diff --git a/src/aro/Builtins.zig b/src/aro/Builtins.zig
index 795c5dcb..eb67a2f3 100644
--- a/src/aro/Builtins.zig
+++ b/src/aro/Builtins.zig
@@ -350,7 +350,7 @@ test Iterator {
 }
 
 test "All builtins" {
-    var comp = Compilation.init(std.testing.allocator);
+    var comp = Compilation.init(std.testing.allocator, std.fs.cwd());
     defer comp.deinit();
     _ = try comp.generateBuiltinMacros(.include_system_defines);
     var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
@@ -373,7 +373,7 @@ test "All builtins" {
 test "Allocation failures" {
     const Test = struct {
         fn testOne(allocator: std.mem.Allocator) !void {
-            var comp = Compilation.init(allocator);
+            var comp = Compilation.init(allocator, std.fs.cwd());
             defer comp.deinit();
             _ = try comp.generateBuiltinMacros(.include_system_defines);
             var arena = std.heap.ArenaAllocator.init(comp.gpa);
diff --git a/src/aro/Compilation.zig b/src/aro/Compilation.zig
index 671939a5..f94e72e2 100644
--- a/src/aro/Compilation.zig
+++ b/src/aro/Compilation.zig
@@ -127,22 +127,27 @@ types: struct {
 } = .{},
 string_interner: StrInt = .{},
 interner: Interner = .{},
+/// If this is not null, the directory containing the specified Source will be searched for includes
+/// Used by MS extensions which allow searching for includes relative to the directory of the main source file.
 ms_cwd_source_id: ?Source.Id = null,
+cwd: std.fs.Dir,
 
-pub fn init(gpa: Allocator) Compilation {
+pub fn init(gpa: Allocator, cwd: std.fs.Dir) Compilation {
     return .{
         .gpa = gpa,
         .diagnostics = Diagnostics.init(gpa),
+        .cwd = cwd,
     };
 }
 
 /// Initialize Compilation with default environment,
 /// pragma handlers and emulation mode set to target.
-pub fn initDefault(gpa: Allocator) !Compilation {
+pub fn initDefault(gpa: Allocator, cwd: std.fs.Dir) !Compilation {
     var comp: Compilation = .{
         .gpa = gpa,
         .environment = try Environment.loadAll(gpa),
         .diagnostics = Diagnostics.init(gpa),
+        .cwd = cwd,
     };
     errdefer comp.deinit();
     try comp.addDefaultPragmaHandlers();
@@ -1090,7 +1095,7 @@ pub fn getCharSignedness(comp: *const Compilation) std.builtin.Signedness {
 pub fn addBuiltinIncludeDir(comp: *Compilation, aro_dir: []const u8) !void {
     var search_path = aro_dir;
     while (std.fs.path.dirname(search_path)) |dirname| : (search_path = dirname) {
-        var base_dir = std.fs.cwd().openDir(dirname, .{}) catch continue;
+        var base_dir = comp.cwd.openDir(dirname, .{}) catch continue;
         defer base_dir.close();
 
         base_dir.access("include/stddef.h", .{}) catch continue;
@@ -1296,7 +1301,7 @@ fn addSourceFromPathExtra(comp: *Compilation, path: []const u8, kind: Source.Kin
         return error.FileNotFound;
     }
 
-    const file = try std.fs.cwd().openFile(path, .{});
+    const file = try comp.cwd.openFile(path, .{});
     defer file.close();
 
     const contents = file.readToEndAlloc(comp.gpa, std.math.maxInt(u32)) catch |err| switch (err) {
@@ -1379,10 +1384,9 @@ pub fn hasInclude(
         return false;
     }
 
-    const cwd = std.fs.cwd();
     if (std.fs.path.isAbsolute(filename)) {
         if (which == .next) return false;
-        return !std.meta.isError(cwd.access(filename, .{}));
+        return !std.meta.isError(comp.cwd.access(filename, .{}));
     }
 
     const cwd_source_id = switch (include_type) {
@@ -1402,7 +1406,7 @@ pub fn hasInclude(
 
     while (try it.nextWithFile(filename, sf_allocator)) |found| {
         defer sf_allocator.free(found.path);
-        if (!std.meta.isError(cwd.access(found.path, .{}))) return true;
+        if (!std.meta.isError(comp.cwd.access(found.path, .{}))) return true;
     }
     return false;
 }
@@ -1422,7 +1426,7 @@ fn getFileContents(comp: *Compilation, path: []const u8, limit: ?u32) ![]const u
         return error.FileNotFound;
     }
 
-    const file = try std.fs.cwd().openFile(path, .{});
+    const file = try comp.cwd.openFile(path, .{});
     defer file.close();
 
     var buf = std.ArrayList(u8).init(comp.gpa);
@@ -1620,7 +1624,7 @@ pub const addDiagnostic = Diagnostics.add;
 test "addSourceFromReader" {
     const Test = struct {
         fn addSourceFromReader(str: []const u8, expected: []const u8, warning_count: u32, splices: []const u32) !void {
-            var comp = Compilation.init(std.testing.allocator);
+            var comp = Compilation.init(std.testing.allocator, std.fs.cwd());
             defer comp.deinit();
 
             var buf_reader = std.io.fixedBufferStream(str);
@@ -1632,7 +1636,7 @@ test "addSourceFromReader" {
         }
 
         fn withAllocationFailures(allocator: std.mem.Allocator) !void {
-            var comp = Compilation.init(allocator);
+            var comp = Compilation.init(allocator, std.fs.cwd());
             defer comp.deinit();
 
             _ = try comp.addSourceFromBuffer("path", "spliced\\\nbuffer\n");
@@ -1674,7 +1678,7 @@ test "addSourceFromReader - exhaustive check for carriage return elimination" {
     const alen = alphabet.len;
     var buf: [alphabet.len]u8 = [1]u8{alphabet[0]} ** alen;
 
-    var comp = Compilation.init(std.testing.allocator);
+    var comp = Compilation.init(std.testing.allocator, std.fs.cwd());
     defer comp.deinit();
 
     var source_count: u32 = 0;
@@ -1702,7 +1706,7 @@ test "ignore BOM at beginning of file" {
 
     const Test = struct {
         fn run(buf: []const u8) !void {
-            var comp = Compilation.init(std.testing.allocator);
+            var comp = Compilation.init(std.testing.allocator, std.fs.cwd());
             defer comp.deinit();
 
             var buf_reader = std.io.fixedBufferStream(buf);
diff --git a/src/aro/Preprocessor.zig b/src/aro/Preprocessor.zig
index 3fd98882..adb034af 100644
--- a/src/aro/Preprocessor.zig
+++ b/src/aro/Preprocessor.zig
@@ -3383,7 +3383,7 @@ test "Preserve pragma tokens sometimes" {
             var buf = std.ArrayList(u8).init(allocator);
             defer buf.deinit();
 
-            var comp = Compilation.init(allocator);
+            var comp = Compilation.init(allocator, std.fs.cwd());
             defer comp.deinit();
 
             try comp.addDefaultPragmaHandlers();
@@ -3443,7 +3443,7 @@ test "destringify" {
             try std.testing.expectEqualStrings(destringified, pp.char_buf.items);
         }
     };
-    var comp = Compilation.init(allocator);
+    var comp = Compilation.init(allocator, std.fs.cwd());
     defer comp.deinit();
     var pp = Preprocessor.init(&comp);
     defer pp.deinit();
@@ -3501,7 +3501,7 @@ test "Include guards" {
         }
 
         fn testIncludeGuard(allocator: std.mem.Allocator, comptime template: []const u8, tok_id: RawToken.Id, expected_guards: u32) !void {
-            var comp = Compilation.init(allocator);
+            var comp = Compilation.init(allocator, std.fs.cwd());
             defer comp.deinit();
             var pp = Preprocessor.init(&comp);
             defer pp.deinit();
diff --git a/src/aro/Tokenizer.zig b/src/aro/Tokenizer.zig
index c7901069..f703940f 100644
--- a/src/aro/Tokenizer.zig
+++ b/src/aro/Tokenizer.zig
@@ -2151,7 +2151,7 @@ test "C23 keywords" {
 }
 
 test "Tokenizer fuzz test" {
-    var comp = Compilation.init(std.testing.allocator);
+    var comp = Compilation.init(std.testing.allocator, std.fs.cwd());
     defer comp.deinit();
 
     const input_bytes = std.testing.fuzzInput(.{});
@@ -2173,7 +2173,7 @@ test "Tokenizer fuzz test" {
 }
 
 fn expectTokensExtra(contents: []const u8, expected_tokens: []const Token.Id, standard: ?LangOpts.Standard) !void {
-    var comp = Compilation.init(std.testing.allocator);
+    var comp = Compilation.init(std.testing.allocator, std.fs.cwd());
     defer comp.deinit();
     if (standard) |provided| {
         comp.langopts.standard = provided;
diff --git a/src/aro/Value.zig b/src/aro/Value.zig
index 4d4c02ba..aad6f0e0 100644
--- a/src/aro/Value.zig
+++ b/src/aro/Value.zig
@@ -67,7 +67,7 @@ test "minUnsignedBits" {
         }
     };
 
-    var comp = Compilation.init(std.testing.allocator);
+    var comp = Compilation.init(std.testing.allocator, std.fs.cwd());
     defer comp.deinit();
     const target_query = try std.Target.Query.parse(.{ .arch_os_abi = "x86_64-linux-gnu" });
     comp.target = try std.zig.system.resolveTargetQuery(target_query);
@@ -102,7 +102,7 @@ test "minSignedBits" {
         }
     };
 
-    var comp = Compilation.init(std.testing.allocator);
+    var comp = Compilation.init(std.testing.allocator, std.fs.cwd());
     defer comp.deinit();
     const target_query = try std.Target.Query.parse(.{ .arch_os_abi = "x86_64-linux-gnu" });
     comp.target = try std.zig.system.resolveTargetQuery(target_query);
diff --git a/src/aro/toolchains/Linux.zig b/src/aro/toolchains/Linux.zig
index f166e9e6..ee24c223 100644
--- a/src/aro/toolchains/Linux.zig
+++ b/src/aro/toolchains/Linux.zig
@@ -424,7 +424,7 @@ test Linux {
     defer arena_instance.deinit();
     const arena = arena_instance.allocator();
 
-    var comp = Compilation.init(std.testing.allocator);
+    var comp = Compilation.init(std.testing.allocator, std.fs.cwd());
     defer comp.deinit();
     comp.environment = .{
         .path = "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
diff --git a/src/main.zig b/src/main.zig
index 3ef6dee9..5bbd6b9e 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -37,7 +37,7 @@ pub fn main() u8 {
     };
     defer gpa.free(aro_name);
 
-    var comp = Compilation.initDefault(gpa) catch |er| switch (er) {
+    var comp = Compilation.initDefault(gpa, std.fs.cwd()) catch |er| switch (er) {
         error.OutOfMemory => {
             std.debug.print("out of memory\n", .{});
             if (fast_exit) process.exit(1);
diff --git a/test/record_runner.zig b/test/record_runner.zig
index f9357a15..1398a602 100644
--- a/test/record_runner.zig
+++ b/test/record_runner.zig
@@ -212,7 +212,7 @@ fn runTestCases(allocator: std.mem.Allocator, test_dir: []const u8, wg: *std.Thr
 fn singleRun(alloc: std.mem.Allocator, test_dir: []const u8, test_case: TestCase, stats: *Stats) !void {
     const path = test_case.path;
 
-    var comp = aro.Compilation.init(alloc);
+    var comp = aro.Compilation.init(alloc, std.fs.cwd());
     defer comp.deinit();
 
     try comp.addDefaultPragmaHandlers();
diff --git a/test/runner.zig b/test/runner.zig
index a1d6cbf7..83283d15 100644
--- a/test/runner.zig
+++ b/test/runner.zig
@@ -54,7 +54,7 @@ fn addCommandLineArgs(comp: *aro.Compilation, file: aro.Source, macro_buf: anyty
 }
 
 fn testOne(allocator: std.mem.Allocator, path: []const u8, test_dir: []const u8) !void {
-    var comp = aro.Compilation.init(allocator);
+    var comp = aro.Compilation.init(allocator, std.fs.cwd());
     defer comp.deinit();
 
     try comp.addDefaultPragmaHandlers();
@@ -159,7 +159,7 @@ pub fn main() !void {
     });
 
     // prepare compiler
-    var initial_comp = aro.Compilation.init(gpa);
+    var initial_comp = aro.Compilation.init(gpa, std.fs.cwd());
     defer initial_comp.deinit();
 
     const cases_include_dir = try std.fs.path.join(gpa, &.{ args[1], "include" });