diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index e8034bf5d1..8cb2fd0430 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -1408,6 +1408,9 @@
 //
 // The -x flag causes tidy to print the commands download executes.
 //
+// Tidy treats all build constraints except 'ignore' as satisfied, including
+// implicit constraints from filenames.
+//
 // See https://golang.org/ref/mod#go-mod-tidy for more about 'go mod tidy'.
 //
 // # Make vendored copy of dependencies
diff --git a/src/cmd/go/internal/modcmd/tidy.go b/src/cmd/go/internal/modcmd/tidy.go
index 2efa33a7c3..fd462f6a75 100644
--- a/src/cmd/go/internal/modcmd/tidy.go
+++ b/src/cmd/go/internal/modcmd/tidy.go
@@ -55,6 +55,9 @@ file.
 
 The -x flag causes tidy to print the commands download executes.
 
+Tidy treats all build constraints except 'ignore' as satisfied, including
+implicit constraints from filenames.
+
 See https://golang.org/ref/mod#go-mod-tidy for more about 'go mod tidy'.
 	`,
 	Run: runTidy,