diff --git a/autoload/ale/lsp.vim b/autoload/ale/lsp.vim
index e01e4eb..72b9442 100644
--- a/autoload/ale/lsp.vim
+++ b/autoload/ale/lsp.vim
@@ -1,6 +1,7 @@
 " Author: w0rp <devw0rp@gmail.com>
 " Description: Language Server Protocol client code
 
+let s:address_info_map = {}
 let g:ale_lsp_next_message_id = 1
 
 function! ale#lsp#GetNextMessageID() abort
@@ -19,75 +20,133 @@ function! ale#lsp#GetNextMessageID() abort
     return l:id
 endfunction
 
-" (method_name, params)
-function! ale#lsp#CreateMessage(method_name, ...) abort
+" Given a List of one or two items, [method_name] or [method_name, params],
+" return a List containing [message_id, message_data]
+function! ale#lsp#CreateMessageData(message) abort
+    let l:is_notification = a:message[0]
+
+    let l:obj = {
+    \   'id': v:null,
+    \   'jsonrpc': '2.0',
+    \   'method': a:message[1],
+    \}
+
+    if !l:is_notification
+        let l:obj.id = ale#lsp#GetNextMessageID()
+    endif
+
+    if len(a:message) > 2
+        let l:obj.params = a:message[2]
+    endif
+
+    let l:body = json_encode(l:obj)
+    let l:data = 'Content-Length: ' . strlen(l:body) . "\r\n\r\n" . l:body
+
+    return [l:is_notification ? 0 : l:obj.id, l:data]
+endfunction
+
+function! ale#lsp#ReadMessageData(data) abort
+    let l:response_list = []
+    let l:remainder = a:data
+
+    while 1
+        " Look for the end of the HTTP headers
+        let l:body_start_index = matchend(l:remainder, "\r\n\r\n")
+
+        if l:body_start_index < 0
+            " No header end was found yet.
+            break
+        endif
+
+        " Parse the Content-Length header.
+        let l:header_data = l:remainder[:l:body_start_index - 4]
+        let l:length_match = matchlist(
+        \   l:header_data,
+        \   '\vContent-Length: *(\d+)'
+        \)
+
+        if empty(l:length_match)
+            throw "Invalid JSON-RPC header:\n" . l:header_data
+        endif
+
+        " Split the body and the remainder of the text.
+        let l:remainder_start_index = l:body_start_index + str2nr(l:length_match[1])
+
+        if len(l:remainder) < l:remainder_start_index
+            " We don't have enough data yet.
+            break
+        endif
+
+        let l:body = l:remainder[l:body_start_index : l:remainder_start_index - 1]
+        let l:remainder = l:remainder[l:remainder_start_index :]
+
+        " Parse the JSON object and add it to the list.
+        call add(l:response_list, json_decode(l:body))
+    endwhile
+
+    return [l:remainder, l:response_list]
+endfunction
+
+function! s:HandleMessage(channel, message) abort
+    let l:channel_info = ch_info(a:channel)
+    let l:address = l:channel_info.hostname . ':' . l:channel_info.port
+    let l:info = s:address_info_map[l:address]
+    let l:info.data .= a:message
+
+    " Parse the objects now if we can, and keep the remaining text.
+    let [l:info.data, l:response_list] = ale#lsp#ReadMessageData(l:info.data)
+
+    " Call our callbacks.
+    for l:response in l:response_list
+        let l:callback = l:info.callback_map.pop(l:response.id)
+        call ale#util#GetFunction(l:callback)(l:response)
+    endfor
+endfunction
+
+" Send a message to the server.
+" A callback can be registered to handle the response.
+" Notifications do not need to be handled.
+" (address, message, callback?)
+function! ale#lsp#SendMessage(address, message, ...) abort
     if a:0 > 1
         throw 'Too many arguments!'
     endif
 
-    let l:obj = {
-    \   'id': ale#lsp#GetNextMessageID(),
-    \   'jsonrpc': '2.0',
-    \   'method': a:method_name,
-    \}
-
-    if a:0 > 0
-        let l:obj.params = a:1
+    if !a:message[0] && a:0 == 0
+        throw 'A callback must be set for messages which are not notifications!'
     endif
 
-    let l:body = json_encode(l:obj)
+    let [l:id, l:data] = ale#lsp#CreateMessageData(a:message)
 
-    return 'Content-Length: ' . strlen(l:body) . "\r\n\r\n" . l:body
-endfunction
+    let l:info = get(s:address_info_map, a:address, {})
 
-function! ale#lsp#ReadMessage(data) abort
-    let l:header_end_index = match(a:data, "\r\n\r\n")
-
-    if l:header_end_index < 0
-        throw 'Invalid messaage: ' . string(a:data)
-    endif
-
-    return json_decode(a:data[l:header_end_index + 4:])
-endfunction
-
-" Constants for message severity codes.
-let s:SEVERITY_ERROR = 1
-let s:SEVERITY_WARNING = 2
-let s:SEVERITY_INFORMATION = 3
-let s:SEVERITY_HINT = 4
-
-" Parse the message for textDocument/publishDiagnostics
-function! ale#lsp#ReadDiagnostics(params) abort
-    let l:filename = a:params.uri
-    let l:loclist = []
-
-    for l:diagnostic in a:params.diagnostics
-        let l:severity = get(l:diagnostic, 'severity', 0)
-        let l:loclist_item = {
-        \   'message': l:diagnostic.message,
-        \   'type': 'E',
-        \   'lnum': l:diagnostic.range.start.line + 1,
-        \   'col': l:diagnostic.range.start.character + 1,
-        \   'end_lnum': l:diagnostic.range.end.line + 1,
-        \   'end_col': l:diagnostic.range.end.character + 1,
+    if empty(l:info)
+        let l:info = {
+        \   'data': '',
+        \   'callback_map': {},
         \}
+        let s:address_info_map[a:address] = l:info
+    endif
 
-        if l:severity == s:SEVERITY_WARNING
-            let l:loclist_item.type = 'W'
-        elseif l:severity == s:SEVERITY_INFORMATION
-            " TODO: Use 'I' here in future.
-            let l:loclist_item.type = 'W'
-        elseif l:severity == s:SEVERITY_HINT
-            " TODO: Use 'H' here in future
-            let l:loclist_item.type = 'W'
-        endif
+    " The ID is 0 when the message is a Notification, which is a JSON-RPC
+    " request for which the server must not return a response.
+    if l:id != 0
+        " Add the callback, which the server will respond to later.
+        let l:info.callback_map[l:id] = a:1
+    endif
 
-        if has_key(l:diagnostic, 'code')
-            let l:loclist_item.nr = l:diagnostic.code
-        endif
+    if !has_key(l:info, 'channel') || ch_status(l:info.channel) !=# 'open'
+        let l:info.channnel = ch_open(a:address, {
+        \   'mode': 'raw',
+        \   'waittime': 0,
+        \   'callback': 's:HandleMessage',
+        \})
+    endif
 
-        call add(l:loclist, l:loclist_item)
-    endfor
+    if ch_status(l:info.channnel) ==# 'fail'
+        throw 'Failed to open channel for: ' . a:address
+    endif
 
-    return [l:filename, l:loclist]
+    " Send the message to the server
+    call ch_sendraw(l:info.channel, l:data)
 endfunction
diff --git a/autoload/ale/lsp/message.vim b/autoload/ale/lsp/message.vim
index d46e68a..937e4f4 100644
--- a/autoload/ale/lsp/message.vim
+++ b/autoload/ale/lsp/message.vim
@@ -1,65 +1,64 @@
 " Author: w0rp <devw0rp@gmail.com>
 " Description: Language Server Protocol message implementations
+"
+" Messages in this movie will be returned in the format
+" [is_notification, method_name, params?]
 
-function! ale#lsp#message#CancelRequest(id) abort
-    return ale#lsp#CreateMessage('$/cancelRequest', {'id': a:id})
-endfunction
-
-function! ale#lsp#message#Initialize(processId, rootUri) abort
+function! ale#lsp#message#Initialize(root_uri) abort
     " TODO: Define needed capabilities.
-    return ale#lsp#CreateMessage('initialize', {
-    \   'processId': a:processId,
-    \   'rootUri': a:rootUri,
+    return [0, 'initialize', {
+    \   'processId': getpid(),
+    \   'rootUri': a:root_uri,
     \   'capabilities': {},
-    \})
+    \}]
 endfunction
 
 function! ale#lsp#message#Initialized() abort
-    return ale#lsp#CreateMessage('initialized')
+    return [1, 'initialized']
 endfunction
 
 function! ale#lsp#message#Shutdown() abort
-    return ale#lsp#CreateMessage('shutdown')
+    return [0, 'shutdown']
 endfunction
 
 function! ale#lsp#message#Exit() abort
-    return ale#lsp#CreateMessage('exit')
+    return [1, 'exit']
 endfunction
 
-function! ale#lsp#message#DidOpen(uri, languageId, version, text) abort
-    return ale#lsp#CreateMessage('textDocument/didOpen', {
+function! ale#lsp#message#DidOpen(uri, language_id, version, text) abort
+    return [1, 'textDocument/didOpen', {
     \   'textDocument': {
     \       'uri': a:uri,
-    \       'languageId': a:languageId,
+    \       'languageId': a:language_id,
     \       'version': a:version,
     \       'text': a:text,
     \   },
-    \})
+    \}]
 endfunction
 
 function! ale#lsp#message#DidChange(uri, version, text) abort
     " For changes, we simply send the full text of the document to the server.
-    return ale#lsp#CreateMessage('textDocument/didChange', {
+    return [1, 'textDocument/didChange', {
     \   'textDocument': {
     \       'uri': a:uri,
     \       'version': a:version,
     \   },
     \   'contentChanges': [{'text': a:text}]
-    \})
+    \}]
 endfunction
 
 function! ale#lsp#message#DidSave(uri) abort
-    return ale#lsp#CreateMessage('textDocument/didSave', {
+    return [1, 'textDocument/didSave', {
     \   'textDocument': {
     \       'uri': a:uri,
     \   },
-    \})
+    \}]
 endfunction
 
 function! ale#lsp#message#DidClose(uri) abort
-    return ale#lsp#CreateMessage('textDocument/didClose', {
+    return [1, 'textDocument/didClose', {
     \   'textDocument': {
     \       'uri': a:uri,
     \   },
-    \})
+    \}]
 endfunction
diff --git a/autoload/ale/lsp/response.vim b/autoload/ale/lsp/response.vim
new file mode 100644
index 0000000..aeb93a5
--- /dev/null
+++ b/autoload/ale/lsp/response.vim
@@ -0,0 +1,44 @@
+" Author: w0rp <devw0rp@gmail.com>
+" Description: Parsing and transforming of LSP server responses.
+
+" Constants for message severity codes.
+let s:SEVERITY_ERROR = 1
+let s:SEVERITY_WARNING = 2
+let s:SEVERITY_INFORMATION = 3
+let s:SEVERITY_HINT = 4
+
+" Parse the message for textDocument/publishDiagnostics
+function! ale#lsp#response#ReadDiagnostics(params) abort
+    let l:filename = a:params.uri
+    let l:loclist = []
+
+    for l:diagnostic in a:params.diagnostics
+        let l:severity = get(l:diagnostic, 'severity', 0)
+        let l:loclist_item = {
+        \   'message': l:diagnostic.message,
+        \   'type': 'E',
+        \   'lnum': l:diagnostic.range.start.line + 1,
+        \   'col': l:diagnostic.range.start.character + 1,
+        \   'end_lnum': l:diagnostic.range.end.line + 1,
+        \   'end_col': l:diagnostic.range.end.character + 1,
+        \}
+
+        if l:severity == s:SEVERITY_WARNING
+            let l:loclist_item.type = 'W'
+        elseif l:severity == s:SEVERITY_INFORMATION
+            " TODO: Use 'I' here in future.
+            let l:loclist_item.type = 'W'
+        elseif l:severity == s:SEVERITY_HINT
+            " TODO: Use 'H' here in future
+            let l:loclist_item.type = 'W'
+        endif
+
+        if has_key(l:diagnostic, 'code')
+            let l:loclist_item.nr = l:diagnostic.code
+        endif
+
+        call add(l:loclist, l:loclist_item)
+    endfor
+
+    return [l:filename, l:loclist]
+endfunction
diff --git a/test/lsp/test_lsp_client_messages.vader b/test/lsp/test_lsp_client_messages.vader
new file mode 100644
index 0000000..de18a4b
--- /dev/null
+++ b/test/lsp/test_lsp_client_messages.vader
@@ -0,0 +1,78 @@
+Execute(ale#lsp#message#Initialize() should return correct messages):
+  AssertEqual
+  \ [
+  \   0,
+  \   'initialize',
+  \   {
+  \    'processId': getpid(),
+  \    'rootUri': '/foo/bar',
+  \    'capabilities': {},
+  \   }
+  \ ],
+  \ ale#lsp#message#Initialize('/foo/bar')
+
+Execute(ale#lsp#message#Initialized() should return correct messages):
+  AssertEqual [1, 'initialized'], ale#lsp#message#Initialized()
+
+Execute(ale#lsp#message#Shutdown() should return correct messages):
+  AssertEqual [0, 'shutdown'], ale#lsp#message#Shutdown()
+
+Execute(ale#lsp#message#Exit() should return correct messages):
+  AssertEqual [1, 'exit'], ale#lsp#message#Exit(),
+
+Execute(ale#lsp#message#DidOpen() should return correct messages):
+  AssertEqual
+  \ [
+  \   1,
+  \   'textDocument/didOpen',
+  \   {
+  \     'textDocument': {
+  \         'uri': '/foo/bar',
+  \         'languageId': 'typescript',
+  \         'version': 123,
+  \         'text': 'foobar',
+  \     },
+  \   }
+  \ ],
+  \ ale#lsp#message#DidOpen('/foo/bar', 'typescript', 123, 'foobar')
+
+Execute(ale#lsp#message#DidChange() should return correct messages):
+  AssertEqual
+  \ [
+  \   1,
+  \   'textDocument/didChange',
+  \   {
+  \     'textDocument': {
+  \         'uri': '/foo/bar',
+  \         'version': 123,
+  \     },
+  \     'contentChanges': [{'text': 'foobar'}],
+  \   }
+  \ ],
+  \ ale#lsp#message#DidChange('/foo/bar', 123, 'foobar')
+
+Execute(ale#lsp#message#DidSave() should return correct messages):
+  AssertEqual
+  \ [
+  \   1,
+  \   'textDocument/didSave',
+  \   {
+  \     'textDocument': {
+  \         'uri': '/foo/bar',
+  \     },
+  \   }
+  \ ],
+  \ ale#lsp#message#DidSave('/foo/bar')
+
+Execute(ale#lsp#message#DidClose() should return correct messages):
+  AssertEqual
+  \ [
+  \   1,
+  \   'textDocument/didClose',
+  \   {
+  \     'textDocument': {
+  \         'uri': '/foo/bar',
+  \     },
+  \   }
+  \ ],
+  \ ale#lsp#message#DidClose('/foo/bar')
diff --git a/test/lsp/test_lsp_connections.vader b/test/lsp/test_lsp_connections.vader
new file mode 100644
index 0000000..36a21bd
--- /dev/null
+++ b/test/lsp/test_lsp_connections.vader
@@ -0,0 +1,111 @@
+Before:
+  let g:ale_lsp_next_message_id = 1
+
+After:
+  unlet! b:data
+
+Execute(GetNextMessageID() should increment appropriately):
+  " We should get the initial ID, and increment a bit.
+  AssertEqual 1, ale#lsp#GetNextMessageID()
+  AssertEqual 2, ale#lsp#GetNextMessageID()
+  AssertEqual 3, ale#lsp#GetNextMessageID()
+
+  " Set the maximum ID.
+  let g:ale_lsp_next_message_id = 9223372036854775807
+
+  " When we hit the maximum ID, the next ID afterwards should be 1.
+  AssertEqual 9223372036854775807, ale#lsp#GetNextMessageID()
+  AssertEqual 1, ale#lsp#GetNextMessageID()
+
+Execute(ale#lsp#CreateMessageData() should create an appropriate message):
+    " 71 is the size in bytes for UTF-8, not the number of characters.
+    AssertEqual
+    \ [
+    \   1,
+    \   "Content-Length: 71\r\n\r\n"
+    \   . '{"id":1,"jsonrpc":"2.0","method":"someMethod","params":{"foo":"barÜ"}}',
+    \ ],
+    \ ale#lsp#CreateMessageData([0, 'someMethod', {'foo': 'barÜ'}])
+    " Check again to ensure that we use the next ID.
+    AssertEqual
+    \ [
+    \   2,
+    \   "Content-Length: 71\r\n\r\n"
+    \   . '{"id":2,"jsonrpc":"2.0","method":"someMethod","params":{"foo":"barÜ"}}',
+    \ ],
+    \ ale#lsp#CreateMessageData([0, 'someMethod', {'foo': 'barÜ'}])
+
+Execute(ale#lsp#CreateMessageData() should create messages without params):
+    AssertEqual
+    \ [
+    \   1,
+    \   "Content-Length: 51\r\n\r\n"
+    \   . '{"id":1,"jsonrpc":"2.0","method":"someOtherMethod"}',
+    \ ],
+    \ ale#lsp#CreateMessageData([0, 'someOtherMethod'])
+
+Execute(ale#lsp#CreateMessageData() should create notifications):
+    AssertEqual
+    \ [
+    \   0,
+    \   "Content-Length: 55\r\n\r\n"
+    \   . '{"id":null,"jsonrpc":"2.0","method":"someNotification"}',
+    \ ],
+    \ ale#lsp#CreateMessageData([1, 'someNotification'])
+    AssertEqual
+    \ [
+    \   0,
+    \   "Content-Length: 78\r\n\r\n"
+    \   . '{"id":null,"jsonrpc":"2.0","method":"someNotification","params":{"foo":"bar"}}',
+    \ ],
+    \ ale#lsp#CreateMessageData([1, 'someNotification', {'foo': 'bar'}])
+
+Execute(ale#lsp#ReadMessageData() should read single whole messages):
+  AssertEqual
+  \  ['', [{'id': 2, 'jsonrpc': '2.0', 'result': {'foo': 'barÜ'}}]],
+  \ ale#lsp#ReadMessageData(
+  \   "Content-Length: 49\r\n\r\n"
+  \   . '{"id":2,"jsonrpc":"2.0","result":{"foo":"barÜ"}}'
+  \ )
+
+Execute(ale#lsp#ReadMessageData() should ignore other headers):
+  AssertEqual
+  \  ['', [{'id': 2, 'jsonrpc': '2.0', 'result': {'foo': 'barÜ'}}]],
+  \ ale#lsp#ReadMessageData(
+  \   "First-Header: 49\r\n"
+  \   . "Content-Length: 49\r\n"
+  \   . "Other-Header: 49\r\n"
+  \   . "\r\n"
+  \   . '{"id":2,"jsonrpc":"2.0","result":{"foo":"barÜ"}}'
+  \ )
+
+Execute(ale#lsp#ReadMessageData() should handle partial messages):
+  let b:data = "Content-Length: 49\r\n\r\n" . '{"id":2,"jsonrpc":"2.0","result":'
+
+  AssertEqual [b:data, []], ale#lsp#ReadMessageData(b:data)
+
+Execute(ale#lsp#ReadMessageData() should handle multiple messages):
+  AssertEqual
+  \  ['', [
+  \   {'id': 2, 'jsonrpc': '2.0', 'result': {'foo': 'barÜ'}},
+  \   {'id': 2, 'jsonrpc': '2.0', 'result': {'foo123': 'barÜ'}},
+  \ ]],
+  \ ale#lsp#ReadMessageData(
+  \   "Content-Length: 49\r\n\r\n"
+  \   . '{"id":2,"jsonrpc":"2.0","result":{"foo":"barÜ"}}'
+  \   . "Content-Length: 52\r\n\r\n"
+  \   . '{"id":2,"jsonrpc":"2.0","result":{"foo123":"barÜ"}}'
+  \ )
+
+Execute(ale#lsp#ReadMessageData() should handle a message with part of a second message):
+  let b:data = "Content-Length: 52\r\n\r\n" . '{"id":2,"jsonrpc":"2.'
+
+  AssertEqual
+  \  [b:data, [
+  \   {'id': 2, 'jsonrpc': '2.0', 'result': {'foo': 'barÜ'}},
+  \ ]],
+  \ ale#lsp#ReadMessageData(
+  \   "Content-Length: 49\r\n\r\n"
+  \   . '{"id":2,"jsonrpc":"2.0","result":{"foo":"barÜ"}}'
+  \   . b:data
+  \ )
diff --git a/test/lsp/test_read_lsp_diagnostics.vader b/test/lsp/test_read_lsp_diagnostics.vader
new file mode 100644
index 0000000..b52da1b
--- /dev/null
+++ b/test/lsp/test_read_lsp_diagnostics.vader
@@ -0,0 +1,121 @@
+Before:
+  function Range(start_line, start_char, end_line, end_char) abort
+    return {
+    \ 'start': {'line': a:start_line, 'character': a:start_char},
+    \ 'end': {'line': a:end_line, 'character': a:end_char},
+    \}
+  endfunction
+
+After:
+  delfunction Range
+
+Execute(ale#lsp#response#ReadDiagnostics() should handle errors):
+  AssertEqual ['filename.ts', [
+  \   {
+  \     'type': 'E',
+  \     'message': 'Something went wrong!',
+  \     'lnum': 3,
+  \     'col': 11,
+  \     'end_lnum': 5,
+  \     'end_col': 16,
+  \     'nr': 'some-error',
+  \   }
+  \ ]],
+  \ ale#lsp#response#ReadDiagnostics({'uri': 'filename.ts', 'diagnostics': [
+  \   {
+  \     'severity': 1,
+  \     'range': Range(2, 10, 4, 15),
+  \     'code': 'some-error',
+  \     'message': 'Something went wrong!',
+  \   },
+  \ ]})
+
+Execute(ale#lsp#response#ReadDiagnostics() should handle warnings):
+  AssertEqual ['filename.ts', [
+  \   {
+  \     'type': 'W',
+  \     'message': 'Something went wrong!',
+  \     'lnum': 2,
+  \     'col': 4,
+  \     'end_lnum': 2,
+  \     'end_col': 4,
+  \     'nr': 'some-warning',
+  \   }
+  \ ]],
+  \ ale#lsp#response#ReadDiagnostics({'uri': 'filename.ts', 'diagnostics': [
+  \   {
+  \     'severity': 2,
+  \     'range': Range(1, 3, 1, 3),
+  \     'code': 'some-warning',
+  \     'message': 'Something went wrong!',
+  \   },
+  \ ]})
+
+Execute(ale#lsp#response#ReadDiagnostics() should treat messages with missing severity as errors):
+  AssertEqual ['filename.ts', [
+  \   {
+  \     'type': 'E',
+  \     'message': 'Something went wrong!',
+  \     'lnum': 3,
+  \     'col': 11,
+  \     'end_lnum': 5,
+  \     'end_col': 16,
+  \     'nr': 'some-error',
+  \   }
+  \ ]],
+  \ ale#lsp#response#ReadDiagnostics({'uri': 'filename.ts', 'diagnostics': [
+  \   {
+  \     'range': Range(2, 10, 4, 15),
+  \     'code': 'some-error',
+  \     'message': 'Something went wrong!',
+  \   },
+  \ ]})
+
+Execute(ale#lsp#response#ReadDiagnostics() should handle messages without codes):
+  AssertEqual ['filename.ts', [
+  \   {
+  \     'type': 'E',
+  \     'message': 'Something went wrong!',
+  \     'lnum': 3,
+  \     'col': 11,
+  \     'end_lnum': 5,
+  \     'end_col': 16,
+  \   }
+  \ ]],
+  \ ale#lsp#response#ReadDiagnostics({'uri': 'filename.ts', 'diagnostics': [
+  \   {
+  \     'range': Range(2, 10, 4, 15),
+  \     'message': 'Something went wrong!',
+  \   },
+  \ ]})
+
+Execute(ale#lsp#response#ReadDiagnostics() should handle multiple messages):
+  AssertEqual ['filename.ts', [
+  \   {
+  \     'type': 'E',
+  \     'message': 'Something went wrong!',
+  \     'lnum': 1,
+  \     'col': 3,
+  \     'end_lnum': 1,
+  \     'end_col': 3,
+  \   },
+  \   {
+  \     'type': 'W',
+  \     'message': 'A warning',
+  \     'lnum': 2,
+  \     'col': 5,
+  \     'end_lnum': 2,
+  \     'end_col': 5,
+  \   },
+  \ ]],
+  \ ale#lsp#response#ReadDiagnostics({'uri': 'filename.ts', 'diagnostics': [
+  \   {
+  \     'range': Range(0, 2, 0, 2),
+  \     'message': 'Something went wrong!',
+  \   },
+  \   {
+  \     'severity': 2,
+  \     'range': Range(1, 4, 1, 4),
+  \     'message': 'A warning',
+  \   },
+  \ ]})
diff --git a/test/test_lsp_client_messages.vader b/test/test_lsp_client_messages.vader
deleted file mode 100644
index d6b398c..0000000
--- a/test/test_lsp_client_messages.vader
+++ /dev/null
@@ -1,254 +0,0 @@
-Before:
-  let g:ale_lsp_next_message_id = 1
-
-  function CheckMessage(message, expected_method_name, ...) abort
-    if a:0 > 1
-        throw 'Too many arguments!'
-    endif
-
-    let l:match = matchlist(a:message, '\v^Content-Length: (\d+)' . "\r\n\r\n" . '(.+)$')
-
-    if empty(l:match)
-      Assert 0, 'Invalid message format: ' . a:message
-    endif
-
-    if strlen(l:match[2]) < str2nr(l:match[1])
-      Assert 0, 'Invalid Content-Length (' . l:match[1] . ') :' . a:message
-    endif
-
-    let l:expected_json = {
-    \   'id': g:ale_lsp_next_message_id - 1,
-    \   'jsonrpc': '2.0',
-    \   'method': a:expected_method_name,
-    \}
-
-    if a:0 > 0
-      let l:expected_json.params = a:1
-    endif
-
-    AssertEqual l:expected_json, json_decode(l:match[2])
-  endfunction
-
-  function Range(start_line, start_char, end_line, end_char) abort
-    return {
-    \ 'start': {'line': a:start_line, 'character': a:start_char},
-    \ 'end': {'line': a:end_line, 'character': a:end_char},
-    \}
-  endfunction
-
-After:
-  delfunction CheckMessage
-  delfunction Range
-
-Execute(GetNextMessageID() should increment appropriately):
-  " We should get the initial ID, and increment a bit.
-  AssertEqual 1, ale#lsp#GetNextMessageID()
-  AssertEqual 2, ale#lsp#GetNextMessageID()
-  AssertEqual 3, ale#lsp#GetNextMessageID()
-
-  " Set the maximum ID.
-  let g:ale_lsp_next_message_id = 9223372036854775807
-
-  " When we hit the maximum ID, the next ID afterwards should be 1.
-  AssertEqual 9223372036854775807, ale#lsp#GetNextMessageID()
-  AssertEqual 1, ale#lsp#GetNextMessageID()
-
-Execute(ale#lsp#CreateMessage() should create an appropriate message):
-    " 71 is the size in bytes for UTF-8, not the number of characters.
-    AssertEqual
-    \   "Content-Length: 71\r\n\r\n"
-    \   . '{"id":1,"jsonrpc":"2.0","method":"someMethod","params":{"foo":"barÜ"}}',
-    \ ale#lsp#CreateMessage('someMethod', {'foo': 'barÜ'})
-    " Check again to ensure that we use the next ID.
-    AssertEqual
-    \   "Content-Length: 71\r\n\r\n"
-    \   . '{"id":2,"jsonrpc":"2.0","method":"someMethod","params":{"foo":"barÜ"}}',
-    \ ale#lsp#CreateMessage('someMethod', {'foo': 'barÜ'})
-
-Execute(ale#lsp#ReadMessage() should read messages correctly):
-  AssertEqual
-  \  {'id': 2, 'jsonrpc': '2.0', 'result': {'foo': 'barÜ'}},
-  \ ale#lsp#ReadMessage(
-  \   "Content-Length: 49\r\n\r\n"
-  \   . '{"id":2,"jsonrpc":"2.0","result":{"foo":"barÜ"}}'
-  \ )
-
-Execute(ale#lsp#message#Initialize() should return correct messages):
-  call CheckMessage(
-  \ ale#lsp#message#Initialize(123, '/foo/bar'),
-  \ 'initialize',
-  \ {
-  \   'processId': 123,
-  \   'rootUri': '/foo/bar',
-  \   'capabilities': {},
-  \ }
-  \)
-
-Execute(ale#lsp#message#Initialized() should return correct messages):
-  call CheckMessage(ale#lsp#message#Initialized(), 'initialized')
-
-Execute(ale#lsp#message#Shutdown() should return correct messages):
-  call CheckMessage(ale#lsp#message#Shutdown(), 'shutdown')
-
-Execute(ale#lsp#message#Exit() should return correct messages):
-  call CheckMessage(ale#lsp#message#Exit(), 'exit')
-
-Execute(ale#lsp#message#DidOpen() should return correct messages):
-  call CheckMessage(
-  \ ale#lsp#message#DidOpen('/foo/bar', 'typescript', 123, 'foobar'),
-  \ 'textDocument/didOpen',
-  \ {
-  \   'textDocument': {
-  \       'uri': '/foo/bar',
-  \       'languageId': 'typescript',
-  \       'version': 123,
-  \       'text': 'foobar',
-  \   },
-  \ }
-  \)
-
-Execute(ale#lsp#message#DidChange() should return correct messages):
-  call CheckMessage(
-  \ ale#lsp#message#DidChange('/foo/bar', 123, 'foobar'),
-  \ 'textDocument/didChange',
-  \ {
-  \   'textDocument': {
-  \       'uri': '/foo/bar',
-  \       'version': 123,
-  \   },
-  \   'contentChanges': [{'text': 'foobar'}]
-  \ }
-  \)
-
-Execute(ale#lsp#message#DidSave() should return correct messages):
-  call CheckMessage(
-  \ ale#lsp#message#DidSave('/foo/bar'),
-  \ 'textDocument/didSave',
-  \ {
-  \   'textDocument': {
-  \       'uri': '/foo/bar',
-  \   },
-  \ }
-  \)
-
-Execute(ale#lsp#message#DidClose() should return correct messages):
-  call CheckMessage(
-  \ ale#lsp#message#DidClose('/foo/bar'),
-  \ 'textDocument/didClose',
-  \ {
-  \   'textDocument': {
-  \       'uri': '/foo/bar',
-  \   },
-  \ }
-  \)
-
-Execute(ale#lsp#ReadDiagnostics() should handle errors):
-  AssertEqual ['filename.ts', [
-  \   {
-  \     'type': 'E',
-  \     'message': 'Something went wrong!',
-  \     'lnum': 3,
-  \     'col': 11,
-  \     'end_lnum': 5,
-  \     'end_col': 16,
-  \     'nr': 'some-error',
-  \   }
-  \ ]],
-  \ ale#lsp#ReadDiagnostics({'uri': 'filename.ts', 'diagnostics': [
-  \   {
-  \     'severity': 1,
-  \     'range': Range(2, 10, 4, 15),
-  \     'code': 'some-error',
-  \     'message': 'Something went wrong!',
-  \   },
-  \ ]})
-
-Execute(ale#lsp#ReadDiagnostics() should handle warnings):
-  AssertEqual ['filename.ts', [
-  \   {
-  \     'type': 'W',
-  \     'message': 'Something went wrong!',
-  \     'lnum': 2,
-  \     'col': 4,
-  \     'end_lnum': 2,
-  \     'end_col': 4,
-  \     'nr': 'some-warning',
-  \   }
-  \ ]],
-  \ ale#lsp#ReadDiagnostics({'uri': 'filename.ts', 'diagnostics': [
-  \   {
-  \     'severity': 2,
-  \     'range': Range(1, 3, 1, 3),
-  \     'code': 'some-warning',
-  \     'message': 'Something went wrong!',
-  \   },
-  \ ]})
-
-Execute(ale#lsp#ReadDiagnostics() should treat messages with missing severity as errors):
-  AssertEqual ['filename.ts', [
-  \   {
-  \     'type': 'E',
-  \     'message': 'Something went wrong!',
-  \     'lnum': 3,
-  \     'col': 11,
-  \     'end_lnum': 5,
-  \     'end_col': 16,
-  \     'nr': 'some-error',
-  \   }
-  \ ]],
-  \ ale#lsp#ReadDiagnostics({'uri': 'filename.ts', 'diagnostics': [
-  \   {
-  \     'range': Range(2, 10, 4, 15),
-  \     'code': 'some-error',
-  \     'message': 'Something went wrong!',
-  \   },
-  \ ]})
-
-Execute(ale#lsp#ReadDiagnostics() should handle messages without codes):
-  AssertEqual ['filename.ts', [
-  \   {
-  \     'type': 'E',
-  \     'message': 'Something went wrong!',
-  \     'lnum': 3,
-  \     'col': 11,
-  \     'end_lnum': 5,
-  \     'end_col': 16,
-  \   }
-  \ ]],
-  \ ale#lsp#ReadDiagnostics({'uri': 'filename.ts', 'diagnostics': [
-  \   {
-  \     'range': Range(2, 10, 4, 15),
-  \     'message': 'Something went wrong!',
-  \   },
-  \ ]})
-
-Execute(ale#lsp#ReadDiagnostics() should handle multiple messages):
-  AssertEqual ['filename.ts', [
-  \   {
-  \     'type': 'E',
-  \     'message': 'Something went wrong!',
-  \     'lnum': 1,
-  \     'col': 3,
-  \     'end_lnum': 1,
-  \     'end_col': 3,
-  \   },
-  \   {
-  \     'type': 'W',
-  \     'message': 'A warning',
-  \     'lnum': 2,
-  \     'col': 5,
-  \     'end_lnum': 2,
-  \     'end_col': 5,
-  \   },
-  \ ]],
-  \ ale#lsp#ReadDiagnostics({'uri': 'filename.ts', 'diagnostics': [
-  \   {
-  \     'range': Range(0, 2, 0, 2),
-  \     'message': 'Something went wrong!',
-  \   },
-  \   {
-  \     'severity': 2,
-  \     'range': Range(1, 4, 1, 4),
-  \     'message': 'A warning',
-  \   },
-  \ ]})