diff --git a/main-settings/src/main/scala/sbt/Def.scala b/main-settings/src/main/scala/sbt/Def.scala
index c75d19960..6a49cd985 100644
--- a/main-settings/src/main/scala/sbt/Def.scala
+++ b/main-settings/src/main/scala/sbt/Def.scala
@@ -404,8 +404,16 @@ object Def extends BuildSyntax with Init with InitializeImplicits:
 
   extension [A1](inline in: TaskKey[A1])
     // implicit def macroPrevious[T](@deprecated("unused", "") in: TaskKey[T]): MacroPrevious[T] = ???
-    inline def previous(using JsonFormat[A1]): Option[A1] =
-      InputWrapper.`wrapInitTask_\u2603\u2603`[Option[A1]](Previous.runtime[A1](in))
+    // Previously, we implemented `.previous` as a plain inline expansion of
+    // `wrapInitTask(Previous.runtime(in))`. For composite types such as `Seq[String]`, the
+    // compiler can rewrite the format argument into a synthetic local like
+    // `{ val x$2$proxy1 = immSeqFormat(StringJsonFormat); Previous.runtime(in)(using x$2$proxy1) }`.
+    // The outer task macro later hoists only `Previous.runtime(...)` as an input dependency,
+    // leaving `x$2$proxy1` behind and causing "used outside the scope where it was defined".
+    // Now we implement `.previous` as a macro so the wrapped tree is already self-contained.
+    // See https://github.com/sbt/sbt/issues/9037.
+    inline def previous(using @scala.annotation.unused format: JsonFormat[A1]): Option[A1] =
+      ${ TaskMacro.previousImpl[A1]('in) }
 
   // The following conversions enable the types Parser[T], Initialize[Parser[T]], and
   // Initialize[State => Parser[T]] to be used in the inputTask macro as an input with an ultimate
diff --git a/main-settings/src/main/scala/sbt/std/TaskMacro.scala b/main-settings/src/main/scala/sbt/std/TaskMacro.scala
index ee10da330..be91e3cfe 100644
--- a/main-settings/src/main/scala/sbt/std/TaskMacro.scala
+++ b/main-settings/src/main/scala/sbt/std/TaskMacro.scala
@@ -17,6 +17,7 @@ import sbt.internal.util.{ SourcePosition, SourcePositionImpl }
 import language.experimental.macros
 import scala.quoted.*
 import sbt.util.BuildWideCacheConfiguration
+import sjsonnew.JsonFormat
 
 object TaskMacro:
   final val AssignInitName = "set"
@@ -120,6 +121,15 @@ object TaskMacro:
     val convert1 = new FullConvert(qctx, 1000)
     convert1.contFlatMap[A1, F, Id](t, convert1.appExpr, None)
 
+  def previousImpl[A1: Type](t: Expr[TaskKey[A1]])(using qctx: Quotes): Expr[Option[A1]] =
+    import qctx.reflect.*
+    Expr.summon[JsonFormat[A1]] match
+      case Some(ev) =>
+        '{
+          InputWrapper.`wrapInitTask_\u2603\u2603`[Option[A1]](Previous.runtime[A1]($t)(using $ev))
+        }
+      case _ => report.errorAndAbort(s"JsonFormat[${Type.show[A1]}] missing")
+
   /** Implementation of := macro for settings. */
   def settingAssignMacroImpl[A1: Type](rec: Expr[Scoped.DefinableSetting[A1]], v: Expr[A1])(using
       qctx: Quotes
diff --git a/main-settings/src/test/scala/sbt/std/TaskPosSpec.scala b/main-settings/src/test/scala/sbt/std/TaskPosSpec.scala
index da3a22327..c82916515 100644
--- a/main-settings/src/test/scala/sbt/std/TaskPosSpec.scala
+++ b/main-settings/src/test/scala/sbt/std/TaskPosSpec.scala
@@ -106,6 +106,23 @@ class TaskPosSpec {
     }
   }
 
+  locally {
+    // .previous should compile for task with complex return type like `Seq[String]`
+    // https://github.com/sbt/sbt/issues/9037
+    import sbt.*, Def.*
+    import sjsonnew.BasicJsonProtocol.given
+    val link = taskKey[Int]("")
+    val fingerprints = taskKey[Seq[String]]("")
+    Def.taskDyn[Int] {
+      val currentFingerprints = (link / fingerprints).value
+      val previousFingerprints = (link / fingerprints).previous
+      Def.task {
+        if previousFingerprints.exists(_ != currentFingerprints) then currentFingerprints.size
+        else 0
+      }
+    }
+  }
+
   locally {
     // missing .value error should not happen inside task dyn
     import sbt.*, Def.*