diff --git a/README.md b/README.md
index 7db566b..562e374 100644
--- a/README.md
+++ b/README.md
@@ -31,8 +31,8 @@ int main() {
     // Print the response content data.
     std::cout << r->content() << std::endl;
 
-  } catch (const webcc::Exception& e) {
-    std::cout << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cout << error << std::endl;
   }
 
   return 0;
diff --git a/autotest/client_autotest.cc b/autotest/client_autotest.cc
index ce44193..b884036 100644
--- a/autotest/client_autotest.cc
+++ b/autotest/client_autotest.cc
@@ -73,8 +73,8 @@ TEST(ClientTest, Get_RequestFunc) {
 
     AssertGet(r);
 
-  } catch (const webcc::Exception& e) {
-    std::cerr << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cerr << error << std::endl;
   }
 }
 
@@ -88,8 +88,8 @@ TEST(ClientTest, Get_Shortcut) {
 
     AssertGet(r);
 
-  } catch (const webcc::Exception& e) {
-    std::cerr << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cerr << error << std::endl;
   }
 }
 
@@ -110,8 +110,8 @@ TEST(ClientTest, Get_SSL) {
 
     AssertGet(r);
 
-  } catch (const webcc::Exception& e) {
-    std::cerr << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cerr << error << std::endl;
   }
 }
 #endif  // WEBCC_ENABLE_SSL
@@ -131,8 +131,8 @@ TEST(ClientTest, Compression_Gzip) {
 
     EXPECT_EQ(true, json["gzipped"].asBool());
 
-  } catch (const webcc::Exception& e) {
-    std::cerr << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cerr << error << std::endl;
   }
 }
 
@@ -147,8 +147,8 @@ TEST(ClientTest, Compression_Deflate) {
 
     EXPECT_EQ(true, json["deflated"].asBool());
 
-  } catch (const webcc::Exception& e) {
-    std::cerr << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cerr << error << std::endl;
   }
 }
 
@@ -201,8 +201,8 @@ TEST(ClientTest, KeepAlive) {
 
     EXPECT_TRUE(iequals(r->GetHeader("Connection"), "Keep-alive"));
 
-  } catch (const webcc::Exception& e) {
-    std::cerr << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cerr << error << std::endl;
   }
 }
 
@@ -226,8 +226,8 @@ TEST(ClientTest, GetImageJpeg) {
 
     // TODO: Verify the response is a valid JPEG image.
 
-  } catch (const webcc::Exception& e) {
-    std::cerr << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cerr << error << std::endl;
   }
 }
 
diff --git a/examples/client_basics.cc b/examples/client_basics.cc
index d5a6fb7..ac1a2e8 100644
--- a/examples/client_basics.cc
+++ b/examples/client_basics.cc
@@ -58,8 +58,8 @@ int main() {
 
 #endif  // WEBCC_ENABLE_SSL
 
-  } catch (const webcc::Exception& e) {
-    std::cout << "Exception: " << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cout << error << std::endl;
   }
 
   return 0;
diff --git a/examples/file_upload_client.cc b/examples/file_upload_client.cc
index 3380ef0..5cb37f8 100644
--- a/examples/file_upload_client.cc
+++ b/examples/file_upload_client.cc
@@ -58,8 +58,8 @@ int main(int argc, char* argv[]) {
 
     std::cout << r->status() << std::endl;
 
-  } catch (const webcc::Exception& e) {
-    std::cout << "Exception: " << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cout << error << std::endl;
   }
 
   return 0;
diff --git a/examples/file_upload_server.cc b/examples/file_upload_server.cc
index f62cb10..5c090d9 100644
--- a/examples/file_upload_server.cc
+++ b/examples/file_upload_server.cc
@@ -55,7 +55,7 @@ int main(int argc, char* argv[]) {
     server.Run();
 
   } catch (const std::exception& e) {
-    std::cerr << "Exception: " << e.what() << std::endl;
+    std::cerr << e.what() << std::endl;
     return 1;
   }
 
diff --git a/examples/github_client.cc b/examples/github_client.cc
index 768a99b..44d648f 100644
--- a/examples/github_client.cc
+++ b/examples/github_client.cc
@@ -67,8 +67,8 @@ void ListEvents(webcc::ClientSession& session) {
 
     PRINT_JSON_STRING(r->content());
 
-  } catch (const webcc::Exception& e) {
-    std::cout << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cout << error << std::endl;
   }
 }
 
@@ -81,8 +81,8 @@ void ListUserFollowers(webcc::ClientSession& session, const std::string& user) {
 
     PRINT_JSON_STRING(r->content());
 
-  } catch (const webcc::Exception& e) {
-    std::cout << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cout << error << std::endl;
   }
 }
 
@@ -100,8 +100,8 @@ void ListAuthUserFollowers(webcc::ClientSession& session,
 
     PRINT_JSON_STRING(r->content());
 
-  } catch (const webcc::Exception& e) {
-    std::cout << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cout << error << std::endl;
   }
 }
 
@@ -124,8 +124,8 @@ void CreateAuthorization(webcc::ClientSession& session,
 
     std::cout << r->content() << std::endl;
 
-  } catch (const webcc::Exception& e) {
-    std::cout << e.what() << std::endl;
+  } catch (const webcc::Error& error) {
+    std::cout << error << std::endl;
   }
 }
 
diff --git a/examples/rest_book_client.cc b/examples/rest_book_client.cc
index f764752..2706495 100644
--- a/examples/rest_book_client.cc
+++ b/examples/rest_book_client.cc
@@ -74,8 +74,8 @@ public:
 
       return true;
 
-    } catch (const webcc::Exception& e) {
-      std::cerr << e.what() << std::endl;
+    } catch (const webcc::Error& error) {
+      std::cerr << error << std::endl;
       return false;
     }
   }
@@ -97,8 +97,8 @@ public:
 
       return !id->empty();
 
-    } catch (const webcc::Exception& e) {
-      std::cerr << e.what() << std::endl;
+    } catch (const webcc::Error& error) {
+      std::cerr << error << std::endl;
       return false;
     }
   }
@@ -122,8 +122,8 @@ public:
 
       return JsonStringToBook(r->content(), book);
 
-    } catch (const webcc::Exception& e) {
-      std::cerr << e.what() << std::endl;
+    } catch (const webcc::Error& error) {
+      std::cerr << error << std::endl;
       return false;
     }
   }
@@ -143,8 +143,8 @@ public:
 
       return true;
 
-    } catch (const webcc::Exception& e) {
-      std::cerr << e.what() << std::endl;
+    } catch (const webcc::Error& error) {
+      std::cerr << error << std::endl;
       return false;
     }
   }
@@ -159,8 +159,8 @@ public:
 
       return true;
 
-    } catch (const webcc::Exception& e) {
-      std::cerr << e.what() << std::endl;
+    } catch (const webcc::Error& error) {
+      std::cerr << error << std::endl;
       return false;
     }
   }
diff --git a/examples/rest_book_server.cc b/examples/rest_book_server.cc
index 1953a89..b9f5575 100644
--- a/examples/rest_book_server.cc
+++ b/examples/rest_book_server.cc
@@ -238,7 +238,7 @@ int main(int argc, char* argv[]) {
     server.Run();
 
   } catch (const std::exception& e) {
-    std::cerr << "Exception: " << e.what() << std::endl;
+    std::cerr << e.what() << std::endl;
     return 1;
   }
 
diff --git a/webcc/client.cc b/webcc/client.cc
index ce4cbc5..5c42eee 100644
--- a/webcc/client.cc
+++ b/webcc/client.cc
@@ -12,43 +12,30 @@ Client::Client()
       buffer_size_(kBufferSize),
       timeout_(kMaxReadSeconds),
       closed_(false),
-      timer_canceled_(false),
-      timed_out_(false),
-      error_(kNoError) {
+      timer_canceled_(false) {
 }
 
-bool Client::Request(RequestPtr request, bool connect) {
-  io_context_.restart();
-
-  response_.reset(new Response{});
-  response_parser_.Init(response_.get());
-
-  closed_ = false;
-  timer_canceled_ = false;
-  timed_out_ = false;
-  error_ = kNoError;
-
-  if (buffer_.size() != buffer_size_) {
-    LOG_VERB("Resize buffer: %u -> %u.", buffer_.size(), buffer_size_);
-    buffer_.resize(buffer_size_);
-  }
+Error Client::Request(RequestPtr request, bool connect) {
+  Restart();
 
   if (connect) {
     // No existing socket connection was specified, create a new one.
-    if ((error_ = Connect(request)) != kNoError) {
-      return false;
+    Connect(request);
+
+    if (error_) {
+      return error_;
     }
   }
 
-  if ((error_ = WriteReqeust(request)) != kNoError) {
-    return false;
-  }
+  WriteReqeust(request);
 
-  if ((error_ = ReadResponse()) != kNoError) {
-    return false;
+  if (error_) {
+    return error_;
   }
 
-  return true;
+  ReadResponse();
+
+  return error_;
 }
 
 void Client::Close() {
@@ -68,22 +55,38 @@ void Client::Close() {
   }
 }
 
-Error Client::Connect(RequestPtr request) {
+void Client::Restart() {
+  io_context_.restart();
+
+  response_.reset(new Response{});
+  response_parser_.Init(response_.get());
+
+  closed_ = false;
+  timer_canceled_ = false;
+  error_ = Error{};
+
+  if (buffer_.size() != buffer_size_) {
+    LOG_VERB("Resize buffer: %u -> %u.", buffer_.size(), buffer_size_);
+    buffer_.resize(buffer_size_);
+  }
+}
+
+void Client::Connect(RequestPtr request) {
   if (request->url().scheme() == "https") {
 #if WEBCC_ENABLE_SSL
     socket_.reset(new SslSocket{ io_context_, ssl_verify_ });
-    return DoConnect(request, "443");
+    DoConnect(request, "443");
 #else
     LOG_ERRO("SSL/HTTPS support is not enabled.");
-    return kSchemaError;
+    return kSyntaxError;
 #endif  // WEBCC_ENABLE_SSL
   } else {
     socket_.reset(new Socket{ io_context_ });
-    return DoConnect(request, "80");
+    DoConnect(request, "80");
   }
 }
 
-Error Client::DoConnect(RequestPtr request, const std::string& default_port) {
+void Client::DoConnect(RequestPtr request, const std::string& default_port) {
   tcp::resolver resolver(io_context_);
 
   std::string port = request->port(default_port);
@@ -94,7 +97,7 @@ Error Client::DoConnect(RequestPtr request, const std::string& default_port) {
   if (ec) {
     LOG_ERRO("Host resolve error (%s): %s, %s.", ec.message().c_str(),
              request->host().c_str(), port.c_str());
-    return kHostResolveError;
+    error_.Set(Error::kResolveError, "Host resolve error");
   }
 
   LOG_VERB("Connect to server...");
@@ -106,15 +109,14 @@ Error Client::DoConnect(RequestPtr request, const std::string& default_port) {
   if (ec) {
     LOG_ERRO("Socket connect error (%s).", ec.message().c_str());
     Close();
-    return kEndpointConnectError;
+    // TODO: Handshake error
+    error_.Set(Error::kConnectError, "Endpoint connect error");
   }
 
   LOG_VERB("Socket connected.");
-
-  return kNoError;
 }
 
-Error Client::WriteReqeust(RequestPtr request) {
+void Client::WriteReqeust(RequestPtr request) {
   LOG_VERB("HTTP request:\n%s", request->Dump(4, "> ").c_str());
 
   // NOTE:
@@ -130,36 +132,30 @@ Error Client::WriteReqeust(RequestPtr request) {
   if (ec) {
     LOG_ERRO("Socket write error (%s).", ec.message().c_str());
     Close();
-    return kSocketWriteError;
+    error_.Set(Error::kSocketWriteError, "Socket write error");
   }
 
   LOG_INFO("Request sent.");
-
-  return kNoError;
 }
 
-Error Client::ReadResponse() {
+void Client::ReadResponse() {
   LOG_VERB("Read response (timeout: %ds)...", timeout_);
 
   timer_.expires_after(std::chrono::seconds(timeout_));
 
   DoWaitTimer();
+  DoReadResponse();
 
-  Error error = kNoError;
-  DoReadResponse(&error);
-
-  if (error == kNoError) {
+  if (!error_) {
     LOG_VERB("HTTP response:\n%s", response_->Dump(4, "> ").c_str());
   }
-
-  return error;
 }
 
-void Client::DoReadResponse(Error* error) {
+void Client::DoReadResponse() {
   boost::system::error_code ec = boost::asio::error::would_block;
 
-  auto handler = [this, &ec, error](boost::system::error_code inner_ec,
-                                    std::size_t length) {
+  auto handler = [this, &ec](boost::system::error_code inner_ec,
+                             std::size_t length) {
     ec = inner_ec;
 
     LOG_VERB("Socket async read handler.");
@@ -170,7 +166,7 @@ void Client::DoReadResponse(Error* error) {
     // TODO: Is it necessary to check `length == 0`?
     if (ec || length == 0) {
       Close();
-      *error = kSocketReadError;
+      error_.Set(Error::kSocketReadError, "Socket read error");
       LOG_ERRO("Socket read error (%s).", ec.message().c_str());
       return;
     }
@@ -180,7 +176,7 @@ void Client::DoReadResponse(Error* error) {
     // Parse the response piece just read.
     if (!response_parser_.Parse(buffer_.data(), length)) {
       Close();
-      *error = kHttpError;
+      error_.Set(Error::kParseError, "HTTP parse error");
       LOG_ERRO("Failed to parse HTTP response.");
       return;
     }
@@ -203,7 +199,7 @@ void Client::DoReadResponse(Error* error) {
     }
 
     if (!closed_) {
-      DoReadResponse(error);
+      DoReadResponse();
     }
   };
 
@@ -238,7 +234,7 @@ void Client::OnTimer(boost::system::error_code ec) {
     // The deadline has passed. The socket is closed so that any outstanding
     // asynchronous operations are canceled.
     LOG_WARN("HTTP client timed out.");
-    timed_out_ = true;
+    error_.set_timeout(true);
     Close();
     return;
   }
diff --git a/webcc/client.h b/webcc/client.h
index c1aa1bb..93ab52e 100644
--- a/webcc/client.h
+++ b/webcc/client.h
@@ -29,7 +29,7 @@ class Client {
 public:
   Client();
 
-  virtual ~Client() = default;
+  ~Client() = default;
 
   Client(const Client&) = delete;
   Client& operator=(const Client&) = delete;
@@ -52,7 +52,7 @@ public:
   }
 
   // Connect to server, send request, wait until response is received.
-  bool Request(RequestPtr request, bool connect = true);
+  Error Request(RequestPtr request, bool connect = true);
 
   // Close the socket.
   void Close();
@@ -61,20 +61,18 @@ public:
 
   bool closed() const { return closed_; }
 
-  bool timed_out() const { return timed_out_; }
-
-  Error error() const { return error_; }
-
 private:
-  Error Connect(RequestPtr request);
+  void Restart();
 
-  Error DoConnect(RequestPtr request, const std::string& default_port);
+  void Connect(RequestPtr request);
 
-  Error WriteReqeust(RequestPtr request);
+  void DoConnect(RequestPtr request, const std::string& default_port);
 
-  Error ReadResponse();
+  void WriteReqeust(RequestPtr request);
 
-  void DoReadResponse(Error* error);
+  void ReadResponse();
+
+  void DoReadResponse();
 
   void DoWaitTimer();
   void OnTimer(boost::system::error_code ec);
@@ -113,10 +111,6 @@ private:
   // Deadline timer canceled.
   bool timer_canceled_;
 
-  // Timeout occurred.
-  bool timed_out_;
-
-  // Error code.
   Error error_;
 };
 
diff --git a/webcc/client_session.cc b/webcc/client_session.cc
index 5e5fc05..90efed9 100644
--- a/webcc/client_session.cc
+++ b/webcc/client_session.cc
@@ -143,11 +143,11 @@ void ClientSession::InitHeaders() {
   //              content coding.
   //
   // A note about "deflate":
-  //   (https://www.zlib.net/zlib_faq.html#faq39)
   // "gzip" is the gzip format, and "deflate" is the zlib format. They should
   // probably have called the second one "zlib" instead to avoid confusion with
   // the raw deflate compressed data format.
   // Simply put, "deflate" is not recommended for HTTP 1.1 encoding.
+  // (https://www.zlib.net/zlib_faq.html#faq39)
 
 #if WEBCC_ENABLE_GZIP
   headers_.Set(kAcceptEncoding, "gzip, deflate");
@@ -161,7 +161,7 @@ void ClientSession::InitHeaders() {
 }
 
 ResponsePtr ClientSession::Send(RequestPtr request) {
-  const ClientPool::Key key{request->url()};
+  const ClientPool::Key key{ request->url() };
 
   // Reuse a pooled connection.
   bool reuse = false;
@@ -179,22 +179,22 @@ ResponsePtr ClientSession::Send(RequestPtr request) {
   client->set_buffer_size(buffer_size_);
   client->set_timeout(timeout_);
 
-  bool ok = client->Request(request, !reuse);
+  Error error = client->Request(request, !reuse);
 
-  if (!ok) {
-    if (reuse && client->error() == kSocketWriteError) {
+  if (error) {
+    if (reuse && error.code() == Error::kSocketWriteError) {
       LOG_WARN("Cannot send request with the reused connection. "
                "The server must have closed it, reconnect and try again.");
-      ok = client->Request(request, true);
+      error = client->Request(request, true);
     }
   }
 
-  if (!ok) {
+  if (error) {
+    // Remove the failed connection from pool.
     if (reuse) {
-      // Remove the failed connection from pool.
       pool_.Remove(key);
     }
-    throw Exception(client->error(), "", client->timed_out());
+    throw error;
   }
 
   // Update connection pool.
diff --git a/webcc/common.cc b/webcc/common.cc
index f1e23af..324413b 100644
--- a/webcc/common.cc
+++ b/webcc/common.cc
@@ -204,7 +204,7 @@ FormPart::FormPart(const std::string& name, const Path& path,
                    const std::string& media_type)
     : name_(name), media_type_(media_type) {
   if (!ReadFile(path, &data_)) {
-    throw Exception(kFileIOError, "Cannot read the file.");
+    throw Error{ Error::kFileError, "Cannot read the file." };
   }
 
   // Determine file name from file path.
diff --git a/webcc/globals.cc b/webcc/globals.cc
index 7402187..e6cb12a 100644
--- a/webcc/globals.cc
+++ b/webcc/globals.cc
@@ -1,9 +1,8 @@
 #include "webcc/globals.h"
 
+#include <iostream>
 #include <map>
 
-#include "webcc/version.h"
-
 namespace webcc {
 
 // -----------------------------------------------------------------------------
@@ -47,37 +46,13 @@ std::string FromExtension(const std::string& extension,
 
 // -----------------------------------------------------------------------------
 
-const char* DescribeError(Error error) {
-  switch (error) {
-    case kSchemaError:
-      return "Schema error";
-    case kHostResolveError:
-      return "Host resolve error";
-    case kEndpointConnectError:
-      return "Endpoint connect error";
-    case kHandshakeError:
-      return "Handshake error";
-    case kSocketReadError:
-      return "Socket read error";
-    case kSocketWriteError:
-      return "Socket write error";
-    case kHttpError:
-      return "HTTP error";
-    case kFileIOError:
-      return "File IO error";
-    default:
-      return "";
-  }
-}
-
-Exception::Exception(Error error, const std::string& details, bool timeout)
-    : error_(error), msg_(DescribeError(error)), timeout_(timeout) {
-  if (!details.empty()) {
-    msg_ += " (" + details + ")";
-  }
-  if (timeout) {
-    msg_ += " (timeout)";
+std::ostream& operator<<(std::ostream& os, const Error& error) {
+  os << std::to_string(static_cast<int>(error.code()));
+  os << ": " << error.message();
+  if (error.timeout()) {
+    os << " (timeout)";
   }
+  return os;
 }
 
 }  // namespace webcc
diff --git a/webcc/globals.h b/webcc/globals.h
index 4e0e14b..aee6ccc 100644
--- a/webcc/globals.h
+++ b/webcc/globals.h
@@ -3,6 +3,7 @@
 
 #include <cassert>
 #include <exception>
+#include <iosfwd>
 #include <string>
 
 #include "webcc/config.h"
@@ -149,56 +150,49 @@ enum class ContentEncoding {
 
 // -----------------------------------------------------------------------------
 
-// Client side error codes.
-enum Error {
-  kNoError = 0,  // i.e., OK
-
-  kSchemaError,  // TODO
-
-  kHostResolveError,
-  kEndpointConnectError,
-  kHandshakeError,  // HTTPS handshake
-  kSocketReadError,
-  kSocketWriteError,
-
-  // HTTP error.
-  // E.g., failed to parse HTTP response (invalid content length, etc.).
-  kHttpError,
-
-  // File read/write error.
-  kFileIOError,
-};
-
-// Return a descriptive message for the given error code.
-const char* DescribeError(Error error);
-
-class Exception : public std::exception {
+// Error (or exception) for the client.
+class Error {
 public:
-  explicit Exception(Error error, const std::string& details = "",
-                     bool timeout = false);
+  enum Code {
+    kOK = 0,
+    kSyntaxError,
+    kResolveError,
+    kConnectError,
+    kHandshakeError,
+    kSocketReadError,
+    kSocketWriteError,
+    kParseError,
+    kFileError,
+  };
 
-  Error error() const {
-    return error_;
+public:
+  Error(Code code = kOK, const std::string& message = "")
+      : code_(code), message_(message), timeout_(false) {
   }
 
-  // Note that `noexcept` is required by GCC.
-  const char* what() const WEBCC_NOEXCEPT override{
-    return msg_.c_str();
-  }
+  Code code() const { return code_; }
 
-  bool timeout() const {
-    return timeout_;
+  const std::string& message() const { return message_; }
+
+  void Set(Code code, const std::string& message) {
+    code_ = code;
+    message_ = message;
   }
 
-private:
-  Error error_;
+  bool timeout() const { return timeout_; }
 
-  std::string msg_;
+  void set_timeout(bool timeout) { timeout_ = timeout; }
 
-  // If the error was caused by timeout or not.
+  operator bool() const { return code_ != kOK; }
+
+private:
+  Code code_;
+  std::string message_;
   bool timeout_;
 };
 
+std::ostream& operator<<(std::ostream& os, const Error& error);
+
 }  // namespace webcc
 
 #endif  // WEBCC_GLOBALS_H_
diff --git a/webcc/request.cc b/webcc/request.cc
index bb143bc..3a735b2 100644
--- a/webcc/request.cc
+++ b/webcc/request.cc
@@ -81,7 +81,7 @@ void Request::CreateStartLine() {
   }
 
   if (url_.host().empty()) {
-    throw Exception(kSchemaError, "Invalid request: host is missing.");
+    throw Error{ Error::kSyntaxError, "Host is missing" };
   }
 
   std::string target = "/" + url_.path();
diff --git a/webcc/rest_service_manager.cc b/webcc/rest_service_manager.cc
index 76229d3..aac4a29 100644
--- a/webcc/rest_service_manager.cc
+++ b/webcc/rest_service_manager.cc
@@ -25,7 +25,7 @@ bool RestServiceManager::AddService(RestServicePtr service,
     item.url_regex.assign(url, flags);
     service_items_.push_back(std::move(item));
     return true;
-  } catch (std::regex_error& e) {
+  } catch (const std::regex_error& e) {
     LOG_ERRO("URL is not a valid regular expression: %s", e.what());
     return false;
   }