diff --git a/src/LSP.zig b/src/LSP.zig
index 2afae60..58012a3 100644
--- a/src/LSP.zig
+++ b/src/LSP.zig
@@ -67,6 +67,45 @@ pub fn send_notification_raw(self: *const Self, method: []const u8, cb: []const
     self.pid.send(.{ "NTFY", method, cb }) catch return error.SendFailed;
 }
 
+pub const ErrorCode = enum(i32) {
+
+    // Defined by JSON-RPC
+    ParseError = -32700,
+    InvalidRequest = -32600,
+    MethodNotFound = -32601,
+    InvalidParams = -32602,
+    InternalError = -32603,
+
+    // Defined by LSP
+    RequestFailed = -32803,
+    ServerCancelled = -32802,
+    ContentModified = -32801,
+    RequestCancelled = -32800,
+};
+
+pub fn send_response(allocator: std.mem.Allocator, to: tp.pid_ref, cbor_id: []const u8, result: anytype) (SendError || OutOfMemoryError)!void {
+    var cb = std.ArrayList(u8).init(allocator);
+    defer cb.deinit();
+    const writer = cb.writer();
+    try cbor.writeArrayHeader(writer, 3);
+    try cbor.writeValue(writer, "RSP");
+    try writer.writeAll(cbor_id);
+    try cbor.writeValue(cb.writer(), result);
+    to.send_raw(.{ .buf = cb.items }) catch return error.SendFailed;
+}
+
+pub fn send_error_response(allocator: std.mem.Allocator, to: tp.pid_ref, cbor_id: []const u8, code: ErrorCode, message: []const u8) (SendError || OutOfMemoryError)!void {
+    var cb = std.ArrayList(u8).init(allocator);
+    defer cb.deinit();
+    const writer = cb.writer();
+    try cbor.writeArrayHeader(writer, 4);
+    try cbor.writeValue(writer, "ERR");
+    try writer.writeAll(cbor_id);
+    try cbor.writeValue(cb.writer(), code);
+    try cbor.writeValue(cb.writer(), message);
+    to.send_raw(.{ .buf = cb.items }) catch return error.SendFailed;
+}
+
 pub fn close(self: *Self) void {
     self.deinit();
 }
@@ -252,6 +291,8 @@ const Process = struct {
         var err: []const u8 = "";
         var code: u32 = 0;
         var cbor_id: []const u8 = "";
+        var error_code: ErrorCode = undefined;
+        var message: []const u8 = "";
 
         if (try cbor.match(m.buf, .{ "REQ", "initialize", tp.extract(&bytes) })) {
             try self.send_request(from, "initialize", bytes);
@@ -262,6 +303,8 @@ const Process = struct {
             }
         } else if (try cbor.match(m.buf, .{ "RSP", tp.extract_cbor(&cbor_id), tp.extract_cbor(&bytes) })) {
             try self.send_response(cbor_id, bytes);
+        } else if (try cbor.match(m.buf, .{ "ERR", tp.extract_cbor(&cbor_id), tp.extract(&error_code), tp.extract(&message) })) {
+            try self.send_error_response(cbor_id, error_code, message);
         } else if (try cbor.match(m.buf, .{ "NTFY", "initialized", tp.extract(&bytes) })) {
             self.state = .running;
             try self.send_notification("initialized", bytes);
@@ -482,6 +525,39 @@ const Process = struct {
         self.write_log("### SEND response:\n{s}\n###\n", .{output.items});
     }
 
+    fn send_error_response(self: *Process, cbor_id: []const u8, error_code: ErrorCode, message: []const u8) (error{Closed} || SendError || cbor.Error || cbor.JsonEncodeError)!void {
+        const sp = if (self.sp) |*sp| sp else return error.Closed;
+
+        var msg = std.ArrayList(u8).init(self.allocator);
+        defer msg.deinit();
+        const msg_writer = msg.writer();
+        try cbor.writeMapHeader(msg_writer, 3);
+        try cbor.writeValue(msg_writer, "jsonrpc");
+        try cbor.writeValue(msg_writer, "2.0");
+        try cbor.writeValue(msg_writer, "id");
+        try msg_writer.writeAll(cbor_id);
+        try cbor.writeValue(msg_writer, "error");
+        try cbor.writeMapHeader(msg_writer, 2);
+        try cbor.writeValue(msg_writer, "code");
+        try cbor.writeValue(msg_writer, @intFromEnum(error_code));
+        try cbor.writeValue(msg_writer, "message");
+        try cbor.writeValue(msg_writer, message);
+
+        const json = try cbor.toJsonAlloc(self.allocator, msg.items);
+        defer self.allocator.free(json);
+        var output = std.ArrayList(u8).init(self.allocator);
+        defer output.deinit();
+        const writer = output.writer();
+        const terminator = "\r\n";
+        const content_length = json.len + terminator.len;
+        try writer.print("Content-Length: {d}\r\nContent-Type: application/vscode-jsonrpc; charset=utf-8\r\n\r\n", .{content_length});
+        _ = try writer.write(json);
+        _ = try writer.write(terminator);
+
+        sp.send(output.items) catch return error.SendFailed;
+        self.write_log("### SEND error response:\n{s}\n###\n", .{output.items});
+    }
+
     fn send_notification(self: *Process, method: []const u8, params_cb: []const u8) Error!void {
         const sp = if (self.sp) |*sp| sp else return error.Closed;
 
diff --git a/src/Project.zig b/src/Project.zig
index 2306d78..f9da66c 100644
--- a/src/Project.zig
+++ b/src/Project.zig
@@ -1494,23 +1494,16 @@ pub fn show_message(self: *Self, _: tp.pid_ref, params_cb: []const u8) !void {
 
 pub fn register_capability(self: *Self, from: tp.pid_ref, cbor_id: []const u8, params_cb: []const u8) ClientError!void {
     _ = params_cb;
-    return self.send_lsp_response(from, cbor_id, null);
+    return LSP.send_response(self.allocator, from, cbor_id, null) catch error.ClientFailed;
 }
 
 pub fn workDoneProgress_create(self: *Self, from: tp.pid_ref, cbor_id: []const u8, params_cb: []const u8) ClientError!void {
     _ = params_cb;
-    return self.send_lsp_response(from, cbor_id, null);
+    return LSP.send_response(self.allocator, from, cbor_id, null) catch error.ClientFailed;
 }
 
-pub fn send_lsp_response(self: *Self, from: tp.pid_ref, cbor_id: []const u8, result: anytype) ClientError!void {
-    var cb = std.ArrayList(u8).init(self.allocator);
-    defer cb.deinit();
-    const writer = cb.writer();
-    try cbor.writeArrayHeader(writer, 3);
-    try cbor.writeValue(writer, "RSP");
-    try writer.writeAll(cbor_id);
-    try cbor.writeValue(cb.writer(), result);
-    from.send_raw(.{ .buf = cb.items }) catch return error.ClientFailed;
+pub fn unsupported_lsp_request(self: *Self, from: tp.pid_ref, cbor_id: []const u8, method: []const u8) ClientError!void {
+    return LSP.send_error_response(self.allocator, from, cbor_id, LSP.ErrorCode.MethodNotFound, method) catch error.ClientFailed;
 }
 
 fn send_lsp_init_request(self: *Self, lsp: *const LSP, project_path: []const u8, project_basename: []const u8, project_uri: []const u8, language_server: []const u8) !void {
diff --git a/src/project_manager.zig b/src/project_manager.zig
index c7eadd5..a179c1e 100644
--- a/src/project_manager.zig
+++ b/src/project_manager.zig
@@ -620,11 +620,11 @@ const Process = struct {
             project.register_capability(from, cbor_id, params_cb)
         else if (std.mem.eql(u8, method, "window/workDoneProgress/create"))
             project.workDoneProgress_create(from, cbor_id, params_cb)
-        else blk: {
+        else {
             const params = try cbor.toJsonAlloc(self.allocator, params_cb);
             defer self.allocator.free(params);
-            self.logger.print_err("lsp", "unsupported LSP request: {s} -> {s}", .{ method, params });
-            break :blk error.Unsupported;
+            self.logger.print("unsupported LSP request: {s} -> {s}", .{ method, params });
+            project.unsupported_lsp_request(from, cbor_id, method) catch {};
         };
     }