diff --git a/.git-blame-ignore-revs b/.git-blame-ignore-revs
index 54c7f29c3..c933772bd 100644
--- a/.git-blame-ignore-revs
+++ b/.git-blame-ignore-revs
@@ -7,3 +7,6 @@ de0c947a407f2d7cbbdd926e33c77842b984f9f6
 
 # reformat with 3.8.6
 90ccd87e19dfa642c8fb789d336a1eba55a6c306
+
+# Scala Steward: Reformat with scalafmt 3.9.0
+eef94e3679cbded0d9ca748009cecb558d38489b
diff --git a/.scalafmt.conf b/.scalafmt.conf
index 5383d6270..e696425e2 100644
--- a/.scalafmt.conf
+++ b/.scalafmt.conf
@@ -1,4 +1,4 @@
-version = "3.8.6"
+version = "3.9.0"
 
 rewrite.rules = [RedundantBraces, SortModifiers, Imports]
 rewrite.redundantBraces.generalExpressions = false
diff --git a/build.sbt b/build.sbt
index a02b72708..91a16c5d4 100644
--- a/build.sbt
+++ b/build.sbt
@@ -207,12 +207,11 @@ lazy val core =
     ,
     autoCompilerPlugins := true,
     addCompilerPlugin("com.lihaoyi" % s"acyclic_${versionString}" % "0.3.16"),
-    scalacOptions += "-P:acyclic:force",
-  )
-  .dependsOn(base, proto)
-  .jsSettings(
-    commonJsSettings,
-  )
+    scalacOptions += "-P:acyclic:force"
+  ).dependsOn(base, proto)
+    .jsSettings(
+      commonJsSettings
+    )
 
 lazy val coreJVM = core.jvm
 lazy val coreJS =
@@ -226,7 +225,13 @@ lazy val cliJS =
       name := "bosatsu-clijs",
       assembly / test := {},
       mainClass := Some("org.bykn.bosatsu.tool.Fs2Main"),
-      libraryDependencies ++= Seq(fs2core.value, fs2io.value, catsEffect.value, http4sCore.value, http4sEmber.value)
+      libraryDependencies ++= Seq(
+        fs2core.value,
+        fs2io.value,
+        catsEffect.value,
+        http4sCore.value,
+        http4sEmber.value
+      )
     )
     .jsSettings(
       scalaJSLinkerConfig ~= { _.withModuleKind(ModuleKind.CommonJSModule) },
diff --git a/project/Dependencies.scala b/project/Dependencies.scala
index 67550ccd7..17f4feb8c 100644
--- a/project/Dependencies.scala
+++ b/project/Dependencies.scala
@@ -14,9 +14,11 @@ object Dependencies {
   lazy val jawnParser = Def.setting("org.typelevel" %%% "jawn-parser" % "1.6.0")
   lazy val jawnAst = Def.setting("org.typelevel" %%% "jawn-ast" % "1.6.0")
   lazy val jython = Def.setting("org.python" % "jython-standalone" % "2.7.4")
-  lazy val http4sBlaze = Def.setting("org.http4s" %% "http4s-blaze-client" % "0.23.17")
+  lazy val http4sBlaze =
+    Def.setting("org.http4s" %% "http4s-blaze-client" % "0.23.17")
   lazy val http4sCore = Def.setting("org.http4s" %%% "http4s-core" % "0.23.30")
-  lazy val http4sEmber = Def.setting("org.http4s" %%% "http4s-ember-client" % "0.23.30")
+  lazy val http4sEmber =
+    Def.setting("org.http4s" %%% "http4s-ember-client" % "0.23.30")
   lazy val munit = Def.setting("org.scalameta" %%% "munit" % "1.0.4")
   lazy val munitScalaCheck =
     Def.setting("org.scalameta" %%% "munit-scalacheck" % "1.0.0")