diff --git a/vlib/v/builder/builder.v b/vlib/v/builder/builder.v
index e66eeb92eba60c..3fc9939a2c56f5 100644
--- a/vlib/v/builder/builder.v
+++ b/vlib/v/builder/builder.v
@@ -538,7 +538,7 @@ pub fn (mut b Builder) print_warnings_and_errors() {
 				if b.pref.json_errors {
 					json_errors << util.JsonError{
 						message: err.message
-						path:    err.file_path
+						path:    os.to_slash(err.file_path)
 						line_nr: err.pos.line_nr + 1
 						col:     err.pos.col + 1
 					}
diff --git a/vlib/v/tests/vls/autocomplete_module_test.v b/vlib/v/tests/vls/autocomplete_module_test.v
index 79f3ceccb03d52..4beceb4bd1d137 100644
--- a/vlib/v/tests/vls/autocomplete_module_test.v
+++ b/vlib/v/tests/vls/autocomplete_module_test.v
@@ -1,10 +1,13 @@
 import os
 import term
 import v.util.diff
+import json
 
 const vroot = os.real_path(@VMODROOT)
 const tmp_dir = os.real_path(os.temp_dir())
 const text_file = os.join_path(vroot, 'vlib', 'v', 'tests', 'vls', 'sample_text.vv')
+// note: windows path separator will cause json decode fail
+const json_errors_text_file = os.to_slash(text_file)
 const mod1_text_file = os.join_path(vroot, 'vlib', 'v', 'tests', 'vls', 'sample_mod1',
 	'sample.v')
 
@@ -45,69 +48,99 @@ const fn_signature_info_for_all_before_last = '{
 }
 '
 
+enum Method {
+	unknown         @['unknown']
+	initialize      @['initialize']
+	initialized     @['initialized']
+	did_open        @['textDocument/didOpen']
+	did_change      @['textDocument/didChange']
+	definition      @['textDocument/definition']
+	completion      @['textDocument/completion']
+	signature_help  @['textDocument/signatureHelp']
+	set_trace       @['$/setTrace']
+	cancel_request  @['$/cancelRequest']
+	shutdown        @['shutdown']
+	exit            @['exit']
+}
+
 struct TestData {
+	method Method
 	cmd    string
 	output string
 }
 
 const test_data = [
 	TestData{
+		method: .completion
 		cmd:    'v -w -check -json-errors -nocolor -vls-mode -line-info "${text_file}:19:3" ${os.quoted_path(text_file)}'
 		output: autocomplete_info_for_mod_sample_mod1
 	},
 	TestData{
+		method: .completion
 		cmd:    'v -w -check -json-errors -nocolor -vls-mode -line-info "${text_file}:20:13" ${os.quoted_path(text_file)}'
 		output: autocomplete_info_for_mod_sample_mod2
 	},
 	TestData{
+		method: .completion
 		cmd:    'v -w -check -json-errors -nocolor -vls-mode -line-info "${text_file}:22:3" ${os.quoted_path(text_file)}'
 		output: autocomplete_info_for_mod_struct
 	},
 	TestData{
+		method: .completion
 		cmd:    'v -w -check -json-errors -nocolor -vls-mode -line-info "${text_file}:23:3" ${os.quoted_path(text_file)}'
 		output: autocomplete_info_for_mod_sample_mod1
 	},
 	TestData{
+		method: .completion
 		cmd:    'v -w -check -json-errors -nocolor -vls-mode -line-info "${text_file}:26:28" ${os.quoted_path(text_file)}'
 		output: autocomplete_info_for_mod_sample_mod1
 	},
 	TestData{
+		method: .signature_help
 		cmd:    'v -w -check -json-errors -nocolor -vls-mode -line-info "${text_file}:25:fn^26" ${os.quoted_path(text_file)}'
 		output: fn_signature_info_for_all_before_last
 	},
 	TestData{
+		method: .completion
 		cmd:    'v -w -check -json-errors -nocolor -vls-mode -line-info "${text_file}:27:9" ${os.quoted_path(text_file)}'
 		output: ''
 	},
 	TestData{
+		method: .completion
 		cmd:    'v -w -check -json-errors -nocolor -vls-mode -line-info "${text_file}:28:9" ${os.quoted_path(text_file)}'
 		output: 'unresolved type, maybe "builtin" was not defined. otherwise this is a bug, should never happen; please report'
 	},
 	TestData{
+		method: .definition
 		cmd:    'v -w -check -json-errors -nocolor -vls-mode -line-info "${text_file}:30:gd^10" ${os.quoted_path(text_file)}'
 		output: '${mod1_text_file}:50:7'
 	},
 	TestData{
+		method: .definition
 		cmd:    'v -w -check -json-errors -nocolor -vls-mode -line-info "${text_file}:31:gd^12" ${os.quoted_path(text_file)}'
 		output: '${mod1_text_file}:8:11'
 	},
 	TestData{
+		method: .definition
 		cmd:    'v -w -check -json-errors -nocolor -vls-mode -line-info "${text_file}:32:gd^11" ${os.quoted_path(text_file)}'
 		output: '${mod1_text_file}:41:9'
 	},
 	TestData{
+		method: .definition
 		cmd:    'v -w -check -json-errors -nocolor -vls-mode -line-info "${text_file}:33:gd^15" ${os.quoted_path(text_file)}'
 		output: '${mod1_text_file}:44:9'
 	},
 	TestData{
+		method: .definition
 		cmd:    'v -w -check -json-errors -nocolor -vls-mode -line-info "${text_file}:34:gd^13" ${os.quoted_path(text_file)}'
 		output: '${mod1_text_file}:19:10'
 	},
 	TestData{
+		method: .did_change
 		cmd:    'v -w -vls-mode -check -json-errors ${os.quoted_path(text_file)}'
 		output: '[
 {
-"path":"${text_file}",
+"path":"${json_errors_text_file}",
 "message":"unexpected token `:=`, expecting `)`",
 "line_nr":26,
 "col":4,
@@ -115,7 +148,7 @@ const test_data = [
 }
 ,
 {
-"path":"${text_file}",
+"path":"${json_errors_text_file}",
 "message":"unexpected name `strings`, expecting `)`",
 "line_nr":27,
 "col":2,
@@ -123,7 +156,7 @@ const test_data = [
 }
 ,
 {
-"path":"${text_file}",
+"path":"${json_errors_text_file}",
 "message":"undefined ident: ``",
 "line_nr":19,
 "col":3,
@@ -131,7 +164,7 @@ const test_data = [
 }
 ,
 {
-"path":"${text_file}",
+"path":"${json_errors_text_file}",
 "message":"undefined ident: ``",
 "line_nr":20,
 "col":13,
@@ -139,7 +172,7 @@ const test_data = [
 }
 ,
 {
-"path":"${text_file}",
+"path":"${json_errors_text_file}",
 "message":"undefined ident: ``",
 "line_nr":23,
 "col":3,
@@ -147,7 +180,7 @@ const test_data = [
 }
 ,
 {
-"path":"${text_file}",
+"path":"${json_errors_text_file}",
 "message":"undefined ident: `j`",
 "line_nr":26,
 "col":2,
@@ -155,7 +188,7 @@ const test_data = [
 }
 ,
 {
-"path":"${text_file}",
+"path":"${json_errors_text_file}",
 "message":"`j` (no value) used as value in argument 1 to `string.all_before_last`",
 "line_nr":26,
 "col":2,
@@ -163,7 +196,7 @@ const test_data = [
 }
 ,
 {
-"path":"${text_file}",
+"path":"${json_errors_text_file}",
 "message":"undefined ident: ``",
 "line_nr":26,
 "col":28,
@@ -171,7 +204,7 @@ const test_data = [
 }
 ,
 {
-"path":"${text_file}",
+"path":"${json_errors_text_file}",
 "message":"`` (no value) used as value in argument 1 to `string.all_before_last`",
 "line_nr":26,
 "col":27,
@@ -179,7 +212,7 @@ const test_data = [
 }
 ,
 {
-"path":"${text_file}",
+"path":"${json_errors_text_file}",
 "message":"`string` has no property ``",
 "line_nr":26,
 "col":11,
@@ -187,7 +220,7 @@ const test_data = [
 }
 ,
 {
-"path":"${text_file}",
+"path":"${json_errors_text_file}",
 "message":"undefined ident: `builtin`",
 "line_nr":28,
 "col":2,
@@ -195,7 +228,7 @@ const test_data = [
 }
 ,
 {
-"path":"${text_file}",
+"path":"${json_errors_text_file}",
 "message":"`builtin` does not return a value",
 "line_nr":28,
 "col":2,
@@ -206,6 +239,43 @@ const test_data = [
 	},
 ]
 
+// copy from `vls`
+struct JsonError {
+	path    string
+	message string
+	line_nr int
+	col     int
+	len     int
+}
+
+struct Detail {
+	kind               int    // The type of item (e.g., Method, Function, Field)
+	label              string // The name of the completion item
+	detail             string // Additional info like the function signature or return type
+	documentation      string // The documentation for the item
+	insert_text        ?string @[json: 'insertText']
+	insert_text_format ?int    @[json: 'insertTextFormat'] // 1 for PlainText, 2 for Snippet
+}
+
+struct JsonVarAC {
+	details []Detail
+}
+
+struct SignatureHelp {
+	signatures       []SignatureInformation
+	active_signature int @[json: 'activeSignature']
+	active_parameter int @[json: 'activeParameter']
+}
+
+struct SignatureInformation {
+	label      string
+	parameters []ParameterInformation
+}
+
+struct ParameterInformation {
+	label string
+}
+
 fn test_main() {
 	mut total_errors := 0
 
@@ -236,6 +306,90 @@ fn test_main() {
 		} else {
 			println('${term.green('OK  ')} ${t.cmd}')
 		}
+
+		// Try to decode the response message and verify
+		// TODO: remove `unresolved type, maybe`
+		if t.output.trim_space().len > 0 && !t.output.starts_with('unresolved type, maybe') {
+			dump(t.output)
+			match t.method {
+				.definition {
+					check_valid_goto_definition(t.output)!
+				}
+				.completion {
+					check_valid_auto_completion(t.output)!
+				}
+				.did_change {
+					check_valid_json_errors(t.output)!
+				}
+				.signature_help {
+					check_valid_fn_signature(t.output)!
+				}
+				else {}
+			}
+		}
 	}
 	assert total_errors == 0
 }
+
+fn check_valid_goto_definition(message string) ! {
+	// `/home/path/aaa.v:19:10`
+	fields := message.split(':')
+	if fields.len >= 3 {
+		path := fields[..fields.len - 2].join(':')
+		line_nr := fields[fields.len - 2].int()
+		col := fields[fields.len - 1].int()
+		if line_nr <= 0 {
+			return error('goto_definition: line_nr should > 0: ${line_nr}')
+		}
+		if col <= 0 {
+			return error('goto_definition: col should > 0: ${col}')
+		}
+		if path.len == 0 {
+			return error('goto_definition: file.len should > 0: ${path}')
+		}
+	} else {
+		return error('goto_definition: goto_definition format error')
+	}
+}
+
+fn check_valid_auto_completion(message string) ! {
+	// {"kind":5,"label":"a","detail":"int","documentation":""},
+	result := json.decode(JsonVarAC, message) or { return error('completion: fail to json decode') }
+	for detail in result.details {
+		if detail.kind <= 0 || detail.kind > 25 {
+			return error('completion: kind should in 1-25 : ${detail.kind}')
+		}
+	}
+}
+
+fn check_valid_json_errors(message string) ! {
+	results := json.decode([]JsonError, message) or {
+		return error('json_errors: fail to json decode')
+	}
+	for result in results {
+		if result.path.len == 0 {
+			return error('json_errors: path.len should > 0')
+		}
+		if result.message.len == 0 {
+			return error('json_errors: message.len should > 0')
+		}
+		if result.line_nr <= 0 {
+			return error('json_errors: line_nr should > 0')
+		}
+		if result.col <= 0 {
+			return error('json_errors: col should > 0')
+		}
+	}
+}
+
+fn check_valid_fn_signature(message string) ! {
+	result := json.decode(SignatureHelp, message) or {
+		return error('fn_signature: fail to json decode')
+	}
+	if result.signatures.len != 1 {
+		return error('fn_signature: signatures.len != 1')
+	}
+	if result.signatures[0].label.len == 0 {
+		return error('fn_signature: label.len == 0')
+	}
+}