diff --git a/priv/repo/migration_application_compatibility.tsv b/priv/repo/migration_application_compatibility.tsv
index c4d0ed0..6a2af8b 100644
--- a/priv/repo/migration_application_compatibility.tsv
+++ b/priv/repo/migration_application_compatibility.tsv
@@ -13,3 +13,4 @@
 20260813194249	application_safe	additive nullable inbound-provider identity columns and a partial unique index are ignored by the previous application
 20260827173037	application_safe	additive discovery rollup tables and maintenance triggers are ignored by the previous application while continuing to follow its writes
 20260827180241	forward_only	pending lifecycle uniqueness can reject duplicate account access or deletion requests that the previous application may still attempt to insert
+20260827225430	application_safe	replacing rollup trigger functions preserves the existing schema and application contract while serializing concurrent counter inserts
diff --git a/priv/repo/migrations/20260827173037_add_discovery_rollups.exs b/priv/repo/migrations/20260827173037_add_discovery_rollups.exs
index 80c7942..f48b69d 100644
--- a/priv/repo/migrations/20260827173037_add_discovery_rollups.exs
+++ b/priv/repo/migrations/20260827173037_add_discovery_rollups.exs
@@ -293,23 +293,63 @@ defmodule WhoNeedHelp.Repo.Migrations.AddDiscoveryRollups do
         GROUP BY level, cell_x, cell_y, category_id, urgency, visible_until_hour
         HAVING sum(delta) <> 0
       )
-      MERGE INTO help_discovery_rollups AS target
-      USING deltas AS source
-      ON target.level = source.level
-        AND target.cell_x = source.cell_x
-        AND target.cell_y = source.cell_y
-        AND target.category_id = source.category_id
-        AND target.urgency = source.urgency
-        AND target.visible_until_hour = source.visible_until_hour
-      WHEN MATCHED AND target.point_count + source.point_count > 0 THEN
-        UPDATE SET point_count = target.point_count + source.point_count
-      WHEN MATCHED THEN DELETE
-      WHEN NOT MATCHED AND source.point_count > 0 THEN
-        INSERT
-          (level, cell_x, cell_y, category_id, urgency, visible_until_hour, point_count)
-        VALUES
-          (source.level, source.cell_x, source.cell_y, source.category_id,
-           source.urgency, source.visible_until_hour, source.point_count);
+      INSERT INTO help_discovery_rollups
+        (level, cell_x, cell_y, category_id, urgency, visible_until_hour, point_count)
+      SELECT
+        level, cell_x, cell_y, category_id, urgency, visible_until_hour, point_count
+      FROM deltas
+      WHERE point_count > 0
+      ON CONFLICT
+        (level, cell_x, cell_y, category_id, urgency, visible_until_hour)
+      DO UPDATE SET
+        point_count = help_discovery_rollups.point_count + EXCLUDED.point_count;
+
+      WITH changes AS (
+        #{changes_sql}
+      ), expanded AS (
+        SELECT
+          changes.delta,
+          level,
+          #{cell_x_sql("changes.public_longitude", "level")},
+          #{cell_y_sql("changes.public_latitude", "level")},
+          changes.category_id,
+          changes.urgency,
+          changes.visible_until_hour
+        FROM changes
+        CROSS JOIN generate_series(0, #{@maximum_rollup_level}) AS level
+      ), deltas AS (
+        SELECT
+          level,
+          cell_x,
+          cell_y,
+          category_id,
+          urgency,
+          visible_until_hour,
+          sum(delta)::bigint AS point_count
+        FROM expanded
+        GROUP BY level, cell_x, cell_y, category_id, urgency, visible_until_hour
+        HAVING sum(delta) < 0
+      ), decremented AS (
+        UPDATE help_discovery_rollups AS target
+        SET point_count = target.point_count + source.point_count
+        FROM deltas AS source
+        WHERE target.level = source.level
+          AND target.cell_x = source.cell_x
+          AND target.cell_y = source.cell_y
+          AND target.category_id = source.category_id
+          AND target.urgency = source.urgency
+          AND target.visible_until_hour = source.visible_until_hour
+        RETURNING target.*
+      )
+      DELETE FROM help_discovery_rollups AS target
+      USING decremented
+      WHERE target.level = decremented.level
+        AND target.cell_x = decremented.cell_x
+        AND target.cell_y = decremented.cell_y
+        AND target.category_id = decremented.category_id
+        AND target.urgency = decremented.urgency
+        AND target.visible_until_hour = decremented.visible_until_hour
+        AND decremented.point_count = 0;
 
       RETURN NULL;
     END
@@ -348,21 +388,57 @@ defmodule WhoNeedHelp.Repo.Migrations.AddDiscoveryRollups do
         GROUP BY level, cell_x, cell_y, category_id, visible_until_hour
         HAVING sum(delta) <> 0
       )
-      MERGE INTO activity_discovery_rollups AS target
-      USING deltas AS source
-      ON target.level = source.level
-        AND target.cell_x = source.cell_x
-        AND target.cell_y = source.cell_y
-        AND target.category_id = source.category_id
-        AND target.visible_until_hour = source.visible_until_hour
-      WHEN MATCHED AND target.point_count + source.point_count > 0 THEN
-        UPDATE SET point_count = target.point_count + source.point_count
-      WHEN MATCHED THEN DELETE
-      WHEN NOT MATCHED AND source.point_count > 0 THEN
-        INSERT (level, cell_x, cell_y, category_id, visible_until_hour, point_count)
-        VALUES
-          (source.level, source.cell_x, source.cell_y, source.category_id,
-           source.visible_until_hour, source.point_count);
+      INSERT INTO activity_discovery_rollups
+        (level, cell_x, cell_y, category_id, visible_until_hour, point_count)
+      SELECT level, cell_x, cell_y, category_id, visible_until_hour, point_count
+      FROM deltas
+      WHERE point_count > 0
+      ON CONFLICT (level, cell_x, cell_y, category_id, visible_until_hour)
+      DO UPDATE SET
+        point_count = activity_discovery_rollups.point_count + EXCLUDED.point_count;
+
+      WITH changes AS (
+        #{changes_sql}
+      ), expanded AS (
+        SELECT
+          changes.delta,
+          level,
+          #{cell_x_sql("changes.public_longitude", "level")},
+          #{cell_y_sql("changes.public_latitude", "level")},
+          changes.category_id,
+          changes.visible_until_hour
+        FROM changes
+        CROSS JOIN generate_series(0, #{@maximum_rollup_level}) AS level
+      ), deltas AS (
+        SELECT
+          level,
+          cell_x,
+          cell_y,
+          category_id,
+          visible_until_hour,
+          sum(delta)::bigint AS point_count
+        FROM expanded
+        GROUP BY level, cell_x, cell_y, category_id, visible_until_hour
+        HAVING sum(delta) < 0
+      ), decremented AS (
+        UPDATE activity_discovery_rollups AS target
+        SET point_count = target.point_count + source.point_count
+        FROM deltas AS source
+        WHERE target.level = source.level
+          AND target.cell_x = source.cell_x
+          AND target.cell_y = source.cell_y
+          AND target.category_id = source.category_id
+          AND target.visible_until_hour = source.visible_until_hour
+        RETURNING target.*
+      )
+      DELETE FROM activity_discovery_rollups AS target
+      USING decremented
+      WHERE target.level = decremented.level
+        AND target.cell_x = decremented.cell_x
+        AND target.cell_y = decremented.cell_y
+        AND target.category_id = decremented.category_id
+        AND target.visible_until_hour = decremented.visible_until_hour
+        AND decremented.point_count = 0;
 
       RETURN NULL;
     END
diff --git a/priv/repo/migrations/20260827225430_make_discovery_rollups_concurrency_safe.exs b/priv/repo/migrations/20260827225430_make_discovery_rollups_concurrency_safe.exs
new file mode 100644
index 0000000..3c533ed
--- /dev/null
+++ b/priv/repo/migrations/20260827225430_make_discovery_rollups_concurrency_safe.exs
@@ -0,0 +1,277 @@
+defmodule WhoNeedHelp.Repo.Migrations.MakeDiscoveryRollupsConcurrencySafe do
+  use Ecto.Migration
+
+  @maximum_rollup_level 12
+
+  def change do
+    for {operation, changes_sql} <- help_operations() do
+      sql = help_trigger_function(operation, changes_sql)
+      execute(sql, sql)
+    end
+
+    for {operation, changes_sql} <- activity_operations() do
+      sql = activity_trigger_function(operation, changes_sql)
+      execute(sql, sql)
+    end
+  end
+
+  defp help_operations do
+    [
+      {"insert", help_change_source("new_rows", 1)},
+      {"delete", help_change_source("old_rows", -1)},
+      {"update",
+       help_change_source("new_rows", 1) <>
+         " UNION ALL " <> help_change_source("old_rows", -1)}
+    ]
+  end
+
+  defp activity_operations do
+    [
+      {"insert", activity_change_source("new_rows", 1)},
+      {"delete", activity_change_source("old_rows", -1)},
+      {"update",
+       activity_change_source("new_rows", 1) <>
+         " UNION ALL " <> activity_change_source("old_rows", -1)}
+    ]
+  end
+
+  defp help_change_source(relation, delta) do
+    """
+    SELECT
+      #{delta}::bigint AS delta,
+      request.public_longitude,
+      request.public_latitude,
+      request.category_id,
+      request.urgency,
+      date_trunc('hour', request.expires_at) AS visible_until_hour
+    FROM #{relation} AS request
+    WHERE request.status = 'open'
+      AND request.hidden_at IS NULL
+      AND request.location_visibility <> 'hidden'
+      AND request.public_longitude IS NOT NULL
+      AND request.public_latitude IS NOT NULL
+      AND request.category_id IS NOT NULL
+      AND request.urgency IS NOT NULL
+      AND request.expires_at IS NOT NULL
+    """
+  end
+
+  defp activity_change_source(relation, delta) do
+    """
+    SELECT
+      #{delta}::bigint AS delta,
+      activity.public_longitude,
+      activity.public_latitude,
+      activity.category_id,
+      date_trunc('hour', least(activity.starts_at, activity.join_deadline))
+        AS visible_until_hour
+    FROM #{relation} AS activity
+    WHERE activity.status = 'open'
+      AND activity.hidden_at IS NULL
+      AND activity.location_visibility = 'approximate_public'
+      AND activity.public_longitude IS NOT NULL
+      AND activity.public_latitude IS NOT NULL
+      AND activity.category_id IS NOT NULL
+      AND activity.starts_at IS NOT NULL
+      AND activity.join_deadline IS NOT NULL
+    """
+  end
+
+  defp help_trigger_function(operation, changes_sql) do
+    """
+    CREATE OR REPLACE FUNCTION wnh_help_discovery_rollups_#{operation}()
+    RETURNS trigger
+    LANGUAGE plpgsql
+    AS $function$
+    BEGIN
+      #{help_positive_deltas_sql(changes_sql)}
+      #{help_negative_deltas_sql(changes_sql)}
+      RETURN NULL;
+    END
+    $function$
+    """
+  end
+
+  defp activity_trigger_function(operation, changes_sql) do
+    """
+    CREATE OR REPLACE FUNCTION wnh_activity_discovery_rollups_#{operation}()
+    RETURNS trigger
+    LANGUAGE plpgsql
+    AS $function$
+    BEGIN
+      #{activity_positive_deltas_sql(changes_sql)}
+      #{activity_negative_deltas_sql(changes_sql)}
+      RETURN NULL;
+    END
+    $function$
+    """
+  end
+
+  defp help_positive_deltas_sql(changes_sql) do
+    """
+    #{help_deltas_cte(changes_sql, "sum(delta) <> 0")}
+    INSERT INTO help_discovery_rollups
+      (level, cell_x, cell_y, category_id, urgency, visible_until_hour, point_count)
+    SELECT level, cell_x, cell_y, category_id, urgency, visible_until_hour, point_count
+    FROM deltas
+    WHERE point_count > 0
+    ON CONFLICT (level, cell_x, cell_y, category_id, urgency, visible_until_hour)
+    DO UPDATE SET
+      point_count = help_discovery_rollups.point_count + EXCLUDED.point_count;
+    """
+  end
+
+  defp help_negative_deltas_sql(changes_sql) do
+    """
+    #{help_deltas_cte(changes_sql, "sum(delta) < 0")}, decremented AS (
+      UPDATE help_discovery_rollups AS target
+      SET point_count = target.point_count + source.point_count
+      FROM deltas AS source
+      WHERE target.level = source.level
+        AND target.cell_x = source.cell_x
+        AND target.cell_y = source.cell_y
+        AND target.category_id = source.category_id
+        AND target.urgency = source.urgency
+        AND target.visible_until_hour = source.visible_until_hour
+      RETURNING target.*
+    )
+    DELETE FROM help_discovery_rollups AS target
+    USING decremented
+    WHERE target.level = decremented.level
+      AND target.cell_x = decremented.cell_x
+      AND target.cell_y = decremented.cell_y
+      AND target.category_id = decremented.category_id
+      AND target.urgency = decremented.urgency
+      AND target.visible_until_hour = decremented.visible_until_hour
+      AND decremented.point_count = 0;
+    """
+  end
+
+  defp activity_positive_deltas_sql(changes_sql) do
+    """
+    #{activity_deltas_cte(changes_sql, "sum(delta) <> 0")}
+    INSERT INTO activity_discovery_rollups
+      (level, cell_x, cell_y, category_id, visible_until_hour, point_count)
+    SELECT level, cell_x, cell_y, category_id, visible_until_hour, point_count
+    FROM deltas
+    WHERE point_count > 0
+    ON CONFLICT (level, cell_x, cell_y, category_id, visible_until_hour)
+    DO UPDATE SET
+      point_count = activity_discovery_rollups.point_count + EXCLUDED.point_count;
+    """
+  end
+
+  defp activity_negative_deltas_sql(changes_sql) do
+    """
+    #{activity_deltas_cte(changes_sql, "sum(delta) < 0")}, decremented AS (
+      UPDATE activity_discovery_rollups AS target
+      SET point_count = target.point_count + source.point_count
+      FROM deltas AS source
+      WHERE target.level = source.level
+        AND target.cell_x = source.cell_x
+        AND target.cell_y = source.cell_y
+        AND target.category_id = source.category_id
+        AND target.visible_until_hour = source.visible_until_hour
+      RETURNING target.*
+    )
+    DELETE FROM activity_discovery_rollups AS target
+    USING decremented
+    WHERE target.level = decremented.level
+      AND target.cell_x = decremented.cell_x
+      AND target.cell_y = decremented.cell_y
+      AND target.category_id = decremented.category_id
+      AND target.visible_until_hour = decremented.visible_until_hour
+      AND decremented.point_count = 0;
+    """
+  end
+
+  defp help_deltas_cte(changes_sql, having) do
+    """
+    WITH changes AS (
+      #{changes_sql}
+    ), expanded AS (
+      SELECT
+        changes.delta,
+        level,
+        #{cell_x_sql("changes.public_longitude", "level")},
+        #{cell_y_sql("changes.public_latitude", "level")},
+        changes.category_id,
+        changes.urgency,
+        changes.visible_until_hour
+      FROM changes
+      CROSS JOIN generate_series(0, #{@maximum_rollup_level}) AS level
+    ), deltas AS (
+      SELECT
+        level,
+        cell_x,
+        cell_y,
+        category_id,
+        urgency,
+        visible_until_hour,
+        sum(delta)::bigint AS point_count
+      FROM expanded
+      GROUP BY level, cell_x, cell_y, category_id, urgency, visible_until_hour
+      HAVING #{having}
+    )
+    """
+  end
+
+  defp activity_deltas_cte(changes_sql, having) do
+    """
+    WITH changes AS (
+      #{changes_sql}
+    ), expanded AS (
+      SELECT
+        changes.delta,
+        level,
+        #{cell_x_sql("changes.public_longitude", "level")},
+        #{cell_y_sql("changes.public_latitude", "level")},
+        changes.category_id,
+        changes.visible_until_hour
+      FROM changes
+      CROSS JOIN generate_series(0, #{@maximum_rollup_level}) AS level
+    ), deltas AS (
+      SELECT
+        level,
+        cell_x,
+        cell_y,
+        category_id,
+        visible_until_hour,
+        sum(delta)::bigint AS point_count
+      FROM expanded
+      GROUP BY level, cell_x, cell_y, category_id, visible_until_hour
+      HAVING #{having}
+    )
+    """
+  end
+
+  defp cell_x_sql(longitude, level) do
+    """
+    least(
+      power(2, #{level})::bigint - 1,
+      greatest(0, floor(((#{longitude} + 180.0) / 360.0) * power(2, #{level})))::bigint
+    ) AS cell_x
+    """
+  end
+
+  defp cell_y_sql(latitude, level) do
+    """
+    least(
+      power(2, #{level})::bigint - 1,
+      greatest(
+        0,
+        floor(
+          (
+            ln(
+              tan(
+                pi() / 4.0 +
+                  radians(least(85.05112878, greatest(-85.05112878, #{latitude}))) / 2.0
+              )
+            ) / pi() + 1.0
+          ) / 2.0 * power(2, #{level})
+        )::bigint
+      )
+    ) AS cell_y
+    """
+  end
+end
diff --git a/test/who_need_help/discovery_rollup_concurrency_test.exs b/test/who_need_help/discovery_rollup_concurrency_test.exs
new file mode 100644
index 0000000..027238c
--- /dev/null
+++ b/test/who_need_help/discovery_rollup_concurrency_test.exs
@@ -0,0 +1,216 @@
+defmodule WhoNeedHelp.DiscoveryRollupConcurrencyTest do
+  use ExUnit.Case, async: false
+
+  import Ecto.Query
+
+  alias Ecto.Adapters.SQL.Sandbox
+  alias WhoNeedHelp.Accounts
+  alias WhoNeedHelp.Activities.Activity
+  alias WhoNeedHelp.Catalog.Category
+  alias WhoNeedHelp.Help.HelpRequest
+  alias WhoNeedHelp.Repo
+
+  @transaction_timeout 10_000
+
+  test "concurrent help-request inserts serialize the first rollup bucket" do
+    %{category: category, user: user} = setup_entities(:help)
+
+    on_exit(fn -> cleanup_entities(category, user) end)
+
+    expires_at = future_hour(48)
+
+    first = fn ->
+      insert_help_request(user.id, category.id, expires_at, "Concurrent request one")
+    end
+
+    second = fn ->
+      insert_help_request(user.id, category.id, expires_at, "Concurrent request two")
+    end
+
+    assert_concurrent_first_bucket(first, second)
+
+    count =
+      Sandbox.unboxed_run(Repo, fn ->
+        rollup_count(
+          "help_discovery_rollups",
+          category.id,
+          expires_at,
+          "urgency = 'now'"
+        )
+      end)
+
+    assert count == 2
+  end
+
+  test "concurrent activity inserts serialize the first rollup bucket" do
+    %{category: category, user: user} = setup_entities(:activity)
+
+    on_exit(fn -> cleanup_entities(category, user) end)
+
+    join_deadline = future_hour(48)
+    starts_at = DateTime.add(join_deadline, 1, :hour)
+
+    first = fn ->
+      insert_activity(user.id, category.id, starts_at, join_deadline, "Concurrent activity one")
+    end
+
+    second = fn ->
+      insert_activity(user.id, category.id, starts_at, join_deadline, "Concurrent activity two")
+    end
+
+    assert_concurrent_first_bucket(first, second)
+
+    count =
+      Sandbox.unboxed_run(Repo, fn ->
+        rollup_count(
+          "activity_discovery_rollups",
+          category.id,
+          join_deadline,
+          "TRUE"
+        )
+      end)
+
+    assert count == 2
+  end
+
+  defp assert_concurrent_first_bucket(first_insert, second_insert) do
+    parent = self()
+
+    first_task =
+      Task.async(fn ->
+        with_unboxed_connection(fn ->
+          Repo.transaction(
+            fn ->
+              first_insert.()
+              send(parent, {:first_inserted, self()})
+
+              receive do
+                :commit_first -> :ok
+              after
+                @transaction_timeout -> raise "timed out waiting to commit first insert"
+              end
+            end,
+            timeout: @transaction_timeout
+          )
+        end)
+      end)
+
+    assert_receive {:first_inserted, first_pid}, @transaction_timeout
+
+    second_task =
+      Task.async(fn ->
+        with_unboxed_connection(fn ->
+          Repo.transaction(second_insert, timeout: @transaction_timeout)
+        end)
+      end)
+
+    assert Task.yield(second_task, 100) == nil
+    send(first_pid, :commit_first)
+
+    assert {:ok, :ok} = Task.await(first_task, @transaction_timeout)
+    assert {:ok, _record} = Task.await(second_task, @transaction_timeout)
+  end
+
+  defp with_unboxed_connection(fun) do
+    :ok = Sandbox.checkout(Repo, sandbox: false)
+
+    try do
+      fun.()
+    after
+      Sandbox.checkin(Repo)
+    end
+  end
+
+  defp setup_entities(mode) do
+    Sandbox.unboxed_run(Repo, fn ->
+      suffix = System.unique_integer([:positive, :monotonic])
+
+      {:ok, user} =
+        Accounts.register_user(%{
+          email: "rollup-concurrency-#{suffix}@example.invalid",
+          display_name: "Rollup concurrency test",
+          terms_accepted: true
+        })
+
+      category =
+        %Category{}
+        |> Category.changeset(%{
+          slug: "rollup-concurrency-#{mode}-#{suffix}",
+          names: %{"en" => "Rollup concurrency test"},
+          mode: mode
+        })
+        |> Repo.insert!()
+
+      %{category: category, user: user}
+    end)
+  end
+
+  defp cleanup_entities(category, user) do
+    Sandbox.unboxed_run(Repo, fn ->
+      Repo.delete_all(from request in HelpRequest, where: request.category_id == ^category.id)
+      Repo.delete_all(from activity in Activity, where: activity.category_id == ^category.id)
+      Repo.delete!(category)
+      Repo.delete!(user)
+    end)
+  end
+
+  defp insert_help_request(user_id, category_id, expires_at, title) do
+    %HelpRequest{}
+    |> Ecto.Changeset.change(%{
+      title: title,
+      description: "A concurrent request used to verify rollup serialization.",
+      location_label: "Concurrency test point",
+      location: %Geo.Point{coordinates: {30.5234, 50.4501}, srid: 4326},
+      location_visibility: :exact_public,
+      location_radius_meters: nil,
+      urgency: :now,
+      status: :open,
+      expires_at: expires_at,
+      requester_id: user_id,
+      category_id: category_id
+    })
+    |> Repo.insert!()
+  end
+
+  defp insert_activity(user_id, category_id, starts_at, join_deadline, title) do
+    %Activity{}
+    |> Ecto.Changeset.change(%{
+      title: title,
+      description: "A concurrent activity used to verify rollup serialization.",
+      location_label: "Concurrency test point",
+      location: %Geo.Point{coordinates: {30.5234, 50.4501}, srid: 4326},
+      location_visibility: :approximate_public,
+      status: :open,
+      starts_at: starts_at,
+      join_deadline: join_deadline,
+      capacity: 4,
+      creator_id: user_id,
+      category_id: category_id
+    })
+    |> Repo.insert!()
+  end
+
+  defp rollup_count(table, category_id, visible_until, additional_predicate) do
+    query = """
+    SELECT point_count
+    FROM #{table}
+    WHERE level = 0
+      AND category_id = $1
+      AND visible_until_hour = $2
+      AND #{additional_predicate}
+    """
+
+    category_id = Ecto.UUID.dump!(category_id)
+    %{rows: [[count]]} = Repo.query!(query, [category_id, visible_until])
+    count
+  end
+
+  defp future_hour(hours) do
+    now = DateTime.utc_now(:second)
+    seconds_to_next_hour = 3_600 - (now.minute * 60 + now.second)
+
+    now
+    |> DateTime.add(seconds_to_next_hour + hours * 3_600, :second)
+    |> DateTime.truncate(:second)
+  end
+end