diff --git a/mlir/include/mlir/Tools/lsp-server-support/Transport.h b/mlir/include/mlir/Tools/lsp-server-support/Transport.h
index ce742be7a941c..44c71058cf717 100644
--- a/mlir/include/mlir/Tools/lsp-server-support/Transport.h
+++ b/mlir/include/mlir/Tools/lsp-server-support/Transport.h
@@ -147,9 +147,15 @@ class MessageHandler {
                     void (ThisT::*handler)(const Param &)) {
     notificationHandlers[method] = [method, handler,
                                     thisPtr](llvm::json::Value rawParams) {
-      llvm::Expected<Param> param = parse<Param>(rawParams, method, "request");
-      if (!param)
-        return llvm::consumeError(param.takeError());
+      llvm::Expected<Param> param =
+          parse<Param>(rawParams, method, "notification");
+      if (!param) {
+        return llvm::consumeError(
+            llvm::handleErrors(param.takeError(), [](const LSPError &lspError) {
+              Logger::error("JSON parsing error: {0}",
+                            lspError.message.c_str());
+            }));
+      }
       (thisPtr->*handler)(*param);
     };
   }
diff --git a/mlir/lib/Tools/lsp-server-support/Transport.cpp b/mlir/lib/Tools/lsp-server-support/Transport.cpp
index 64dea35614c07..339c5f3825165 100644
--- a/mlir/lib/Tools/lsp-server-support/Transport.cpp
+++ b/mlir/lib/Tools/lsp-server-support/Transport.cpp
@@ -51,12 +51,12 @@ class Reply {
 
 Reply::Reply(const llvm::json::Value &id, llvm::StringRef method,
              JSONTransport &transport, std::mutex &transportOutputMutex)
-    : id(id), transport(&transport),
+    : method(method), id(id), transport(&transport),
       transportOutputMutex(transportOutputMutex) {}
 
 Reply::Reply(Reply &&other)
-    : replied(other.replied.load()), id(std::move(other.id)),
-      transport(other.transport),
+    : method(other.method), replied(other.replied.load()),
+      id(std::move(other.id)), transport(other.transport),
       transportOutputMutex(other.transportOutputMutex) {
   other.transport = nullptr;
 }
diff --git a/mlir/unittests/CMakeLists.txt b/mlir/unittests/CMakeLists.txt
index 6fad249a0b2fb..6d8aa290e82f2 100644
--- a/mlir/unittests/CMakeLists.txt
+++ b/mlir/unittests/CMakeLists.txt
@@ -20,6 +20,7 @@ add_subdirectory(Support)
 add_subdirectory(Rewrite)
 add_subdirectory(TableGen)
 add_subdirectory(Target)
+add_subdirectory(Tools)
 add_subdirectory(Transforms)
 
 if(MLIR_ENABLE_EXECUTION_ENGINE)
diff --git a/mlir/unittests/Tools/CMakeLists.txt b/mlir/unittests/Tools/CMakeLists.txt
new file mode 100644
index 0000000000000..a97588d928668
--- /dev/null
+++ b/mlir/unittests/Tools/CMakeLists.txt
@@ -0,0 +1 @@
+add_subdirectory(lsp-server-support)
diff --git a/mlir/unittests/Tools/lsp-server-support/CMakeLists.txt b/mlir/unittests/Tools/lsp-server-support/CMakeLists.txt
new file mode 100644
index 0000000000000..3aa8b9c4bc773
--- /dev/null
+++ b/mlir/unittests/Tools/lsp-server-support/CMakeLists.txt
@@ -0,0 +1,6 @@
+add_mlir_unittest(MLIRLspServerSupportTests
+  Transport.cpp
+)
+target_link_libraries(MLIRLspServerSupportTests
+  PRIVATE
+  MLIRLspServerSupportLib)
diff --git a/mlir/unittests/Tools/lsp-server-support/Transport.cpp b/mlir/unittests/Tools/lsp-server-support/Transport.cpp
new file mode 100644
index 0000000000000..a086964cd3660
--- /dev/null
+++ b/mlir/unittests/Tools/lsp-server-support/Transport.cpp
@@ -0,0 +1,134 @@
+//===- Transport.cpp - LSP JSON transport unit tests ----------------------===//
+//
+// 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 "mlir/Tools/lsp-server-support/Transport.h"
+#include "mlir/Tools/lsp-server-support/Logging.h"
+#include "mlir/Tools/lsp-server-support/Protocol.h"
+#include "llvm/Support/FileSystem.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+using namespace mlir;
+using namespace mlir::lsp;
+using namespace testing;
+
+namespace {
+
+TEST(TransportTest, SendReply) {
+  std::string out;
+  llvm::raw_string_ostream os(out);
+  JSONTransport transport(nullptr, os);
+  MessageHandler handler(transport);
+
+  transport.reply(1989, nullptr);
+  EXPECT_THAT(out, HasSubstr("\"id\":1989"));
+  EXPECT_THAT(out, HasSubstr("\"result\":null"));
+}
+
+class TransportInputTest : public Test {
+  llvm::SmallVector<char> inputPath;
+  std::FILE *in = nullptr;
+  std::string output = "";
+  llvm::raw_string_ostream os;
+  std::optional<JSONTransport> transport = std::nullopt;
+  std::optional<MessageHandler> messageHandler = std::nullopt;
+
+protected:
+  TransportInputTest() : os(output) {}
+
+  void SetUp() override {
+    std::error_code ec =
+        llvm::sys::fs::createTemporaryFile("lsp-unittest", "json", inputPath);
+    ASSERT_FALSE(ec) << "Could not create temporary file: " << ec.message();
+
+    in = std::fopen(inputPath.data(), "r");
+    ASSERT_TRUE(in) << "Could not open temporary file: "
+                    << std::strerror(errno);
+    transport.emplace(in, os, JSONStreamStyle::Delimited);
+    messageHandler.emplace(*transport);
+  }
+
+  void TearDown() override {
+    EXPECT_EQ(std::fclose(in), 0)
+        << "Could not close temporary file FD: " << std::strerror(errno);
+    std::error_code ec =
+        llvm::sys::fs::remove(inputPath, /*IgnoreNonExisting=*/false);
+    EXPECT_FALSE(ec) << "Could not remove temporary file '" << inputPath.data()
+                     << "': " << ec.message();
+  }
+
+  void writeInput(StringRef buffer) {
+    std::error_code ec;
+    llvm::raw_fd_ostream os(inputPath.data(), ec);
+    ASSERT_FALSE(ec) << "Could not write to '" << inputPath.data()
+                     << "': " << ec.message();
+    os << buffer;
+    os.close();
+  }
+
+  StringRef getOutput() const { return output; }
+  MessageHandler &getMessageHandler() { return *messageHandler; }
+
+  void runTransport() {
+    bool gotEOF = false;
+    llvm::Error err = llvm::handleErrors(
+        transport->run(*messageHandler), [&](const llvm::ECError &ecErr) {
+          gotEOF = ecErr.convertToErrorCode() == std::errc::io_error;
+        });
+    llvm::consumeError(std::move(err));
+    EXPECT_TRUE(gotEOF);
+  }
+};
+
+TEST_F(TransportInputTest, RequestWithInvalidParams) {
+  struct Handler {
+    void onMethod(const TextDocumentItem &params,
+                  mlir::lsp::Callback<TextDocumentIdentifier> callback) {}
+  } handler;
+  getMessageHandler().method("invalid-params-request", &handler,
+                             &Handler::onMethod);
+
+  writeInput("{\"jsonrpc\":\"2.0\",\"id\":92,"
+             "\"method\":\"invalid-params-request\",\"params\":{}}\n");
+  runTransport();
+  EXPECT_THAT(getOutput(), HasSubstr("error"));
+  EXPECT_THAT(getOutput(), HasSubstr("missing value at (root).uri"));
+}
+
+TEST_F(TransportInputTest, NotificationWithInvalidParams) {
+  // JSON parsing errors are only reported via error logging. As a result, this
+  // test can't make any expectations -- but it prints the output anyway, by way
+  // of demonstration.
+  Logger::setLogLevel(Logger::Level::Error);
+
+  struct Handler {
+    void onNotification(const TextDocumentItem &params) {}
+  } handler;
+  getMessageHandler().notification("invalid-params-notification", &handler,
+                                   &Handler::onNotification);
+
+  writeInput("{\"jsonrpc\":\"2.0\",\"method\":\"invalid-params-notification\","
+             "\"params\":{}}\n");
+  runTransport();
+}
+
+TEST_F(TransportInputTest, MethodNotFound) {
+  writeInput("{\"jsonrpc\":\"2.0\",\"id\":29,\"method\":\"ack\"}\n");
+  runTransport();
+  EXPECT_THAT(getOutput(), HasSubstr("\"id\":29"));
+  EXPECT_THAT(getOutput(), HasSubstr("\"error\""));
+  EXPECT_THAT(getOutput(), HasSubstr("\"message\":\"method not found: ack\""));
+}
+
+TEST_F(TransportInputTest, OutgoingNotification) {
+  auto notifyFn = getMessageHandler().outgoingNotification<CompletionList>(
+      "outgoing-notification");
+  notifyFn(CompletionList{});
+  EXPECT_THAT(getOutput(), HasSubstr("\"method\":\"outgoing-notification\""));
+}
+} // namespace