diff --git a/app/controllers/tournaments_controller.rb b/app/controllers/tournaments_controller.rb
index f878c77..d5dd499 100644
--- a/app/controllers/tournaments_controller.rb
+++ b/app/controllers/tournaments_controller.rb
@@ -205,12 +205,13 @@ class TournamentsController < ApplicationController
   private
 
   def timer_params
-    {
-      timestamp: params[:timestamp],
-      timer_mode: params[:timer_mode],
+    timer_params = {
       timer_reason: params[:timer_reason],
       timer_reason_text: params[:timer_reason_text]
     }
+    timer_params[:timestamp] = params[:timestamp] if params.key?(:timestamp)
+    timer_params[:timer_mode] = params[:timer_mode] if params.key?(:timer_mode)
+    timer_params
   end
 
   def serialize_timer_state(tournament)
@@ -371,6 +372,9 @@ class TournamentsController < ApplicationController
     timestamp = params[:timestamp]
     timestamp_seconds = params[:timestamp_seconds]
     timer_mode = params[:timer_mode]
+    timer_reason_param_present = params.key?(:timer_reason)
+    timer_reason_text_param_present = params.key?(:timer_reason_text)
+    timer_reason_update_requested = timer_reason_param_present || timer_reason_text_param_present
     normalize_timer_reason_params
     timer_reason = params[:timer_reason]
     timer_reason_text = params[:timer_reason_text]
@@ -384,7 +388,12 @@ class TournamentsController < ApplicationController
       return render_timer_validation_error('Invalid timer_mode')
     end
 
-    timer_reason_error = validate_timer_reason_params(timer_reason, timer_reason_text)
+    timer_reason_error = validate_timer_reason_params(
+      timer_reason,
+      timer_reason_text,
+      timer_reason_param_present,
+      timer_reason_text_param_present
+    )
     return render_timer_validation_error(timer_reason_error) if timer_reason_error
 
     # throw error if both timestamp and timestamp_seconds are present
@@ -428,6 +437,8 @@ class TournamentsController < ApplicationController
       params[:timestamp] = Time.zone.now
     elsif timer_mode.present?
       return render_timer_validation_error('timer_mode requires timestamp, timestamp_seconds, or clear_timer')
+    elsif timer_reason_update_requested
+      return render_timer_validation_error('timer_reason requires active timer') unless timer_active?
     else
       return render_timer_validation_error('timestamp, timestamp_seconds, or clear_timer is required')
     end
@@ -456,8 +467,17 @@ class TournamentsController < ApplicationController
     params[:timer_reason_text] = nil
   end
 
-  def validate_timer_reason_params(timer_reason, timer_reason_text)
+  def validate_timer_reason_params(
+    timer_reason,
+    timer_reason_text,
+    timer_reason_param_present,
+    timer_reason_text_param_present
+  )
+    if timer_reason_text_param_present && !timer_reason_param_present
+      return 'timer_reason is required when timer_reason_text is provided'
+    end
     return if timer_reason.blank? && timer_reason_text.blank?
+
     return 'timer_reason_text is only allowed when timer_reason is custom' if timer_reason.blank?
     return 'Invalid timer_reason' unless Tournament::TIMER_REASONS.include?(timer_reason)
 
@@ -470,6 +490,10 @@ class TournamentsController < ApplicationController
     'timer_reason_text is only allowed when timer_reason is custom'
   end
 
+  def timer_active?
+    @tournament.timestamp.present? && @tournament.timer_mode.present?
+  end
+
   def render_timer_validation_error(message)
     render json: { error: message }, status: :unprocessable_content
   end
diff --git a/doc/timer_api.md b/doc/timer_api.md
index f1d0622..b60447f 100644
--- a/doc/timer_api.md
+++ b/doc/timer_api.md
@@ -185,6 +185,39 @@ Useful for:
 - rehydrating synced follower state
 - advanced admin tooling
 
+### Change timer reason without restarting
+
+After a timer is active, owner may change only the reason fields.
+Backend keeps current `timestamp` and `timer_mode` unchanged.
+
+Set a predefined reason:
+
+```json
+{
+  "timer_reason": "playoff_start"
+}
+```
+
+Set a custom reason:
+
+```json
+{
+  "timer_reason": "custom",
+  "timer_reason_text": "Opening ceremony ran long"
+}
+```
+
+Remove reason:
+
+```json
+{
+  "timer_reason": null
+}
+```
+
+Response shape is the normal timer state with the same active `timestamp` and
+`timer_mode`, plus updated reason fields.
+
 ### Clear timer
 
 Request:
@@ -215,6 +248,8 @@ Backend rejects:
 - `timer_mode` by itself unless it is `countup`
 - `clear_timer` mixed with timer values
 - invalid `timer_reason`
+- reason-only updates when no timer is active
+- `timer_reason_text` without `timer_reason`
 - `timer_reason = "custom"` without `timer_reason_text`
 - `timer_reason_text` with any non-custom `timer_reason`
 
@@ -338,6 +373,7 @@ Implemented coverage:
 - API surface timer lifecycle:
   - countdown
   - countup
+  - reason-only set / change / remove
   - clear
   - validation
   - [spec/e2e/http/api_surface_spec.rb](../spec/e2e/http/api_surface_spec.rb)
diff --git a/spec/controllers/tournaments_controller_spec.rb b/spec/controllers/tournaments_controller_spec.rb
index 4f396b3..9f76c7e 100644
--- a/spec/controllers/tournaments_controller_spec.rb
+++ b/spec/controllers/tournaments_controller_spec.rb
@@ -307,6 +307,53 @@ RSpec.describe TournamentsController, type: :controller do
       expect(deserialize_response(response)[:timer_reason_text]).to be_nil
     end
 
+    it 'updates timer reason without changing active timer clock' do
+      original_timestamp = Time.utc(2030, 4, 13, 18, 0, 0)
+      @tournament.update!(
+        timestamp: original_timestamp,
+        timer_mode: 'countdown',
+        timer_reason: 'tournament_start'
+      )
+
+      patch :set_timer, params: {
+        id: @tournament.to_param,
+        timer_reason: 'custom',
+        timer_reason_text: 'Schedule pause'
+      }
+
+      expect(response).to be_successful
+      expect(@tournament.reload.timestamp).to eq(original_timestamp)
+      expect(@tournament.timer_mode).to eq('countdown')
+      expect(@tournament.timer_reason).to eq('custom')
+      expect(@tournament.timer_reason_text).to eq('Schedule pause')
+      expect(deserialize_response(response)[:timestamp]).to eq(original_timestamp.iso8601)
+      expect(deserialize_response(response)[:timer_mode]).to eq('countdown')
+      expect(deserialize_response(response)[:timer_reason]).to eq('custom')
+      expect(deserialize_response(response)[:timer_reason_text]).to eq('Schedule pause')
+
+      patch :set_timer, params: {
+        id: @tournament.to_param,
+        timer_reason: ''
+      }
+
+      expect(response).to be_successful
+      expect(@tournament.reload.timestamp).to eq(original_timestamp)
+      expect(@tournament.timer_mode).to eq('countdown')
+      expect(@tournament.timer_reason).to be_nil
+      expect(@tournament.timer_reason_text).to be_nil
+      expect(deserialize_response(response)[:timestamp]).to eq(original_timestamp.iso8601)
+      expect(deserialize_response(response)[:timer_mode]).to eq('countdown')
+      expect(deserialize_response(response)[:timer_reason]).to be_nil
+      expect(deserialize_response(response)[:timer_reason_text]).to be_nil
+    end
+
+    it 'rejects timer reason updates when no timer is active' do
+      patch :set_timer, params: { id: @tournament.to_param, timer_reason: 'match_end' }
+
+      expect(response).to have_http_status(:unprocessable_content)
+      expect(deserialize_response(response)[:error]).to eq('timer_reason requires active timer')
+    end
+
     it 'rejects timestamp_seconds for countup mode' do
       patch :set_timer, params: { id: @tournament.to_param, timer_mode: 'countup', timestamp_seconds: 120 }
 
diff --git a/spec/e2e/http/api_surface_spec.rb b/spec/e2e/http/api_surface_spec.rb
index f8600d7..45a1531 100644
--- a/spec/e2e/http/api_surface_spec.rb
+++ b/spec/e2e/http/api_surface_spec.rb
@@ -139,6 +139,13 @@ RSpec.describe 'Backend API surface HTTP E2E' do
                                          timer_reason: 'tournament_start'
                                        })
     timer_show = anonymous_client.get("/tournaments/#{tournament.fetch(:id)}/timer")
+    timer_reason_custom = owner_client.patch("/tournaments/#{tournament.fetch(:id)}/timer", body: {
+                                               timer_reason: 'custom',
+                                               timer_reason_text: 'Pause ran long'
+                                             })
+    timer_reason_remove = owner_client.patch("/tournaments/#{tournament.fetch(:id)}/timer", body: {
+                                               timer_reason: nil
+                                             })
     timer_countup = owner_client.patch("/tournaments/#{tournament.fetch(:id)}/timer", body: {
                                          timer_mode: 'countup',
                                          timer_reason: 'custom',
@@ -194,6 +201,16 @@ RSpec.describe 'Backend API surface HTTP E2E' do
     expect(timer_show.dig(:json, :timestamp)).to eq(timer_seconds.dig(:json, :timestamp))
     expect(timer_show.dig(:json, :timer_mode)).to eq('countdown')
     expect(timer_show.dig(:json, :timer_reason)).to eq('tournament_start')
+    expect(timer_reason_custom[:status]).to eq(200)
+    expect(timer_reason_custom.dig(:json, :timestamp)).to eq(timer_seconds.dig(:json, :timestamp))
+    expect(timer_reason_custom.dig(:json, :timer_mode)).to eq('countdown')
+    expect(timer_reason_custom.dig(:json, :timer_reason)).to eq('custom')
+    expect(timer_reason_custom.dig(:json, :timer_reason_text)).to eq('Pause ran long')
+    expect(timer_reason_remove[:status]).to eq(200)
+    expect(timer_reason_remove.dig(:json, :timestamp)).to eq(timer_seconds.dig(:json, :timestamp))
+    expect(timer_reason_remove.dig(:json, :timer_mode)).to eq('countdown')
+    expect(timer_reason_remove.dig(:json, :timer_reason)).to be_nil
+    expect(timer_reason_remove.dig(:json, :timer_reason_text)).to be_nil
     expect(timer_countup[:status]).to eq(200)
     expect(timer_countup.dig(:json, :timestamp)).to be_a(String)
     expect(timer_countup.dig(:json, :timer_mode)).to eq('countup')
diff --git a/spec/e2e/http/tournament_timer_follow_sync_websocket_spec.rb b/spec/e2e/http/tournament_timer_follow_sync_websocket_spec.rb
index 4112807..5b101d6 100644
--- a/spec/e2e/http/tournament_timer_follow_sync_websocket_spec.rb
+++ b/spec/e2e/http/tournament_timer_follow_sync_websocket_spec.rb
@@ -55,6 +55,35 @@ RSpec.describe 'Tournament timer follower sync websocket HTTP E2E' do
     expect(countdown_payload.fetch('timer_reason')).to eq('match_end')
     expect(countdown_payload.fetch('timer_reason_text')).to be_nil
 
+    reason_update = source_client.patch("/tournaments/#{source.fetch(:id)}/timer", body: {
+                                          timer_reason: 'custom',
+                                          timer_reason_text: 'Follower reason after start'
+                                        })
+    expect(reason_update[:status]).to eq(200)
+    sync_now_if_requested!(source.fetch(:id))
+
+    reason_payload = wait_for_timer_payload_matching!(cable_client) do |payload|
+      payload.fetch('timestamp') == countdown.dig(:json, :timestamp) &&
+        payload.fetch('timer_mode') == 'countdown' &&
+        payload.fetch('timer_reason') == 'custom' &&
+        payload.fetch('timer_reason_text') == 'Follower reason after start'
+    end
+    expect(reason_payload.fetch('timestamp')).to eq(countdown.dig(:json, :timestamp))
+
+    reason_clear = source_client.patch("/tournaments/#{source.fetch(:id)}/timer", body: {
+                                         timer_reason: nil
+                                       })
+    expect(reason_clear[:status]).to eq(200)
+    sync_now_if_requested!(source.fetch(:id))
+
+    reason_cleared_payload = wait_for_timer_payload_matching!(cable_client) do |payload|
+      payload.fetch('timestamp') == countdown.dig(:json, :timestamp) &&
+        payload.fetch('timer_mode') == 'countdown' &&
+        payload.fetch('timer_reason').nil? &&
+        payload.fetch('timer_reason_text').nil?
+    end
+    expect(reason_cleared_payload.fetch('timestamp')).to eq(countdown.dig(:json, :timestamp))
+
     countup = source_client.patch("/tournaments/#{source.fetch(:id)}/timer", body: {
                                     timer_mode: 'countup',
                                     timer_reason: 'custom',
@@ -121,10 +150,16 @@ RSpec.describe 'Tournament timer follower sync websocket HTTP E2E' do
   end
 
   def wait_for_timer_state!(cable_client, timer_mode:, timeout: 20)
+    wait_for_timer_payload_matching!(cable_client, timeout: timeout) do |payload|
+      payload.fetch('timer_mode') == timer_mode
+    end
+  end
+
+  def wait_for_timer_payload_matching!(cable_client, timeout: 20)
     deadline = Time.now + timeout
     loop do
       payload = cable_client.wait_for_timer_payload!(timeout: [deadline - Time.now, 0.1].max)
-      return payload if payload.fetch('timer_mode') == timer_mode
+      return payload if yield(payload)
 
       raise "follower timer websocket payload not received within #{timeout}s" if Time.now >= deadline
     end
diff --git a/spec/e2e/http/tournament_timer_websocket_spec.rb b/spec/e2e/http/tournament_timer_websocket_spec.rb
index 73913bb..69fead4 100644
--- a/spec/e2e/http/tournament_timer_websocket_spec.rb
+++ b/spec/e2e/http/tournament_timer_websocket_spec.rb
@@ -45,6 +45,33 @@ RSpec.describe 'Tournament timer websocket HTTP E2E' do
     expect(countdown_payload.fetch('timer_reason')).to eq('playoff_start')
     expect(countdown_payload.fetch('timer_reason_text')).to be_nil
 
+    reason_update = client.patch("/tournaments/#{tournament.fetch(:id)}/timer", body: {
+                                   timer_reason: 'custom',
+                                   timer_reason_text: 'Reason changed after start'
+                                 })
+    expect(reason_update[:status]).to eq(200)
+
+    reason_payload = wait_for_timer_payload_matching!(cable_client) do |payload|
+      payload.fetch('timestamp') == countdown.dig(:json, :timestamp) &&
+        payload.fetch('timer_mode') == 'countdown' &&
+        payload.fetch('timer_reason') == 'custom' &&
+        payload.fetch('timer_reason_text') == 'Reason changed after start'
+    end
+    expect(reason_payload.fetch('timestamp')).to eq(countdown.dig(:json, :timestamp))
+
+    reason_clear = client.patch("/tournaments/#{tournament.fetch(:id)}/timer", body: {
+                                  timer_reason: nil
+                                })
+    expect(reason_clear[:status]).to eq(200)
+
+    reason_cleared_payload = wait_for_timer_payload_matching!(cable_client) do |payload|
+      payload.fetch('timestamp') == countdown.dig(:json, :timestamp) &&
+        payload.fetch('timer_mode') == 'countdown' &&
+        payload.fetch('timer_reason').nil? &&
+        payload.fetch('timer_reason_text').nil?
+    end
+    expect(reason_cleared_payload.fetch('timestamp')).to eq(countdown.dig(:json, :timestamp))
+
     countup = client.patch("/tournaments/#{tournament.fetch(:id)}/timer", body: {
                              timer_mode: 'countup',
                              timer_reason: 'custom',
@@ -90,10 +117,16 @@ RSpec.describe 'Tournament timer websocket HTTP E2E' do
   end
 
   def wait_for_timer_state!(cable_client, timer_mode:, timeout: 20)
+    wait_for_timer_payload_matching!(cable_client, timeout: timeout) do |payload|
+      payload.fetch('timer_mode') == timer_mode
+    end
+  end
+
+  def wait_for_timer_payload_matching!(cable_client, timeout: 20)
     deadline = Time.now + timeout
     loop do
       payload = cable_client.wait_for_timer_payload!(timeout: [deadline - Time.now, 0.1].max)
-      return payload if payload.fetch('timer_mode') == timer_mode
+      return payload if yield(payload)
 
       raise "timer websocket payload not received within #{timeout}s" if Time.now >= deadline
     end
diff --git a/tasks.py b/tasks.py
index 2e5efe6..4ce0978 100644
--- a/tasks.py
+++ b/tasks.py
@@ -633,10 +633,17 @@ def _start_server(command, env, log_path="/tmp/turniere-e2e-server.log"):
     return process, log_handle
 
 
+def _server_env(env=None):
+    server_env = _env(GIT_COMMIT_SHA=_git_commit_sha())
+    if _rails_env(env) == "test":
+        server_env["WEB_CONCURRENCY"] = os.environ.get("WEB_CONCURRENCY", "1")
+    return server_env
+
+
 def _start_test_server(host=SERVER_HOST, port=SERVER_PORT):
     return _start_server(
         f"{_bundle_exec('rails', 's')} -e test -b {host} -p {port}",
-        _env(GIT_COMMIT_SHA=_git_commit_sha()),
+        _server_env("test"),
     )
 
 
@@ -704,7 +711,7 @@ def server(c, host=SERVER_HOST, port=SERVER_PORT, env=None):
     """Start the backend locally in development."""
     c.run(
         f"RAILS_ENV={_rails_env(env)} {_bundle_exec('rails', 's')} -b {host} -p {port}",
-        env=_env(GIT_COMMIT_SHA=_git_commit_sha()),
+        env=_server_env(env),
         pty=True,
     )