diff --git a/ui/build/config.go b/ui/build/config.go
index 36119f0..b38f230 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -236,6 +236,21 @@
 	return nil
 }
 
+func defaultBazelProdMode(cfg *configImpl) bool {
+	// Envirnoment flag to disable Bazel for users which experience
+	// broken bazel-handled builds, or significant performance regressions.
+	if cfg.IsBazelMixedBuildForceDisabled() {
+		return false
+	}
+	// Darwin-host builds are currently untested with Bazel.
+	if runtime.GOOS == "darwin" {
+		return false
+	}
+	// TODO(b/255364055): Flip this to true to enable bazel-mode by default
+	// for all users that don't opt out with BUILD_BROKEN_DISABLE_BAZEL.
+	return false
+}
+
 func NewConfig(ctx Context, args ...string) Config {
 	ret := &configImpl{
 		environ:       OsEnvironment(),
@@ -774,6 +789,9 @@
 			c.arguments = append(c.arguments, arg)
 		}
 	}
+	if (!c.bazelProdMode) && (!c.bazelDevMode) && (!c.bazelStagingMode) {
+		c.bazelProdMode = defaultBazelProdMode(c)
+	}
 }
 
 func (c *configImpl) configureLocale(ctx Context) {
diff --git a/ui/build/config_test.go b/ui/build/config_test.go
index 968544b..940d85c 100644
--- a/ui/build/config_test.go
+++ b/ui/build/config_test.go
@@ -90,7 +90,9 @@
 				t.Fatal(err)
 			})
 
+			env := Environment([]string{})
 			c := &configImpl{
+				environ:   &env,
 				parallel:  -1,
 				keepGoing: -1,
 			}