diff --git a/.build.yml b/.build.yml
index 14eb5c1..de41c4e 100644
--- a/.build.yml
+++ b/.build.yml
@@ -10,9 +10,14 @@ tasks:
       curl -L https://github.com/bazelbuild/bazelisk/releases/download/v1.9.0/bazelisk-linux-amd64 \
         -o bazel
       chmod +x bazel
+  - test_default: |
+      cd bazel-zig-cc; ../bazel run --toolchain_resolution_debug=true \
+        //test:gognu
   - test_musl: |
-      cd bazel-zig-cc
-      ../bazel run --platforms @com_github_ziglang_zig//:platform_linux-x86_64-musl //test:gomusl
+      cd bazel-zig-cc; ../bazel run --toolchain_resolution_debug=true \
+        --platforms @com_github_ziglang_zig//:platform_linux-x86_64-musl \
+        //test:gomusl
   - test_gnu: |
-      cd bazel-zig-cc
-      ../bazel run --platforms @com_github_ziglang_zig//:platform_linux-x86_64-gnu //test:gognu
+      cd bazel-zig-cc; ../bazel run --toolchain_resolution_debug=true \
+        --platforms @com_github_ziglang_zig//:platform_linux-x86_64-gnu \
+        //test:gognu
diff --git a/README.md b/README.md
index 44b36ef..86d48fe 100644
--- a/README.md
+++ b/README.md
@@ -6,12 +6,24 @@ libraries:
 - glibc 2.19
 - musl
 
+glibc 2.19 is the default. That means, glibc 2.19 toolchain is registered with
+these basic constraints:
+
+```
+[
+    "@platforms//os:linux",
+    "@platforms//cpu:x86_64",
+]
+```
+
 # Testing
 
 ## cgo + glibc 2.19
 
+Using the default toolchain:
+
 ```
-$ bazel build //test:gognu
+$ bazel build --toolchain_resolution_debug=true //test:gognu
 ...
 $ ../bazel-bin/test/gognu_/gognu
 hello, world
@@ -19,10 +31,15 @@ $ file ../bazel-bin/test/gognu_/gognu
 ../bazel-bin/test/gognu_/gognu: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.0.0, Go BuildID=redacted, with debug_info, not stripped
 ```
 
+Explicitly specifying `-gnu`:
+```
+$ bazel run --platforms @com_github_ziglang_zig//:platform_x86_64-linux-gnu //test:gognu
+```
+
 ## cgo + musl
 
 ```
-$ bazel build --platforms @com_github_ziglang_zig//:platform_linux-x86_64-musl :gomusl
+$ bazel build --platforms @com_github_ziglang_zig//:platform_x86_64-linux-musl :gomusl
 ...
 $ file ../bazel-out/k8-fastbuild-ST-d17813c235ce/bin/test/gomusl_/gomusl
 ../bazel-out/k8-fastbuild-ST-d17813c235ce/bin/test/gomusl_/gomusl: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, Go BuildID=redacted, with debug_info, not stripped
@@ -36,6 +53,7 @@ hello, world
 $ docker run -ti --rm -v $(pwd):/x -w /x debian:buster-slim
 # apt update && apt install curl ca-certificates -y && curl -L https://github.com/bazelbuild/bazelisk/releases/download/v1.7.5/bazelisk-linux-amd64 > /usr/local/bin/bazel && chmod +x /usr/local/bin/bazel
 # export CC=/usr/bin/false
-# bazel run --platforms @com_github_ziglang_zig//:platform_linux-x86_64-musl //test:gomusl
-# bazel run --platforms @com_github_ziglang_zig//:platform_linux-x86_64-gnu //test:gognu
 ```
+
+And run the `bazel build` commands above. Take a look at `.build.yml` and see
+how CI does it.
diff --git a/toolchain/defs.bzl b/toolchain/defs.bzl
index f26a425..687e4ad 100644
--- a/toolchain/defs.bzl
+++ b/toolchain/defs.bzl
@@ -32,7 +32,10 @@ TARGET_CONFIGS = [
         linkopts=[],
         copts=[],
         bazel_target_cpu="darwin",
-        constraint_values=["@platforms//os:macos", "@platforms//cpu:x86_64"],
+        constraint_values=[
+            "@platforms//os:macos",
+            "@platforms//cpu:x86_64",
+        ],
         tool_paths={"ld": "ld64.lld"},
     ),
     struct(
@@ -50,7 +53,6 @@ TARGET_CONFIGS = [
         constraint_values=[
             "@platforms//os:linux",
             "@platforms//cpu:x86_64",
-            ":gnu_2_19",
         ],
         tool_paths={"ld": "ld.lld"},
     ),
@@ -122,11 +124,6 @@ zig_build_macro(absolute_path={absolute_path}, zig_include_root={zig_include_roo
 
 constraint_setting(name = "libc")
 
-constraint_value(
-    name = "gnu_2_19",
-    constraint_setting = ":libc",
-)
-
 constraint_value(
     name = "musl",
     constraint_setting = ":libc",
@@ -134,20 +131,27 @@ constraint_value(
 
 
 platform(
-    name = "platform_linux-x86_64-musl",
+    name = "platform_x86_64-macos-gnu",
     constraint_values = [
-        "@platforms//os:linux",
+        "@platforms//os:macos",
         "@platforms//cpu:x86_64",
-        ":musl",
     ],
 )
 
 platform(
-    name = "platform_linux-x86_64-gnu",
+    name = "platform_x86_64-linux-gnu",
     constraint_values = [
         "@platforms//os:linux",
         "@platforms//cpu:x86_64",
-        ":gnu_2_19",
+    ],
+)
+
+platform(
+    name = "platform_x86_64-linux-musl",
+    constraint_values = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+        ":musl",
     ],
 )