diff --git a/src/ColumnCalculator.php b/src/ColumnCalculator.php
index 87dddcfc..90502640 100644
--- a/src/ColumnCalculator.php
+++ b/src/ColumnCalculator.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types = 1);
 
 namespace LanguageServer;
 
diff --git a/src/LanguageServer.php b/src/LanguageServer.php
index 04206fd8..83c2167e 100644
--- a/src/LanguageServer.php
+++ b/src/LanguageServer.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types = 1);
 
 namespace LanguageServer;
 
diff --git a/src/PhpDocument.php b/src/PhpDocument.php
index 424afd37..b97f6c35 100644
--- a/src/PhpDocument.php
+++ b/src/PhpDocument.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types = 1);
 
 namespace LanguageServer;
 
diff --git a/src/Project.php b/src/Project.php
index d62a8cf5..50632157 100644
--- a/src/Project.php
+++ b/src/Project.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types = 1);
 
 namespace LanguageServer;
 
diff --git a/src/Server/TextDocument.php b/src/Server/TextDocument.php
index 38c8ea31..ba7f482c 100644
--- a/src/Server/TextDocument.php
+++ b/src/Server/TextDocument.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types = 1);
 
 namespace LanguageServer\Server;
 
diff --git a/src/Server/Workspace.php b/src/Server/Workspace.php
index 36248445..cbd0da44 100644
--- a/src/Server/Workspace.php
+++ b/src/Server/Workspace.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types = 1);
 
 namespace LanguageServer\Server;
 
diff --git a/src/utils.php b/src/utils.php
index 8d45cab0..3086efdb 100644
--- a/src/utils.php
+++ b/src/utils.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types = 1);
 
 namespace LanguageServer;