Add built-in plugin directory support

docs/features/plugin-directories.md

@@ -240,6 +240,41 @@ let client = Client::start(
 
 > The example above uses an stdio runtime connection — the default when the SDK bundles the CLI. If you connect to an external runtime via a URL (`forUri` / `ForUri`), pass `--plugin-dir` to the long-running CLI server when you start it; the SDK does not forward `--plugin-dir` to runtimes it didn't spawn.
 
+## Trusted host-bundled plugin directories
+
+Applications that ship their own trusted plugins can register them as a client startup option. The SDK sends the complete ordered set after connecting and verifying the protocol, before `start` returns or any session can be created. Paths must be absolute; leaving the option unset or empty makes no RPC call.
+
+<!-- docs-validate: hidden -->
+```typescript
+import { CopilotClient } from "@github/copilot-sdk";
+
+async function main() {
+  const client = new CopilotClient({
+    builtinPluginDirectories: [
+      "/opt/my-app/copilot-plugins/core",
+      "/opt/my-app/copilot-plugins/github",
+    ],
+  });
+  await client.start();
+}
+
+main();
+```
+<!-- /docs-validate: hidden -->
+
+The equivalent option in each SDK is:
+
+| SDK | Startup option |
+|---|---|
+| Node.js / TypeScript | `builtinPluginDirectories: string[]` |
+| Python | `builtin_plugin_directories=[...]` |
+| Go | `BuiltinPluginDirectories: []string{...}` |
+| .NET | `BuiltinPluginDirectories = [...]` |
+| Java | `.setBuiltinPluginDirectories(List.of(Path.of(...)))` |
+| Rust | `.with_builtin_plugin_directories([...])` |
+
+This is a trust boundary for plugins bundled and controlled by the host application. It is distinct from `--plugin-dir`, which is a CLI process launch argument for explicitly loading ordinary plugin directories. The startup option also works when connecting to an existing runtime because it is sent over JSON-RPC rather than forwarded as a process argument.
+
 ## What a plugin can contribute
 
 Loading a plugin directory makes its extensions visible to every session created by the client. The runtime merges plugin-provided extensions with anything you register inline:

dotnet/src/Client.cs

@@ -76,6 +76,7 @@ public sealed partial class CopilotClient : IDisposable, IAsyncDisposable
     private readonly ILogger _logger;
     private readonly int? _optionsPort;
     private readonly string? _optionsHost;
+    private readonly string[] _builtinPluginDirectories;
     private readonly Func<CancellationToken, Task<IList<ModelInfo>>>? _onListModels;
     private readonly List<LifecycleSubscription> _lifecycleHandlers = [];
 
@@ -138,6 +139,14 @@ public CopilotClient(CopilotClientOptions? options = null)
     {
         _options = options ?? new();
         _connection = _options.Connection ?? ResolveDefaultConnection(_options);
+        _builtinPluginDirectories = _options.BuiltinPluginDirectories?.ToArray() ?? [];
+        foreach (var path in _builtinPluginDirectories.Where(path => !IsFullyQualifiedPath(path)))
+        {
+            throw new ArgumentException(
+                $"{nameof(CopilotClientOptions)}.{nameof(CopilotClientOptions.BuiltinPluginDirectories)} " +
+                $"must contain only absolute paths: {path}",
+                nameof(options));
+        }
 
         switch (_connection)
         {
@@ -317,6 +326,26 @@ private static Uri ParseRuntimeUrl(string url)
         return new Uri(url);
     }
 
+    private static bool IsFullyQualifiedPath(string path)
+    {
+        if (string.IsNullOrEmpty(path) || !Path.IsPathRooted(path))
+        {
+            return false;
+        }
+#if NETSTANDARD2_0
+        if (Path.DirectorySeparatorChar != '\\')
+        {
+            return true;
+        }
+
+        bool IsSeparator(char value) => value == '\\' || value == '/';
+        return (path.Length >= 3 && path[1] == ':' && IsSeparator(path[2]))
+            || (path.Length >= 2 && IsSeparator(path[0]) && IsSeparator(path[1]));
+#else
+        return Path.IsPathFullyQualified(path);
+#endif
+    }
+
     /// <summary>
     /// Starts the Copilot client and connects to the server.
     /// </summary>
@@ -423,6 +452,13 @@ async Task<Connection> StartCoreAsync(CancellationToken ct)
                     "CopilotClient.StartAsync protocol verification complete. Elapsed={Elapsed}",
                     startTimestamp);
 
+                if (_builtinPluginDirectories.Length > 0)
+                {
+                    var request = new BuiltinPluginDirectoriesRequest(_builtinPluginDirectories);
+                    await InvokeRpcAsync<JsonElement>(
+                        connection.Rpc, "plugins.builtin.set", [request], null, ct);
+                }
+
                 var sessionFsTimestamp = Stopwatch.GetTimestamp();
                 await ConfigureSessionFsAsync(ct);
                 if (_options.SessionFs is not null)
@@ -2946,6 +2982,9 @@ internal record ConnectHandshakeRequest(
         string? Token,
         [property: JsonPropertyName("enableGitHubTelemetryForwarding")] bool? EnableGitHubTelemetryForwarding = null);
 
+    internal record BuiltinPluginDirectoriesRequest(
+        string[] Paths);
+
     internal record SetForegroundSessionRequest(
         string SessionId);
 
@@ -2981,6 +3020,7 @@ internal record HooksInvokeResponse(
     [JsonSerializable(typeof(GetSessionMetadataRequest))]
     [JsonSerializable(typeof(GetSessionMetadataResponse))]
     [JsonSerializable(typeof(ConnectHandshakeRequest))]
+    [JsonSerializable(typeof(BuiltinPluginDirectoriesRequest))]
     [JsonSerializable(typeof(McpOAuthTokenStorageMode))]
     [JsonSerializable(typeof(EmbeddingCacheStorageMode))]
     [JsonSerializable(typeof(ModelCapabilitiesOverride))]

dotnet/src/Types.cs

@@ -309,6 +309,7 @@ private CopilotClientOptions(CopilotClientOptions? other)
         Connection = other.Connection;
         WorkingDirectory = other.WorkingDirectory;
         BaseDirectory = other.BaseDirectory;
+        BuiltinPluginDirectories = other.BuiltinPluginDirectories is null ? null : [.. other.BuiltinPluginDirectories];
         Environment = other.Environment;
         GitHubToken = other.GitHubToken;
         Logger = other.Logger;
@@ -358,6 +359,13 @@ private CopilotClientOptions(CopilotClientOptions? other)
     /// </summary>
     public string? BaseDirectory { get; set; }
 
+    /// <summary>
+    /// Absolute paths to trusted plugin directories bundled by the host.
+    /// When non-empty, the complete set is registered with the runtime during
+    /// startup before sessions can be created.
+    /// </summary>
+    public IList<string>? BuiltinPluginDirectories { get; set; }
+
     /// <summary>
     /// Log level for the Copilot runtime. Use the well-known values on
     /// <see cref="CopilotLogLevel"/> (<see cref="CopilotLogLevel.None"/>,

dotnet/test/Unit/CloneTests.cs

@@ -20,6 +20,7 @@ public void CopilotClientOptions_Clone_CopiesAllProperties()
             GitHubToken = "ghp_test",
             UseLoggedInUser = false,
             BaseDirectory = "/custom/copilot/home",
+            BuiltinPluginDirectories = ["/plugins/core", "/plugins/github"],
             EnableRemoteSessions = true,
             SessionIdleTimeoutSeconds = 600,
         };
@@ -33,6 +34,8 @@ public void CopilotClientOptions_Clone_CopiesAllProperties()
         Assert.Equal(original.GitHubToken, clone.GitHubToken);
         Assert.Equal(original.UseLoggedInUser, clone.UseLoggedInUser);
         Assert.Equal(original.BaseDirectory, clone.BaseDirectory);
+        Assert.Equal(original.BuiltinPluginDirectories, clone.BuiltinPluginDirectories);
+        Assert.NotSame(original.BuiltinPluginDirectories, clone.BuiltinPluginDirectories);
         Assert.Equal(original.EnableRemoteSessions, clone.EnableRemoteSessions);
         Assert.Equal(original.SessionIdleTimeoutSeconds, clone.SessionIdleTimeoutSeconds);
     }

dotnet/test/Unit/GitHubTelemetryTests.cs

@@ -17,6 +17,60 @@ namespace GitHub.Copilot.Test.Unit;
 
 public sealed class GitHubTelemetryTests
 {
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public async Task BuiltinPluginDirectories_Default_Or_Empty_Does_Not_Call_Rpc(bool useEmpty)
+    {
+        await using var server = await FakeTelemetryServer.StartAsync();
+        await using var client = new CopilotClient(new CopilotClientOptions
+        {
+            Connection = RuntimeConnection.ForUri(server.Url),
+            BuiltinPluginDirectories = useEmpty ? [] : null,
+        });
+
+        await client.StartAsync();
+
+        Assert.Equal(0, server.BuiltinPluginSetCount);
+    }
+
+    [Fact]
+    public async Task BuiltinPluginDirectories_Are_Set_Once_Before_Start_Completes()
+    {
+        var paths = new[]
+        {
+            Path.GetFullPath(Path.Join("plugins", "core")),
+            Path.GetFullPath(Path.Join("plugins", "github")),
+        };
+        await using var server = await FakeTelemetryServer.StartAsync();
+        await using var client = new CopilotClient(new CopilotClientOptions
+        {
+            Connection = RuntimeConnection.ForUri(server.Url),
+            BuiltinPluginDirectories = paths,
+        });
+
+        await client.StartAsync();
+
+        Assert.Equal(1, server.BuiltinPluginSetCount);
+        var payload = server.LastBuiltinPluginParams
+            ?? throw new InvalidOperationException("plugins.builtin.set was not captured.");
+        Assert.Collection(
+            payload.GetProperty("paths").EnumerateArray(),
+            value => Assert.Equal(paths[0], value.GetString()),
+            value => Assert.Equal(paths[1], value.GetString()));
+    }
+
+    [Fact]
+    public void BuiltinPluginDirectories_Reject_Relative_Paths()
+    {
+        var exception = Assert.Throws<ArgumentException>(() => new CopilotClient(new CopilotClientOptions
+        {
+            BuiltinPluginDirectories = ["plugins/core"],
+        }));
+
+        Assert.Contains("absolute paths", exception.Message);
+    }
+
     [Fact]
     public async Task CreateSession_Opts_Into_Forwarding_When_Handler_Provided()
     {
@@ -266,6 +320,10 @@ public string Url
 
         public JsonElement? LastConnectParams { get; private set; }
 
+        public JsonElement? LastBuiltinPluginParams { get; private set; }
+
+        public int BuiltinPluginSetCount { get; private set; }
+
         public static Task<FakeTelemetryServer> StartAsync()
         {
             var listener = new TcpListener(IPAddress.Loopback, 0);
@@ -347,6 +405,7 @@ private async Task HandleRequestAsync(Stream stream, JsonElement request, Cancel
             object? result = method switch
             {
                 "connect" => CaptureConnect(request),
+                "plugins.builtin.set" => CaptureBuiltinPluginDirectories(request),
                 "session.create" => CaptureCreate(request),
                 "session.resume" => CaptureResume(request),
                 "session.send" => new Dictionary<string, object?> { ["messageId"] = "message-1" },
@@ -375,6 +434,13 @@ private async Task HandleRequestAsync(Stream stream, JsonElement request, Cancel
             };
         }
 
+        private Dictionary<string, object?> CaptureBuiltinPluginDirectories(JsonElement request)
+        {
+            BuiltinPluginSetCount++;
+            LastBuiltinPluginParams = request.TryGetProperty("params", out var p) ? p.Clone() : null;
+            return new Dictionary<string, object?>();
+        }
+
         private Dictionary<string, object?> CaptureCreate(JsonElement request)
         {
             LastCreateParams = request.TryGetProperty("params", out var p) ? p.Clone() : null;

go/client.go

@@ -38,6 +38,7 @@ import (
 	"net"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"regexp"
 	"strconv"
 	"strings"
@@ -225,6 +226,12 @@ func NewClient(options *ClientOptions) *Client {
 	if options != nil {
 		opts = *options
 	}
+	for _, path := range opts.BuiltinPluginDirectories {
+		if !filepath.IsAbs(path) {
+			panic(fmt.Sprintf("BuiltinPluginDirectories must contain only absolute paths: %s", path))
+		}
+	}
+	opts.BuiltinPluginDirectories = append([]string(nil), opts.BuiltinPluginDirectories...)
 
 	// Resolve the connection. An explicit connection always wins; otherwise
 	// honor the same process/environment override as the other SDKs.
@@ -453,6 +460,21 @@ func (c *Client) Start(ctx context.Context) error {
 		return errors.Join(err, killErr)
 	}
 
+	if len(c.options.BuiltinPluginDirectories) > 0 {
+		if _, err := c.client.Request(ctx, "plugins.builtin.set", map[string]any{
+			"paths": c.options.BuiltinPluginDirectories,
+		}); err != nil {
+			c.client.Stop()
+			c.client = nil
+			c.conn = nil
+			c.RPC = nil
+			c.internalRPC = nil
+			killErr := c.killProcess()
+			c.state = stateError
+			return errors.Join(err, killErr)
+		}
+	}
+
 	// If a session filesystem provider was configured, register it.
 	if c.options.SessionFS != nil {
 		req := &rpc.SessionFSSetProviderRequest{