diff --git a/Userland/DevTools/HackStudio/LanguageServers/Cpp/CppComprehensionEngine.cpp b/Userland/DevTools/HackStudio/LanguageServers/Cpp/CppComprehensionEngine.cpp
index fd07b4f370..84c776faff 100644
--- a/Userland/DevTools/HackStudio/LanguageServers/Cpp/CppComprehensionEngine.cpp
+++ b/Userland/DevTools/HackStudio/LanguageServers/Cpp/CppComprehensionEngine.cpp
@@ -569,11 +569,16 @@ OwnPtr<CppComprehensionEngine::DocumentData> CppComprehensionEngine::create_docu
     document_data->m_preprocessor = make<Preprocessor>(document_data->m_filename, document_data->text());
     document_data->preprocessor().set_ignore_unsupported_keywords(true);
     document_data->preprocessor().set_keep_include_statements(true);
-    auto tokens = document_data->preprocessor().process_and_lex();
 
-    Preprocessor::Definitions preprocessor_definitions;
-    for (auto item : document_data->preprocessor().definitions())
-        preprocessor_definitions.set(move(item.key), move(item.value));
+    document_data->preprocessor().definitions_in_header_callback = [this](StringView include_path) -> Preprocessor::Definitions {
+        auto included_document = get_or_create_document_data(document_path_from_include_path(include_path));
+        if (!included_document)
+            return {};
+
+        return included_document->preprocessor().definitions();
+    };
+
+    auto tokens = document_data->preprocessor().process_and_lex();
 
     for (auto include_path : document_data->preprocessor().included_paths()) {
         auto include_fullpath = document_path_from_include_path(include_path);
@@ -585,12 +590,9 @@ OwnPtr<CppComprehensionEngine::DocumentData> CppComprehensionEngine::create_docu
 
         for (auto& header : included_document->m_available_headers)
             document_data->m_available_headers.set(header);
-
-        for (auto item : included_document->parser().preprocessor_definitions())
-            preprocessor_definitions.set(move(item.key), move(item.value));
     }
 
-    document_data->m_parser = make<Parser>(move(tokens), filename, move(preprocessor_definitions));
+    document_data->m_parser = make<Parser>(move(tokens), filename, document_data->preprocessor().definitions());
 
     auto root = document_data->parser().parse();
 
diff --git a/Userland/Libraries/LibCpp/Parser.cpp b/Userland/Libraries/LibCpp/Parser.cpp
index da0d479fda..4bcc5ecd67 100644
--- a/Userland/Libraries/LibCpp/Parser.cpp
+++ b/Userland/Libraries/LibCpp/Parser.cpp
@@ -15,7 +15,7 @@
 
 namespace Cpp {
 
-Parser::Parser(Vector<Token> const& tokens, const String& filename, Preprocessor::Definitions&& definitions)
+Parser::Parser(Vector<Token> const& tokens, const String& filename, Preprocessor::Definitions const& definitions)
     : m_preprocessor_definitions(move(definitions))
     , m_filename(filename)
 {
diff --git a/Userland/Libraries/LibCpp/Parser.h b/Userland/Libraries/LibCpp/Parser.h
index b21eca15bc..34e70bd7a6 100644
--- a/Userland/Libraries/LibCpp/Parser.h
+++ b/Userland/Libraries/LibCpp/Parser.h
@@ -18,7 +18,7 @@ class Parser final {
     AK_MAKE_NONCOPYABLE(Parser);
 
 public:
-    explicit Parser(Vector<Token> const& tokens, const String& filename, Preprocessor::Definitions&& = {});
+    explicit Parser(Vector<Token> const& tokens, const String& filename, Preprocessor::Definitions const& = {});
     ~Parser() = default;
 
     NonnullRefPtr<TranslationUnit> parse();
diff --git a/Userland/Libraries/LibCpp/Preprocessor.cpp b/Userland/Libraries/LibCpp/Preprocessor.cpp
index 85d5f143e4..cb7be70f7a 100644
--- a/Userland/Libraries/LibCpp/Preprocessor.cpp
+++ b/Userland/Libraries/LibCpp/Preprocessor.cpp
@@ -108,7 +108,12 @@ void Preprocessor::handle_preprocessor_keyword(const StringView& keyword, Generi
 {
     if (keyword == "include") {
         consume_whitespace(line_lexer);
-        m_included_paths.append(line_lexer.consume_all());
+        auto include_path = line_lexer.consume_all();
+        m_included_paths.append(include_path);
+        if (definitions_in_header_callback) {
+            for (auto& def : definitions_in_header_callback(include_path))
+                m_definitions.set(def.key, def.value);
+        }
         return;
     }
 
diff --git a/Userland/Libraries/LibCpp/Preprocessor.h b/Userland/Libraries/LibCpp/Preprocessor.h
index e3dd45aa40..6459a7582c 100644
--- a/Userland/Libraries/LibCpp/Preprocessor.h
+++ b/Userland/Libraries/LibCpp/Preprocessor.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/FlyString.h>
+#include <AK/Function.h>
 #include <AK/HashMap.h>
 #include <AK/Optional.h>
 #include <AK/String.h>
@@ -36,6 +37,8 @@ public:
     void set_ignore_unsupported_keywords(bool ignore) { m_options.ignore_unsupported_keywords = ignore; }
     void set_keep_include_statements(bool keep) { m_options.keep_include_statements = keep; }
 
+    Function<Definitions(StringView)> definitions_in_header_callback { nullptr };
+
 private:
     using PreprocessorKeyword = StringView;
     PreprocessorKeyword handle_preprocessor_line(StringView const&);