diff --git a/clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp b/clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp
--- a/clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp
+++ b/clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp
@@ -298,7 +298,14 @@
         .getValueOr(tooling::CompileCommand())
         .CommandLine;
   };
-  EXPECT_THAT(Command("x/foo.cc"), IsEmpty());
+
+  // Since we use real FS and place our files somewhere in /tmp/, an existing
+  // /tmp/compile_flags.txt or /tmp/compile_commands.json could affect the test.
+  // By creating this file we prevent CDB from looking outside of the test
+  // directory.
+  FS.write("compile_flags.txt", "-DFOO");
+
+  EXPECT_THAT(Command("x/foo.cc"), Contains("-DFOO"));
   const char *const CDB =
       R"cdb(
       [
@@ -315,9 +322,11 @@
       ]
       )cdb";
   FS.write("x/build/compile_commands.json", CDB);
-  EXPECT_THAT(Command("x/foo.cc"), Contains("-DXYZZY"));
-  EXPECT_THAT(Command("bar.cc"), IsEmpty())
-      << "x/build/compile_flags.json only applicable to x/";
+  EXPECT_THAT(Command("x/foo.cc"),
+              AllOf(Contains("-DXYZZY"), Not(Contains("-DFOO"))));
+  EXPECT_THAT(Command("bar.cc"),
+              AllOf(Contains("-DFOO"), Not(Contains("-DXYZZY"))))
+      << "x/build/compile_commands.json only applicable to x/";
 }
 
 TEST(GlobalCompilationDatabaseTest, NonCanonicalFilenames) {