Let the host adjust the headers an MCP connection is opened with

src/Infrastructure/BotSharp.Abstraction/MCP/Services/IMcpClientHeaderProvider.cs

@@ -0,0 +1,30 @@
+namespace BotSharp.Abstraction.MCP.Services;
+
+/// <summary>
+/// An optional host hook over the HTTP headers used to open one MCP server connection.
+/// </summary>
+/// <remarks>
+/// Nothing registers this by default. With no implementation registered the headers configured
+/// under <c>MCP:McpServerConfigs</c> are used verbatim, which is the only behaviour there was
+/// before the hook existed — a host that does not implement it sees no change at all.
+/// <para>
+/// It exists so a host can call a server as whoever is driving the conversation instead of with
+/// one fixed credential. That decision belongs to the host: it is the only side that knows what
+/// a caller's credential is and which servers may be shown it.
+/// </para>
+/// </remarks>
+public interface IMcpClientHeaderProvider
+{
+    /// <summary>
+    /// Answers the headers to send to <paramref name="serverId"/>.
+    /// </summary>
+    /// <param name="configured">
+    /// The headers from configuration. This dictionary is shared for the lifetime of the process,
+    /// so an implementation that changes a header MUST copy it rather than write into it.
+    /// </param>
+    /// <returns>
+    /// The headers to send. Returning <paramref name="configured"/> unchanged is the no-op answer,
+    /// and is the answer expected for any server the implementation does not recognise.
+    /// </returns>
+    Dictionary<string, string>? GetHeaders(string serverId, Dictionary<string, string>? configured);
+}

src/Infrastructure/BotSharp.Core/MCP/Managers/McpClientManager.cs

@@ -34,7 +34,7 @@ public McpClientManager(
                 {
                     Name = config.Name,
                     Endpoint = new Uri(config.HttpConfig.EndPoint),
-                    AdditionalHeaders = config.HttpConfig.AdditionalHeaders,
+                    AdditionalHeaders = ResolveHeaders(config.Id, config.HttpConfig.AdditionalHeaders),
                     ConnectionTimeout = config.HttpConfig.ConnectionTimeout
                 });
             }
@@ -44,7 +44,7 @@ public McpClientManager(
                 {
                     Name = config.Name,
                     Endpoint = new Uri(config.SseConfig.EndPoint),
-                    AdditionalHeaders = config.SseConfig.AdditionalHeaders,
+                    AdditionalHeaders = ResolveHeaders(config.Id, config.SseConfig.AdditionalHeaders),
                     ConnectionTimeout = config.SseConfig.ConnectionTimeout
                 });
             }
@@ -74,6 +74,21 @@ public McpClientManager(
         }
     }
 
+    /// <summary>
+    /// The headers to open a connection with: the ones from configuration, unless the host has
+    /// registered an <see cref="IMcpClientHeaderProvider"/> that wants to adjust them.
+    /// </summary>
+    /// <remarks>
+    /// No provider is registered by default, and a provider is free to answer with what it was
+    /// given, so a host without one — or with one that does not recognise this server — gets the
+    /// configured headers back untouched.
+    /// </remarks>
+    private Dictionary<string, string>? ResolveHeaders(string serverId, Dictionary<string, string>? configured)
+    {
+        var provider = _services.GetService<IMcpClientHeaderProvider>();
+        return provider == null ? configured : provider.GetHeaders(serverId, configured);
+    }
+
     public void Dispose()
     {