diff --git a/README.md b/README.md
index 2214a49..e33359e 100644
--- a/README.md
+++ b/README.md
@@ -190,6 +190,7 @@ Each `[[job]]` entry defines one sync operation.
 | `mode` | string | yes | `"copy"` (add files) or `"sync"` (mirror, may delete) |
 | `backup_path` | string | no | Remote path prefix for archived deleted files (sync mode only) |
 | `backup_retention_days` | int | no | Days to keep archived files before cleanup |
+| `allow_destructive` | bool | no | Required when `mode = "sync"` and `backup_path` is empty. Acknowledges that deleted local files will be permanently removed from the remote on sync. Default: `false`. |
 | `filter_file` | string | no | Per-job filter file (overrides default) |
 
 Rclone jobs also accept all tuning fields from `[defaults.rclone]` as per-job overrides (`transfers`, `bwlimit`, etc.).
diff --git a/config.example.toml b/config.example.toml
index 4993a7d..a52a25c 100644
--- a/config.example.toml
+++ b/config.example.toml
@@ -60,6 +60,10 @@ delete = true
 
 # Rclone job: sync documents to multiple cloud remotes
 # Remotes must already be configured in rclone (run `rclone config`).
+# Note: sync mode deletes remote files when local files are removed.
+# Either set backup_path (shown here) to archive deletions for later
+# recovery, or set allow_destructive = true to opt in to permanent
+# deletion.
 [[job]]
 name = "documents-to-cloud"
 engine = "rclone"
@@ -68,6 +72,9 @@ remotes = ["my_gdrive", "my_s3"]
 mode = "sync"
 backup_path = "_archive"
 backup_retention_days = 365
+# allow_destructive = true   # required only if backup_path is empty;
+                             # acknowledges that deleted local files will
+                             # be permanently removed from the remote.
 
 # Rclone job: copy media (bandwidth-limited)
 [[job]]
diff --git a/internal/config/config.go b/internal/config/config.go
index cc488ea..290230e 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -104,7 +104,13 @@ type Job struct {
 	Mode                string   `toml:"mode"`
 	BackupPath          string   `toml:"backup_path"`
 	BackupRetentionDays int      `toml:"backup_retention_days"`
-	FilterFile          string   `toml:"filter_file"`
+	// AllowDestructive acknowledges that an rclone job with mode="sync"
+	// and empty backup_path permanently deletes remote files when their
+	// local counterparts are removed. Required for that configuration to
+	// pass validation. Ignored on all other configurations (copy mode,
+	// sync with backup_path set, rsync jobs).
+	AllowDestructive bool   `toml:"allow_destructive"`
+	FilterFile       string `toml:"filter_file"`
 
 	// Rclone per-job tuning overrides
 	Transfers       int    `toml:"transfers"`
@@ -346,6 +352,9 @@ func validateRcloneJob(job Job) error {
 		}
 		remoteSeen[r] = true
 	}
+	if err := validateDestructiveSync(job); err != nil {
+		return err
+	}
 	return validateMaxRuntime(job)
 }
 
@@ -369,3 +378,19 @@ func validateMaxRuntime(job Job) error {
 	}
 	return nil
 }
+
+// validateDestructiveSync rejects rclone sync jobs that would permanently
+// delete remote files without archival. Passes when mode is not "sync",
+// when backup_path is set (deletions are archived), or when the job
+// acknowledges the risk via AllowDestructive=true. The error message names
+// both resolution paths so the user can fix the config without consulting
+// documentation.
+func validateDestructiveSync(job Job) error {
+	if job.Mode != ModeSync || job.BackupPath != "" || job.AllowDestructive {
+		return nil
+	}
+	return fmt.Errorf(
+		`job %q: mode=%q without backup_path permanently deletes remote files when their local counterparts are removed. Set backup_path to archive deletions, or set allow_destructive=true to acknowledge this`,
+		job.Name, ModeSync,
+	)
+}
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index c2f27ce..4038190 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -699,3 +699,129 @@ max_runtime = "-1h"
 		t.Errorf("error %q should mention job name", err.Error())
 	}
 }
+
+func TestValidate_RcloneSyncWithBackupPath_Passes(t *testing.T) {
+	tomlData := `
+[[job]]
+name = "docs-safe"
+engine = "rclone"
+source = "/tmp/docs"
+remotes = ["gdrive"]
+mode = "sync"
+backup_path = "_archive"
+`
+	cfg, err := config.LoadBytes([]byte(tomlData))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if cfg.Jobs[0].AllowDestructive {
+		t.Error("AllowDestructive = true, want false (zero value)")
+	}
+}
+
+func TestValidate_RcloneSyncNoBackupPath_Rejected(t *testing.T) {
+	_, err := config.LoadFile(testdataPath("config_sync_no_backup_path.toml"))
+	if err == nil {
+		t.Fatal("expected error for sync without backup_path or opt-in, got nil")
+	}
+	msg := err.Error()
+	if !strings.Contains(msg, "unsafe-sync") {
+		t.Errorf("error %q should mention the job name", msg)
+	}
+	if !strings.Contains(msg, "allow_destructive") {
+		t.Errorf("error %q should mention allow_destructive", msg)
+	}
+	if !strings.Contains(msg, "backup_path") {
+		t.Errorf("error %q should mention backup_path", msg)
+	}
+	if !strings.Contains(msg, "mode=") {
+		t.Errorf("error %q should name the offending mode via the mode= prefix", msg)
+	}
+}
+
+func TestValidate_RcloneSyncAllowDestructive_Passes(t *testing.T) {
+	cfg, err := config.LoadFile(testdataPath("config_sync_allow_destructive.toml"))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if !cfg.Jobs[0].AllowDestructive {
+		t.Error("AllowDestructive = false, want true")
+	}
+}
+
+func TestValidate_RcloneCopyNoBackupPath_Passes(t *testing.T) {
+	tomlData := `
+[[job]]
+name = "copy-job"
+engine = "rclone"
+source = "/tmp/docs"
+remotes = ["gdrive"]
+mode = "copy"
+`
+	if _, err := config.LoadBytes([]byte(tomlData)); err != nil {
+		t.Errorf("copy mode with empty backup_path should pass, got: %v", err)
+	}
+}
+
+func TestValidate_RcloneSyncWithBothBackupPathAndAllowDestructive_Passes(t *testing.T) {
+	tomlData := `
+[[job]]
+name = "belt-and-suspenders"
+engine = "rclone"
+source = "/tmp/docs"
+remotes = ["gdrive"]
+mode = "sync"
+backup_path = "_archive"
+allow_destructive = true
+`
+	cfg, err := config.LoadBytes([]byte(tomlData))
+	if err != nil {
+		t.Fatalf("sync with both backup_path and allow_destructive should pass, got: %v", err)
+	}
+	if !cfg.Jobs[0].AllowDestructive {
+		t.Error("AllowDestructive = false, want true")
+	}
+	if cfg.Jobs[0].BackupPath == "" {
+		t.Error("BackupPath should be set")
+	}
+}
+
+func TestValidate_RsyncJobWithAllowDestructive_Accepted(t *testing.T) {
+	tomlData := `
+[[job]]
+name = "rsync-job"
+engine = "rsync"
+sources = ["/tmp/src"]
+destination = "/tmp/dst"
+allow_destructive = true
+`
+	cfg, err := config.LoadBytes([]byte(tomlData))
+	if err != nil {
+		t.Fatalf("allow_destructive on rsync job should be silently accepted, got: %v", err)
+	}
+	if !cfg.Jobs[0].AllowDestructive {
+		t.Error("AllowDestructive field should still unmarshal to true on rsync jobs")
+	}
+}
+
+func TestValidate_RcloneSyncExplicitFalse_Rejected(t *testing.T) {
+	// Pins that an explicit `allow_destructive = false` in TOML behaves
+	// identically to omitting the key. Guards against a future TOML
+	// library change that could distinguish zero-value from unset.
+	tomlData := `
+[[job]]
+name = "explicit-false-sync"
+engine = "rclone"
+source = "/tmp/docs"
+remotes = ["gdrive"]
+mode = "sync"
+allow_destructive = false
+`
+	_, err := config.LoadBytes([]byte(tomlData))
+	if err == nil {
+		t.Fatal("expected error for sync with explicit allow_destructive=false, got nil")
+	}
+	if !strings.Contains(err.Error(), "allow_destructive") {
+		t.Errorf("error %q should mention allow_destructive", err.Error())
+	}
+}
diff --git a/testdata/config_sync_allow_destructive.toml b/testdata/config_sync_allow_destructive.toml
new file mode 100644
index 0000000..2350ba5
--- /dev/null
+++ b/testdata/config_sync_allow_destructive.toml
@@ -0,0 +1,7 @@
+[[job]]
+name = "intentional-destructive"
+engine = "rclone"
+source = "/tmp/docs"
+remotes = ["gdrive"]
+mode = "sync"
+allow_destructive = true
diff --git a/testdata/config_sync_no_backup_path.toml b/testdata/config_sync_no_backup_path.toml
new file mode 100644
index 0000000..b160a94
--- /dev/null
+++ b/testdata/config_sync_no_backup_path.toml
@@ -0,0 +1,6 @@
+[[job]]
+name = "unsafe-sync"
+engine = "rclone"
+source = "/tmp/docs"
+remotes = ["gdrive"]
+mode = "sync"