yep

Shogi.UI/Pages/Home/GameBoard.razor

@@ -1,6 +1,9 @@
-@using Shogi.Contracts.Types;
+@using Shogi.Contracts.Api
+@using Shogi.Contracts.Types;
+@using System.Text.RegularExpressions;
 @inject IShogiApi ShogiApi
 @inject AccountState Account;
+@inject PromotePrompt PromotePrompt;
 
 <article class="game-board">
     <!-- Game board -->
@@ -42,6 +45,15 @@
             <span>H</span>
             <span>I</span>
         </div>
+        <!-- Promote prompt -->
+        <div class="promote-prompt">
+            <p>Do you wish to promote?</p>
+            <div>
+                <button type="button">Yes</button>
+                <button type="button">No</button>
+                <button type="button">Cancel</button>
+            </div>
+        </div>
     </section>
     <!-- Side board -->
     @if (session != null)
@@ -67,15 +79,15 @@
                 }
             </div>
         </aside>
-
     }
 </article>
 
 @code {
+    static readonly string[] Files = new[] { "A", "B", "C", "D", "E", "F", "G", "H", "I" };
+
     [Parameter]
     public string? SessionName { get; set; }
 
-    static readonly string[] Files = new[] { "A", "B", "C", "D", "E", "F", "G", "H", "I" };
     WhichPlayer Perspective => Account.User?.Id == session?.Player1
         ? WhichPlayer.Player1
         : WhichPlayer.Player2;
@@ -114,8 +126,22 @@
         }
     }
 
-    void OnClickTile(Piece? piece, string position)
+    bool ShouldPromptForPromotion(string position)
     {
+        if (Perspective == WhichPlayer.Player1 && Regex.IsMatch(position, ".[7-9]"))
+        {
+            return true;
+        }
+        if (Perspective == WhichPlayer.Player2 && Regex.IsMatch(position, ".[1-3]"))
+        {
+            return true;
+        }
+        return false;
+    }
+    async void OnClickTile(Piece? piece, string position)
+    {
+        if (SessionName == null) return;
+
         if (selectedPosition == null)
         {
             selectedPosition = position;
@@ -128,12 +154,23 @@
         }
         else if (piece != null)
         {
-            ShogiApi.PostMove(SessionName!, new Contracts.Api.MovePieceCommand
+            if (ShouldPromptForPromotion(position) || ShouldPromptForPromotion(selectedPosition))
             {
-                From = selectedPosition,
-                To = position,
-                IsP
-            });
+                PromotePrompt.Show(SessionName, new MovePieceCommand
+                    {
+                        From = selectedPosition,
+                        To = position
+                    });
+            }
+            else
+            {
+                await ShogiApi.PostMove(SessionName, new MovePieceCommand
+                    {
+                        From = selectedPosition,
+                        IsPromotion = false,
+                        To = position
+                    });
+            }
         }
     }
     void OnClickHand(Piece piece)

Shogi.UI/Pages/Home/PromotePrompt.cs

@@ -0,0 +1,58 @@
+using Shogi.Contracts.Api;
+using Shogi.UI.Pages.Home.Api;
+
+namespace Shogi.UI.Pages.Home;
+
+public class PromotePrompt
+{
+  private readonly IShogiApi shogiApi;
+  private string? sessionName;
+  private MovePieceCommand? command;
+
+  public PromotePrompt(IShogiApi shogiApi)
+  {
+    this.shogiApi = shogiApi;
+    IsVisible = false;
+    OnClickCancel = Hide;
+  }
+
+  public bool IsVisible { get; private set; }
+  public Action OnClickCancel;
+  public Func<Task>? OnClickNo;
+  public Func<Task>? OnClickYes;
+
+  public void Show(string sessionName, MovePieceCommand command)
+  {
+    this.sessionName = sessionName;
+    this.command = command;
+    IsVisible = true;
+    OnClickNo = Move;
+    OnClickYes = MoveAndPromote;
+  }
+
+  public void Hide()
+  {
+    IsVisible = false;
+    OnClickNo = null;
+    OnClickYes = null;
+  }
+
+  private Task Move()
+  {
+    if (command != null && sessionName != null)
+    {
+      command.IsPromotion = false;
+      return shogiApi.PostMove(sessionName, command);
+    }
+    return Task.CompletedTask;
+  }
+  private Task MoveAndPromote()
+  {
+    if (command != null && sessionName != null)
+    {
+      command.IsPromotion = true;
+      return shogiApi.PostMove(sessionName, command);
+    }
+    return Task.CompletedTask;
+  }
+}