diff --git a/ui/build/ninja.go b/ui/build/ninja.go
index def0783..f5f637f 100644
--- a/ui/build/ninja.go
+++ b/ui/build/ninja.go
@@ -241,6 +241,9 @@
 			"SOONG_USE_N2",
 			"RUST_BACKTRACE",
 			"RUST_LOG",
+
+			// SOONG_USE_PARTIAL_COMPILE only determines which half of the rule we execute.
+			"SOONG_USE_PARTIAL_COMPILE",
 		}, config.BuildBrokenNinjaUsesEnvVars()...)...)
 	}