish("staff/booking/changed", changed_payload_shrunk)
+  sleep 1.5
+
+  # Even though the action is "metadata_changed", the time genuinely
+  # changed so visitors must be notified.
+  system(:Mailer)[:send_count].should eq 4
+  system(:Mailer)[:last_to].should eq "visitor@external.com"
+  system(:Mailer)[:last_template].should eq ["visitor_invited", "booking_changed"]
+  system(:Mailer)[:last_args]["event_title"].should eq "Shrunk Window Meeting"
+
+  # ------------------------------------------------------------------
+  # Test 6c: booking_changed with end-time-only change.
+  #          Start time and zones are the same, only the end time moved
+  #          earlier (e.g. 5pm → 3pm).  Visitors should still be notified.
+  # ------------------------------------------------------------------
+
+  changed_payload_end_only = {
+    action:                 "metadata_changed",
+    id:                     107_i64,
+    booking_type:           "desk",
+    booking_start:          now + 3600,
+    booking_end:            now + 10800,
+    timezone:               "GMT",
+    resource_id:            "desk-1",
+    resource_ids:           ["desk-1"],
+    user_email:             "host@example.com",
+    title:                  "End Time Only Change",
+    zones:                  ["zone-building", "zone-room"],
+    previous_booking_start: now + 3600,
+    previous_booking_end:   now + 14400,
+  }.to_json
+
+  publish("staff/booking/changed", changed_payload_end_only)
+  sleep 1.5
+
+  system(:Mailer)[:send_count].should eq 5
+  system(:Mailer)[:last_to].should eq "visitor@external.com"
+  system(:Mailer)[:last_template].should eq ["visitor_invited", "booking_changed"]
+  system(:Mailer)[:last_args]["event_title"].should eq "End Time Only Change"
+
+  # ==================================================================
+  # booking_host_changed_event tests
+  # ==================================================================
+
+  # ------------------------------------------------------------------
+  # Test 7: booking_host_changed — sends email to previous host
+  # ------------------------------------------------------------------
+
+  host_changed_payload = {
+    action:              "host_changed",
+    booking_id:          200_i64,
+    resource_id:         "desk-1",
+    resource_ids:        ["desk-1"],
+    event_title:         "Team Standup",
+    event_summary:       "Team Standup Description",
+    event_starting:      now + 3600,
+    previous_host_email: "old-host@example.com",
+    new_host_email:      "new-host@example.com",
+    zones:               ["zone-building", "zone-room"],
+  }.to_json
+
+  publish("staff/booking/host_changed", host_changed_payload)
+  sleep 1.5
+
+  # Email should be sent to the previous host
+  system(:Mailer)[:send_count].should eq 6
+  system(:Mailer)[:last_to].should eq "old-host@example.com"
+  system(:Mailer)[:last_template].should eq ["visitor_invited", "notify_original_host"]
+
+  # Verify all template args
+  args7 = system(:Mailer)[:last_args]
+  args7["previous_host_email"].should eq "old-host@example.com"
+  args7["previous_host_name"].should eq "Host User"
+  args7["new_host_email"].should eq "new-host@example.com"
+  args7["new_host_name"].should eq "Host User"
+  args7["building_name"].should eq "Main Building"
+  args7["event_title"].should eq "Team Standup"
+  args7["event_date"].should_not be_nil
+  args7["event_time"].should_not be_nil
+
+  # ------------------------------------------------------------------
+  # Test 8: booking_host_changed — wrong zone is ignored
+  # ------------------------------------------------------------------
+
+  host_changed_wrong_zone = {
+    action:              "host_changed",
+    booking_id:          201_i64,
+    resource_id:         "desk-1",
+    resource_ids:        ["desk-1"],
+    event_title:         "Wrong Zone Meeting",
+    event_summary:       "Wrong Zone Meeting",
+    event_starting:      now + 3600,
+    previous_host_email: "old-host@example.com",
+    new_host_email:      "new-host@example.com",
+    zones:               ["zone-other-building"],
+  }.to_json
+
+  publish("staff/booking/host_changed", host_changed_wrong_zone)
+  sleep 0.5
+
+  # Count should not have increased — event was for a different building
+  system(:Mailer)[:send_count].should eq 6
+
+  # ------------------------------------------------------------------
+  # Test 9: booking_host_changed — nil zones skips zone filter
+  # ------------------------------------------------------------------
+
+  host_changed_no_zones = {
+    action:              "host_changed",
+    booking_id:          202_i64,
+    resource_id:         "desk-1",
+    resource_ids:        ["desk-1"],
+    event_title:         "No Zone Meeting",
+    event_summary:       "No Zone Meeting",
+    event_starting:      now + 7200,
+    previous_host_email: "old-host2@example.com",
+    new_host_email:      "new-host2@example.com",
+  }.to_json
+
+  publish("staff/booking/host_changed", host_changed_no_zones)
+  sleep 1.5
+
+  # When zones are nil, zone filtering is skipped — email should be sent
+  system(:Mailer)[:send_count].should eq 7
+  system(:Mailer)[:last_to].should eq "old-host2@example.com"
+  system(:Mailer)[:last_template].should eq ["visitor_invited", "notify_original_host"]
+
+  # ------------------------------------------------------------------
+  # Test 10: booking_host_changed — event_title nil falls back to
+  #          event_summary
+  # ------------------------------------------------------------------
+
+  host_changed_no_title = {
+    action:              "host_changed",
+    booking_id:          203_i64,
+    resource_id:         "desk-1",
+    resource_ids:        ["desk-1"],
+    event_summary:       "Fallback Summary Title",
+    event_starting:      now + 3600,
+    previous_host_email: "old-host3@example.com",
+    new_host_email:      "new-host3@example.com",
+    zones:               ["zone-building"],
+  }.to_json
+
+  publish("staff/booking/host_changed", host_changed_no_title)
+  sleep 1.5
+
+  system(:Mailer)[:send_count].should eq 8
+  system(:Mailer)[:last_to].should eq "old-host3@example.com"
+
+  args10 = system(:Mailer)[:last_args]
+  # event_title is nil in the payload, so it falls back to event_summary
+  args10["event_title"].should eq "Fallback Summary Title"
+
+  # ------------------------------------------------------------------
+  # Test 10b: booking_host_changed — both event_title and event_summary
+  #           are null (booking has no title or description).  Must not
+  #           crash during deserialisation.
+  # ------------------------------------------------------------------
+
+  host_changed_nil_summary = {
+    action:              "host_changed",
+    booking_id:          204_i64,
+    resource_id:         "desk-1",
+    resource_ids:        ["desk-1"],
+    event_starting:      now + 3600,
+    previous_host_email: "old-host4@example.com",
+    new_host_email:      "new-host4@example.com",
+    zones:               ["zone-building"],
+  }.to_json
+
+  publish("staff/booking/host_changed", host_changed_nil_summary)
+  sleep 1.5
+
+  # Email should still be sent — event_title falls back to nil gracefully
+  system(:Mailer)[:send_count].should eq 9
+  system(:Mailer)[:last_to].should eq "old-host4@example.com"
+
+  args10b = system(:Mailer)[:last_args]
+  args10b["event_title"].raw.should be_nil
+
+  # ==================================================================
+  # event_changed_event tests (staff/event/changed)
+  # ==================================================================
+
+  # ------------------------------------------------------------------
+  # Test 11: event_changed with time change — sends booking_changed
+  #          emails to all visitors on the event
+  # ------------------------------------------------------------------
+
+  event_changed_time = {
+    action:               "update",
+    system_id:            "sys-room1",
+    event_id:             "evt-100",
+    event_ical_uid:       "ical-100",
+    host:                 "host@example.com",
+    resource:             "room1@example.com",
+    title:                "Quarterly Review",
+    event_start:          now + 7200,
+    event_end:            now + 10800,
+    zones:                ["zone-building", "zone-room"],
+    previous_event_start: now + 3600,
+    previous_event_end:   now + 7200,
+  }.to_json
+
+  publish("staff/event/changed", event_changed_time)
+  sleep 1.5
+
+  # Visitor should receive a booking_changed email
+  system(:Mailer)[:send_count].should eq 10
+  system(:Mailer)[:last_to].should eq "visitor@external.com"
+  system(:Mailer)[:last_template].should eq ["visitor_invited", "booking_changed"]
+
+  args11 = system(:Mailer)[:last_args]
+  args11["host_name"].should eq "Host User"
+  args11["host_email"].should eq "host@example.com"
+  args11["event_title"].should eq "Quarterly Review"
+  args11["building_name"].should eq "Main Building"
+  # previous dates should be present
+  args11["previous_event_date"].should_not be_nil
+  args11["previous_event_time"].should_not be_nil
+
+  # ------------------------------------------------------------------
+  # Test 12: event_changed with location change (system_id differs) —
+  #          sends booking_changed emails to visitors
+  # ------------------------------------------------------------------
+
+  event_changed_location = {
+    action:             "update",
+    system_id:          "sys-room1",
+    event_id:           "evt-101",
+    event_ical_uid:     "ical-101",
+    host:               "host@example.com",
+    resource:           "room1@example.com",
+    title:              "Sprint Planning",
+    event_start:        now + 3600,
+    event_end:          now + 7200,
+    zones:              ["zone-building", "zone-room"],
+    previous_system_id: "sys-old-room",
+  }.to_json
+
+  publish("staff/event/changed", event_changed_location)
+  sleep 1.5
+
+  system(:Mailer)[:send_count].should eq 11
+  system(:Mailer)[:last_to].should eq "visitor@external.com"
+  system(:Mailer)[:last_template].should eq ["visitor_invited", "booking_changed"]
+
+  args12 = system(:Mailer)[:last_args]
+  args12["event_title"].should eq "Sprint Planning"
+
+  # ------------------------------------------------------------------
+  # Test 13: event_changed with host change — sends host-change
+  #          notification to the previous host
+  # ------------------------------------------------------------------
+
+  event_changed_host = {
+    action:              "update",
+    system_id:           "sys-room1",
+    event_id:            "evt-102",
+    event_ical_uid:      "ical-102",
+    host:                "new-organiser@example.com",
+    resource:            "room1@example.com",
+    title:               "Design Review",
+    event_start:         now + 3600,
+    event_end:           now + 7200,
+    zones:               ["zone-building"],
+    previous_host_email: "old-organiser@example.com",
+  }.to_json
+
+  publish("staff/event/changed", event_changed_host)
+  sleep 1.5
+
+  system(:Mailer)[:send_count].should eq 12
+  system(:Mailer)[:last_to].should eq "old-organiser@example.com"
+  system(:Mailer)[:last_template].should eq ["visitor_invited", "notify_original_host"]
+
+  args13 = system(:Mailer)[:last_args]
+  args13["previous_host_email"].should eq "old-organiser@example.com"
+  args13["new_host_email"].should eq "new-organiser@example.com"
+  args13["event_title"].should eq "Design Review"
+
+  # ------------------------------------------------------------------
+  # Test 14: event_changed — action "create" is ignored (no previous
+  #          values to compare)
+  # ------------------------------------------------------------------
+
+  event_created_payload = {
+    action:         "create",
+    system_id:      "sys-room1",
+    event_id:       "evt-103",
+    event_ical_uid: "ical-103",
+    host:           "host@example.com",
+    resource:       "room1@example.com",
+    title:          "New Meeting",
+    event_start:    now + 3600,
+    event_end:      now + 7200,
+    zones:          ["zone-building"],
+  }.to_json
+
+  publish("staff/event/changed", event_created_payload)
+  sleep 0.5
+
+  # No email — create events have no previous state to diff against
+  system(:Mailer)[:send_count].should eq 12
+
+  # ------------------------------------------------------------------
+  # Test 15: event_changed — wrong zone is ignored
+  # ------------------------------------------------------------------
+
+  event_changed_wrong_zone = {
+    action:               "update",
+    system_id:            "sys-room1",
+    event_id:             "evt-104",
+    event_ical_uid:       "ical-104",
+    host:                 "host@example.com",
+    resource:             "room1@example.com",
+    title:                "Offsite Meeting",
+    event_start:          now + 7200,
+    event_end:            now + 10800,
+    zones:                ["zone-other-building"],
+    previous_event_start: now + 3600,
+    previous_event_end:   now + 7200,
+  }.to_json
+
+  publish("staff/event/changed", event_changed_wrong_zone)
+  sleep 0.5
+
+  system(:Mailer)[:send_count].should eq 12
+
+  # ------------------------------------------------------------------
+  # Test 16: event_changed — no actual changes (previous == current)
+  #          does not send email
+  # ------------------------------------------------------------------
+
+  event_changed_no_diff = {
+    action:               "update",
+    system_id:            "sys-room1",
+    event_id:             "evt-105",
+    event_ical_uid:       "ical-105",
+    host:                 "host@example.com",
+    resource:             "room1@example.com",
+    title:                "Unchanged Meeting",
+    event_start:          now + 3600,
+    event_end:            now + 7200,
+    zones:                ["zone-building"],
+    previous_event_start: now + 3600,
+    previous_event_end:   now + 7200,
+  }.to_json
+
+  publish("staff/event/changed", event_changed_no_diff)
+  sleep 0.5
+
+  system(:Mailer)[:send_count].should eq 12
+
+  # ------------------------------------------------------------------
+  # Test 17: event_changed with end-time-only change.
+  #          Start time and system_id are the same, only the end time
+  #          moved earlier.  Visitors should still be notified.
+  # ------------------------------------------------------------------
+
+  event_changed_end_only = {
+    action:               "update",
+    system_id:            "sys-room1",
+    event_id:             "evt-106",
+    event_ical_uid:       "ical-106",
+    host:                 "host@example.com",
+    resource:             "room1@example.com",
+    title:                "End Time Only Event",
+    event_start:          now + 3600,
+    event_end:            now + 7200,
+    zones:                ["zone-building", "zone-room"],
+    previous_event_start: now + 3600,
+    previous_event_end:   now + 10800,
+  }.to_json
+
+  publish("staff/event/changed", event_changed_end_only)
+  sleep 1.5
+
+  system(:Mailer)[:send_count].should eq 13
+  system(:Mailer)[:last_to].should eq "visitor@external.com"
+  system(:Mailer)[:last_template].should eq ["visitor_invited", "booking_changed"]
+  system(:Mailer)[:last_args]["event_title"].should eq "End Time Only Event"
+
+  # ------------------------------------------------------------------
+  # Test 18: event_changed with location change — previous_room_name and
+  #          previous_building_name show the PREVIOUS location, not the
+  #          current one. previous_system_id differs from system_id.
+  # ------------------------------------------------------------------
+
+  event_changed_prev_location = {
+    action:             "update",
+    system_id:          "sys-room1",
+    event_id:           "evt-108",
+    event_ical_uid:     "ical-108",
+    host:               "host@example.com",
+    resource:           "room1@example.com",
+    title:              "Location Change Meeting",
+    event_start:        now + 3600,
+    event_end:          now + 7200,
+    zones:              ["zone-building", "zone-room"],
+    previous_system_id: "sys-old-room",
+  }.to_json
+
+  publish("staff/event/changed", event_changed_prev_location)
+  sleep 1.5
+
+  system(:Mailer)[:send_count].should eq 14
+  system(:Mailer)[:last_to].should eq "visitor@external.com"
+  system(:Mailer)[:last_template].should eq ["visitor_invited", "booking_changed"]
+
+  args18 = system(:Mailer)[:last_args]
+  args18["event_title"].should eq "Location Change Meeting"
+  # previous location should come from the previous system (sys-old-room), NOT the current
+  args18["previous_room_name"].should eq "Old Conference Room 202"
+  args18["previous_building_name"].should eq "Previous Building"
+  # current location should remain correct
+  args18["room_name"].should eq "Client Floor"
+  args18["building_name"].should eq "Main Building"
+
+  # ------------------------------------------------------------------
+  # Test 18b: event_changed with an unresolvable previous_system_id.
+  #           get_room_details retries 4× with 1-second delays before
+  #           giving up, so previous_room_name must fall back to "unknown"
+  #           rather than silently showing the current room.
+  # Note: this test requires a longer sleep to accommodate the retries.
+  # ------------------------------------------------------------------
+
+  event_changed_error_system = {
+    action:             "update",
+    system_id:          "sys-room1",
+    event_id:           "evt-109",
+    event_ical_uid:     "ical-109",
+    host:               "host@example.com",
+    resource:           "room1@example.com",
+    title:              "Error System Meeting",
+    event_start:        now + 3600,
+    event_end:          now + 7200,
+    zones:              ["zone-building", "zone-room"],
+    previous_system_id: "sys-error",
+  }.to_json
+
+  count_before_18b = system(:Mailer)[:send_count].as_i
+  publish("staff/event/changed", event_changed_error_system)
+  sleep 6.0 # allow for 4× 1-second retries inside get_room_details
+
+  system(:Mailer)[:send_count].should eq count_before_18b + 1
+  system(:Mailer)[:last_to].should eq "visitor@external.com"
+  args18b = system(:Mailer)[:last_args]
+  args18b["event_title"].should eq "Error System Meeting"
+  args18b["previous_room_name"].should eq "unknown"
+
+  # ------------------------------------------------------------------
+  # Test 19: booking_changed with action "approved" that contains
+  #          previous_* field differences must NOT send an email.
+  #          Only "changed" and "metadata_changed" actions are relevant.
+  # ------------------------------------------------------------------
+
+  approved_payload_with_diff = {
+    action:                 "approved",
+    id:                     108_i64,
+    booking_type:           "desk",
+    booking_start:          now + 7200,
+    booking_end:            now + 10800,
+    timezone:               "GMT",
+    resource_id:            "desk-1",
+    resource_ids:           ["desk-1"],
+    user_email:             "host@example.com",
+    title:                  "Approved Booking",
+    zones:                  ["zone-building", "zone-room"],
+    previous_booking_start: now + 3600,
+    previous_booking_end:   now + 7200,
+  }.to_json
+
+  count_before_approved = system(:Mailer)[:send_count].as_i
+  publish("staff/booking/changed", approved_payload_with_diff)
+  sleep 0.5
+
+  # "approved" is not a visitor-notification action — no email should be sent
+  system(:Mailer)[:send_count].should eq count_before_approved
 end
diff --git a/drivers/place/visitor_models.cr b/drivers/place/visitor_models.cr
index 6681374dc75..fb9cff831c0 100644
--- a/drivers/place/visitor_models.cr
+++ b/drivers/place/visitor_models.cr
@@ -92,7 +92,7 @@ module Place
     property resource_id : String
     property resource_ids : Array(String)
     property event_title : String?
-    property event_summary : String
+    property event_summary : String?
     property event_starting : Int64
     property previous_host_email : String
     property new_host_email : String
@@ -126,4 +126,27 @@ module Place
     property previous_booking_end : Int64?
     property previous_zones : Array(String)?
   end
+
+  # Standalone model for the staff/event/changed channel.
+  # Used to notify visitors when calendar event details they care about have changed.
+  class EventChanged
+    include JSON::Serializable
+
+    property action : String
+    property system_id : String
+    property event_id : String
+    property event_ical_uid : String?
+    property host : String
+    property resource : String?
+    property title : String?
+    property event_start : Int64
+    property event_end : Int64
+    property zones : Array(String)?
+
+    # Previous values — only present when action is "update" and the meta was persisted.
+    property previous_event_start : Int64?
+    property previous_event_end : Int64?
+    property previous_system_id : String?
+    property previous_host_email : String?
+  end
 end
diff --git a/harness b/harness
index 6be5a1b5050..9e08ba8b630 100755
--- a/harness
+++ b/harness
@@ -22,6 +22,15 @@ restore_git_worktree() {
 # that points to an absolute path the container cannot see. Detect this and
 # temporarily replace it with a self-contained repo for the duration of the run.
 setup_git_for_harness() {
+  # Recover from a previous run that was killed before the EXIT trap could fire.
+  # If the backup exists, the current .git is a leftover temp repo — restore the
+  # real worktree pointer before proceeding.
+  if [ -f "${PWD}/.git.worktree-bak" ]; then
+    echo '░░░ Stale worktree backup found — restoring from previous interrupted run...'
+    rm -rf "${PWD}/.git"
+    mv "${PWD}/.git.worktree-bak" "${PWD}/.git"
+  fi
+
   if [ -f "${PWD}/.git" ]; then
     echo '░░░ Git worktree detected, creating temporary repo for harness...'
     _WORKTREE_BACKUP="${PWD}/.git.worktree-bak"