$ git diff
diff --git a/base/BUILD.gn b/base/BUILD.gn
index 463fa3e30aae..bf58e7a6e3e4 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -110,6 +110,12 @@ if (is_android) {
   }
 }
 
+action("foo") {
+  script = "//build/gn_helpers.py"
+  args = [ "-D", "values=a=0$0x0Ab=1" ]
+  outputs = [ "$root_out_dir/output" ]
+}
+
 # Base and everything it depends on should be a static library rather than
 # a source set. Base is more of a "library" in the classic sense in that many
 # small parts of it are used in many different contexts. This combined with a
Nicos-MacBook-Pro:src thakis$ gn gen out/gn
Done. Made 8922 targets from 1524 files in 5922ms
$ ~/src/ninja/ninja -C out/gn base_unittests
ninja: Entering directory `out/gn'
ninja: error: toolchain.ninja:184: unexpected indent

$ sed -n 182,186p out/gn/toolchain.ninja 
  command = python ../../build/gn_helpers.py -D values=a=0\
b=1
  description = ACTION //base:foo(//build/toolchain/mac:clang_x64)
  restat = 1
  pool = build_toolchain_action_pool



Newlines must be escaped by '$' right in front of the newline in ninja. Looks like gn did write a '\' there, so maybe it attempted to do the right thing and failed, or maybe that's just the escaping for bash.

My guess is that the $0xFF notation is newer than gn's escaping code and since newlines are the only thing needing escaping on the rhs of a command line, no ninja-level escaping code was added at that point.