diff --git a/build/arduino-builder-linux32-1.6.0.tar.bz2.sha b/build/arduino-builder-linux32-1.6.0.tar.bz2.sha
deleted file mode 100644
index b63b01678..000000000
--- a/build/arduino-builder-linux32-1.6.0.tar.bz2.sha
+++ /dev/null
@@ -1 +0,0 @@
-3facc1477b1089e8ba6904a891134c16610daab1
diff --git a/build/arduino-builder-linux32-1.6.1.tar.bz2.sha b/build/arduino-builder-linux32-1.6.1.tar.bz2.sha
new file mode 100644
index 000000000..a603d7693
--- /dev/null
+++ b/build/arduino-builder-linux32-1.6.1.tar.bz2.sha
@@ -0,0 +1 @@
+e034c072a81f00a831dd602a5e7beabc2cd0f1a0
diff --git a/build/arduino-builder-linux64-1.6.0.tar.bz2.sha b/build/arduino-builder-linux64-1.6.0.tar.bz2.sha
deleted file mode 100644
index f9d1e95bf..000000000
--- a/build/arduino-builder-linux64-1.6.0.tar.bz2.sha
+++ /dev/null
@@ -1 +0,0 @@
-2cd8f03c0070c726c1c906338b4221a50dc75e78
diff --git a/build/arduino-builder-linux64-1.6.1.tar.bz2.sha b/build/arduino-builder-linux64-1.6.1.tar.bz2.sha
new file mode 100644
index 000000000..60d019e6d
--- /dev/null
+++ b/build/arduino-builder-linux64-1.6.1.tar.bz2.sha
@@ -0,0 +1 @@
+57c1f3ffc849aed119207d8fd8a130a967d0e6c8
diff --git a/build/arduino-builder-linuxaarch64-1.6.0.tar.bz2.sha b/build/arduino-builder-linuxaarch64-1.6.0.tar.bz2.sha
deleted file mode 100644
index 5f15209fe..000000000
--- a/build/arduino-builder-linuxaarch64-1.6.0.tar.bz2.sha
+++ /dev/null
@@ -1 +0,0 @@
-9a3f0a0c9b7759972bbe6e75d43b2a4760b94c12
diff --git a/build/arduino-builder-linuxaarch64-1.6.1.tar.bz2.sha b/build/arduino-builder-linuxaarch64-1.6.1.tar.bz2.sha
new file mode 100644
index 000000000..431a1f1da
--- /dev/null
+++ b/build/arduino-builder-linuxaarch64-1.6.1.tar.bz2.sha
@@ -0,0 +1 @@
+b54ad6ec630d66521a84075c763e8325c6f82591
diff --git a/build/arduino-builder-linuxarm-1.6.0.tar.bz2.sha b/build/arduino-builder-linuxarm-1.6.0.tar.bz2.sha
deleted file mode 100644
index 55d809120..000000000
--- a/build/arduino-builder-linuxarm-1.6.0.tar.bz2.sha
+++ /dev/null
@@ -1 +0,0 @@
-b5896d835b1c81459652e2deafe194bd1986b3dc
diff --git a/build/arduino-builder-linuxarm-1.6.1.tar.bz2.sha b/build/arduino-builder-linuxarm-1.6.1.tar.bz2.sha
new file mode 100644
index 000000000..1620c757d
--- /dev/null
+++ b/build/arduino-builder-linuxarm-1.6.1.tar.bz2.sha
@@ -0,0 +1 @@
+cb378bc29edeb90ad52d0544e02862a137b5f54c
diff --git a/build/arduino-builder-macosx-1.6.0-signed.tar.bz2.sha b/build/arduino-builder-macosx-1.6.0-signed.tar.bz2.sha
deleted file mode 100644
index 9c47c863d..000000000
--- a/build/arduino-builder-macosx-1.6.0-signed.tar.bz2.sha
+++ /dev/null
@@ -1 +0,0 @@
-7606aa6d79b96ec76c02ee34719222eee807190c
diff --git a/build/arduino-builder-macosx-1.6.1-signed.tar.bz2.sha b/build/arduino-builder-macosx-1.6.1-signed.tar.bz2.sha
new file mode 100644
index 000000000..328593fd5
--- /dev/null
+++ b/build/arduino-builder-macosx-1.6.1-signed.tar.bz2.sha
@@ -0,0 +1 @@
+e1335e27ac3b92421c18977884eb558c07af76ab
diff --git a/build/arduino-builder-windows-1.6.0.zip.sha b/build/arduino-builder-windows-1.6.0.zip.sha
deleted file mode 100644
index 06302ed70..000000000
--- a/build/arduino-builder-windows-1.6.0.zip.sha
+++ /dev/null
@@ -1 +0,0 @@
-c2236b5a2a16657f42a2ba1381f240febdc61f7f
diff --git a/build/arduino-builder-windows-1.6.1.zip.sha b/build/arduino-builder-windows-1.6.1.zip.sha
new file mode 100644
index 000000000..9a72545fb
--- /dev/null
+++ b/build/arduino-builder-windows-1.6.1.zip.sha
@@ -0,0 +1 @@
+b341957539bc149300607ec03366e3910e2ac0a5
diff --git a/build/build.xml b/build/build.xml
index 3f0961bb5..b4e0ef8ea 100644
--- a/build/build.xml
+++ b/build/build.xml
@@ -99,7 +99,7 @@
 
   <property name="portable" value="false" />
 
-  <property name="ARDUINO-BUILDER-VERSION" value="1.6.0" />
+  <property name="ARDUINO-BUILDER-VERSION" value="1.6.1" />
   <property name="LIBLISTSERIAL-VERSION" value="1.4.2-2" />
   <property name="AVRGCC-VERSION" value="7.3.0-atmel3.6.1-arduino7" />
   <property name="AVRDUDE-VERSION" value="6.3.0-arduino17" />
diff --git a/build/shared/revisions.txt b/build/shared/revisions.txt
index c82b90388..a4a40da00 100644
--- a/build/shared/revisions.txt
+++ b/build/shared/revisions.txt
@@ -1,4 +1,7 @@
-ARDUINO 1.8.15 2021.??.??
+ARDUINO 1.8.15 2021.05.13
+
+HOTFIX:
+* Fixed regression in library discovery (failing to compile bundled libs, like SD or Ethernet, on some platforms)
 
 
 ARDUINO 1.8.14 2021.05.12