[lldb] Add Model Context Protocol (MCP) support to LLDB (#143628)

This PR adds an MCP (Model Context Protocol ) server to LLDB. For motivation and background, please refer to the corresponding RFC: https://discourse.llvm.org/t/rfc-adding-mcp-support-to-lldb/86798 I implemented this as a new kind of plugin. The idea is that we could support multiple protocol servers (e.g. if we want to support DAP from within LLDB). This also introduces a corresponding top-level command (`protocol-server`) with two subcommands to `start` and `stop` the server. ``` (lldb) protocol-server start MCP tcp://localhost:1234 MCP server started with connection listeners: connection://[::1]:1234, connection://[127.0.0.1]:1234 ``` The MCP sever supports one tool (`lldb_command`) which executes a command, but can easily be extended with more commands.
2025-06-20 10:48:04 -05:00 · 2025-06-20 10:48:04 -05:00 · 9524bfb270
commit 9524bfb270
parent 887222e352
33 changed files with 1803 additions and 23 deletions
--- a/lldb/cmake/modules/LLDBConfig.cmake
+++ b/lldb/cmake/modules/LLDBConfig.cmake
@ -67,6 +67,7 @@ add_optional_dependency(LLDB_ENABLE_FBSDVMCORE "Enable libfbsdvmcore support in

 option(LLDB_USE_ENTITLEMENTS "When codesigning, use entitlements if available" ON)
 option(LLDB_BUILD_FRAMEWORK "Build LLDB.framework (Darwin only)" OFF)
+option(LLDB_ENABLE_PROTOCOL_SERVERS "Enable protocol servers (e.g. MCP) in LLDB" ON)
 option(LLDB_NO_INSTALL_DEFAULT_RPATH "Disable default RPATH settings in binaries" OFF)
 option(LLDB_USE_SYSTEM_DEBUGSERVER "Use the system's debugserver for testing (Darwin only)." OFF)
 option(LLDB_SKIP_STRIP "Whether to skip stripping of binaries when installing lldb." OFF)
--- a/lldb/include/lldb/Core/Debugger.h
+++ b/lldb/include/lldb/Core/Debugger.h
@ -602,6 +602,10 @@ public:
  void FlushProcessOutput(Process &process, bool flush_stdout,
                          bool flush_stderr);

+  void AddProtocolServer(lldb::ProtocolServerSP protocol_server_sp);
+  void RemoveProtocolServer(lldb::ProtocolServerSP protocol_server_sp);
+  lldb::ProtocolServerSP GetProtocolServer(llvm::StringRef protocol) const;
+
  SourceManager::SourceFileCache &GetSourceFileCache() {
    return m_source_file_cache;
  }
@ -772,6 +776,8 @@ protected:
  mutable std::mutex m_progress_reports_mutex;
  /// @}

+  llvm::SmallVector<lldb::ProtocolServerSP> m_protocol_servers;
+
  std::mutex m_destroy_callback_mutex;
  lldb::callback_token_t m_destroy_callback_next_token = 0;
  struct DestroyCallbackInfo {
--- a/lldb/include/lldb/Core/PluginManager.h
+++ b/lldb/include/lldb/Core/PluginManager.h
@ -321,6 +321,17 @@ public:
  static void AutoCompleteProcessName(llvm::StringRef partial_name,
                                      CompletionRequest &request);

+  // Protocol
+  static bool RegisterPlugin(llvm::StringRef name, llvm::StringRef description,
+                             ProtocolServerCreateInstance create_callback);
+
+  static bool UnregisterPlugin(ProtocolServerCreateInstance create_callback);
+
+  static llvm::StringRef GetProtocolServerPluginNameAtIndex(uint32_t idx);
+
+  static ProtocolServerCreateInstance
+  GetProtocolCreateCallbackForPluginName(llvm::StringRef name);
+
  // Register Type Provider
  static bool RegisterPlugin(llvm::StringRef name, llvm::StringRef description,
                             RegisterTypeBuilderCreateInstance create_callback);
--- a/lldb/include/lldb/Core/ProtocolServer.h
+++ b/lldb/include/lldb/Core/ProtocolServer.h
@ -0,0 +1,39 @@
+//===-- ProtocolServer.h --------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_CORE_PROTOCOLSERVER_H
+#define LLDB_CORE_PROTOCOLSERVER_H
+
+#include "lldb/Core/PluginInterface.h"
+#include "lldb/Host/Socket.h"
+#include "lldb/lldb-private-interfaces.h"
+
+namespace lldb_private {
+
+class ProtocolServer : public PluginInterface {
+public:
+  ProtocolServer() = default;
+  virtual ~ProtocolServer() = default;
+
+  static lldb::ProtocolServerSP Create(llvm::StringRef name,
+                                       Debugger &debugger);
+
+  struct Connection {
+    Socket::SocketProtocol protocol;
+    std::string name;
+  };
+
+  virtual llvm::Error Start(Connection connection) = 0;
+  virtual llvm::Error Stop() = 0;
+
+  virtual Socket *GetSocket() const = 0;
+};
+
+} // namespace lldb_private
+
+#endif
--- a/lldb/include/lldb/Interpreter/CommandOptionArgumentTable.h
+++ b/lldb/include/lldb/Interpreter/CommandOptionArgumentTable.h
@ -315,6 +315,7 @@ static constexpr CommandObject::ArgumentTableEntry g_argument_table[] = {
    { lldb::eArgTypeCPUName, "cpu-name", lldb::CompletionType::eNoCompletion, {}, { nullptr, false }, "The name of a CPU." },
    { lldb::eArgTypeCPUFeatures, "cpu-features", lldb::CompletionType::eNoCompletion, {}, { nullptr, false }, "The CPU feature string." },
    { lldb::eArgTypeManagedPlugin, "managed-plugin", lldb::CompletionType::eNoCompletion, {}, { nullptr, false }, "Plugins managed by the PluginManager" },
+    { lldb::eArgTypeProtocol, "protocol", lldb::CompletionType::eNoCompletion, {}, { nullptr, false }, "The name of the protocol." },
    // clang-format on
 };

--- a/lldb/include/lldb/lldb-enumerations.h
+++ b/lldb/include/lldb/lldb-enumerations.h
@ -664,6 +664,7 @@ enum CommandArgumentType {
  eArgTypeCPUName,
  eArgTypeCPUFeatures,
  eArgTypeManagedPlugin,
+  eArgTypeProtocol,
  eArgTypeLastArg // Always keep this entry as the last entry in this
                  // enumeration!!
 };
--- a/lldb/include/lldb/lldb-forward.h
+++ b/lldb/include/lldb/lldb-forward.h
@ -164,13 +164,13 @@ class PersistentExpressionState;
 class Platform;
 class Process;
 class ProcessAttachInfo;
-class ProcessLaunchInfo;
 class ProcessInfo;
 class ProcessInstanceInfo;
 class ProcessInstanceInfoMatch;
 class ProcessLaunchInfo;
 class ProcessModID;
 class Property;
+class ProtocolServer;
 class Queue;
 class QueueImpl;
 class QueueItem;
@ -391,6 +391,7 @@ typedef std::shared_ptr<lldb_private::Platform> PlatformSP;
 typedef std::shared_ptr<lldb_private::Process> ProcessSP;
 typedef std::shared_ptr<lldb_private::ProcessAttachInfo> ProcessAttachInfoSP;
 typedef std::shared_ptr<lldb_private::ProcessLaunchInfo> ProcessLaunchInfoSP;
+typedef std::shared_ptr<lldb_private::ProtocolServer> ProtocolServerSP;
 typedef std::weak_ptr<lldb_private::Process> ProcessWP;
 typedef std::shared_ptr<lldb_private::RegisterCheckpoint> RegisterCheckpointSP;
 typedef std::shared_ptr<lldb_private::RegisterContext> RegisterContextSP;
--- a/lldb/include/lldb/lldb-private-interfaces.h
+++ b/lldb/include/lldb/lldb-private-interfaces.h
@ -81,6 +81,8 @@ typedef lldb::PlatformSP (*PlatformCreateInstance)(bool force,
 typedef lldb::ProcessSP (*ProcessCreateInstance)(
    lldb::TargetSP target_sp, lldb::ListenerSP listener_sp,
    const FileSpec *crash_file_path, bool can_connect);
+typedef lldb::ProtocolServerSP (*ProtocolServerCreateInstance)(
+    Debugger &debugger);
 typedef lldb::RegisterTypeBuilderSP (*RegisterTypeBuilderCreateInstance)(
    Target &target);
 typedef lldb::ScriptInterpreterSP (*ScriptInterpreterCreateInstance)(
--- a/lldb/source/Commands/CMakeLists.txt
+++ b/lldb/source/Commands/CMakeLists.txt
@ -23,6 +23,7 @@ add_lldb_library(lldbCommands NO_PLUGIN_DEPENDENCIES
  CommandObjectPlatform.cpp
  CommandObjectPlugin.cpp
  CommandObjectProcess.cpp
+  CommandObjectProtocolServer.cpp
  CommandObjectQuit.cpp
  CommandObjectRegexCommand.cpp
  CommandObjectRegister.cpp
--- a/lldb/source/Commands/CommandObjectProtocolServer.cpp
+++ b/lldb/source/Commands/CommandObjectProtocolServer.cpp
@ -0,0 +1,176 @@
+//===-- CommandObjectProtocolServer.cpp
+//----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "CommandObjectProtocolServer.h"
+#include "lldb/Core/PluginManager.h"
+#include "lldb/Core/ProtocolServer.h"
+#include "lldb/Host/Socket.h"
+#include "lldb/Interpreter/CommandInterpreter.h"
+#include "lldb/Interpreter/CommandReturnObject.h"
+#include "lldb/Utility/UriParser.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/FormatAdapters.h"
+
+using namespace llvm;
+using namespace lldb;
+using namespace lldb_private;
+
+#define LLDB_OPTIONS_mcp
+#include "CommandOptions.inc"
+
+static std::vector<llvm::StringRef> GetSupportedProtocols() {
+  std::vector<llvm::StringRef> supported_protocols;
+  size_t i = 0;
+
+  for (llvm::StringRef protocol_name =
+           PluginManager::GetProtocolServerPluginNameAtIndex(i++);
+       !protocol_name.empty();
+       protocol_name = PluginManager::GetProtocolServerPluginNameAtIndex(i++)) {
+    supported_protocols.push_back(protocol_name);
+  }
+
+  return supported_protocols;
+}
+
+class CommandObjectProtocolServerStart : public CommandObjectParsed {
+public:
+  CommandObjectProtocolServerStart(CommandInterpreter &interpreter)
+      : CommandObjectParsed(interpreter, "protocol-server start",
+                            "start protocol server",
+                            "protocol-server start <protocol> <connection>") {
+    AddSimpleArgumentList(lldb::eArgTypeProtocol, eArgRepeatPlain);
+    AddSimpleArgumentList(lldb::eArgTypeConnectURL, eArgRepeatPlain);
+  }
+
+  ~CommandObjectProtocolServerStart() override = default;
+
+protected:
+  void DoExecute(Args &args, CommandReturnObject &result) override {
+    if (args.GetArgumentCount() < 1) {
+      result.AppendError("no protocol specified");
+      return;
+    }
+
+    llvm::StringRef protocol = args.GetArgumentAtIndex(0);
+    std::vector<llvm::StringRef> supported_protocols = GetSupportedProtocols();
+    if (llvm::find(supported_protocols, protocol) ==
+        supported_protocols.end()) {
+      result.AppendErrorWithFormatv(
+          "unsupported protocol: {0}. Supported protocols are: {1}", protocol,
+          llvm::join(GetSupportedProtocols(), ", "));
+      return;
+    }
+
+    if (args.GetArgumentCount() < 2) {
+      result.AppendError("no connection specified");
+      return;
+    }
+    llvm::StringRef connection_uri = args.GetArgumentAtIndex(1);
+
+    ProtocolServerSP server_sp = GetDebugger().GetProtocolServer(protocol);
+    if (!server_sp)
+      server_sp = ProtocolServer::Create(protocol, GetDebugger());
+
+    const char *connection_error =
+        "unsupported connection specifier, expected 'accept:///path' or "
+        "'listen://[host]:port', got '{0}'.";
+    auto uri = lldb_private::URI::Parse(connection_uri);
+    if (!uri) {
+      result.AppendErrorWithFormatv(connection_error, connection_uri);
+      return;
+    }
+
+    std::optional<Socket::ProtocolModePair> protocol_and_mode =
+        Socket::GetProtocolAndMode(uri->scheme);
+    if (!protocol_and_mode || protocol_and_mode->second != Socket::ModeAccept) {
+      result.AppendErrorWithFormatv(connection_error, connection_uri);
+      return;
+    }
+
+    ProtocolServer::Connection connection;
+    connection.protocol = protocol_and_mode->first;
+    connection.name =
+        formatv("[{0}]:{1}", uri->hostname.empty() ? "0.0.0.0" : uri->hostname,
+                uri->port.value_or(0));
+
+    if (llvm::Error error = server_sp->Start(connection)) {
+      result.AppendErrorWithFormatv("{0}", llvm::fmt_consume(std::move(error)));
+      return;
+    }
+
+    GetDebugger().AddProtocolServer(server_sp);
+
+    if (Socket *socket = server_sp->GetSocket()) {
+      std::string address =
+          llvm::join(socket->GetListeningConnectionURI(), ", ");
+      result.AppendMessageWithFormatv(
+          "{0} server started with connection listeners: {1}", protocol,
+          address);
+    }
+  }
+};
+
+class CommandObjectProtocolServerStop : public CommandObjectParsed {
+public:
+  CommandObjectProtocolServerStop(CommandInterpreter &interpreter)
+      : CommandObjectParsed(interpreter, "protocol-server stop",
+                            "stop protocol server",
+                            "protocol-server stop <protocol>") {
+    AddSimpleArgumentList(lldb::eArgTypeProtocol, eArgRepeatPlain);
+  }
+
+  ~CommandObjectProtocolServerStop() override = default;
+
+protected:
+  void DoExecute(Args &args, CommandReturnObject &result) override {
+    if (args.GetArgumentCount() < 1) {
+      result.AppendError("no protocol specified");
+      return;
+    }
+
+    llvm::StringRef protocol = args.GetArgumentAtIndex(0);
+    std::vector<llvm::StringRef> supported_protocols = GetSupportedProtocols();
+    if (llvm::find(supported_protocols, protocol) ==
+        supported_protocols.end()) {
+      result.AppendErrorWithFormatv(
+          "unsupported protocol: {0}. Supported protocols are: {1}", protocol,
+          llvm::join(GetSupportedProtocols(), ", "));
+      return;
+    }
+
+    Debugger &debugger = GetDebugger();
+
+    ProtocolServerSP server_sp = debugger.GetProtocolServer(protocol);
+    if (!server_sp) {
+      result.AppendError(
+          llvm::formatv("no {0} protocol server running", protocol).str());
+      return;
+    }
+
+    if (llvm::Error error = server_sp->Stop()) {
+      result.AppendErrorWithFormatv("{0}", llvm::fmt_consume(std::move(error)));
+      return;
+    }
+
+    debugger.RemoveProtocolServer(server_sp);
+  }
+};
+
+CommandObjectProtocolServer::CommandObjectProtocolServer(
+    CommandInterpreter &interpreter)
+    : CommandObjectMultiword(interpreter, "protocol-server",
+                             "Start and stop a protocol server.",
+                             "protocol-server") {
+  LoadSubCommand("start", CommandObjectSP(new CommandObjectProtocolServerStart(
+                              interpreter)));
+  LoadSubCommand("stop", CommandObjectSP(
+                             new CommandObjectProtocolServerStop(interpreter)));
+}
+
+CommandObjectProtocolServer::~CommandObjectProtocolServer() = default;
--- a/lldb/source/Commands/CommandObjectProtocolServer.h
+++ b/lldb/source/Commands/CommandObjectProtocolServer.h
@ -0,0 +1,25 @@
+//===-- CommandObjectProtocolServer.h
+//------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_SOURCE_COMMANDS_COMMANDOBJECTPROTOCOLSERVER_H
+#define LLDB_SOURCE_COMMANDS_COMMANDOBJECTPROTOCOLSERVER_H
+
+#include "lldb/Interpreter/CommandObjectMultiword.h"
+
+namespace lldb_private {
+
+class CommandObjectProtocolServer : public CommandObjectMultiword {
+public:
+  CommandObjectProtocolServer(CommandInterpreter &interpreter);
+  ~CommandObjectProtocolServer() override;
+};
+
+} // namespace lldb_private
+
+#endif // LLDB_SOURCE_COMMANDS_COMMANDOBJECTMCP_H
--- a/lldb/source/Core/CMakeLists.txt
+++ b/lldb/source/Core/CMakeLists.txt
@ -46,6 +46,7 @@ add_lldb_library(lldbCore NO_PLUGIN_DEPENDENCIES
  Opcode.cpp
  PluginManager.cpp
  Progress.cpp
+  ProtocolServer.cpp
  Statusline.cpp
  RichManglingContext.cpp
  SearchFilter.cpp
--- a/lldb/source/Core/Debugger.cpp
+++ b/lldb/source/Core/Debugger.cpp
@ -16,6 +16,7 @@
 #include "lldb/Core/ModuleSpec.h"
 #include "lldb/Core/PluginManager.h"
 #include "lldb/Core/Progress.h"
+#include "lldb/Core/ProtocolServer.h"
 #include "lldb/Core/StreamAsynchronousIO.h"
 #include "lldb/Core/Telemetry.h"
 #include "lldb/DataFormatters/DataVisualization.h"
@ -2375,3 +2376,26 @@ llvm::ThreadPoolInterface &Debugger::GetThreadPool() {
         "Debugger::GetThreadPool called before Debugger::Initialize");
  return *g_thread_pool;
 }
+
+void Debugger::AddProtocolServer(lldb::ProtocolServerSP protocol_server_sp) {
+  assert(protocol_server_sp &&
+         GetProtocolServer(protocol_server_sp->GetPluginName()) == nullptr);
+  m_protocol_servers.push_back(protocol_server_sp);
+}
+
+void Debugger::RemoveProtocolServer(lldb::ProtocolServerSP protocol_server_sp) {
+  auto it = llvm::find(m_protocol_servers, protocol_server_sp);
+  if (it != m_protocol_servers.end())
+    m_protocol_servers.erase(it);
+}
+
+lldb::ProtocolServerSP
+Debugger::GetProtocolServer(llvm::StringRef protocol) const {
+  for (ProtocolServerSP protocol_server_sp : m_protocol_servers) {
+    if (!protocol_server_sp)
+      continue;
+    if (protocol_server_sp->GetPluginName() == protocol)
+      return protocol_server_sp;
+  }
+  return nullptr;
+}
--- a/lldb/source/Core/PluginManager.cpp
+++ b/lldb/source/Core/PluginManager.cpp
@ -1152,6 +1152,38 @@ void PluginManager::AutoCompleteProcessName(llvm::StringRef name,
  }
 }

+#pragma mark ProtocolServer
+
+typedef PluginInstance<ProtocolServerCreateInstance> ProtocolServerInstance;
+typedef PluginInstances<ProtocolServerInstance> ProtocolServerInstances;
+
+static ProtocolServerInstances &GetProtocolServerInstances() {
+  static ProtocolServerInstances g_instances;
+  return g_instances;
+}
+
+bool PluginManager::RegisterPlugin(
+    llvm::StringRef name, llvm::StringRef description,
+    ProtocolServerCreateInstance create_callback) {
+  return GetProtocolServerInstances().RegisterPlugin(name, description,
+                                                     create_callback);
+}
+
+bool PluginManager::UnregisterPlugin(
+    ProtocolServerCreateInstance create_callback) {
+  return GetProtocolServerInstances().UnregisterPlugin(create_callback);
+}
+
+llvm::StringRef
+PluginManager::GetProtocolServerPluginNameAtIndex(uint32_t idx) {
+  return GetProtocolServerInstances().GetNameAtIndex(idx);
+}
+
+ProtocolServerCreateInstance
+PluginManager::GetProtocolCreateCallbackForPluginName(llvm::StringRef name) {
+  return GetProtocolServerInstances().GetCallbackForName(name);
+}
+
 #pragma mark RegisterTypeBuilder

 struct RegisterTypeBuilderInstance
--- a/lldb/source/Core/ProtocolServer.cpp
+++ b/lldb/source/Core/ProtocolServer.cpp
@ -0,0 +1,21 @@
+//===-- ProtocolServer.cpp ------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "lldb/Core/ProtocolServer.h"
+#include "lldb/Core/PluginManager.h"
+
+using namespace lldb_private;
+using namespace lldb;
+
+ProtocolServerSP ProtocolServer::Create(llvm::StringRef name,
+                                        Debugger &debugger) {
+  if (ProtocolServerCreateInstance create_callback =
+          PluginManager::GetProtocolCreateCallbackForPluginName(name))
+    return create_callback(debugger);
+  return nullptr;
+}
--- a/lldb/source/Interpreter/CommandInterpreter.cpp
+++ b/lldb/source/Interpreter/CommandInterpreter.cpp
@ -30,6 +30,7 @@
 #include "Commands/CommandObjectPlatform.h"
 #include "Commands/CommandObjectPlugin.h"
 #include "Commands/CommandObjectProcess.h"
+#include "Commands/CommandObjectProtocolServer.h"
 #include "Commands/CommandObjectQuit.h"
 #include "Commands/CommandObjectRegexCommand.h"
 #include "Commands/CommandObjectRegister.h"
@ -574,6 +575,7 @@ void CommandInterpreter::LoadCommandDictionary() {
  REGISTER_COMMAND_OBJECT("platform", CommandObjectPlatform);
  REGISTER_COMMAND_OBJECT("plugin", CommandObjectPlugin);
  REGISTER_COMMAND_OBJECT("process", CommandObjectMultiwordProcess);
+  REGISTER_COMMAND_OBJECT("protocol-server", CommandObjectProtocolServer);
  REGISTER_COMMAND_OBJECT("quit", CommandObjectQuit);
  REGISTER_COMMAND_OBJECT("register", CommandObjectRegister);
  REGISTER_COMMAND_OBJECT("scripting", CommandObjectMultiwordScripting);
--- a/lldb/source/Plugins/CMakeLists.txt
+++ b/lldb/source/Plugins/CMakeLists.txt
@ -27,6 +27,10 @@ add_subdirectory(TraceExporter)
 add_subdirectory(TypeSystem)
 add_subdirectory(UnwindAssembly)

+if(LLDB_ENABLE_PROTOCOL_SERVERS)
+  add_subdirectory(Protocol)
+endif()
+
 set(LLDB_STRIPPED_PLUGINS)
 get_property(LLDB_ALL_PLUGINS GLOBAL PROPERTY LLDB_PLUGINS)

--- a/lldb/source/Plugins/Protocol/CMakeLists.txt
+++ b/lldb/source/Plugins/Protocol/CMakeLists.txt
@ -0,0 +1 @@
+add_subdirectory(MCP)
--- a/lldb/source/Plugins/Protocol/MCP/CMakeLists.txt
+++ b/lldb/source/Plugins/Protocol/MCP/CMakeLists.txt
@ -0,0 +1,13 @@
+add_lldb_library(lldbPluginProtocolServerMCP PLUGIN
+  MCPError.cpp
+  Protocol.cpp
+  ProtocolServerMCP.cpp
+  Tool.cpp
+
+  LINK_COMPONENTS
+    Support
+
+  LINK_LIBS
+    lldbHost
+    lldbUtility
+)
--- a/lldb/source/Plugins/Protocol/MCP/MCPError.cpp
+++ b/lldb/source/Plugins/Protocol/MCP/MCPError.cpp
@ -0,0 +1,34 @@
+//===-- MCPError.cpp ------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "MCPError.h"
+#include "llvm/Support/Error.h"
+#include "llvm/Support/raw_ostream.h"
+#include <system_error>
+
+namespace lldb_private::mcp {
+
+char MCPError::ID;
+
+MCPError::MCPError(std::string message, int64_t error_code)
+    : m_message(message), m_error_code(error_code) {}
+
+void MCPError::log(llvm::raw_ostream &OS) const { OS << m_message; }
+
+std::error_code MCPError::convertToErrorCode() const {
+  return llvm::inconvertibleErrorCode();
+}
+
+protocol::Error MCPError::toProtcolError() const {
+  protocol::Error error;
+  error.error.code = m_error_code;
+  error.error.message = m_message;
+  return error;
+}
+
+} // namespace lldb_private::mcp
--- a/lldb/source/Plugins/Protocol/MCP/MCPError.h
+++ b/lldb/source/Plugins/Protocol/MCP/MCPError.h
@ -0,0 +1,33 @@
+//===-- MCPError.h --------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Protocol.h"
+#include "llvm/Support/Error.h"
+#include <string>
+
+namespace lldb_private::mcp {
+
+class MCPError : public llvm::ErrorInfo<MCPError> {
+public:
+  static char ID;
+
+  MCPError(std::string message, int64_t error_code);
+
+  void log(llvm::raw_ostream &OS) const override;
+  std::error_code convertToErrorCode() const override;
+
+  const std::string &getMessage() const { return m_message; }
+
+  protocol::Error toProtcolError() const;
+
+private:
+  std::string m_message;
+  int64_t m_error_code;
+};
+
+} // namespace lldb_private::mcp
--- a/lldb/source/Plugins/Protocol/MCP/Protocol.cpp
+++ b/lldb/source/Plugins/Protocol/MCP/Protocol.cpp
@ -0,0 +1,214 @@
+//===- Protocol.cpp -------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Protocol.h"
+#include "llvm/Support/JSON.h"
+
+using namespace llvm;
+
+namespace lldb_private::mcp::protocol {
+
+static bool mapRaw(const json::Value &Params, StringLiteral Prop,
+                   std::optional<json::Value> &V, json::Path P) {
+  const auto *O = Params.getAsObject();
+  if (!O) {
+    P.report("expected object");
+    return false;
+  }
+  const json::Value *E = O->get(Prop);
+  if (E)
+    V = std::move(*E);
+  return true;
+}
+
+llvm::json::Value toJSON(const Request &R) {
+  json::Object Result{{"jsonrpc", "2.0"}, {"id", R.id}, {"method", R.method}};
+  if (R.params)
+    Result.insert({"params", R.params});
+  return Result;
+}
+
+bool fromJSON(const llvm::json::Value &V, Request &R, llvm::json::Path P) {
+  llvm::json::ObjectMapper O(V, P);
+  if (!O || !O.map("id", R.id) || !O.map("method", R.method))
+    return false;
+  return mapRaw(V, "params", R.params, P);
+}
+
+llvm::json::Value toJSON(const ErrorInfo &EI) {
+  llvm::json::Object Result{{"code", EI.code}, {"message", EI.message}};
+  if (EI.data)
+    Result.insert({"data", EI.data});
+  return Result;
+}
+
+bool fromJSON(const llvm::json::Value &V, ErrorInfo &EI, llvm::json::Path P) {
+  llvm::json::ObjectMapper O(V, P);
+  return O && O.map("code", EI.code) && O.map("message", EI.message) &&
+         O.mapOptional("data", EI.data);
+}
+
+llvm::json::Value toJSON(const Error &E) {
+  return json::Object{{"jsonrpc", "2.0"}, {"id", E.id}, {"error", E.error}};
+}
+
+bool fromJSON(const llvm::json::Value &V, Error &E, llvm::json::Path P) {
+  llvm::json::ObjectMapper O(V, P);
+  return O && O.map("id", E.id) && O.map("error", E.error);
+}
+
+llvm::json::Value toJSON(const Response &R) {
+  llvm::json::Object Result{{"jsonrpc", "2.0"}, {"id", R.id}};
+  if (R.result)
+    Result.insert({"result", R.result});
+  if (R.error)
+    Result.insert({"error", R.error});
+  return Result;
+}
+
+bool fromJSON(const llvm::json::Value &V, Response &R, llvm::json::Path P) {
+  llvm::json::ObjectMapper O(V, P);
+  if (!O || !O.map("id", R.id) || !O.map("error", R.error))
+    return false;
+  return mapRaw(V, "result", R.result, P);
+}
+
+llvm::json::Value toJSON(const Notification &N) {
+  llvm::json::Object Result{{"jsonrpc", "2.0"}, {"method", N.method}};
+  if (N.params)
+    Result.insert({"params", N.params});
+  return Result;
+}
+
+bool fromJSON(const llvm::json::Value &V, Notification &N, llvm::json::Path P) {
+  llvm::json::ObjectMapper O(V, P);
+  if (!O || !O.map("method", N.method))
+    return false;
+  auto *Obj = V.getAsObject();
+  if (!Obj)
+    return false;
+  if (auto *Params = Obj->get("params"))
+    N.params = *Params;
+  return true;
+}
+
+llvm::json::Value toJSON(const ToolCapability &TC) {
+  return llvm::json::Object{{"listChanged", TC.listChanged}};
+}
+
+bool fromJSON(const llvm::json::Value &V, ToolCapability &TC,
+              llvm::json::Path P) {
+  llvm::json::ObjectMapper O(V, P);
+  return O && O.map("listChanged", TC.listChanged);
+}
+
+llvm::json::Value toJSON(const Capabilities &C) {
+  return llvm::json::Object{{"tools", C.tools}};
+}
+
+bool fromJSON(const llvm::json::Value &V, Capabilities &C, llvm::json::Path P) {
+  llvm::json::ObjectMapper O(V, P);
+  return O && O.map("tools", C.tools);
+}
+
+llvm::json::Value toJSON(const TextContent &TC) {
+  return llvm::json::Object{{"type", "text"}, {"text", TC.text}};
+}
+
+bool fromJSON(const llvm::json::Value &V, TextContent &TC, llvm::json::Path P) {
+  llvm::json::ObjectMapper O(V, P);
+  return O && O.map("text", TC.text);
+}
+
+llvm::json::Value toJSON(const TextResult &TR) {
+  return llvm::json::Object{{"content", TR.content}, {"isError", TR.isError}};
+}
+
+bool fromJSON(const llvm::json::Value &V, TextResult &TR, llvm::json::Path P) {
+  llvm::json::ObjectMapper O(V, P);
+  return O && O.map("content", TR.content) && O.map("isError", TR.isError);
+}
+
+llvm::json::Value toJSON(const ToolDefinition &TD) {
+  llvm::json::Object Result{{"name", TD.name}};
+  if (TD.description)
+    Result.insert({"description", TD.description});
+  if (TD.inputSchema)
+    Result.insert({"inputSchema", TD.inputSchema});
+  return Result;
+}
+
+bool fromJSON(const llvm::json::Value &V, ToolDefinition &TD,
+              llvm::json::Path P) {
+
+  llvm::json::ObjectMapper O(V, P);
+  if (!O || !O.map("name", TD.name) ||
+      !O.mapOptional("description", TD.description))
+    return false;
+  return mapRaw(V, "inputSchema", TD.inputSchema, P);
+}
+
+llvm::json::Value toJSON(const Message &M) {
+  return std::visit([](auto &M) { return toJSON(M); }, M);
+}
+
+bool fromJSON(const llvm::json::Value &V, Message &M, llvm::json::Path P) {
+  const auto *O = V.getAsObject();
+  if (!O) {
+    P.report("expected object");
+    return false;
+  }
+
+  if (const json::Value *V = O->get("jsonrpc")) {
+    if (V->getAsString().value_or("") != "2.0") {
+      P.report("unsupported JSON RPC version");
+      return false;
+    }
+  } else {
+    P.report("not a valid JSON RPC message");
+    return false;
+  }
+
+  // A message without an ID is a Notification.
+  if (!O->get("id")) {
+    protocol::Notification N;
+    if (!fromJSON(V, N, P))
+      return false;
+    M = std::move(N);
+    return true;
+  }
+
+  if (O->get("error")) {
+    protocol::Error E;
+    if (!fromJSON(V, E, P))
+      return false;
+    M = std::move(E);
+    return true;
+  }
+
+  if (O->get("result")) {
+    protocol::Response R;
+    if (!fromJSON(V, R, P))
+      return false;
+    M = std::move(R);
+    return true;
+  }
+
+  if (O->get("method")) {
+    protocol::Request R;
+    if (!fromJSON(V, R, P))
+      return false;
+    M = std::move(R);
+    return true;
+  }
+
+  P.report("unrecognized message type");
+  return false;
+}
+
+} // namespace lldb_private::mcp::protocol
--- a/lldb/source/Plugins/Protocol/MCP/Protocol.h
+++ b/lldb/source/Plugins/Protocol/MCP/Protocol.h
@ -0,0 +1,128 @@
+//===- Protocol.h ---------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains POD structs based on the MCP specification at
+// https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/2024-11-05/schema.json
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_PLUGINS_PROTOCOL_MCP_PROTOCOL_H
+#define LLDB_PLUGINS_PROTOCOL_MCP_PROTOCOL_H
+
+#include "llvm/Support/JSON.h"
+#include <optional>
+#include <string>
+#include <variant>
+
+namespace lldb_private::mcp::protocol {
+
+static llvm::StringLiteral kVersion = "2024-11-05";
+
+/// A request that expects a response.
+struct Request {
+  uint64_t id = 0;
+  std::string method;
+  std::optional<llvm::json::Value> params;
+};
+
+llvm::json::Value toJSON(const Request &);
+bool fromJSON(const llvm::json::Value &, Request &, llvm::json::Path);
+
+struct ErrorInfo {
+  int64_t code = 0;
+  std::string message;
+  std::optional<std::string> data;
+};
+
+llvm::json::Value toJSON(const ErrorInfo &);
+bool fromJSON(const llvm::json::Value &, ErrorInfo &, llvm::json::Path);
+
+struct Error {
+  uint64_t id = 0;
+  ErrorInfo error;
+};
+
+llvm::json::Value toJSON(const Error &);
+bool fromJSON(const llvm::json::Value &, Error &, llvm::json::Path);
+
+struct Response {
+  uint64_t id = 0;
+  std::optional<llvm::json::Value> result;
+  std::optional<ErrorInfo> error;
+};
+
+llvm::json::Value toJSON(const Response &);
+bool fromJSON(const llvm::json::Value &, Response &, llvm::json::Path);
+
+/// A notification which does not expect a response.
+struct Notification {
+  std::string method;
+  std::optional<llvm::json::Value> params;
+};
+
+llvm::json::Value toJSON(const Notification &);
+bool fromJSON(const llvm::json::Value &, Notification &, llvm::json::Path);
+
+struct ToolCapability {
+  /// Whether this server supports notifications for changes to the tool list.
+  bool listChanged = false;
+};
+
+llvm::json::Value toJSON(const ToolCapability &);
+bool fromJSON(const llvm::json::Value &, ToolCapability &, llvm::json::Path);
+
+/// Capabilities that a server may support. Known capabilities are defined here,
+/// in this schema, but this is not a closed set: any server can define its own,
+/// additional capabilities.
+struct Capabilities {
+  /// Present if the server offers any tools to call.
+  ToolCapability tools;
+};
+
+llvm::json::Value toJSON(const Capabilities &);
+bool fromJSON(const llvm::json::Value &, Capabilities &, llvm::json::Path);
+
+/// Text provided to or from an LLM.
+struct TextContent {
+  /// The text content of the message.
+  std::string text;
+};
+
+llvm::json::Value toJSON(const TextContent &);
+bool fromJSON(const llvm::json::Value &, TextContent &, llvm::json::Path);
+
+struct TextResult {
+  std::vector<TextContent> content;
+  bool isError = false;
+};
+
+llvm::json::Value toJSON(const TextResult &);
+bool fromJSON(const llvm::json::Value &, TextResult &, llvm::json::Path);
+
+struct ToolDefinition {
+  /// Unique identifier for the tool.
+  std::string name;
+
+  /// Human-readable description.
+  std::optional<std::string> description;
+
+  // JSON Schema for the tool's parameters.
+  std::optional<llvm::json::Value> inputSchema;
+};
+
+llvm::json::Value toJSON(const ToolDefinition &);
+bool fromJSON(const llvm::json::Value &, ToolDefinition &, llvm::json::Path);
+
+using Message = std::variant<Request, Response, Notification, Error>;
+
+bool fromJSON(const llvm::json::Value &, Message &, llvm::json::Path);
+llvm::json::Value toJSON(const Message &);
+
+} // namespace lldb_private::mcp::protocol
+
+#endif
--- a/lldb/source/Plugins/Protocol/MCP/ProtocolServerMCP.cpp
+++ b/lldb/source/Plugins/Protocol/MCP/ProtocolServerMCP.cpp
@ -0,0 +1,327 @@
+//===- ProtocolServerMCP.cpp ----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "ProtocolServerMCP.h"
+#include "MCPError.h"
+#include "lldb/Core/PluginManager.h"
+#include "lldb/Utility/LLDBLog.h"
+#include "lldb/Utility/Log.h"
+#include "llvm/ADT/StringExtras.h"
+#include "llvm/Support/Threading.h"
+#include <thread>
+#include <variant>
+
+using namespace lldb_private;
+using namespace lldb_private::mcp;
+using namespace llvm;
+
+LLDB_PLUGIN_DEFINE(ProtocolServerMCP)
+
+static constexpr size_t kChunkSize = 1024;
+
+ProtocolServerMCP::ProtocolServerMCP(Debugger &debugger)
+    : ProtocolServer(), m_debugger(debugger) {
+  AddRequestHandler("initialize",
+                    std::bind(&ProtocolServerMCP::InitializeHandler, this,
+                              std::placeholders::_1));
+  AddRequestHandler("tools/list",
+                    std::bind(&ProtocolServerMCP::ToolsListHandler, this,
+                              std::placeholders::_1));
+  AddRequestHandler("tools/call",
+                    std::bind(&ProtocolServerMCP::ToolsCallHandler, this,
+                              std::placeholders::_1));
+  AddNotificationHandler(
+      "notifications/initialized", [](const protocol::Notification &) {
+        LLDB_LOG(GetLog(LLDBLog::Host), "MCP initialization complete");
+      });
+  AddTool(std::make_unique<LLDBCommandTool>(
+      "lldb_command", "Run an lldb command.", m_debugger));
+}
+
+ProtocolServerMCP::~ProtocolServerMCP() { llvm::consumeError(Stop()); }
+
+void ProtocolServerMCP::Initialize() {
+  PluginManager::RegisterPlugin(GetPluginNameStatic(),
+                                GetPluginDescriptionStatic(), CreateInstance);
+}
+
+void ProtocolServerMCP::Terminate() {
+  PluginManager::UnregisterPlugin(CreateInstance);
+}
+
+lldb::ProtocolServerSP ProtocolServerMCP::CreateInstance(Debugger &debugger) {
+  return std::make_shared<ProtocolServerMCP>(debugger);
+}
+
+llvm::StringRef ProtocolServerMCP::GetPluginDescriptionStatic() {
+  return "MCP Server.";
+}
+
+llvm::Expected<protocol::Response>
+ProtocolServerMCP::Handle(protocol::Request request) {
+  auto it = m_request_handlers.find(request.method);
+  if (it != m_request_handlers.end()) {
+    llvm::Expected<protocol::Response> response = it->second(request);
+    if (!response)
+      return response;
+    response->id = request.id;
+    return *response;
+  }
+
+  return make_error<MCPError>(
+      llvm::formatv("no handler for request: {0}", request.method).str(), 1);
+}
+
+void ProtocolServerMCP::Handle(protocol::Notification notification) {
+  auto it = m_notification_handlers.find(notification.method);
+  if (it != m_notification_handlers.end()) {
+    it->second(notification);
+    return;
+  }
+
+  LLDB_LOG(GetLog(LLDBLog::Host), "MPC notification: {0} ({1})",
+           notification.method, notification.params);
+}
+
+void ProtocolServerMCP::AcceptCallback(std::unique_ptr<Socket> socket) {
+  LLDB_LOG(GetLog(LLDBLog::Host), "New MCP client ({0}) connected",
+           m_clients.size() + 1);
+
+  lldb::IOObjectSP io_sp = std::move(socket);
+  auto client_up = std::make_unique<Client>();
+  client_up->io_sp = io_sp;
+  Client *client = client_up.get();
+
+  Status status;
+  auto read_handle_up = m_loop.RegisterReadObject(
+      io_sp,
+      [this, client](MainLoopBase &loop) {
+        if (Error error = ReadCallback(*client)) {
+          LLDB_LOG_ERROR(GetLog(LLDBLog::Host), std::move(error), "{0}");
+          client->read_handle_up.reset();
+        }
+      },
+      status);
+  if (status.Fail())
+    return;
+
+  client_up->read_handle_up = std::move(read_handle_up);
+  m_clients.emplace_back(std::move(client_up));
+}
+
+llvm::Error ProtocolServerMCP::ReadCallback(Client &client) {
+  char chunk[kChunkSize];
+  size_t bytes_read = sizeof(chunk);
+  if (Status status = client.io_sp->Read(chunk, bytes_read); status.Fail())
+    return status.takeError();
+  client.buffer.append(chunk, bytes_read);
+
+  for (std::string::size_type pos;
+       (pos = client.buffer.find('\n')) != std::string::npos;) {
+    llvm::Expected<std::optional<protocol::Message>> message =
+        HandleData(StringRef(client.buffer.data(), pos));
+    client.buffer = client.buffer.erase(0, pos + 1);
+    if (!message)
+      return message.takeError();
+
+    if (*message) {
+      std::string Output;
+      llvm::raw_string_ostream OS(Output);
+      OS << llvm::formatv("{0}", toJSON(**message)) << '\n';
+      size_t num_bytes = Output.size();
+      return client.io_sp->Write(Output.data(), num_bytes).takeError();
+    }
+  }
+
+  return llvm::Error::success();
+}
+
+llvm::Error ProtocolServerMCP::Start(ProtocolServer::Connection connection) {
+  std::lock_guard<std::mutex> guard(m_server_mutex);
+
+  if (m_running)
+    return llvm::createStringError("server already running");
+
+  Status status;
+  m_listener = Socket::Create(connection.protocol, status);
+  if (status.Fail())
+    return status.takeError();
+
+  status = m_listener->Listen(connection.name, /*backlog=*/5);
+  if (status.Fail())
+    return status.takeError();
+
+  std::string address =
+      llvm::join(m_listener->GetListeningConnectionURI(), ", ");
+  auto handles =
+      m_listener->Accept(m_loop, std::bind(&ProtocolServerMCP::AcceptCallback,
+                                           this, std::placeholders::_1));
+  if (llvm::Error error = handles.takeError())
+    return error;
+
+  m_listen_handlers = std::move(*handles);
+  m_loop_thread = std::thread([=] {
+    llvm::set_thread_name(
+        llvm::formatv("debugger-{0}.mcp.runloop", m_debugger.GetID()));
+    m_loop.Run();
+  });
+
+  return llvm::Error::success();
+}
+
+llvm::Error ProtocolServerMCP::Stop() {
+  {
+    std::lock_guard<std::mutex> guard(m_server_mutex);
+    m_running = false;
+  }
+
+  // Stop the main loop.
+  m_loop.AddPendingCallback(
+      [](MainLoopBase &loop) { loop.RequestTermination(); });
+
+  // Wait for the main loop to exit.
+  if (m_loop_thread.joinable())
+    m_loop_thread.join();
+
+  {
+    std::lock_guard<std::mutex> guard(m_server_mutex);
+    m_listener.reset();
+    m_listen_handlers.clear();
+    m_clients.clear();
+  }
+
+  return llvm::Error::success();
+}
+
+llvm::Expected<std::optional<protocol::Message>>
+ProtocolServerMCP::HandleData(llvm::StringRef data) {
+  auto message = llvm::json::parse<protocol::Message>(/*JSON=*/data);
+  if (!message)
+    return message.takeError();
+
+  if (const protocol::Request *request =
+          std::get_if<protocol::Request>(&(*message))) {
+    llvm::Expected<protocol::Response> response = Handle(*request);
+
+    // Handle failures by converting them into an Error message.
+    if (!response) {
+      protocol::Error protocol_error;
+      llvm::handleAllErrors(
+          response.takeError(),
+          [&](const MCPError &err) { protocol_error = err.toProtcolError(); },
+          [&](const llvm::ErrorInfoBase &err) {
+            protocol_error.error.code = -1;
+            protocol_error.error.message = err.message();
+          });
+      protocol_error.id = request->id;
+      return protocol_error;
+    }
+
+    return *response;
+  }
+
+  if (const protocol::Notification *notification =
+          std::get_if<protocol::Notification>(&(*message))) {
+    Handle(*notification);
+    return std::nullopt;
+  }
+
+  if (std::get_if<protocol::Error>(&(*message)))
+    return llvm::createStringError("unexpected MCP message: error");
+
+  if (std::get_if<protocol::Response>(&(*message)))
+    return llvm::createStringError("unexpected MCP message: response");
+
+  llvm_unreachable("all message types handled");
+}
+
+protocol::Capabilities ProtocolServerMCP::GetCapabilities() {
+  protocol::Capabilities capabilities;
+  capabilities.tools.listChanged = true;
+  return capabilities;
+}
+
+void ProtocolServerMCP::AddTool(std::unique_ptr<Tool> tool) {
+  std::lock_guard<std::mutex> guard(m_server_mutex);
+
+  if (!tool)
+    return;
+  m_tools[tool->GetName()] = std::move(tool);
+}
+
+void ProtocolServerMCP::AddRequestHandler(llvm::StringRef method,
+                                          RequestHandler handler) {
+  std::lock_guard<std::mutex> guard(m_server_mutex);
+  m_request_handlers[method] = std::move(handler);
+}
+
+void ProtocolServerMCP::AddNotificationHandler(llvm::StringRef method,
+                                               NotificationHandler handler) {
+  std::lock_guard<std::mutex> guard(m_server_mutex);
+  m_notification_handlers[method] = std::move(handler);
+}
+
+llvm::Expected<protocol::Response>
+ProtocolServerMCP::InitializeHandler(const protocol::Request &request) {
+  protocol::Response response;
+  response.result.emplace(llvm::json::Object{
+      {"protocolVersion", protocol::kVersion},
+      {"capabilities", GetCapabilities()},
+      {"serverInfo",
+       llvm::json::Object{{"name", kName}, {"version", kVersion}}}});
+  return response;
+}
+
+llvm::Expected<protocol::Response>
+ProtocolServerMCP::ToolsListHandler(const protocol::Request &request) {
+  protocol::Response response;
+
+  llvm::json::Array tools;
+  for (const auto &tool : m_tools)
+    tools.emplace_back(toJSON(tool.second->GetDefinition()));
+
+  response.result.emplace(llvm::json::Object{{"tools", std::move(tools)}});
+
+  return response;
+}
+
+llvm::Expected<protocol::Response>
+ProtocolServerMCP::ToolsCallHandler(const protocol::Request &request) {
+  protocol::Response response;
+
+  if (!request.params)
+    return llvm::createStringError("no tool parameters");
+
+  const json::Object *param_obj = request.params->getAsObject();
+  if (!param_obj)
+    return llvm::createStringError("no tool parameters");
+
+  const json::Value *name = param_obj->get("name");
+  if (!name)
+    return llvm::createStringError("no tool name");
+
+  llvm::StringRef tool_name = name->getAsString().value_or("");
+  if (tool_name.empty())
+    return llvm::createStringError("no tool name");
+
+  auto it = m_tools.find(tool_name);
+  if (it == m_tools.end())
+    return llvm::createStringError(llvm::formatv("no tool \"{0}\"", tool_name));
+
+  const json::Value *args = param_obj->get("arguments");
+  if (!args)
+    return llvm::createStringError("no tool arguments");
+
+  llvm::Expected<protocol::TextResult> text_result = it->second->Call(*args);
+  if (!text_result)
+    return text_result.takeError();
+
+  response.result.emplace(toJSON(*text_result));
+
+  return response;
+}
--- a/lldb/source/Plugins/Protocol/MCP/ProtocolServerMCP.h
+++ b/lldb/source/Plugins/Protocol/MCP/ProtocolServerMCP.h
@ -0,0 +1,100 @@
+//===- ProtocolServerMCP.h ------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_PLUGINS_PROTOCOL_MCP_PROTOCOLSERVERMCP_H
+#define LLDB_PLUGINS_PROTOCOL_MCP_PROTOCOLSERVERMCP_H
+
+#include "Protocol.h"
+#include "Tool.h"
+#include "lldb/Core/ProtocolServer.h"
+#include "lldb/Host/MainLoop.h"
+#include "lldb/Host/Socket.h"
+#include "llvm/ADT/StringMap.h"
+#include <thread>
+
+namespace lldb_private::mcp {
+
+class ProtocolServerMCP : public ProtocolServer {
+public:
+  ProtocolServerMCP(Debugger &debugger);
+  virtual ~ProtocolServerMCP() override;
+
+  virtual llvm::Error Start(ProtocolServer::Connection connection) override;
+  virtual llvm::Error Stop() override;
+
+  static void Initialize();
+  static void Terminate();
+
+  static llvm::StringRef GetPluginNameStatic() { return "MCP"; }
+  static llvm::StringRef GetPluginDescriptionStatic();
+
+  static lldb::ProtocolServerSP CreateInstance(Debugger &debugger);
+
+  llvm::StringRef GetPluginName() override { return GetPluginNameStatic(); }
+
+  Socket *GetSocket() const override { return m_listener.get(); }
+
+protected:
+  using RequestHandler = std::function<llvm::Expected<protocol::Response>(
+      const protocol::Request &)>;
+  using NotificationHandler =
+      std::function<void(const protocol::Notification &)>;
+
+  void AddTool(std::unique_ptr<Tool> tool);
+  void AddRequestHandler(llvm::StringRef method, RequestHandler handler);
+  void AddNotificationHandler(llvm::StringRef method,
+                              NotificationHandler handler);
+
+private:
+  void AcceptCallback(std::unique_ptr<Socket> socket);
+
+  llvm::Expected<std::optional<protocol::Message>>
+  HandleData(llvm::StringRef data);
+
+  llvm::Expected<protocol::Response> Handle(protocol::Request request);
+  void Handle(protocol::Notification notification);
+
+  llvm::Expected<protocol::Response>
+  InitializeHandler(const protocol::Request &);
+  llvm::Expected<protocol::Response>
+  ToolsListHandler(const protocol::Request &);
+  llvm::Expected<protocol::Response>
+  ToolsCallHandler(const protocol::Request &);
+
+  protocol::Capabilities GetCapabilities();
+
+  llvm::StringLiteral kName = "lldb-mcp";
+  llvm::StringLiteral kVersion = "0.1.0";
+
+  Debugger &m_debugger;
+
+  bool m_running = false;
+
+  MainLoop m_loop;
+  std::thread m_loop_thread;
+
+  std::unique_ptr<Socket> m_listener;
+  std::vector<MainLoopBase::ReadHandleUP> m_listen_handlers;
+
+  struct Client {
+    lldb::IOObjectSP io_sp;
+    MainLoopBase::ReadHandleUP read_handle_up;
+    std::string buffer;
+  };
+  llvm::Error ReadCallback(Client &client);
+  std::vector<std::unique_ptr<Client>> m_clients;
+
+  std::mutex m_server_mutex;
+  llvm::StringMap<std::unique_ptr<Tool>> m_tools;
+
+  llvm::StringMap<RequestHandler> m_request_handlers;
+  llvm::StringMap<NotificationHandler> m_notification_handlers;
+};
+} // namespace lldb_private::mcp
+
+#endif
--- a/lldb/source/Plugins/Protocol/MCP/Tool.cpp
+++ b/lldb/source/Plugins/Protocol/MCP/Tool.cpp
@ -0,0 +1,81 @@
+//===- Tool.cpp -----------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Tool.h"
+#include "lldb/Interpreter/CommandInterpreter.h"
+#include "lldb/Interpreter/CommandReturnObject.h"
+
+using namespace lldb_private::mcp;
+using namespace llvm;
+
+struct LLDBCommandToolArguments {
+  std::string arguments;
+};
+
+bool fromJSON(const llvm::json::Value &V, LLDBCommandToolArguments &A,
+              llvm::json::Path P) {
+  llvm::json::ObjectMapper O(V, P);
+  return O && O.map("arguments", A.arguments);
+}
+
+Tool::Tool(std::string name, std::string description)
+    : m_name(std::move(name)), m_description(std::move(description)) {}
+
+protocol::ToolDefinition Tool::GetDefinition() const {
+  protocol::ToolDefinition definition;
+  definition.name = m_name;
+  definition.description.emplace(m_description);
+
+  if (std::optional<llvm::json::Value> input_schema = GetSchema())
+    definition.inputSchema = *input_schema;
+
+  return definition;
+}
+
+LLDBCommandTool::LLDBCommandTool(std::string name, std::string description,
+                                 Debugger &debugger)
+    : Tool(std::move(name), std::move(description)), m_debugger(debugger) {}
+
+llvm::Expected<protocol::TextResult>
+LLDBCommandTool::Call(const llvm::json::Value &args) {
+  llvm::json::Path::Root root;
+
+  LLDBCommandToolArguments arguments;
+  if (!fromJSON(args, arguments, root))
+    return root.getError();
+
+  // FIXME: Disallow certain commands and their aliases.
+  CommandReturnObject result(/*colors=*/false);
+  m_debugger.GetCommandInterpreter().HandleCommand(arguments.arguments.c_str(),
+                                                   eLazyBoolYes, result);
+
+  std::string output;
+  llvm::StringRef output_str = result.GetOutputString();
+  if (!output_str.empty())
+    output += output_str.str();
+
+  std::string err_str = result.GetErrorString();
+  if (!err_str.empty()) {
+    if (!output.empty())
+      output += '\n';
+    output += err_str;
+  }
+
+  mcp::protocol::TextResult text_result;
+  text_result.content.emplace_back(mcp::protocol::TextContent{{output}});
+  text_result.isError = !result.Succeeded();
+  return text_result;
+}
+
+std::optional<llvm::json::Value> LLDBCommandTool::GetSchema() const {
+  llvm::json::Object str_type{{"type", "string"}};
+  llvm::json::Object properties{{"arguments", std::move(str_type)}};
+  llvm::json::Object schema{{"type", "object"},
+                            {"properties", std::move(properties)}};
+  return schema;
+}
--- a/lldb/source/Plugins/Protocol/MCP/Tool.h
+++ b/lldb/source/Plugins/Protocol/MCP/Tool.h
@ -0,0 +1,56 @@
+//===- Tool.h -------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_PLUGINS_PROTOCOL_MCP_TOOL_H
+#define LLDB_PLUGINS_PROTOCOL_MCP_TOOL_H
+
+#include "Protocol.h"
+#include "lldb/Core/Debugger.h"
+#include "llvm/Support/JSON.h"
+#include <string>
+
+namespace lldb_private::mcp {
+
+class Tool {
+public:
+  Tool(std::string name, std::string description);
+  virtual ~Tool() = default;
+
+  virtual llvm::Expected<protocol::TextResult>
+  Call(const llvm::json::Value &args) = 0;
+
+  virtual std::optional<llvm::json::Value> GetSchema() const {
+    return std::nullopt;
+  }
+
+  protocol::ToolDefinition GetDefinition() const;
+
+  const std::string &GetName() { return m_name; }
+
+private:
+  std::string m_name;
+  std::string m_description;
+};
+
+class LLDBCommandTool : public mcp::Tool {
+public:
+  LLDBCommandTool(std::string name, std::string description,
+                  Debugger &debugger);
+  ~LLDBCommandTool() = default;
+
+  virtual llvm::Expected<protocol::TextResult>
+  Call(const llvm::json::Value &args) override;
+
+  virtual std::optional<llvm::json::Value> GetSchema() const override;
+
+private:
+  Debugger &m_debugger;
+};
+} // namespace lldb_private::mcp
+
+#endif
--- a/lldb/unittests/CMakeLists.txt
+++ b/lldb/unittests/CMakeLists.txt
@ -78,6 +78,10 @@ add_subdirectory(Utility)
 add_subdirectory(Thread)
 add_subdirectory(ValueObject)

+if(LLDB_ENABLE_PROTOCOL_SERVERS)
+  add_subdirectory(Protocol)
+endif()
+
 if(LLDB_CAN_USE_DEBUGSERVER AND LLDB_TOOL_DEBUGSERVER_BUILD AND NOT LLDB_USE_SYSTEM_DEBUGSERVER)
  add_subdirectory(debugserver)
 endif()
--- a/lldb/unittests/DAP/ProtocolTypesTest.cpp
+++ b/lldb/unittests/DAP/ProtocolTypesTest.cpp
@ -9,6 +9,7 @@
 #include "Protocol/ProtocolTypes.h"
 #include "Protocol/ProtocolEvents.h"
 #include "Protocol/ProtocolRequests.h"
+#include "TestingSupport/TestUtilities.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/JSON.h"
 #include "llvm/Testing/Support/Error.h"
@ -20,6 +21,7 @@ using namespace llvm;
 using namespace lldb;
 using namespace lldb_dap;
 using namespace lldb_dap::protocol;
+using lldb_private::roundtripJSON;
 using llvm::json::parse;
 using llvm::json::Value;

@ -28,15 +30,6 @@ static std::string pp(const json::Value &E) {
  return formatv("{0:2}", E).str();
 }

-template <typename T> static llvm::Expected<T> roundtrip(const T &input) {
-  llvm::json::Value value = toJSON(input);
-  llvm::json::Path::Root root;
-  T output;
-  if (!fromJSON(value, output, root))
-    return root.getError();
-  return output;
-}
-
 TEST(ProtocolTypesTest, ExceptionBreakpointsFilter) {
  ExceptionBreakpointsFilter filter;
  filter.filter = "testFilter";
@ -47,7 +40,7 @@ TEST(ProtocolTypesTest, ExceptionBreakpointsFilter) {
  filter.conditionDescription = "Condition for test filter";

  llvm::Expected<ExceptionBreakpointsFilter> deserialized_filter =
-      roundtrip(filter);
+      roundtripJSON(filter);
  ASSERT_THAT_EXPECTED(deserialized_filter, llvm::Succeeded());

  EXPECT_EQ(filter.filter, deserialized_filter->filter);
@ -66,7 +59,7 @@ TEST(ProtocolTypesTest, Source) {
  source.sourceReference = 12345;
  source.presentationHint = Source::eSourcePresentationHintEmphasize;

-  llvm::Expected<Source> deserialized_source = roundtrip(source);
+  llvm::Expected<Source> deserialized_source = roundtripJSON(source);
  ASSERT_THAT_EXPECTED(deserialized_source, llvm::Succeeded());

  EXPECT_EQ(source.name, deserialized_source->name);
@ -83,7 +76,7 @@ TEST(ProtocolTypesTest, ColumnDescriptor) {
  column.type = eColumnTypeString;
  column.width = 20;

-  llvm::Expected<ColumnDescriptor> deserialized_column = roundtrip(column);
+  llvm::Expected<ColumnDescriptor> deserialized_column = roundtripJSON(column);
  ASSERT_THAT_EXPECTED(deserialized_column, llvm::Succeeded());

  EXPECT_EQ(column.attributeName, deserialized_column->attributeName);
@ -101,7 +94,7 @@ TEST(ProtocolTypesTest, BreakpointMode) {
  mode.appliesTo = {eBreakpointModeApplicabilitySource,
                    eBreakpointModeApplicabilityException};

-  llvm::Expected<BreakpointMode> deserialized_mode = roundtrip(mode);
+  llvm::Expected<BreakpointMode> deserialized_mode = roundtripJSON(mode);
  ASSERT_THAT_EXPECTED(deserialized_mode, llvm::Succeeded());

  EXPECT_EQ(mode.mode, deserialized_mode->mode);
@ -125,7 +118,8 @@ TEST(ProtocolTypesTest, Breakpoint) {
  breakpoint.offset = 4;
  breakpoint.reason = BreakpointReason::eBreakpointReasonPending;

-  llvm::Expected<Breakpoint> deserialized_breakpoint = roundtrip(breakpoint);
+  llvm::Expected<Breakpoint> deserialized_breakpoint =
+      roundtripJSON(breakpoint);
  ASSERT_THAT_EXPECTED(deserialized_breakpoint, llvm::Succeeded());

  EXPECT_EQ(breakpoint.id, deserialized_breakpoint->id);
@ -157,7 +151,7 @@ TEST(ProtocolTypesTest, SourceBreakpoint) {
  source_breakpoint.mode = "hardware";

  llvm::Expected<SourceBreakpoint> deserialized_source_breakpoint =
-      roundtrip(source_breakpoint);
+      roundtripJSON(source_breakpoint);
  ASSERT_THAT_EXPECTED(deserialized_source_breakpoint, llvm::Succeeded());

  EXPECT_EQ(source_breakpoint.line, deserialized_source_breakpoint->line);
@ -178,7 +172,7 @@ TEST(ProtocolTypesTest, FunctionBreakpoint) {
  function_breakpoint.hitCondition = "3";

  llvm::Expected<FunctionBreakpoint> deserialized_function_breakpoint =
-      roundtrip(function_breakpoint);
+      roundtripJSON(function_breakpoint);
  ASSERT_THAT_EXPECTED(deserialized_function_breakpoint, llvm::Succeeded());

  EXPECT_EQ(function_breakpoint.name, deserialized_function_breakpoint->name);
@ -196,7 +190,7 @@ TEST(ProtocolTypesTest, DataBreakpoint) {
  data_breakpoint_info.hitCondition = "10";

  llvm::Expected<DataBreakpoint> deserialized_data_breakpoint_info =
-      roundtrip(data_breakpoint_info);
+      roundtripJSON(data_breakpoint_info);
  ASSERT_THAT_EXPECTED(deserialized_data_breakpoint_info, llvm::Succeeded());

  EXPECT_EQ(data_breakpoint_info.dataId,
@ -233,9 +227,9 @@ TEST(ProtocolTypesTest, Capabilities) {
                                   {eBreakpointModeApplicabilitySource}}};
  capabilities.lldbExtVersion = "1.0.0";

-  // Perform roundtrip serialization and deserialization.
+  // Perform roundtripJSON serialization and deserialization.
  llvm::Expected<Capabilities> deserialized_capabilities =
-      roundtrip(capabilities);
+      roundtripJSON(capabilities);
  ASSERT_THAT_EXPECTED(deserialized_capabilities, llvm::Succeeded());

  // Verify supported features.
@ -316,7 +310,7 @@ TEST(ProtocolTypesTest, Scope) {
  source.presentationHint = Source::eSourcePresentationHintNormal;
  scope.source = source;

-  llvm::Expected<Scope> deserialized_scope = roundtrip(scope);
+  llvm::Expected<Scope> deserialized_scope = roundtripJSON(scope);
  ASSERT_THAT_EXPECTED(deserialized_scope, llvm::Succeeded());
  EXPECT_EQ(scope.name, deserialized_scope->name);
  EXPECT_EQ(scope.presentationHint, deserialized_scope->presentationHint);
@ -755,7 +749,7 @@ TEST(ProtocolTypesTest, StepInTarget) {
  target.endLine = 32;
  target.endColumn = 23;

-  llvm::Expected<StepInTarget> deserialized_target = roundtrip(target);
+  llvm::Expected<StepInTarget> deserialized_target = roundtripJSON(target);
  ASSERT_THAT_EXPECTED(deserialized_target, llvm::Succeeded());

  EXPECT_EQ(target.id, deserialized_target->id);
@ -797,4 +791,4 @@ TEST(ProtocolTypesTest, ReadMemoryResponseBody) {
      R"({ "address": "0xdeadbeef", "data": "aGVsbG8gd29ybGQh", "unreadableBytes": 1})");
  ASSERT_THAT_EXPECTED(expected, llvm::Succeeded());
  EXPECT_EQ(pp(*expected), pp(response));
-}
+}
--- a/lldb/unittests/Protocol/CMakeLists.txt
+++ b/lldb/unittests/Protocol/CMakeLists.txt
@ -0,0 +1,12 @@
+add_lldb_unittest(ProtocolTests
+  ProtocolMCPTest.cpp
+  ProtocolMCPServerTest.cpp
+
+  LINK_LIBS
+    lldbCore
+    lldbUtility
+    lldbHost
+    lldbPluginPlatformMacOSX
+    lldbPluginProtocolServerMCP
+    LLVMTestingSupport
+  )
--- a/lldb/unittests/Protocol/ProtocolMCPServerTest.cpp
+++ b/lldb/unittests/Protocol/ProtocolMCPServerTest.cpp
@ -0,0 +1,291 @@
+//===-- ProtocolServerMCPTest.cpp -----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Plugins/Platform/MacOSX/PlatformRemoteMacOSX.h"
+#include "Plugins/Protocol/MCP/ProtocolServerMCP.h"
+#include "TestingSupport/Host/SocketTestUtilities.h"
+#include "TestingSupport/SubsystemRAII.h"
+#include "lldb/Core/ProtocolServer.h"
+#include "lldb/Host/FileSystem.h"
+#include "lldb/Host/HostInfo.h"
+#include "lldb/Host/JSONTransport.h"
+#include "lldb/Host/Socket.h"
+#include "llvm/Testing/Support/Error.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+using namespace lldb;
+using namespace lldb_private;
+using namespace lldb_private::mcp::protocol;
+
+namespace {
+class TestProtocolServerMCP : public lldb_private::mcp::ProtocolServerMCP {
+public:
+  using ProtocolServerMCP::AddNotificationHandler;
+  using ProtocolServerMCP::AddRequestHandler;
+  using ProtocolServerMCP::AddTool;
+  using ProtocolServerMCP::GetSocket;
+  using ProtocolServerMCP::ProtocolServerMCP;
+};
+
+class TestJSONTransport : public lldb_private::JSONRPCTransport {
+public:
+  using JSONRPCTransport::JSONRPCTransport;
+  using JSONRPCTransport::ReadImpl;
+  using JSONRPCTransport::WriteImpl;
+};
+
+/// Test tool that returns it argument as text.
+class TestTool : public mcp::Tool {
+public:
+  using mcp::Tool::Tool;
+
+  virtual llvm::Expected<mcp::protocol::TextResult>
+  Call(const llvm::json::Value &args) override {
+    std::string argument;
+    if (const json::Object *args_obj = args.getAsObject()) {
+      if (const json::Value *s = args_obj->get("arguments")) {
+        argument = s->getAsString().value_or("");
+      }
+    }
+
+    mcp::protocol::TextResult text_result;
+    text_result.content.emplace_back(mcp::protocol::TextContent{{argument}});
+    return text_result;
+  }
+};
+
+/// Test tool that returns an error.
+class ErrorTool : public mcp::Tool {
+public:
+  using mcp::Tool::Tool;
+
+  virtual llvm::Expected<mcp::protocol::TextResult>
+  Call(const llvm::json::Value &args) override {
+    return llvm::createStringError("error");
+  }
+};
+
+/// Test tool that fails but doesn't return an error.
+class FailTool : public mcp::Tool {
+public:
+  using mcp::Tool::Tool;
+
+  virtual llvm::Expected<mcp::protocol::TextResult>
+  Call(const llvm::json::Value &args) override {
+    mcp::protocol::TextResult text_result;
+    text_result.content.emplace_back(mcp::protocol::TextContent{{"failed"}});
+    text_result.isError = true;
+    return text_result;
+  }
+};
+
+class ProtocolServerMCPTest : public ::testing::Test {
+public:
+  SubsystemRAII<FileSystem, HostInfo, PlatformRemoteMacOSX, Socket> subsystems;
+  DebuggerSP m_debugger_sp;
+
+  lldb::IOObjectSP m_io_sp;
+  std::unique_ptr<TestJSONTransport> m_transport_up;
+  std::unique_ptr<TestProtocolServerMCP> m_server_up;
+
+  static constexpr llvm::StringLiteral k_localhost = "localhost";
+
+  llvm::Error Write(llvm::StringRef message) {
+    return m_transport_up->WriteImpl(llvm::formatv("{0}\n", message).str());
+  }
+
+  llvm::Expected<std::string> Read() {
+    return m_transport_up->ReadImpl(std::chrono::milliseconds(100));
+  }
+
+  void SetUp() {
+    // Create a debugger.
+    ArchSpec arch("arm64-apple-macosx-");
+    Platform::SetHostPlatform(
+        PlatformRemoteMacOSX::CreateInstance(true, &arch));
+    m_debugger_sp = Debugger::CreateInstance();
+
+    // Create & start the server.
+    ProtocolServer::Connection connection;
+    connection.protocol = Socket::SocketProtocol::ProtocolTcp;
+    connection.name = llvm::formatv("{0}:0", k_localhost).str();
+    m_server_up = std::make_unique<TestProtocolServerMCP>(*m_debugger_sp);
+    m_server_up->AddTool(std::make_unique<TestTool>("test", "test tool"));
+    ASSERT_THAT_ERROR(m_server_up->Start(connection), llvm::Succeeded());
+
+    // Connect to the server over a TCP socket.
+    auto connect_socket_up = std::make_unique<TCPSocket>(true);
+    ASSERT_THAT_ERROR(connect_socket_up
+                          ->Connect(llvm::formatv("{0}:{1}", k_localhost,
+                                                  static_cast<TCPSocket *>(
+                                                      m_server_up->GetSocket())
+                                                      ->GetLocalPortNumber())
+                                        .str())
+                          .ToError(),
+                      llvm::Succeeded());
+
+    // Set up JSON transport for the client.
+    m_io_sp = std::move(connect_socket_up);
+    m_transport_up = std::make_unique<TestJSONTransport>(m_io_sp, m_io_sp);
+  }
+
+  void TearDown() {
+    // Stop the server.
+    ASSERT_THAT_ERROR(m_server_up->Stop(), llvm::Succeeded());
+  }
+};
+
+} // namespace
+
+TEST_F(ProtocolServerMCPTest, Intialization) {
+  llvm::StringLiteral request =
+      R"json({"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"claude-ai","version":"0.1.0"}},"jsonrpc":"2.0","id":0})json";
+  llvm::StringLiteral response =
+      R"json({"jsonrpc":"2.0","id":0,"result":{"capabilities":{"tools":{"listChanged":true}},"protocolVersion":"2024-11-05","serverInfo":{"name":"lldb-mcp","version":"0.1.0"}}})json";
+
+  ASSERT_THAT_ERROR(Write(request), llvm::Succeeded());
+
+  llvm::Expected<std::string> response_str = Read();
+  ASSERT_THAT_EXPECTED(response_str, llvm::Succeeded());
+
+  llvm::Expected<json::Value> response_json = json::parse(*response_str);
+  ASSERT_THAT_EXPECTED(response_json, llvm::Succeeded());
+
+  llvm::Expected<json::Value> expected_json = json::parse(response);
+  ASSERT_THAT_EXPECTED(expected_json, llvm::Succeeded());
+
+  EXPECT_EQ(*response_json, *expected_json);
+}
+
+TEST_F(ProtocolServerMCPTest, ToolsList) {
+  llvm::StringLiteral request =
+      R"json({"method":"tools/list","params":{},"jsonrpc":"2.0","id":1})json";
+  llvm::StringLiteral response =
+      R"json({"id":1,"jsonrpc":"2.0","result":{"tools":[{"description":"test tool","name":"test"},{"description":"Run an lldb command.","inputSchema":{"properties":{"arguments":{"type":"string"}},"type":"object"},"name":"lldb_command"}]}})json";
+
+  ASSERT_THAT_ERROR(Write(request), llvm::Succeeded());
+
+  llvm::Expected<std::string> response_str = Read();
+  ASSERT_THAT_EXPECTED(response_str, llvm::Succeeded());
+
+  llvm::Expected<json::Value> response_json = json::parse(*response_str);
+  ASSERT_THAT_EXPECTED(response_json, llvm::Succeeded());
+
+  llvm::Expected<json::Value> expected_json = json::parse(response);
+  ASSERT_THAT_EXPECTED(expected_json, llvm::Succeeded());
+
+  EXPECT_EQ(*response_json, *expected_json);
+}
+
+TEST_F(ProtocolServerMCPTest, ResourcesList) {
+  llvm::StringLiteral request =
+      R"json({"method":"resources/list","params":{},"jsonrpc":"2.0","id":2})json";
+  llvm::StringLiteral response =
+      R"json({"error":{"code":1,"message":"no handler for request: resources/list"},"id":2,"jsonrpc":"2.0"})json";
+
+  ASSERT_THAT_ERROR(Write(request), llvm::Succeeded());
+
+  llvm::Expected<std::string> response_str = Read();
+  ASSERT_THAT_EXPECTED(response_str, llvm::Succeeded());
+
+  llvm::Expected<json::Value> response_json = json::parse(*response_str);
+  ASSERT_THAT_EXPECTED(response_json, llvm::Succeeded());
+
+  llvm::Expected<json::Value> expected_json = json::parse(response);
+  ASSERT_THAT_EXPECTED(expected_json, llvm::Succeeded());
+
+  EXPECT_EQ(*response_json, *expected_json);
+}
+
+TEST_F(ProtocolServerMCPTest, ToolsCall) {
+  llvm::StringLiteral request =
+      R"json({"method":"tools/call","params":{"name":"test","arguments":{"arguments":"foo"}},"jsonrpc":"2.0","id":11})json";
+  llvm::StringLiteral response =
+      R"json({"id":11,"jsonrpc":"2.0","result":{"content":[{"text":"foo","type":"text"}],"isError":false}})json";
+
+  ASSERT_THAT_ERROR(Write(request), llvm::Succeeded());
+
+  llvm::Expected<std::string> response_str = Read();
+  ASSERT_THAT_EXPECTED(response_str, llvm::Succeeded());
+
+  llvm::Expected<json::Value> response_json = json::parse(*response_str);
+  ASSERT_THAT_EXPECTED(response_json, llvm::Succeeded());
+
+  llvm::Expected<json::Value> expected_json = json::parse(response);
+  ASSERT_THAT_EXPECTED(expected_json, llvm::Succeeded());
+
+  EXPECT_EQ(*response_json, *expected_json);
+}
+
+TEST_F(ProtocolServerMCPTest, ToolsCallError) {
+  m_server_up->AddTool(std::make_unique<ErrorTool>("error", "error tool"));
+
+  llvm::StringLiteral request =
+      R"json({"method":"tools/call","params":{"name":"error","arguments":{"arguments":"foo"}},"jsonrpc":"2.0","id":11})json";
+  llvm::StringLiteral response =
+      R"json({"error":{"code":-1,"message":"error"},"id":11,"jsonrpc":"2.0"})json";
+
+  ASSERT_THAT_ERROR(Write(request), llvm::Succeeded());
+
+  llvm::Expected<std::string> response_str = Read();
+  ASSERT_THAT_EXPECTED(response_str, llvm::Succeeded());
+
+  llvm::Expected<json::Value> response_json = json::parse(*response_str);
+  ASSERT_THAT_EXPECTED(response_json, llvm::Succeeded());
+
+  llvm::Expected<json::Value> expected_json = json::parse(response);
+  ASSERT_THAT_EXPECTED(expected_json, llvm::Succeeded());
+
+  EXPECT_EQ(*response_json, *expected_json);
+}
+
+TEST_F(ProtocolServerMCPTest, ToolsCallFail) {
+  m_server_up->AddTool(std::make_unique<FailTool>("fail", "fail tool"));
+
+  llvm::StringLiteral request =
+      R"json({"method":"tools/call","params":{"name":"fail","arguments":{"arguments":"foo"}},"jsonrpc":"2.0","id":11})json";
+  llvm::StringLiteral response =
+      R"json({"id":11,"jsonrpc":"2.0","result":{"content":[{"text":"failed","type":"text"}],"isError":true}})json";
+
+  ASSERT_THAT_ERROR(Write(request), llvm::Succeeded());
+
+  llvm::Expected<std::string> response_str = Read();
+  ASSERT_THAT_EXPECTED(response_str, llvm::Succeeded());
+
+  llvm::Expected<json::Value> response_json = json::parse(*response_str);
+  ASSERT_THAT_EXPECTED(response_json, llvm::Succeeded());
+
+  llvm::Expected<json::Value> expected_json = json::parse(response);
+  ASSERT_THAT_EXPECTED(expected_json, llvm::Succeeded());
+
+  EXPECT_EQ(*response_json, *expected_json);
+}
+
+TEST_F(ProtocolServerMCPTest, NotificationInitialized) {
+  bool handler_called = false;
+  std::condition_variable cv;
+  std::mutex mutex;
+
+  m_server_up->AddNotificationHandler(
+      "notifications/initialized",
+      [&](const mcp::protocol::Notification &notification) {
+        {
+          std::lock_guard<std::mutex> lock(mutex);
+          handler_called = true;
+        }
+        cv.notify_all();
+      });
+  llvm::StringLiteral request =
+      R"json({"method":"notifications/initialized","jsonrpc":"2.0"})json";
+
+  ASSERT_THAT_ERROR(Write(request), llvm::Succeeded());
+
+  std::unique_lock<std::mutex> lock(mutex);
+  cv.wait(lock, [&] { return handler_called; });
+}
--- a/lldb/unittests/Protocol/ProtocolMCPTest.cpp
+++ b/lldb/unittests/Protocol/ProtocolMCPTest.cpp
@ -0,0 +1,135 @@
+//===-- ProtocolMCPTest.cpp -----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Plugins/Protocol/MCP/Protocol.h"
+#include "TestingSupport/TestUtilities.h"
+#include "llvm/Testing/Support/Error.h"
+#include "gtest/gtest.h"
+
+using namespace lldb;
+using namespace lldb_private;
+using namespace lldb_private::mcp::protocol;
+
+TEST(ProtocolMCPTest, Request) {
+  Request request;
+  request.id = 1;
+  request.method = "foo";
+  request.params = llvm::json::Object{{"key", "value"}};
+
+  llvm::Expected<Request> deserialized_request = roundtripJSON(request);
+  ASSERT_THAT_EXPECTED(deserialized_request, llvm::Succeeded());
+
+  EXPECT_EQ(request.id, deserialized_request->id);
+  EXPECT_EQ(request.method, deserialized_request->method);
+  EXPECT_EQ(request.params, deserialized_request->params);
+}
+
+TEST(ProtocolMCPTest, Response) {
+  Response response;
+  response.id = 1;
+  response.result = llvm::json::Object{{"key", "value"}};
+
+  llvm::Expected<Response> deserialized_response = roundtripJSON(response);
+  ASSERT_THAT_EXPECTED(deserialized_response, llvm::Succeeded());
+
+  EXPECT_EQ(response.id, deserialized_response->id);
+  EXPECT_EQ(response.result, deserialized_response->result);
+}
+
+TEST(ProtocolMCPTest, Notification) {
+  Notification notification;
+  notification.method = "notifyMethod";
+  notification.params = llvm::json::Object{{"key", "value"}};
+
+  llvm::Expected<Notification> deserialized_notification =
+      roundtripJSON(notification);
+  ASSERT_THAT_EXPECTED(deserialized_notification, llvm::Succeeded());
+
+  EXPECT_EQ(notification.method, deserialized_notification->method);
+  EXPECT_EQ(notification.params, deserialized_notification->params);
+}
+
+TEST(ProtocolMCPTest, ToolCapability) {
+  ToolCapability tool_capability;
+  tool_capability.listChanged = true;
+
+  llvm::Expected<ToolCapability> deserialized_tool_capability =
+      roundtripJSON(tool_capability);
+  ASSERT_THAT_EXPECTED(deserialized_tool_capability, llvm::Succeeded());
+
+  EXPECT_EQ(tool_capability.listChanged,
+            deserialized_tool_capability->listChanged);
+}
+
+TEST(ProtocolMCPTest, Capabilities) {
+  ToolCapability tool_capability;
+  tool_capability.listChanged = true;
+
+  Capabilities capabilities;
+  capabilities.tools = tool_capability;
+
+  llvm::Expected<Capabilities> deserialized_capabilities =
+      roundtripJSON(capabilities);
+  ASSERT_THAT_EXPECTED(deserialized_capabilities, llvm::Succeeded());
+
+  EXPECT_EQ(capabilities.tools.listChanged,
+            deserialized_capabilities->tools.listChanged);
+}
+
+TEST(ProtocolMCPTest, TextContent) {
+  TextContent text_content;
+  text_content.text = "Sample text";
+
+  llvm::Expected<TextContent> deserialized_text_content =
+      roundtripJSON(text_content);
+  ASSERT_THAT_EXPECTED(deserialized_text_content, llvm::Succeeded());
+
+  EXPECT_EQ(text_content.text, deserialized_text_content->text);
+}
+
+TEST(ProtocolMCPTest, TextResult) {
+  TextContent text_content1;
+  text_content1.text = "Text 1";
+
+  TextContent text_content2;
+  text_content2.text = "Text 2";
+
+  TextResult text_result;
+  text_result.content = {text_content1, text_content2};
+  text_result.isError = true;
+
+  llvm::Expected<TextResult> deserialized_text_result =
+      roundtripJSON(text_result);
+  ASSERT_THAT_EXPECTED(deserialized_text_result, llvm::Succeeded());
+
+  EXPECT_EQ(text_result.isError, deserialized_text_result->isError);
+  ASSERT_EQ(text_result.content.size(),
+            deserialized_text_result->content.size());
+  EXPECT_EQ(text_result.content[0].text,
+            deserialized_text_result->content[0].text);
+  EXPECT_EQ(text_result.content[1].text,
+            deserialized_text_result->content[1].text);
+}
+
+TEST(ProtocolMCPTest, ToolDefinition) {
+  ToolDefinition tool_definition;
+  tool_definition.name = "ToolName";
+  tool_definition.description = "Tool Description";
+  tool_definition.inputSchema =
+      llvm::json::Object{{"schemaKey", "schemaValue"}};
+
+  llvm::Expected<ToolDefinition> deserialized_tool_definition =
+      roundtripJSON(tool_definition);
+  ASSERT_THAT_EXPECTED(deserialized_tool_definition, llvm::Succeeded());
+
+  EXPECT_EQ(tool_definition.name, deserialized_tool_definition->name);
+  EXPECT_EQ(tool_definition.description,
+            deserialized_tool_definition->description);
+  EXPECT_EQ(tool_definition.inputSchema,
+            deserialized_tool_definition->inputSchema);
+}
--- a/lldb/unittests/TestingSupport/TestUtilities.h
+++ b/lldb/unittests/TestingSupport/TestUtilities.h
@ -59,6 +59,15 @@ private:

  std::string Buffer;
 };
+
+template <typename T> static llvm::Expected<T> roundtripJSON(const T &input) {
+  llvm::json::Value value = toJSON(input);
+  llvm::json::Path::Root root;
+  T output;
+  if (!fromJSON(value, output, root))
+    return root.getError();
+  return output;
+}
 } // namespace lldb_private

 #endif