Add session lifecycle hooks for observability

dotnet/src/Client.cs

@@ -59,6 +59,24 @@ public partial class CopilotClient : IDisposable, IAsyncDisposable
     private readonly int? _optionsPort;
     private readonly string? _optionsHost;
 
+    /// <summary>
+    /// Occurs when a new session is created.
+    /// </summary>
+    /// <remarks>
+    /// Subscribe to this event to hook into session events globally.
+    /// The handler receives the newly created <see cref="CopilotSession"/> instance.
+    /// </remarks>
+    public event Action<CopilotSession>? SessionCreated;
+
+    /// <summary>
+    /// Occurs when a session is destroyed.
+    /// </summary>
+    /// <remarks>
+    /// Subscribe to this event to perform cleanup when sessions end.
+    /// The handler receives the session ID of the destroyed session.
+    /// </remarks>
+    public event Action<string>? SessionDestroyed;
+
     /// <summary>
     /// Creates a new instance of <see cref="CopilotClient"/>.
     /// </summary>
@@ -362,6 +380,27 @@ public async Task<CopilotSession> CreateSessionAsync(SessionConfig? config = nul
             throw new InvalidOperationException($"Session {response.SessionId} already exists");
         }
 
+        session.OnDisposed = (id) =>
+        {
+            _sessions.TryRemove(id, out _);
+            try
+            {
+                SessionDestroyed?.Invoke(id);
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine($"Exception in SessionDestroyed event handler: {ex}");
+            }
+        };
+        try
+        {
+            SessionCreated?.Invoke(session);
+        }
+        catch (Exception ex)
+        {
+            Debug.WriteLine($"Exception in SessionCreated event handler: {ex}");
+        }
+
         return session;
     }
 
@@ -416,6 +455,28 @@ public async Task<CopilotSession> ResumeSessionAsync(string sessionId, ResumeSes
 
         // Replace any existing session entry to ensure new config (like permission handler) is used
         _sessions[response.SessionId] = session;
+
+        session.OnDisposed = (id) =>
+        {
+            _sessions.TryRemove(id, out _);
+            try
+            {
+                SessionDestroyed?.Invoke(id);
+            }
+            catch (System.Exception ex)
+            {
+                Debug.WriteLine($"Exception in SessionDestroyed event handler: {ex}");
+            }
+        };
+        try
+        {
+            SessionCreated?.Invoke(session);
+        }
+        catch (System.Exception ex)
+        {
+            Debug.WriteLine($"Exception in SessionCreated event handler: {ex}");
+        }
+
         return session;
     }
 

dotnet/src/Session.cs

@@ -64,6 +64,12 @@ public partial class CopilotSession : IAsyncDisposable
     /// </value>
     public string? WorkspacePath { get; }
 
+    /// <summary>
+    /// Internal callback invoked when the session is disposed.
+    /// Used by CopilotClient to fire the SessionDestroyed event.
+    /// </summary>
+    internal Action<string>? OnDisposed { get; set; }
+
     /// <summary>
     /// Initializes a new instance of the <see cref="CopilotSession"/> class.
     /// </summary>
@@ -431,6 +437,15 @@ await _rpc.InvokeWithCancellationAsync<object>(
         {
             _permissionHandlerLock.Release();
         }
+
+        try
+        {
+            OnDisposed?.Invoke(SessionId);
+        }
+        catch (Exception)
+        {
+            // Swallow exceptions from disposal callbacks to avoid failing DisposeAsync.
+        }
     }
 
     private class OnDisposeCall(Action callback) : IDisposable