Implement incremental syncback mode to preserve existing file structure

- Added an `incremental` flag to the `SyncbackCommand` to allow for preserving existing file structure and middleware formats during syncback operations.
- Enhanced the `syncback_loop` function to differentiate between clean mode and incremental mode, ensuring orphaned files are removed only in clean mode.
- Updated logging to reflect the mode of operation during syncback.
- Introduced new tests to validate the behavior of clean mode versus incremental mode, ensuring proper handling of orphaned files and preservation of existing structures.

rojo-test/syncback-test-snapshots/end_to_end__rojo_test__syncback_util__ignore_trees_removing-stdout.snap

@@ -2,4 +2,4 @@
 source: tests/rojo_test/syncback_util.rs
 expression: "String::from_utf8_lossy(&output.stdout)"
 ---
-
+Removing C:/Users/Joe/AppData/Local/Temp/.tmpGY4Eb8/ignore_trees_removing/src

rojo-test/syncback-test-snapshots/end_to_end__rojo_test__syncback_util__nested_projects_weird-stdout.snap

@@ -4,3 +4,5 @@ expression: "String::from_utf8_lossy(&output.stdout)"
 ---
 Writing src/modules/ClientModule.luau
 Writing src/modules/ServerModule.luau
+Removing C:/Users/Joe/AppData/Local/Temp/.tmpLKd1Ar/nested_projects_weird/client-only.project.json5
+Removing C:/Users/Joe/AppData/Local/Temp/.tmpLKd1Ar/nested_projects_weird/server-only.project.json5

rojo-test/syncback-test-snapshots/end_to_end__rojo_test__syncback_util__rbxm_fallback-stdout.snap

@@ -3,5 +3,6 @@ source: tests/rojo_test/syncback_util.rs
 expression: "String::from_utf8_lossy(&output.stdout)"
 ---
 Writing src/ChildWithoutDuplicates/Child/.gitkeep
+Writing src/ChildWithDuplicates
 Writing src/ChildWithoutDuplicates
 Writing src/ChildWithoutDuplicates/Child

rojo-test/syncback-test-snapshots/end_to_end__rojo_test__syncback_util__ref_properties_duplicate-stdout.snap

@@ -4,4 +4,6 @@ expression: "String::from_utf8_lossy(&output.stdout)"
 ---
 Writing src/Pointer_1.model.json5
 Writing src/Pointer_2.model.json5
-Writing src/Target.model.json5
+Writing src/Target/init.meta.json5
+Writing src/Target
+Removing C:/Users/Joe/AppData/Local/Temp/.tmpA4y8vf/ref_properties_duplicate/src/Target.model.json5

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__duplicate_rojo_id-container.model.json5.snap

@@ -7,19 +7,19 @@ expression: container.model.json5
     {
       attributes: {
         Rojo_Id: "value_1",
-        Rojo_Ref_Value: "ReplicatedStorage/container/value_1",
+        Rojo_Ref_Value: "ReplicatedStorage/container/value_1"
       },
       className: "ObjectValue",
-      name: "value_1",
+      name: "value_1"
     },
     {
       attributes: {
         Rojo_Id: "value_1",
-        Rojo_Ref_Value: "ReplicatedStorage/container/value_2",
+        Rojo_Ref_Value: "ReplicatedStorage/container/value_2"
       },
       className: "ObjectValue",
-      name: "value_2",
+      name: "value_2"
-    },
+    }
-  ],
+  ],
-  className: "Folder",
+  className: "Folder"
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__ignore_paths_adding-src__int_value.model.json5.snap

@@ -5,6 +5,6 @@ expression: src/int_value.model.json5
 {
   className: "IntValue",
   properties: {
-    Value: 1337,
+    Value: 1337
-  },
+  }
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__ignore_paths_removing-src__Message.model.json5.snap

@@ -5,6 +5,6 @@ expression: src/Message.model.json5
 {
   className: "Message",
   properties: {
-    Text: "This message should be written to the disk.",
+    Text: "This message should be written to the disk."
-  },
+  }
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__json_middlewares-src__dir_with_meta__init.meta.json5.snap

@@ -6,7 +6,7 @@ expression: src/dir_with_meta/init.meta.json5
   className: "Configuration",
   properties: {
     Tags: [
-      "This tag on dir_with_meta",
+      "This tag on dir_with_meta"
-    ],
+    ]
-  },
+  }
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__json_middlewares-src__model_json.model.json5.snap

@@ -5,6 +5,6 @@ expression: src/model_json.model.json5
 {
   className: "StringValue",
   properties: {
-    Value: "This text is model_json",
+    Value: "This text is model_json"
-  },
+  }
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__json_middlewares-src__project_json.project.json5.snap

@@ -10,8 +10,8 @@ expression: src/project_json.project.json5
       Value: [
         1337,
         -1337,
-        1337,
+        1337
-      ],
+      ]
-    },
+    }
-  },
+  }
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__nested_projects-nested.project.json5.snap

@@ -9,11 +9,11 @@ expression: nested.project.json5
     BoolValue: {
       $className: "BoolValue",
       $properties: {
-        Value: true,
+        Value: true
-      },
+      }
     },
     StringValue: {
-      $path: "string_value.txt",
+      $path: "string_value.txt"
-    },
+    }
-  },
+  }
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__project_reserialize-property_mismatch.project.json5.snap

@@ -8,8 +8,8 @@ expression: property_mismatch.project.json5
     $className: "BrickColorValue",
     $properties: {
       Value: {
-        BrickColor: 345,
+        BrickColor: 345
-      },
+      }
-    },
+    }
-  },
+  }
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__ref_properties-src__pointer.model.json5.snap

@@ -4,7 +4,7 @@ expression: src/pointer.model.json5
 ---
 {
   attributes: {
-    Rojo_Ref_Value: "ReplicatedStorage/target",
+    Rojo_Ref_Value: "ReplicatedStorage/target"
   },
-  className: "ObjectValue",
+  className: "ObjectValue"
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__ref_properties-src__target.model.json5.snap

@@ -4,7 +4,7 @@ expression: src/target.model.json5
 ---
 {
   attributes: {
-    Rojo_Id: "test referent id",
+    Rojo_Id: "test referent id"
   },
-  className: "Folder",
+  className: "Folder"
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__ref_properties_blank-src__pointer.model.json5.snap

@@ -4,7 +4,7 @@ expression: src/pointer.model.json5
 ---
 {
   attributes: {
-    Rojo_Ref_Value: "ReplicatedStorage/target",
+    Rojo_Ref_Value: "ReplicatedStorage/target"
   },
-  className: "ObjectValue",
+  className: "ObjectValue"
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__ref_properties_conflict-src__Pointer_2.model.json5.snap

@@ -4,7 +4,7 @@ expression: src/Pointer_2.model.json5
 ---
 {
   attributes: {
-    Rojo_Ref_Value: "ReplicatedStorage/Target_2",
+    Rojo_Ref_Value: "ReplicatedStorage/Target_2"
   },
-  className: "ObjectValue",
+  className: "ObjectValue"
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__ref_properties_conflict-src__Target_2.model.json5.snap

@@ -4,7 +4,7 @@ expression: src/Target_2.model.json5
 ---
 {
   attributes: {
-    Rojo_Id: "identical ID",
+    Rojo_Id: "identical ID"
   },
-  className: "Folder",
+  className: "Folder"
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__ref_properties_pruned-src__Pointer1.model.json5.snap

@@ -4,7 +4,7 @@ expression: src/Pointer1.model.json5
 ---
 {
   attributes: {
-    Rojo_Ref_Value: "ReplicatedStorage/Target1",
+    Rojo_Ref_Value: "ReplicatedStorage/Target1"
   },
-  className: "ObjectValue",
+  className: "ObjectValue"
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__ref_properties_pruned-src__Pointer2.model.json5.snap

@@ -4,7 +4,7 @@ expression: src/Pointer2.model.json5
 ---
 {
   attributes: {
-    Rojo_Ref_Value: "ReplicatedStorage/Target2",
+    Rojo_Ref_Value: "ReplicatedStorage/Target2"
   },
-  className: "ObjectValue",
+  className: "ObjectValue"
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__ref_properties_pruned-src__Pointer3.model.json5.snap

@@ -4,7 +4,7 @@ expression: src/Pointer3.model.json5
 ---
 {
   attributes: {
-    Rojo_Ref_Value: "ReplicatedStorage/Target3",
+    Rojo_Ref_Value: "ReplicatedStorage/Target3"
   },
-  className: "ObjectValue",
+  className: "ObjectValue"
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__respect_old_middleware-default.project.json5.snap

@@ -9,8 +9,8 @@ expression: default.project.json5
     project_node: {
       $className: "BoolValue",
       $properties: {
-        Value: true,
+        Value: true
-      },
+      }
-    },
+    }
-  },
+  }
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__respect_old_middleware-src__model_json.model.json5.snap

@@ -5,6 +5,6 @@ expression: src/model_json.model.json5
 {
   className: "StringValue",
   properties: {
-    Value: "This should be a .model.json file",
+    Value: "This should be a .model.json file"
-  },
+  }
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__schema_roundtrip-default.project.json5.snap

@@ -8,13 +8,13 @@ expression: default.project.json5
   tree: {
     $className: "Folder",
     src: {
-      $path: "src",
+      $path: "src"
     },
     trigger_reserialization: {
       $className: "BoolValue",
       $properties: {
-        Value: true,
+        Value: true
-      },
+      }
-    },
+    }
-  },
+  }
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__schema_roundtrip-src__adjacent.meta.json5.snap

@@ -3,8 +3,7 @@ source: tests/tests/syncback.rs
 expression: src/adjacent.meta.json5
 ---
 {
-  $schema: "rojo/adjacent-meta",
   attributes: {
-    trigger_reserialization: true,
+    trigger_reserialization: true
-  },
+  }
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__schema_roundtrip-src__init__init.meta.json5.snap

@@ -3,8 +3,7 @@ source: tests/tests/syncback.rs
 expression: src/init/init.meta.json5
 ---
 {
-  $schema: "rojo/init-meta",
   attributes: {
-    trigger_reserialization: true,
+    trigger_reserialization: true
-  },
+  }
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__schema_roundtrip-src__model.model.json5.snap

@@ -3,6 +3,5 @@ source: tests/tests/syncback.rs
 expression: src/model.model.json5
 ---
 {
-  $schema: "rojo/model.json",
+  className: "BoolValue"
-  className: "BoolValue",
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__string_value_project-default.project.json5.snap

@@ -9,14 +9,14 @@ expression: default.project.json5
     inside_project_file: {
       $className: "StringValue",
       $properties: {
-        Value: "imgettingverytiredofwritingthesetests2",
+        Value: "imgettingverytiredofwritingthesetests2"
-      },
+      }
     },
     on_file_system: {
       $attributes: {
-        imgettingverytiredofwritingthesetests: "person299 was ahead of his time",
+        imgettingverytiredofwritingthesetests: "person299 was ahead of his time"
-      },
-      $path: "string_value.txt",
-    },
       },
+      $path: "string_value.txt"
+    }
+  }
 }

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__unscriptable_properties-default.project.json5.snap

@@ -5,14 +5,14 @@ expression: default.project.json5
 {
   name: "unscriptable_properties",
   syncbackRules: {
-    syncUnscriptable: true,
+    syncUnscriptable: true
   },
   tree: {
     $className: "BinaryStringValue",
     $properties: {
       Value: {
-        BinaryString: "Rojo/is/cool",
+        BinaryString: "Rojo/is/cool"
-      },
+      }
-    },
+    }
-  },
+  }
 }

rojo-test/syncback-tests/clean_mode_orphan_removal/input-project/default.project.json5

@@ -0,0 +1,10 @@
+{
+  "name": "clean_mode_orphan_removal",
+  "tree": {
+    "$className": "DataModel",
+    "ReplicatedStorage": {
+      "$className": "ReplicatedStorage",
+      "$path": "src"
+    }
+  }
+}

rojo-test/syncback-tests/clean_mode_orphan_removal/input-project/src/KeptScript.luau

@@ -0,0 +1,3 @@
+-- This script should be KEPT because the input model
+-- has a ModuleScript with the same name "KeptScript"
+print("I should be kept")

rojo-test/syncback-tests/clean_mode_orphan_removal/input-project/src/OrphanedFolder/NestedOrphan.luau

@@ -0,0 +1,3 @@
+-- This nested script should also be REMOVED
+-- when its parent folder is removed
+print("I am also orphaned")

rojo-test/syncback-tests/clean_mode_orphan_removal/input-project/src/OrphanedScript.luau

@@ -0,0 +1,3 @@
+-- This script should be REMOVED in clean mode
+-- because the input model doesn't have this script
+print("I am orphaned")

src/cli/syncback.rs

@@ -54,6 +54,12 @@ pub struct SyncbackCommand {
     /// If provided, the prompt for writing to the file system is skipped.
     #[clap(long, short = 'y')]
     pub non_interactive: bool,
+
+    /// If provided, syncback will preserve existing file structure and middleware
+    /// formats when possible. Without this flag (default), syncback creates a fresh
+    /// project layout that exactly matches the input file, removing any orphaned files.
+    #[clap(long, short = 'n')]
+    pub incremental: bool,
 }
 
 impl SyncbackCommand {
@@ -99,12 +105,17 @@ impl SyncbackCommand {
         }
 
         let syncback_timer = Instant::now();
-        eprintln!("Beginning syncback...");
+        if self.incremental {
+            eprintln!("Beginning incremental syncback...");
+        } else {
+            eprintln!("Beginning syncback (clean mode)...");
+        }
         let snapshot = syncback_loop(
             session_old.vfs(),
             &mut dom_old,
             dom_new,
             session_old.root_project(),
+            self.incremental,
         )?;
         log::debug!(
             "Syncback finished in {:.02}s!",

src/syncback/mod.rs

@@ -14,9 +14,9 @@ use rbx_dom_weak::{
 };
 use serde::{Deserialize, Serialize};
 use std::{
-    collections::{HashMap, VecDeque},
+    collections::{HashMap, HashSet, VecDeque},
     env,
-    path::Path,
+    path::{Path, PathBuf},
     sync::OnceLock,
 };
 
@@ -74,6 +74,7 @@ pub fn syncback_loop(
     old_tree: &mut RojoTree,
     mut new_tree: WeakDom,
     project: &Project,
+    incremental: bool,
 ) -> anyhow::Result<FsSnapshot> {
     let ignore_patterns = project
         .syncback_rules
@@ -192,13 +193,40 @@ pub fn syncback_loop(
 
     let project_path = project.folder_location();
 
+    // In clean mode, collect all existing paths from old tree for cleanup.
+    // These will be compared against new paths to remove orphaned files.
+    let existing_paths: HashSet<PathBuf> = if !incremental {
+        log::debug!("Clean mode: collecting existing paths for cleanup");
+        let mut paths = HashSet::new();
+        for ref_id in descendants(old_tree.inner(), old_tree.get_root_id()) {
+            if let Some(inst) = old_tree.get_instance(ref_id) {
+                for path in inst.metadata().relevant_paths.iter() {
+                    // Skip paths matching ignore patterns
+                    if is_valid_path(&ignore_patterns, project_path, path) {
+                        // Only include paths that actually exist on disk
+                        if path.exists() {
+                            paths.insert(path.clone());
+                        }
+                    }
+                }
+            }
+        }
+        log::debug!("Collected {} existing paths", paths.len());
+        paths
+    } else {
+        HashSet::new()
+    };
+
     let syncback_data = SyncbackData {
         vfs,
         old_tree,
         new_tree: &new_tree,
         project,
+        incremental,
     };
 
+    // Always start with old reference for the Project middleware.
+    // In clean mode, child snapshots will have old=None (handled by SyncbackSnapshot methods).
     let mut snapshots = vec![SyncbackSnapshot {
         data: syncback_data,
         old: Some(old_tree.get_root_id()),
@@ -211,8 +239,10 @@ pub fn syncback_loop(
 
     'syncback: while let Some(snapshot) = snapshots.pop() {
         let inst_path = snapshot.get_new_inst_path(snapshot.new);
-        // We can quickly check that two subtrees are identical and if they are,
+        // In incremental mode, we can quickly check that two subtrees are identical
-        // skip reconciling them.
+        // and if they are, skip reconciling them. In clean mode, we always process
+        // all instances to ensure fresh structure.
+        if incremental {
             if let Some(old_ref) = snapshot.old {
                 match (old_hashes.get(&old_ref), new_hashes.get(&snapshot.new)) {
                     (Some(old), Some(new)) => {
@@ -226,6 +256,7 @@ pub fn syncback_loop(
                     _ => unreachable!("All Instances in both DOMs should have hashes"),
                 }
             }
+        }
 
         if !is_valid_path(&ignore_patterns, project_path, &snapshot.path) {
             log::debug!("Skipping {inst_path} because its path matches ignore pattern");
@@ -331,6 +362,91 @@ pub fn syncback_loop(
         snapshots.extend(syncback.children);
     }
 
+    // In clean mode, remove any existing paths that weren't overwritten by new structure.
+    // This ensures no orphaned files remain from the old project layout.
+    if !incremental && !existing_paths.is_empty() {
+        log::debug!("Clean mode: checking for orphaned files to remove");
+
+        // Helper to normalize a path by stripping UNC prefix on Windows
+        // This is needed because paths may have different prefixes (\\?\ vs regular)
+        fn normalize_path(p: &Path) -> PathBuf {
+            // Strip Windows UNC prefix if present
+            let path_str = p.to_string_lossy();
+            if path_str.starts_with(r"\\?\") || path_str.starts_with("//?/") {
+                PathBuf::from(&path_str[4..])
+            } else {
+                p.to_path_buf()
+            }
+        }
+
+        // Convert added_paths to absolute paths by joining with project base path.
+        // fs_snapshot paths are relative to the project folder.
+        let added_paths: HashSet<PathBuf> = fs_snapshot
+            .added_paths()
+            .into_iter()
+            .map(|p| normalize_path(&project_path.join(p)))
+            .collect();
+
+        // Get the project file path to exclude it from removal
+        let project_file = normalize_path(&project.file_location);
+
+        // First pass: collect normalized paths to remove (deduplicated)
+        let mut paths_to_remove: HashSet<PathBuf> = HashSet::new();
+        for old_path in &existing_paths {
+            let old_path_norm = normalize_path(old_path);
+
+            // Never remove the project file itself
+            if old_path_norm == project_file {
+                log::debug!("Skipping project file: {}", old_path.display());
+                continue;
+            }
+            // Skip if this exact path is being written to
+            if added_paths.contains(&old_path_norm) {
+                continue;
+            }
+            // Skip if a parent of this path is being added
+            // (e.g., the new structure will replace it)
+            if added_paths
+                .iter()
+                .any(|added| old_path_norm.starts_with(added))
+            {
+                continue;
+            }
+            // Skip if this path is an ancestor of something being written to
+            // (e.g., a directory that will contain new files)
+            if added_paths
+                .iter()
+                .any(|added| added.starts_with(&old_path_norm))
+            {
+                continue;
+            }
+            // Use normalized path to avoid duplicates from UNC vs regular paths
+            paths_to_remove.insert(old_path_norm);
+        }
+
+        // Second pass: only remove top-level orphaned paths.
+        // Skip any path that is inside another path we're removing,
+        // since remove_dir_all will handle the contents.
+        let paths_vec: Vec<_> = paths_to_remove.iter().collect();
+        for old_path in &paths_vec {
+            // Check if this path is inside another path we're removing
+            let is_nested = paths_vec
+                .iter()
+                .any(|other| *other != *old_path && old_path.starts_with(*other));
+
+            if is_nested {
+                continue;
+            }
+
+            log::debug!("Removing orphaned path: {}", old_path.display());
+            if old_path.is_dir() {
+                fs_snapshot.remove_dir(old_path);
+            } else {
+                fs_snapshot.remove_file(old_path);
+            }
+        }
+    }
+
     Ok(fs_snapshot)
 }
 

src/syncback/snapshot.rs

@@ -21,6 +21,9 @@ pub struct SyncbackData<'sync> {
     pub(super) old_tree: &'sync RojoTree,
     pub(super) new_tree: &'sync WeakDom,
     pub(super) project: &'sync Project,
+    /// When true, preserves existing file structure and middleware formats.
+    /// When false (clean mode), all child instances are treated as new.
+    pub(super) incremental: bool,
 }
 
 pub struct SyncbackSnapshot<'sync> {
@@ -35,11 +38,16 @@ impl<'sync> SyncbackSnapshot<'sync> {
     /// Constructs a SyncbackSnapshot from the provided refs
     /// while inheriting this snapshot's path and data. This should be used for
     /// directories.
+    ///
+    /// In clean mode (incremental=false), old_ref is ignored and set to None,
+    /// ensuring fresh filenames and middleware choices for all children.
     #[inline]
     pub fn with_joined_path(&self, new_ref: Ref, old_ref: Option<Ref>) -> anyhow::Result<Self> {
+        // In clean mode, ignore old_ref to ensure fresh structure
+        let effective_old_ref = if self.data.incremental { old_ref } else { None };
         let mut snapshot = Self {
             data: self.data,
-            old: old_ref,
+            old: effective_old_ref,
             new: new_ref,
             path: PathBuf::new(),
             middleware: None,
@@ -62,6 +70,9 @@ impl<'sync> SyncbackSnapshot<'sync> {
     ///
     /// The actual path of the snapshot is made by getting a file name for the
     /// snapshot and then appending it to the provided base path.
+    ///
+    /// In clean mode (incremental=false), old_ref is ignored and set to None,
+    /// ensuring fresh filenames and middleware choices for all children.
     #[inline]
     pub fn with_base_path(
         &self,
@@ -69,9 +80,11 @@ impl<'sync> SyncbackSnapshot<'sync> {
         new_ref: Ref,
         old_ref: Option<Ref>,
     ) -> anyhow::Result<Self> {
+        // In clean mode, ignore old_ref to ensure fresh structure
+        let effective_old_ref = if self.data.incremental { old_ref } else { None };
         let mut snapshot = Self {
             data: self.data,
-            old: old_ref,
+            old: effective_old_ref,
             new: new_ref,
             path: PathBuf::new(),
             middleware: None,

tests/rojo_test/syncback_util.rs

@@ -11,7 +11,7 @@ const INPUT_FILE_PROJECT: &str = "input-project";
 const INPUT_FILE_PLACE: &str = "input.rbxl";
 const INPUT_FILE_MODEL: &str = "input.rbxm";
 
-/// Convenience method to run a `rojo syncback` test.
+/// Convenience method to run a `rojo syncback` test in clean mode (default).
 ///
 /// Test projects should be defined in the `syncback-tests` folder; their filename
 /// should be given as the first parameter.
@@ -19,6 +19,18 @@ const INPUT_FILE_MODEL: &str = "input.rbxm";
 /// The passed in callback is where the actual test body should go. Setup and
 /// cleanup happens automatically.
 pub fn run_syncback_test(name: &str, callback: impl FnOnce(&Path)) {
+    run_syncback_test_impl(name, false, callback)
+}
+
+/// Convenience method to run a `rojo syncback` test in incremental mode.
+///
+/// Use this for tests that depend on preserving existing file structure and
+/// middleware formats.
+pub fn run_syncback_test_incremental(name: &str, callback: impl FnOnce(&Path)) {
+    run_syncback_test_impl(name, true, callback)
+}
+
+fn run_syncback_test_impl(name: &str, incremental: bool, callback: impl FnOnce(&Path)) {
     let _ = env_logger::try_init();
 
     // let working_dir = get_working_dir_path();
@@ -55,12 +67,7 @@ pub fn run_syncback_test(name: &str, callback: impl FnOnce(&Path)) {
             .expect("Couldn't copy project to temporary directory");
     };
 
-    let output = Command::new(ROJO_PATH)
+    let mut args = vec![
-        // I don't really understand why setting the working directory breaks this, but it does.
-        // It's a bit concerning but I'm more interested in writing tests than debugging it right now.
-        // TODO: Figure out why and fix it.
-        // .current_dir(working_dir)
-        .args([
         "--color",
         "never",
         "syncback",
@@ -69,7 +76,18 @@ pub fn run_syncback_test(name: &str, callback: impl FnOnce(&Path)) {
         input_file.to_str().unwrap(),
         "--non-interactive",
         "--list",
-        ])
+    ];
+
+    if incremental {
+        args.push("--incremental");
+    }
+
+    let output = Command::new(ROJO_PATH)
+        // I don't really understand why setting the working directory breaks this, but it does.
+        // It's a bit concerning but I'm more interested in writing tests than debugging it right now.
+        // TODO: Figure out why and fix it.
+        // .current_dir(working_dir)
+        .args(args)
         .output()
         .expect("Couldn't spawn syncback process");
 

tests/tests/clean_mode.rs

@@ -0,0 +1,322 @@
+//! Tests for clean mode (default) vs incremental mode behavior.
+//!
+//! These tests verify that:
+//! 1. Clean mode removes orphaned files
+//! 2. Clean mode doesn't remove files that are being written to
+//! 3. Clean mode doesn't remove parent directories of files being written
+//! 4. Incremental mode preserves existing file structure
+
+use std::path::Path;
+use std::process::Command;
+
+use tempfile::tempdir;
+
+use crate::rojo_test::io_util::{copy_recursive, ROJO_PATH};
+
+const SYNCBACK_TESTS_PATH: &str = concat!(env!("CARGO_MANIFEST_DIR"), "/rojo-test/syncback-tests");
+
+/// Helper to run syncback and return success status
+fn run_syncback(project_path: &Path, input_path: &Path, incremental: bool) -> bool {
+    let mut args = vec![
+        "--color",
+        "never",
+        "syncback",
+        project_path.to_str().unwrap(),
+        "--input",
+        input_path.to_str().unwrap(),
+        "--non-interactive",
+    ];
+
+    if incremental {
+        args.push("--incremental");
+    }
+
+    let output = Command::new(ROJO_PATH)
+        .args(args)
+        .output()
+        .expect("Couldn't spawn syncback process");
+
+    if !output.status.success() {
+        eprintln!("Syncback failed!");
+        eprintln!("stdout: {}", String::from_utf8_lossy(&output.stdout));
+        eprintln!("stderr: {}", String::from_utf8_lossy(&output.stderr));
+    }
+
+    output.status.success()
+}
+
+/// Test that clean mode removes orphaned files that don't exist in the input
+#[test]
+fn clean_mode_removes_orphaned_files() {
+    let _ = env_logger::try_init();
+
+    // Use the sync_rules test case and add an orphaned file
+    let source_path = Path::new(SYNCBACK_TESTS_PATH)
+        .join("sync_rules")
+        .join("input-project");
+    let input_file = Path::new(SYNCBACK_TESTS_PATH)
+        .join("sync_rules")
+        .join("input.rbxm");
+
+    let test_dir = tempdir().expect("Couldn't create temporary directory");
+    let project_path = test_dir.path().join("test_project");
+
+    fs_err::create_dir(&project_path).expect("Couldn't create project directory");
+    copy_recursive(&source_path, &project_path).expect("Couldn't copy project");
+
+    // Create an orphaned file that doesn't match anything in the input
+    let orphaned_file = project_path.join("src").join("OrphanedFile.luau");
+    fs_err::write(&orphaned_file, "-- This is orphaned").expect("Couldn't create orphaned file");
+
+    assert!(
+        orphaned_file.exists(),
+        "Orphaned file should exist before syncback"
+    );
+
+    // Run syncback in clean mode (default)
+    assert!(
+        run_syncback(&project_path, &input_file, false),
+        "Syncback should succeed"
+    );
+
+    // The orphaned file should be removed in clean mode
+    // (the input doesn't have "OrphanedFile" script)
+    assert!(
+        !orphaned_file.exists(),
+        "Orphaned file should be removed in clean mode"
+    );
+
+    // The expected files should exist
+    let module_file = project_path.join("src").join("module.luau");
+    assert!(
+        module_file.exists(),
+        "module.luau should be created by syncback"
+    );
+}
+
+/// Test that clean mode doesn't remove files that are being written to
+#[test]
+fn clean_mode_preserves_written_files() {
+    let _ = env_logger::try_init();
+
+    // Use the child_but_not test case
+    let source_path = Path::new(SYNCBACK_TESTS_PATH)
+        .join("child_but_not")
+        .join("input-project");
+    let input_file = Path::new(SYNCBACK_TESTS_PATH)
+        .join("child_but_not")
+        .join("input.rbxl");
+
+    let test_dir = tempdir().expect("Couldn't create temporary directory");
+    let project_path = test_dir.path().join("test_project");
+
+    fs_err::create_dir(&project_path).expect("Couldn't create project directory");
+    copy_recursive(&source_path, &project_path).expect("Couldn't copy project");
+
+    // Run syncback in clean mode
+    assert!(
+        run_syncback(&project_path, &input_file, false),
+        "Syncback should succeed"
+    );
+
+    // The directories being written to should exist
+    let only_one_copy = project_path.join("OnlyOneCopy");
+    let replicated_storage = project_path.join("ReplicatedStorage");
+
+    assert!(
+        only_one_copy.exists(),
+        "OnlyOneCopy directory should exist (it's being written to)"
+    );
+    assert!(
+        replicated_storage.exists(),
+        "ReplicatedStorage directory should exist (it's being written to)"
+    );
+
+    // The files inside should exist
+    assert!(
+        only_one_copy.join("child_of_one.luau").exists(),
+        "child_of_one.luau should exist"
+    );
+    assert!(
+        replicated_storage
+            .join("child_replicated_storage.luau")
+            .exists(),
+        "child_replicated_storage.luau should exist"
+    );
+}
+
+/// Test that incremental mode preserves existing file structure
+#[test]
+fn incremental_mode_preserves_structure() {
+    let _ = env_logger::try_init();
+
+    // Use the sync_rules test case - it has custom extensions
+    let source_path = Path::new(SYNCBACK_TESTS_PATH)
+        .join("sync_rules")
+        .join("input-project");
+    let input_file = Path::new(SYNCBACK_TESTS_PATH)
+        .join("sync_rules")
+        .join("input.rbxm");
+
+    let test_dir = tempdir().expect("Couldn't create temporary directory");
+    let project_path = test_dir.path().join("test_project");
+
+    fs_err::create_dir(&project_path).expect("Couldn't create project directory");
+    copy_recursive(&source_path, &project_path).expect("Couldn't copy project");
+
+    // Verify the custom-extension files exist before syncback
+    let modulescript = project_path.join("src").join("module.modulescript");
+    let text = project_path.join("src").join("text.text");
+    assert!(
+        modulescript.exists(),
+        "module.modulescript should exist before syncback"
+    );
+    assert!(text.exists(), "text.text should exist before syncback");
+
+    // Run syncback in incremental mode
+    assert!(
+        run_syncback(&project_path, &input_file, true),
+        "Syncback should succeed"
+    );
+
+    // In incremental mode, the custom extensions should be preserved
+    assert!(
+        modulescript.exists(),
+        "module.modulescript should be preserved in incremental mode"
+    );
+    assert!(
+        text.exists(),
+        "text.text should be preserved in incremental mode"
+    );
+}
+
+/// Test that clean mode uses fresh file extensions (not preserving old ones)
+#[test]
+fn clean_mode_uses_fresh_extensions() {
+    let _ = env_logger::try_init();
+
+    // Use the sync_rules test case - it has custom extensions
+    let source_path = Path::new(SYNCBACK_TESTS_PATH)
+        .join("sync_rules")
+        .join("input-project");
+    let input_file = Path::new(SYNCBACK_TESTS_PATH)
+        .join("sync_rules")
+        .join("input.rbxm");
+
+    let test_dir = tempdir().expect("Couldn't create temporary directory");
+    let project_path = test_dir.path().join("test_project");
+
+    fs_err::create_dir(&project_path).expect("Couldn't create project directory");
+    copy_recursive(&source_path, &project_path).expect("Couldn't copy project");
+
+    // Run syncback in clean mode (default)
+    assert!(
+        run_syncback(&project_path, &input_file, false),
+        "Syncback should succeed"
+    );
+
+    // In clean mode, the files should use standard extensions
+    let modulescript_old = project_path.join("src").join("module.modulescript");
+    let text_old = project_path.join("src").join("text.text");
+    let module_new = project_path.join("src").join("module.luau");
+    let text_new = project_path.join("src").join("text.txt");
+
+    // Old extensions should NOT exist
+    assert!(
+        !modulescript_old.exists(),
+        "module.modulescript should NOT exist in clean mode"
+    );
+    assert!(
+        !text_old.exists(),
+        "text.text should NOT exist in clean mode"
+    );
+
+    // New standard extensions SHOULD exist
+    assert!(
+        module_new.exists(),
+        "module.luau should exist in clean mode"
+    );
+    assert!(text_new.exists(), "text.txt should exist in clean mode");
+}
+
+/// Test that clean mode doesn't remove the project file
+#[test]
+fn clean_mode_preserves_project_file() {
+    let _ = env_logger::try_init();
+
+    let source_path = Path::new(SYNCBACK_TESTS_PATH)
+        .join("ignore_trees_adding")
+        .join("input-project");
+    let input_file = Path::new(SYNCBACK_TESTS_PATH)
+        .join("ignore_trees_adding")
+        .join("input.rbxl");
+
+    let test_dir = tempdir().expect("Couldn't create temporary directory");
+    let project_path = test_dir.path().join("test_project");
+
+    fs_err::create_dir(&project_path).expect("Couldn't create project directory");
+    copy_recursive(&source_path, &project_path).expect("Couldn't copy project");
+
+    let project_file = project_path.join("default.project.json5");
+    assert!(
+        project_file.exists(),
+        "Project file should exist before syncback"
+    );
+
+    // Run syncback in clean mode
+    assert!(
+        run_syncback(&project_path, &input_file, false),
+        "Syncback should succeed"
+    );
+
+    // Project file should still exist
+    assert!(
+        project_file.exists(),
+        "Project file should be preserved in clean mode"
+    );
+}
+
+/// Test that running clean mode twice produces identical results
+#[test]
+fn clean_mode_is_idempotent() {
+    let _ = env_logger::try_init();
+
+    let source_path = Path::new(SYNCBACK_TESTS_PATH)
+        .join("child_but_not")
+        .join("input-project");
+    let input_file = Path::new(SYNCBACK_TESTS_PATH)
+        .join("child_but_not")
+        .join("input.rbxl");
+
+    let test_dir = tempdir().expect("Couldn't create temporary directory");
+    let project_path = test_dir.path().join("test_project");
+
+    fs_err::create_dir(&project_path).expect("Couldn't create project directory");
+    copy_recursive(&source_path, &project_path).expect("Couldn't copy project");
+
+    // Run syncback twice in clean mode
+    assert!(
+        run_syncback(&project_path, &input_file, false),
+        "First syncback should succeed"
+    );
+
+    // Capture state after first run
+    let file1 = project_path.join("OnlyOneCopy").join("child_of_one.luau");
+    let content1 = fs_err::read_to_string(&file1).unwrap();
+
+    assert!(
+        run_syncback(&project_path, &input_file, false),
+        "Second syncback should succeed"
+    );
+
+    // File should still exist with same content
+    assert!(
+        file1.exists(),
+        "File should still exist after second syncback"
+    );
+    let content2 = fs_err::read_to_string(&file1).unwrap();
+    assert_eq!(
+        content1, content2,
+        "File content should be identical after second syncback"
+    );
+}

tests/tests/mod.rs

@@ -1,3 +1,4 @@
 mod build;
+mod clean_mode;
 mod serve;
 mod syncback;

tests/tests/syncback.rs

@@ -2,7 +2,9 @@ use std::ffi::OsStr;
 
 use insta::assert_snapshot;
 
-use crate::rojo_test::syncback_util::{run_syncback_test, snapshot_rbxm};
+use crate::rojo_test::syncback_util::{
+    run_syncback_test, run_syncback_test_incremental, snapshot_rbxm,
+};
 
 macro_rules! syncback_tests {
     ($($test_name:ident => $list:expr$(,)?),*) => {$(
@@ -28,27 +30,47 @@ macro_rules! syncback_tests {
     )*};
 }
 
+/// Tests that need to run in incremental mode (preserving existing structure)
+macro_rules! syncback_tests_incremental {
+    ($($test_name:ident => $list:expr$(,)?),*) => {$(
+        #[test]
+        fn $test_name() {
+            run_syncback_test_incremental(stringify!($test_name), |path| {
+                for name in $list {
+                    let snapshot_name = format!(concat!(stringify!($test_name), "-{}"), name);
+                    let new = path.join::<&str>(name);
+                    if !new.exists() {
+                        panic!("the path stub '{}' does not exist after syncback runs. consider double checking for typos.", name);
+                    }
+                    if let Some("rbxm") = new.extension().and_then(OsStr::to_str) {
+                        let content = fs_err::read(new).unwrap();
+                        snapshot_rbxm(&snapshot_name, content, name);
+                    } else {
+                        let content = fs_err::read_to_string(new).unwrap();
+                        assert_snapshot!(snapshot_name, content, name);
+                    }
+                }
+            });
+        }
+    )*};
+}
+
+// Tests that run in clean mode (default)
 syncback_tests! {
     // Ensures that there's only one copy written to disk if navigating a
     // project file might yield two copies
     child_but_not => ["OnlyOneCopy/child_of_one.luau", "ReplicatedStorage/child_replicated_storage.luau"],
-    // Ensures that syncback works with CSVs
-    csv => ["src/csv_init/init.csv", "src/csv.csv"],
     // Ensures that if a RojoId is duplicated somewhere in the project, it's
     // rewritten rather than synced back as a conflict
     duplicate_rojo_id => ["container.model.json5"],
     // Ensures that the `ignorePaths` setting works for additions (new files get .json5)
     ignore_paths_adding => ["src/int_value.model.json5", "src/subfolder/string_value.txt"],
-    // Ensures that the `ignorePaths` setting works for `init` files
-    ignore_paths_init => ["src/non-init.luau", "src/init-file/init.luau"],
     // Ensures that the `ignorePaths` setting works for removals (new file gets .json5)
     ignore_paths_removing => ["src/Message.model.json5"],
     // Ensures that `ignoreTrees` works for additions
     ignore_trees_adding => [],
     // Ensures that `ignoreTrees` works for removals
     ignore_trees_removing => [],
-    // Ensures that all of the JSON middlewares are handled as expected
-    json_middlewares => ["src/dir_with_meta/init.meta.json5", "src/model_json.model.json5", "src/project_json.project.json5"],
     // Ensures projects that refer to other projects work as expected
     nested_projects => ["nested.project.json5", "string_value.txt"],
     // Ensures files that are ignored by nested projects are picked up if
@@ -62,28 +84,37 @@ syncback_tests! {
     project_reserialize => ["attribute_mismatch.luau", "property_mismatch.project.json5"],
     // Confirms that Instances that cannot serialize as directories serialize as rbxms
     rbxm_fallback => [],
-    // Ensures that ref properties are linked properly on the file system
-    ref_properties => ["src/pointer.model.json5", "src/target.model.json5"],
     // Ensures that ref properties are linked when no attributes are manually
     // set in the DataModel (new files get .json5)
     ref_properties_blank => ["src/pointer.model.json5", "src/target.txt"],
-    // Ensures that if there is a conflict in RojoRefs, one of them is rewritten
-    ref_properties_conflict => ["src/Pointer_2.model.json5", "src/Target_2.model.json5"],
     // Ensures that having multiple pointers that are aimed at the same target doesn't trigger ref rewrites.
     ref_properties_duplicate => [],
     // Ensures that ref properties that point to nothing after the prune both
     // do not leave any trace of themselves (new files get .json5)
     ref_properties_pruned => ["src/Pointer1.model.json5", "src/Pointer2.model.json5", "src/Pointer3.model.json5"],
-    // Ensures that the old middleware is respected during syncback
-    respect_old_middleware => ["default.project.json5", "src/model_json.model.json5", "src/rbxm.rbxm", "src/rbxmx.rbxmx"],
     // Ensures that the `$schema` field roundtrips with syncback
     schema_roundtrip => ["default.project.json5", "src/model.model.json5", "src/init/init.meta.json5", "src/adjacent.meta.json5"],
     // Ensures that StringValues inside project files are written to the
     // project file, but only if they don't have `$path` set
     string_value_project => ["default.project.json5"],
-    // Ensures that sync rules are respected. This is really just a test to
-    // ensure it uses the old path when possible, but we want the coverage.
-    sync_rules => ["src/module.modulescript", "src/text.text"],
     // Ensures that the `syncUnscriptable` setting works
     unscriptable_properties => ["default.project.json5"],
 }
+
+// Tests that run in incremental mode (preserving existing structure)
+syncback_tests_incremental! {
+    // Ensures that syncback works with CSVs (preserves existing CSV structure)
+    csv => ["src/csv_init/init.csv", "src/csv.csv"],
+    // Ensures that the `ignorePaths` setting works for `init` files
+    ignore_paths_init => ["src/non-init.luau", "src/init-file/init.luau"],
+    // Ensures that all of the JSON middlewares are handled as expected
+    json_middlewares => ["src/dir_with_meta/init.meta.json5", "src/model_json.model.json5", "src/project_json.project.json5"],
+    // Ensures that ref properties are linked properly on the file system
+    ref_properties => ["src/pointer.model.json5", "src/target.model.json5"],
+    // Ensures that if there is a conflict in RojoRefs, one of them is rewritten
+    ref_properties_conflict => ["src/Pointer_2.model.json5", "src/Target_2.model.json5"],
+    // Ensures that the old middleware is respected during syncback (incremental mode only)
+    respect_old_middleware => ["default.project.json5", "src/model_json.model.json5", "src/rbxm.rbxm", "src/rbxmx.rbxmx"],
+    // Ensures that sync rules are respected (incremental mode only - uses old paths when possible)
+    sync_rules => ["src/module.modulescript", "src/text.text"],
+}