diff --git a/backend/internal/apierr/errors.go b/backend/internal/apierr/errors.go
new file mode 100644
index 0000000..d620221
--- /dev/null
+++ b/backend/internal/apierr/errors.go
@@ -0,0 +1,23 @@
+// Package apierr translates upstream (Garage Admin API, S3/MinIO) errors into
+// backend API error responses with correct HTTP status codes and stable codes.
+package apierr
+
+import "fmt"
+
+// UpstreamError is a typed error describing a failure reported by an upstream
+// service. Returned from the services layer and consumed by handlers via Map
+// or Respond.
+type UpstreamError struct {
+	HTTPStatus int
+	Code       string            // upstream code, e.g. "BucketNotEmpty", "NoSuchKey"
+	Message    string            // human-readable upstream message
+	Source     string            // "garage" or "s3"
+	Details    map[string]string // optional: region, path, bucket, key
+}
+
+func (e *UpstreamError) Error() string {
+	if e.Code == "" {
+		return fmt.Sprintf("%s: %s", e.Source, e.Message)
+	}
+	return fmt.Sprintf("%s %s: %s", e.Source, e.Code, e.Message)
+}
diff --git a/backend/internal/apierr/errors_test.go b/backend/internal/apierr/errors_test.go
new file mode 100644
index 0000000..fc0e045
--- /dev/null
+++ b/backend/internal/apierr/errors_test.go
@@ -0,0 +1,40 @@
+package apierr
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestUpstreamError_Error(t *testing.T) {
+	e := &UpstreamError{
+		HTTPStatus: 409,
+		Code:       "BucketNotEmpty",
+		Message:    "Tried to delete a non-empty bucket",
+		Source:     "garage",
+	}
+	got := e.Error()
+	want := "garage BucketNotEmpty: Tried to delete a non-empty bucket"
+	if got != want {
+		t.Fatalf("Error() = %q, want %q", got, want)
+	}
+}
+
+func TestUpstreamError_ErrorWithoutCode(t *testing.T) {
+	e := &UpstreamError{HTTPStatus: 500, Source: "garage", Message: "server went boom"}
+	got := e.Error()
+	want := "garage: server went boom"
+	if got != want {
+		t.Fatalf("Error() = %q, want %q", got, want)
+	}
+}
+
+func TestUpstreamError_ErrorsAs(t *testing.T) {
+	var base error = &UpstreamError{HTTPStatus: 404, Code: "NoSuchBucket", Source: "garage"}
+	var target *UpstreamError
+	if !errors.As(base, &target) {
+		t.Fatal("errors.As should have matched *UpstreamError")
+	}
+	if target.Code != "NoSuchBucket" {
+		t.Fatalf("Code = %q, want NoSuchBucket", target.Code)
+	}
+}