diff --git a/apps/language_server/lib/language_server/build.ex b/apps/language_server/lib/language_server/build.ex
index e8d050cc2..7830c657f 100644
--- a/apps/language_server/lib/language_server/build.ex
+++ b/apps/language_server/lib/language_server/build.ex
@@ -108,7 +108,8 @@ defmodule ElixirLS.LanguageServer.Build do
     %Mix.Task.Compiler.Diagnostic{
       compiler_name: "ElixirLS",
       file: Path.absname(System.get_env("MIX_EXS") || "mix.exs"),
-      position: nil,
+      # 0 means unknown
+      position: 0,
       message: msg,
       severity: :error,
       details: error
@@ -278,17 +279,15 @@ defmodule ElixirLS.LanguageServer.Build do
     :ok
   end
 
-  defp range(position, nil) when is_integer(position) do
-    line = position - 1
-
-    # we don't care about utf16 positions here as we send 0
-    %{
-      "start" => %{"line" => line, "character" => 0},
-      "end" => %{"line" => line, "character" => 0}
-    }
-  end
+  # for details see
+  # https://hexdocs.pm/mix/1.13.4/Mix.Task.Compiler.Diagnostic.html#t:position/0
+  # https://microsoft.github.io/language-server-protocol/specifications/specification-3-16/#diagnostic
 
-  defp range(position, source_file) when is_integer(position) do
+  # position is a 1 based line number
+  # we return a range of trimmed text in that line
+  defp range(position, source_file)
+       when is_integer(position) and position >= 1 and is_binary(source_file) do
+    # line is 1 based
     line = position - 1
     text = Enum.at(SourceFile.lines(source_file), line) || ""
 
@@ -307,12 +306,64 @@ defmodule ElixirLS.LanguageServer.Build do
     }
   end
 
-  defp range(_, nil) do
-    # we don't care about utf16 positions here as we send 0
-    %{"start" => %{"line" => 0, "character" => 0}, "end" => %{"line" => 0, "character" => 0}}
+  # position is a 1 based line number and 0 based character cursor (UTF8)
+  # we return a 0 length range exactly at that location
+  defp range({line_start, char_start}, source_file)
+       when line_start >= 1 and is_binary(source_file) do
+    lines = SourceFile.lines(source_file)
+    # line is 1 based
+    start_line = Enum.at(lines, line_start - 1)
+    # SourceFile.elixir_character_to_lsp assumes char to be 1 based but it's 0 based bere
+    character = SourceFile.elixir_character_to_lsp(start_line, char_start + 1)
+
+    %{
+      "start" => %{
+        "line" => line_start - 1,
+        "character" => character
+      },
+      "end" => %{
+        "line" => line_start - 1,
+        "character" => character
+      }
+    }
   end
 
-  defp range(_, source_file) do
+  # position is a range defined by 1 based line numbers and 0 based character cursors (UTF8)
+  # we return exactly that range
+  defp range({line_start, char_start, line_end, char_end}, source_file)
+       when line_start >= 1 and line_end >= 1 and is_binary(source_file) do
+    lines = SourceFile.lines(source_file)
+    # line is 1 based
+    start_line = Enum.at(lines, line_start - 1)
+    end_line = Enum.at(lines, line_end - 1)
+
+    # SourceFile.elixir_character_to_lsp assumes char to be 1 based but it's 0 based bere
+    start_char = SourceFile.elixir_character_to_lsp(start_line, char_start + 1)
+    end_char = SourceFile.elixir_character_to_lsp(end_line, char_end + 1)
+
+    %{
+      "start" => %{
+        "line" => line_start - 1,
+        "character" => start_char
+      },
+      "end" => %{
+        "line" => line_end - 1,
+        "character" => end_char
+      }
+    }
+  end
+
+  # position is 0 which means unknown
+  # we return the full file range
+  defp range(0, source_file) when is_binary(source_file) do
     SourceFile.full_range(source_file)
   end
+
+  # source file is unknown
+  # we discard any position information as it is meaningless
+  # unfortunately LSP does not allow `null` range so we need to return something
+  defp range(_, nil) do
+    # we don't care about utf16 positions here as we send 0
+    %{"start" => %{"line" => 0, "character" => 0}, "end" => %{"line" => 0, "character" => 0}}
+  end
 end
diff --git a/apps/language_server/lib/language_server/server.ex b/apps/language_server/lib/language_server/server.ex
index c9bf9261f..d79aaad27 100644
--- a/apps/language_server/lib/language_server/server.ex
+++ b/apps/language_server/lib/language_server/server.ex
@@ -1043,13 +1043,29 @@ defmodule ElixirLS.LanguageServer.Server do
     Dialyzer.check_support() == :ok and build_enabled?(state) and state.dialyzer_sup != nil
   end
 
+  defp safely_read_file(file) do
+    case File.read(file) do
+      {:ok, text} ->
+        text
+
+      {:error, reason} ->
+        IO.warn("Couldn't read file #{file}: #{inspect(reason)}")
+        nil
+    end
+  end
+
   defp publish_diagnostics(new_diagnostics, old_diagnostics, source_files) do
     files =
       Enum.uniq(Enum.map(new_diagnostics, & &1.file) ++ Enum.map(old_diagnostics, & &1.file))
 
     for file <- files,
         uri = SourceFile.path_to_uri(file),
-        do: Build.publish_file_diagnostics(uri, new_diagnostics, Map.get(source_files, uri))
+        do:
+          Build.publish_file_diagnostics(
+            uri,
+            new_diagnostics,
+            Map.get_lazy(source_files, uri, fn -> safely_read_file(file) end)
+          )
   end
 
   defp show_version_warnings do
diff --git a/apps/language_server/test/dialyzer_test.exs b/apps/language_server/test/dialyzer_test.exs
index 082a3f06b..fa4d516af 100644
--- a/apps/language_server/test/dialyzer_test.exs
+++ b/apps/language_server/test/dialyzer_test.exs
@@ -41,8 +41,8 @@ defmodule ElixirLS.LanguageServer.DialyzerTest do
                  %{
                    "message" => error_message1,
                    "range" => %{
-                     "end" => %{"character" => 0, "line" => _},
-                     "start" => %{"character" => 0, "line" => _}
+                     "end" => %{"character" => 12, "line" => 1},
+                     "start" => %{"character" => 2, "line" => 1}
                    },
                    "severity" => 2,
                    "source" => "ElixirLS Dialyzer"
@@ -50,8 +50,8 @@ defmodule ElixirLS.LanguageServer.DialyzerTest do
                  %{
                    "message" => error_message2,
                    "range" => %{
-                     "end" => %{"character" => 0, "line" => _},
-                     "start" => %{"character" => 0, "line" => _}
+                     "end" => %{"character" => 17, "line" => 2},
+                     "start" => %{"character" => 4, "line" => 2}
                    },
                    "severity" => 2,
                    "source" => "ElixirLS Dialyzer"
@@ -175,8 +175,8 @@ defmodule ElixirLS.LanguageServer.DialyzerTest do
                  %{
                    "message" => error_message1,
                    "range" => %{
-                     "end" => %{"character" => 0, "line" => _},
-                     "start" => %{"character" => 0, "line" => _}
+                     "end" => %{"character" => 12, "line" => 1},
+                     "start" => %{"character" => 2, "line" => 1}
                    },
                    "severity" => 2,
                    "source" => "ElixirLS Dialyzer"
@@ -184,8 +184,8 @@ defmodule ElixirLS.LanguageServer.DialyzerTest do
                  %{
                    "message" => error_message2,
                    "range" => %{
-                     "end" => %{"character" => 0, "line" => _},
-                     "start" => %{"character" => 0, "line" => _}
+                     "end" => %{"character" => 17, "line" => 2},
+                     "start" => %{"character" => 4, "line" => 2}
                    },
                    "severity" => 2,
                    "source" => "ElixirLS Dialyzer"
@@ -229,8 +229,8 @@ defmodule ElixirLS.LanguageServer.DialyzerTest do
                  %{
                    "message" => error_message1,
                    "range" => %{
-                     "end" => %{"character" => 0, "line" => _},
-                     "start" => %{"character" => 0, "line" => _}
+                     "end" => %{"character" => 12, "line" => 1},
+                     "start" => %{"character" => 2, "line" => 1}
                    },
                    "severity" => 2,
                    "source" => "ElixirLS Dialyzer"
@@ -238,8 +238,8 @@ defmodule ElixirLS.LanguageServer.DialyzerTest do
                  %{
                    "message" => error_message2,
                    "range" => %{
-                     "end" => %{"character" => 0, "line" => _},
-                     "start" => %{"character" => 0, "line" => _}
+                     "end" => %{"character" => 17, "line" => 2},
+                     "start" => %{"character" => 4, "line" => 2}
                    },
                    "severity" => 2,
                    "source" => "ElixirLS Dialyzer"
@@ -274,8 +274,8 @@ defmodule ElixirLS.LanguageServer.DialyzerTest do
                  %{
                    "message" => error_message1,
                    "range" => %{
-                     "end" => %{"character" => 0, "line" => _},
-                     "start" => %{"character" => 0, "line" => _}
+                     "end" => %{"character" => 12, "line" => 1},
+                     "start" => %{"character" => 2, "line" => 1}
                    },
                    "severity" => 2,
                    "source" => "ElixirLS Dialyzer"
@@ -283,8 +283,8 @@ defmodule ElixirLS.LanguageServer.DialyzerTest do
                  %{
                    "message" => _error_message2,
                    "range" => %{
-                     "end" => %{"character" => 0, "line" => _},
-                     "start" => %{"character" => 0, "line" => _}
+                     "end" => %{"character" => 17, "line" => 2},
+                     "start" => %{"character" => 4, "line" => 2}
                    },
                    "severity" => 2,
                    "source" => "ElixirLS Dialyzer"
@@ -320,8 +320,8 @@ defmodule ElixirLS.LanguageServer.DialyzerTest do
                  %{
                    "message" => error_message1,
                    "range" => %{
-                     "end" => %{"character" => 0, "line" => _},
-                     "start" => %{"character" => 0, "line" => _}
+                     "end" => %{"character" => 22, "line" => 1},
+                     "start" => %{"character" => 2, "line" => 1}
                    },
                    "severity" => 2,
                    "source" => "ElixirLS Dialyzer"
@@ -329,8 +329,8 @@ defmodule ElixirLS.LanguageServer.DialyzerTest do
                  %{
                    "message" => error_message2,
                    "range" => %{
-                     "end" => %{"character" => 0, "line" => _},
-                     "start" => %{"character" => 0, "line" => _}
+                     "end" => %{"character" => 22, "line" => 2},
+                     "start" => %{"character" => 4, "line" => 2}
                    },
                    "severity" => 2,
                    "source" => "ElixirLS Dialyzer"