diff --git a/pw_toolchain/dummy/BUILD.gn b/pw_toolchain/dummy/BUILD.gn
index f7e6d81..5d4cf74 100644
--- a/pw_toolchain/dummy/BUILD.gn
+++ b/pw_toolchain/dummy/BUILD.gn
@@ -23,4 +23,41 @@
     }
     description = "stamp {{output}}"
   }
+
+  # If the user tries to build a target with the default toolchain, run a script
+  # printing out the error.
+  _bad_toolchain_command = "python " + rebase_path("bad_toolchain.py")
+
+  tool("asm") {
+    command = _bad_toolchain_command
+    outputs =
+        [ "{{source_out_dir}}/{{target_output_name}}.{{source_file_part}}.o" ]
+  }
+
+  tool("cc") {
+    command = _bad_toolchain_command
+    outputs =
+        [ "{{source_out_dir}}/{{target_output_name}}.{{source_file_part}}.o" ]
+  }
+
+  tool("cxx") {
+    command = _bad_toolchain_command
+    outputs =
+        [ "{{source_out_dir}}/{{target_output_name}}.{{source_file_part}}.o" ]
+  }
+
+  tool("link") {
+    command = _bad_toolchain_command
+    outputs = [ "{{output_dir}}/{{target_output_name}}{{output_extension}}" ]
+  }
+
+  tool("alink") {
+    command = _bad_toolchain_command
+    outputs = [ "{{output_dir}}/{{target_output_name}}{{output_extension}}" ]
+  }
+
+  tool("solink") {
+    command = _bad_toolchain_command
+    outputs = [ "{{output_dir}}/{{target_output_name}}{{output_extension}}" ]
+  }
 }
diff --git a/pw_toolchain/dummy/bad_toolchain.py b/pw_toolchain/dummy/bad_toolchain.py
new file mode 100644
index 0000000..c87150d
--- /dev/null
+++ b/pw_toolchain/dummy/bad_toolchain.py
@@ -0,0 +1,92 @@
+# Copyright 2020 The Pigweed Authors
+#
+# 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
+#
+#     https://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.
+"""Emits an error telling the user not to use the default toolchain."""
+
+import sys
+
+_ERROR_MESSAGE = """
+Error: trying to build a target with the default toolchain
+
+  This occurs when a GN target is listed as a dependency outside of a toolchain
+  group (such as host_clang or stm32f429i) in the root BUILD.gn file.
+
+  Make sure that your top-level targets are always instantiated with a toolchain
+  and that no dependencies are pulled in through the default toolchain.
+
+  group("my_target_wrapper") {
+    deps = [ ":my_target(//path/to/my/toolchain)" ]
+  }
+
+  group("my_target") {
+    deps = []
+    if (current_toolchain != default_toolchain) {
+      deps += [ "//my_application:image" ]
+    }
+  }
+
+  If you are developing in Pigweed itself, list your build target under one of
+  the predefined groups in //BUILD.gn. For example,
+
+    # apps is an existing group intended for building application images.
+    group("apps") {
+      deps = [
+        ...
+        "your_target:here",
+      ]
+    }
+
+  Other predefined groups include host_tools, pw_modules, and pw_module_tests.
+
+  If you want to add a custom group instead of using an existing one, it must be
+  defined alongside the predefined groups, within the toolchain condition block:
+
+    if (current_toolchain != default_toolchain) {
+      group("apps") {
+        ...
+      }
+
+      # Other predefined groups...
+
+      group("my_custom_group") {
+        deps = [ "//path/to:my_target" ]
+      }
+    }
+
+  To include your custom group in the build, add it to the pigweed_default group
+  to have it compile for every supported Pigweed target.
+
+    group("pigweed_default") {
+      deps = []
+
+      if (current_toolchain != default_toolchain) {
+        # Standard Pigweed dependencies...
+
+        # Add your group here.
+        deps += [ ":my_custom_group" ]
+      }
+    }
+
+  For more details on the Pigweed build structure and how to configure custom
+  build targets or toolchains, please refer to "Build system" in the Pigweed
+  documentation.
+"""
+
+
+def main() -> int:
+    print(_ERROR_MESSAGE, file=sys.stderr)
+    return 1
+
+
+if __name__ == '__main__':
+    sys.exit(main())