diff --git a/base/BUILD.gn b/base/BUILD.gn
index 509aa373690d..8787dc42d72b 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -34,6 +34,7 @@ if (!is_starboard) {
 }
 import("//testing/libfuzzer/fuzzer_test.gni")
 import("//testing/test.gni")
+import("//starboard/build/config/modular/helper_variables.gni")
 
 declare_args() {
   # Indicates if the Location object contains the source code information
@@ -1582,7 +1583,7 @@ component("base") {
     # TODO(b/206642994): see if we can remove this condition. It's added for now
     # because linking atomic leads to linker errors for evergreen on arm-hardfp,
     # and it's apparently not really needed (i.e., we can build without it).
-    if (!is_starboard || !sb_is_modular) {
+    if (!is_starboard || !(sb_is_modular && current_toolchain == cobalt_toolchain)) {
       libs += [ "atomic" ]
     }
   }
diff --git a/build/toolchain/gcc_toolchain.gni b/build/toolchain/gcc_toolchain.gni
index 1c1b364b1ec2..d80b2f3552f1 100644
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -11,6 +11,7 @@ import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/rbe.gni")
 import("//build/toolchain/toolchain.gni")
+import("//starboard/build/config/modular/helper_variables.gni")
 
 if (is_nacl) {
   # To keep NaCl variables out of builds that don't include NaCl, all
diff --git a/cobalt/BUILD.gn b/cobalt/BUILD.gn
index 8fea180c0c24..a41ca3623147 100644
--- a/cobalt/BUILD.gn
+++ b/cobalt/BUILD.gn
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import("//starboard/build/config/modular/helper_variables.gni")
+
 group("gn_all") {
   testonly = true
   deps = [
diff --git a/cobalt/media/sandbox/BUILD.gn b/cobalt/media/sandbox/BUILD.gn
index 19be83551b45..789c09f3d092 100644
--- a/cobalt/media/sandbox/BUILD.gn
+++ b/cobalt/media/sandbox/BUILD.gn
@@ -15,6 +15,8 @@
 # This is a sample sandbox application for experimenting with the Cobalt
 # media/renderer interface.
 
+import("//starboard/build/config/modular/helper_variables.gni")
+
 target(final_executable_type, "media_sandbox") {
   sources = [ "media2_sandbox.cc" ]
 
diff --git a/cobalt/network/BUILD.gn b/cobalt/network/BUILD.gn
index 1a4fd75969b0..e39e50e53a33 100644
--- a/cobalt/network/BUILD.gn
+++ b/cobalt/network/BUILD.gn
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import("//starboard/build/config/modular/helper_variables.gni")
+
 static_library("network") {
   has_pedantic_warnings = true
 
diff --git a/starboard/BUILD.gn b/starboard/BUILD.gn
index c0a010de11e2..50693fb0a8e3 100644
--- a/starboard/BUILD.gn
+++ b/starboard/BUILD.gn
@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import("//starboard/build/config/modular/helper_variables.gni")
 import("//starboard/build/config/starboard_target_type.gni")
 
 group("gn_all") {
@@ -57,11 +58,6 @@ group("gn_all") {
     deps += [ "//starboard/benchmark" ]
   }
 
-  if (is_modular_toolchain && is_cobalt_toolchain) {
-    assert(sb_is_modular,
-           "sb_is_modular should be set when building with cobalt_toolchain")
-  }
-
   if (!sb_is_evergreen) {
     deps += [ "//third_party/lz4_lib:lz4" ]
 
@@ -89,7 +85,7 @@ group("starboard") {
     "//starboard/client_porting/eztime",
   ]
 
-  if (sb_is_modular) {
+  if (sb_is_modular && current_toolchain == cobalt_toolchain) {
     public_deps += [
       "//third_party/llvm-project/compiler-rt:compiler_rt",
       "//third_party/llvm-project/libcxx:cxx",
@@ -192,7 +188,7 @@ if (current_toolchain == starboard_toolchain) {
     } else {
       public_deps += [ "//third_party/crashpad/wrapper:wrapper_stub" ]
     }
-    if (!sb_is_modular) {
+    if (!(sb_is_modular && current_toolchain == cobalt_toolchain)) {
       public_deps += [ "//$starboard_path:starboard_platform" ]
     }
   }
diff --git a/starboard/build/config/BUILD.gn b/starboard/build/config/BUILD.gn
index a9653497f346..395ceb5790d9 100644
--- a/starboard/build/config/BUILD.gn
+++ b/starboard/build/config/BUILD.gn
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import("//build/config/compiler/compiler.gni")
+import("//starboard/build/config/modular/helper_variables.gni")
 
 config("base") {
   defines = [ "USE_COBALT_CUSTOMIZATIONS" ]
@@ -104,6 +105,7 @@ config("target") {
       # however.
       # For windows based platforms this flag causes a linker error while building with cobalt toolchain
       if (!(host_os == "win" && sb_is_modular &&
+            current_toolchain == cobalt_toolchain &&
             current_toolchain == cobalt_toolchain)) {
         cflags = [ "-fPIC" ]
       }
@@ -154,7 +156,7 @@ config("starboard") {
     }
 
     if (sb_is_modular) {
-      defines += [ "SB_IS_MODULAR=1" ]
+      defines += [ "SB_IS_MODULAR" ]
     }
 
     if (sb_is_evergreen) {
@@ -229,7 +231,8 @@ config("warnings_as_errors") {
 # override flags specified in a platform's "platform_configuration" config,
 # which is where these particular flags would otherwise naturally fit.
 config("default_compiler_flags") {
-  if (is_starboardized_toolchain && sb_is_modular && target_cpu == "arm") {
+  if (is_starboardized_toolchain && sb_is_modular &&
+      current_toolchain == cobalt_toolchain && target_cpu == "arm") {
     cflags = [ "-mfpu=vfpv3" ]
     asmflags = cflags
   }
diff --git a/starboard/build/config/base_configuration.gni b/starboard/build/config/base_configuration.gni
index 3c60204516c2..8e19f4c331e6 100644
--- a/starboard/build/config/base_configuration.gni
+++ b/starboard/build/config/base_configuration.gni
@@ -44,9 +44,6 @@ declare_args() {
   # Directory path to static contents' data.
   sb_static_contents_output_data_dir = "$root_out_dir/content/data"
 
-  # Whether this is a modular build.
-  sb_is_modular = false
-
   # Whether this is an Evergreen build.
   sb_is_evergreen = false
 
diff --git a/starboard/build/config/modular/helper_variables.gni b/starboard/build/config/modular/helper_variables.gni
new file mode 100644
index 000000000000..2f37e61164dc
--- /dev/null
+++ b/starboard/build/config/modular/helper_variables.gni
@@ -0,0 +1,15 @@
+# Copyright 2023 The Cobalt Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+sb_is_modular = sb_is_evergreen || is_modular_toolchain
diff --git a/starboard/evergreen/shared/platform_configuration/configuration.gni b/starboard/evergreen/shared/platform_configuration/configuration.gni
index c76baea77b99..f3aab3370c76 100644
--- a/starboard/evergreen/shared/platform_configuration/configuration.gni
+++ b/starboard/evergreen/shared/platform_configuration/configuration.gni
@@ -14,8 +14,6 @@
 
 import("//starboard/build/config/base_configuration.gni")
 
-sb_is_modular = true
-
 sb_is_evergreen = true
 
 cobalt_font_package = "empty"
diff --git a/starboard/extension/BUILD.gn b/starboard/extension/BUILD.gn
index 5212664b2e01..6319447c1cb9 100644
--- a/starboard/extension/BUILD.gn
+++ b/starboard/extension/BUILD.gn
@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import("//starboard/build/config/modular/helper_variables.gni")
 target(gtest_target_type, "extension_test") {
   testonly = true
   has_pedantic_warnings = true
@@ -26,7 +27,7 @@ target(gtest_target_type, "extension_test") {
     "//testing/gmock",
     "//testing/gtest",
   ]
-  if (sb_is_modular) {
+  if (sb_is_modular && current_toolchain == cobalt_toolchain) {
     deps += cobalt_platform_dependencies
   }
 }
diff --git a/starboard/linux/shared/platform_configuration/BUILD.gn b/starboard/linux/shared/platform_configuration/BUILD.gn
index dcdaa48c857e..ad379e6021cf 100644
--- a/starboard/linux/shared/platform_configuration/BUILD.gn
+++ b/starboard/linux/shared/platform_configuration/BUILD.gn
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import("//starboard/build/config/modular/helper_variables.gni")
+
 config("compiler_flags") {
   cflags = []
   cflags_c = []
diff --git a/third_party/boringssl/BUILD.gn b/third_party/boringssl/BUILD.gn
index f90940c03088..201b95f2713d 100644
--- a/third_party/boringssl/BUILD.gn
+++ b/third_party/boringssl/BUILD.gn
@@ -1,5 +1,6 @@
 import("//starboard/sabi/sabi.gni")
 import("//third_party/boringssl/buildfiles.gni")
+import("//starboard/build/config/modular/helper_variables.gni")
 
 declare_args() {
   asm_target_arch = target_cpu
@@ -102,7 +103,7 @@ static_library("crypto") {
     configs -= [ "//starboard/build/config:size" ]
     configs += [ "//starboard/build/config:speed" ]
 
-    if (sb_is_modular) {
+    if (sb_is_modular && current_toolchain == cobalt_toolchain) {
       calling_convention = sabi_variables.calling_convention
       if ((calling_convention != "aarch64" && calling_convention != "eabi" &&
            calling_convention != "sysv" && calling_convention != "windows") ||
diff --git a/third_party/libjpeg-turbo/BUILD.gn b/third_party/libjpeg-turbo/BUILD.gn
index 43731152d8f5..35a8d07679cd 100644
--- a/third_party/libjpeg-turbo/BUILD.gn
+++ b/third_party/libjpeg-turbo/BUILD.gn
@@ -5,6 +5,7 @@
 # Do not use the targets in this file unless you need a certain libjpeg
 # implementation. Use the meta target //third_party:jpeg instead.
 
+import("//starboard/build/config/modular/helper_variables.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 if (current_cpu == "arm" || current_cpu == "arm64") {
   import("//build/config/arm.gni")
@@ -205,7 +206,7 @@ static_library("simd") {
     }
     configs += [ "//build/config/compiler:optimize_speed" ]
 
-    if (is_starboard && sb_is_modular && current_cpu == "arm") {
+    if (is_starboard && sb_is_modular && current_toolchain == cobalt_toolchain && current_cpu == "arm") {
       configs += [ ":simd_arm_config" ]
     }
   } else {
@@ -316,7 +317,7 @@ static_library("libjpeg") {
   # TODO(b/236258535): Enable neon instructions once the code is updated
   # to use SbCPUFeatures() at runtime.
   if ((is_msan && (current_cpu == "x86" || current_cpu == "x64")) ||
-      (is_starboard && sb_is_modular)) {
+      (is_starboard && sb_is_modular && current_toolchain == cobalt_toolchain)) {
     sources += [ "jsimd_none.c" ]
   } else {
     public_deps += [ ":simd" ]
diff --git a/third_party/zlib/BUILD.gn b/third_party/zlib/BUILD.gn
index 83cb3275eadf..f474e07998cb 100644
--- a/third_party/zlib/BUILD.gn
+++ b/third_party/zlib/BUILD.gn
@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//starboard/build/config/modular/helper_variables.gni")
+
 import("//build/config/compiler/compiler.gni")
 if (current_cpu == "arm" || current_cpu == "arm64") {
   import("//build/config/arm.gni")
@@ -25,7 +27,7 @@ if (current_cpu == "arm" || current_cpu == "arm64") {
 # TODO(b/206642994): look into whether we should condition on the
 # floating_point_fpu SABI variable, rather than sb_is_modular, here as we did
 # in third_party/zlib/zlib.gyp.
-if (is_starboard && sb_is_modular && target_cpu == "arm" &&
+if (is_starboard && sb_is_modular && current_toolchain == cobalt_toolchain && target_cpu == "arm" &&
     use_arm_neon_optimizations) {
   config("zlib_neon_fpu_config") {
     cflags = [ "-mfpu=neon" ]
@@ -80,7 +82,7 @@ source_set("zlib_adler32_simd") {
     configs += [ "//starboard/build/config:speed" ]
   }
 
-  if (is_starboard && sb_is_modular && target_cpu == "arm" &&
+  if (is_starboard && sb_is_modular && current_toolchain == cobalt_toolchain && target_cpu == "arm" &&
       use_arm_neon_optimizations) {
     configs += [ ":zlib_neon_fpu_config" ]
   }
@@ -161,7 +163,7 @@ if (use_arm_neon_optimizations) {
       deps = [ "//starboard:starboard_headers_only" ]
     }
 
-    if (is_starboard && sb_is_modular && target_cpu == "arm" &&
+    if (is_starboard && sb_is_modular && current_toolchain == cobalt_toolchain && target_cpu == "arm" &&
         use_arm_neon_optimizations) {
       configs += [ ":zlib_neon_fpu_config" ]
     }
@@ -236,7 +238,7 @@ source_set("zlib_inflate_chunk_simd") {
     configs += [ "//starboard/build/config:speed" ]
   }
 
-  if (is_starboard && sb_is_modular && target_cpu == "arm" &&
+  if (is_starboard && sb_is_modular && current_toolchain == cobalt_toolchain && target_cpu == "arm" &&
       use_arm_neon_optimizations) {
     configs += [ ":zlib_neon_fpu_config" ]
   }