diff --git a/Utilities/bootstrap b/Utilities/bootstrap
index 2da5c06d725..fcaacba094e 100755
--- a/Utilities/bootstrap
+++ b/Utilities/bootstrap
@@ -745,6 +745,8 @@ def get_swiftpm_env_cmd(args):
         env_cmd.append("SWIFTPM_LLBUILD_FWK=1")
     env_cmd.append("SWIFTCI_USE_LOCAL_DEPS=1")
     env_cmd.append("SWIFTPM_MACOS_DEPLOYMENT_TARGET=%s" % g_macos_deployment_target)
+    # Disable Swift 6 mode in swift-syntax to work around rdar://126952308
+    env_cmd.append("SWIFTSYNTAX_DISABLE_SWIFT_6_MODE=1")
 
     if not '-macosx' in args.build_target and args.command == 'install':
         env_cmd.append("SWIFTCI_INSTALL_RPATH_OS=%s" % args.platform_path.group(2))
diff --git a/Utilities/build-using-self b/Utilities/build-using-self
index 1d6a1552993..57a7eed54c2 100755
--- a/Utilities/build-using-self
+++ b/Utilities/build-using-self
@@ -20,6 +20,8 @@ echo "Current directory is ${PWD}"
 
 CONFIGURATION=debug
 export SWIFTCI_IS_SELF_HOSTED=1
+# Disable Swift 6 mode in swift-syntax to work around rdar://126952308
+export SWIFTSYNTAX_DISABLE_SWIFT_6_MODE=1
 
 set -x