github
diff --git a/‎dotnet/src/Client.cs‎
Lines changed: 3 additions & 1 deletion b/‎dotnet/src/Client.cs‎
Lines changed: 3 additions & 1 deletion
diff --git a/‎dotnet/src/Session.cs‎
Lines changed: 15 additions & 0 deletions b/‎dotnet/src/Session.cs‎
Lines changed: 15 additions & 0 deletions
diff --git a/‎dotnet/src/Types.cs‎
Lines changed: 93 additions & 9 deletions b/‎dotnet/src/Types.cs‎
Lines changed: 93 additions & 9 deletions
diff --git a/‎dotnet/test/E2E/HookLifecycleAndOutputE2ETests.cs‎
Lines changed: 4 additions & 4 deletions b/‎dotnet/test/E2E/HookLifecycleAndOutputE2ETests.cs‎
Lines changed: 4 additions & 4 deletions
@@ -528,6 +528,7 @@ public async Task<CopilotSession> CreateSessionAsync(SessionConfig config, Cance
 
         var hasHooks = config.Hooks != null && (
             config.Hooks.OnPreToolUse != null ||
+            config.Hooks.OnPreMcpToolCall != null ||
             config.Hooks.OnPostToolUse != null ||
             config.Hooks.OnUserPromptSubmitted != null ||
             config.Hooks.OnSessionStart != null ||
@@ -688,6 +689,7 @@ public async Task<CopilotSession> ResumeSessionAsync(string sessionId, ResumeSes
 
         var hasHooks = config.Hooks != null && (
             config.Hooks.OnPreToolUse != null ||
+            config.Hooks.OnPreMcpToolCall != null ||
             config.Hooks.OnPostToolUse != null ||
             config.Hooks.OnUserPromptSubmitted != null ||
             config.Hooks.OnSessionStart != null ||
@@ -1231,7 +1233,7 @@ private async Task ConfigureSessionFsAsync(CancellationToken cancellationToken)
         }
 
         await Rpc.SessionFs.SetProviderAsync(
-            _options.SessionFs.InitialCwd,
+            _options.SessionFs.InitialWorkingDirectory,
             _options.SessionFs.SessionStatePath,
             _options.SessionFs.Conventions,
             _options.SessionFs.Capabilities,
 
@@ -1245,6 +1245,11 @@ internal void RegisterHooks(SessionHooks hooks)
                         JsonSerializer.Deserialize(input.GetRawText(), SessionJsonContext.Default.PreToolUseHookInput)!,
                         invocation)
                     : null,
+                "preMcpToolCall" => hooks.OnPreMcpToolCall != null
+                    ? SerializeHookOutput(await hooks.OnPreMcpToolCall(
+                        JsonSerializer.Deserialize(input.GetRawText(), SessionJsonContext.Default.PreMcpToolCallHookInput)!,
+                        invocation))
+                    : null,
                 "postToolUse" => hooks.OnPostToolUse != null
                     ? await hooks.OnPostToolUse(
                         JsonSerializer.Deserialize(input.GetRawText(), SessionJsonContext.Default.PostToolUseHookInput)!,
@@ -1283,6 +1288,14 @@ internal void RegisterHooks(SessionHooks hooks)
         }
     }
 
+    /// <summary>
+    /// Pre-serializes a hook output to JsonElement so that the <c>object?</c> typed
+    /// <see cref="CopilotClient.HooksInvokeResponse.Output"/> property writes the
+    /// correct JSON without relying on polymorphic type resolution.
+    /// </summary>
+    private static JsonElement? SerializeHookOutput(PreMcpToolCallHookOutput? output) =>
+        output is null ? null : JsonSerializer.SerializeToElement(output, SessionJsonContext.Default.PreMcpToolCallHookOutput);
+
     /// <summary>
     /// Registers transform callbacks for system message sections.
     /// </summary>
@@ -1607,6 +1620,8 @@ internal void ThrowIfDisposed()
     [JsonSerializable(typeof(GetMessagesResponse))]
     [JsonSerializable(typeof(PostToolUseHookInput))]
     [JsonSerializable(typeof(PostToolUseHookOutput))]
+    [JsonSerializable(typeof(PreMcpToolCallHookInput))]
+    [JsonSerializable(typeof(PreMcpToolCallHookOutput))]
     [JsonSerializable(typeof(PreToolUseHookInput))]
     [JsonSerializable(typeof(PreToolUseHookOutput))]
     [JsonSerializable(typeof(SendMessageRequest))]
 
@@ -1,4 +1,4 @@
-/*---------------------------------------------------------------------------------------------
+/*---------------------------------------------------------------------------------------------
  *  Copyright (c) Microsoft Corporation. All rights reserved.
  *--------------------------------------------------------------------------------------------*/
 
@@ -396,7 +396,8 @@ public sealed class SessionFsConfig
     /// <summary>
     /// Initial working directory for sessions (user's project directory).
     /// </summary>
-    public required string InitialCwd { get; init; }
+    [JsonPropertyName("initialCwd")]
+    public required string InitialWorkingDirectory { get; init; }
 
     /// <summary>
     /// Path within each session's SessionFs where the runtime stores
@@ -1215,7 +1216,7 @@ public sealed class PreToolUseHookInput
     /// Current working directory of the session.
     /// </summary>
     [JsonPropertyName("cwd")]
-    public string Cwd { get; set; } = string.Empty;
+    public string WorkingDirectory { get; set; } = string.Empty;
 
     /// <summary>
     /// Name of the tool about to be executed.
@@ -1271,6 +1272,83 @@ public sealed class PreToolUseHookOutput
     public bool? SuppressOutput { get; set; }
 }
 
+/// <summary>
+/// Input for a pre-MCP-tool-call hook.
+/// </summary>
+public sealed class PreMcpToolCallHookInput
+{
+    /// <summary>
+    /// The runtime session ID of the session that triggered the hook.
+    /// </summary>
+    [JsonPropertyName("sessionId")]
+    public string SessionId { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Unix timestamp in milliseconds when the hook was triggered.
+    /// </summary>
+    [JsonPropertyName("timestamp")]
+    [JsonConverter(typeof(UnixMillisecondsDateTimeOffsetConverter))]
+    public DateTimeOffset Timestamp { get; set; }
+
+    /// <summary>
+    /// Current working directory of the session.
+    /// </summary>
+    [JsonPropertyName("cwd")]
+    public string WorkingDirectory { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Name of the MCP server being called.
+    /// </summary>
+    [JsonPropertyName("serverName")]
+    public string ServerName { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Name of the MCP tool being called.
+    /// </summary>
+    [JsonPropertyName("toolName")]
+    public string ToolName { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Arguments for the MCP tool call.
+    /// </summary>
+    [JsonPropertyName("arguments")]
+    public JsonElement? Arguments { get; set; }
+
+    /// <summary>
+    /// Tool call ID, if available.
+    /// </summary>
+    [JsonPropertyName("toolCallId")]
+    public string? ToolCallId { get; set; }
+
+    /// <summary>
+    /// MCP request metadata, if present.
+    /// </summary>
+    [JsonPropertyName("_meta")]
+    public IDictionary<string, JsonElement>? Meta { get; set; }
+}
+
+/// <summary>
+/// Output for a pre-MCP-tool-call hook.
+/// </summary>
+/// <remarks>
+/// <para>The <see cref="MetaToUse"/> property controls outgoing MCP request metadata:</para>
+/// <list type="bullet">
+/// <item><description>Return <c>null</c> from the hook handler: preserve existing <c>_meta</c> (no-op).</description></item>
+/// <item><description>Return a <see cref="PreMcpToolCallHookOutput"/> with <see cref="MetaToUse"/> left as <c>null</c>: omit <c>_meta</c> from the request.</description></item>
+/// <item><description>Return a <see cref="PreMcpToolCallHookOutput"/> with <see cref="MetaToUse"/> set to a <see cref="JsonElement"/> object: replace <c>_meta</c> with that object.</description></item>
+/// </list>
+/// </remarks>
+public sealed class PreMcpToolCallHookOutput
+{
+    /// <summary>
+    /// Hook-controlled metadata to use for the outgoing MCP request.
+    /// See class remarks for semantics.
+    /// </summary>
+    [JsonPropertyName("metaToUse")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public JsonElement? MetaToUse { get; set; }
+}
+
 /// <summary>
 /// Input for a post-tool-use hook.
 /// </summary>
@@ -1293,7 +1371,7 @@ public sealed class PostToolUseHookInput
     /// Current working directory of the session.
     /// </summary>
     [JsonPropertyName("cwd")]
-    public string Cwd { get; set; } = string.Empty;
+    public string WorkingDirectory { get; set; } = string.Empty;
 
     /// <summary>
     /// Name of the tool that was executed.
@@ -1360,7 +1438,7 @@ public sealed class UserPromptSubmittedHookInput
     /// Current working directory of the session.
     /// </summary>
     [JsonPropertyName("cwd")]
-    public string Cwd { get; set; } = string.Empty;
+    public string WorkingDirectory { get; set; } = string.Empty;
 
     /// <summary>
     /// The user's prompt text.
@@ -1415,7 +1493,7 @@ public sealed class SessionStartHookInput
     /// Current working directory of the session.
     /// </summary>
     [JsonPropertyName("cwd")]
-    public string Cwd { get; set; } = string.Empty;
+    public string WorkingDirectory { get; set; } = string.Empty;
 
     /// <summary>
     /// Source of the session start.
@@ -1475,7 +1553,7 @@ public sealed class SessionEndHookInput
     /// Current working directory of the session.
     /// </summary>
     [JsonPropertyName("cwd")]
-    public string Cwd { get; set; } = string.Empty;
+    public string WorkingDirectory { get; set; } = string.Empty;
 
     /// <summary>
     /// Reason for session end.
@@ -1549,7 +1627,7 @@ public sealed class ErrorOccurredHookInput
     /// Current working directory of the session.
     /// </summary>
     [JsonPropertyName("cwd")]
-    public string Cwd { get; set; } = string.Empty;
+    public string WorkingDirectory { get; set; } = string.Empty;
 
     /// <summary>
     /// Error message describing what went wrong.
@@ -1621,6 +1699,11 @@ public sealed class SessionHooks
     /// </summary>
     public Func<PreToolUseHookInput, HookInvocation, Task<PreToolUseHookOutput?>>? OnPreToolUse { get; set; }
 
+    /// <summary>
+    /// Handler called before an MCP tool is called.
+    /// </summary>
+    public Func<PreMcpToolCallHookInput, HookInvocation, Task<PreMcpToolCallHookOutput?>>? OnPreMcpToolCall { get; set; }
+
     /// <summary>
     /// Handler called after a tool has been executed.
     /// </summary>
@@ -2518,7 +2601,8 @@ public MessageOptions Clone()
 public sealed class SessionContext
 {
     /// <summary>Working directory where the session was created.</summary>
-    public string Cwd { get; set; } = string.Empty;
+    [JsonPropertyName("cwd")]
+    public string WorkingDirectory { get; set; } = string.Empty;
     /// <summary>Git repository root (if in a git repo).</summary>
     public string? GitRoot { get; set; }
     /// <summary>GitHub repository in "owner/repo" format.</summary>
 
@@ -45,7 +45,7 @@ public async Task Should_Invoke_OnSessionStart_Hook_On_New_Session()
         Assert.NotEmpty(sessionStartInputs);
         Assert.Equal("new", sessionStartInputs[0].Source);
         Assert.True(sessionStartInputs[0].Timestamp > DateTimeOffset.UnixEpoch);
-        Assert.False(string.IsNullOrEmpty(sessionStartInputs[0].Cwd));
+        Assert.False(string.IsNullOrEmpty(sessionStartInputs[0].WorkingDirectory));
 
         await session.DisposeAsync();
     }
@@ -73,7 +73,7 @@ public async Task Should_Invoke_OnUserPromptSubmitted_Hook_When_Sending_A_Messag
         Assert.NotEmpty(userPromptInputs);
         Assert.Contains("Say hello", userPromptInputs[0].Prompt);
         Assert.True(userPromptInputs[0].Timestamp > DateTimeOffset.UnixEpoch);
-        Assert.False(string.IsNullOrEmpty(userPromptInputs[0].Cwd));
+        Assert.False(string.IsNullOrEmpty(userPromptInputs[0].WorkingDirectory));
 
         await session.DisposeAsync();
     }
@@ -118,7 +118,7 @@ public async Task Should_Invoke_OnErrorOccurred_Hook_When_Error_Occurs()
                 {
                     Assert.Equal(session!.SessionId, invocation.SessionId);
                     Assert.True(input.Timestamp > DateTimeOffset.UnixEpoch);
-                    Assert.False(string.IsNullOrEmpty(input.Cwd));
+                    Assert.False(string.IsNullOrEmpty(input.WorkingDirectory));
                     Assert.False(string.IsNullOrEmpty(input.Error));
                     Assert.Contains(input.ErrorContext, ValidErrorContexts);
                     return Task.FromResult<ErrorOccurredHookOutput?>(null);
@@ -188,7 +188,7 @@ public async Task Should_Invoke_SessionStart_Hook()
 
         Assert.NotEmpty(inputs);
         Assert.Equal("new", inputs[0].Source);
-        Assert.False(string.IsNullOrEmpty(inputs[0].Cwd));
+        Assert.False(string.IsNullOrEmpty(inputs[0].WorkingDirectory));
     }
 
     [Fact]