Merge branch 'feature/init-name-resolution'

Instead of bailing when children have duplicate filesystem names,
syncback now resolves collisions by appending incrementing suffixes
(e.g. Foo, Foo1, Foo2). This handles both init-renamed children and
any other name collisions. Meta stem derivation is now path-based
to correctly handle collision suffixes and dotted names.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

rojo-test/syncback-test-snapshots/end_to_end__rojo_test__syncback_util__rbxm_fallback-src__ChildWithDuplicates.rbxm.snap

@@ -1,73 +0,0 @@
----
-source: tests/rojo_test/syncback_util.rs
-expression: src/ChildWithDuplicates.rbxm
----
-num_types: 1
-num_instances: 3
-chunks:
-  - Inst:
-      type_id: 0
-      type_name: Folder
-      object_format: 0
-      referents:
-        - 0
-        - 1
-        - 2
-  - Prop:
-      type_id: 0
-      prop_name: AttributesSerialize
-      prop_type: String
-      values:
-        - ""
-        - ""
-        - ""
-  - Prop:
-      type_id: 0
-      prop_name: Capabilities
-      prop_type: SecurityCapabilities
-      values:
-        - 0
-        - 0
-        - 0
-  - Prop:
-      type_id: 0
-      prop_name: Name
-      prop_type: String
-      values:
-        - DuplicateChild
-        - DuplicateChild
-        - ChildWithDuplicates
-  - Prop:
-      type_id: 0
-      prop_name: DefinesCapabilities
-      prop_type: Bool
-      values:
-        - false
-        - false
-        - false
-  - Prop:
-      type_id: 0
-      prop_name: SourceAssetId
-      prop_type: Int64
-      values:
-        - -1
-        - -1
-        - -1
-  - Prop:
-      type_id: 0
-      prop_name: Tags
-      prop_type: String
-      values:
-        - ""
-        - ""
-        - ""
-  - Prnt:
-      version: 0
-      links:
-        - - 0
-          - 2
-        - - 1
-          - 2
-        - - 2
-          - -1
-  - End

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

@@ -1,9 +1,12 @@
 ---
 source: tests/rojo_test/syncback_util.rs
+assertion_line: 101
 expression: "String::from_utf8_lossy(&output.stdout)"
 ---
-Writing src/ChildWithDuplicates.rbxm
+Writing src/ChildWithDuplicates/DuplicateChild/.gitkeep
+Writing src/ChildWithDuplicates/DuplicateChild1/.gitkeep
 Writing src/ChildWithoutDuplicates/Child/.gitkeep
+Writing src/ChildWithDuplicates/DuplicateChild
+Writing src/ChildWithDuplicates/DuplicateChild1
 Writing src/ChildWithoutDuplicates
 Writing src/ChildWithoutDuplicates/Child
-Removing src/ChildWithDuplicates

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__rbxm_fallback-src__ChildWithDuplicates__DuplicateChild1__.gitkeep.snap

@@ -0,0 +1,6 @@
+---
+source: tests/tests/syncback.rs
+assertion_line: 31
+expression: src/ChildWithDuplicates/DuplicateChild1/.gitkeep
+---
+

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__rbxm_fallback-src__ChildWithDuplicates__DuplicateChild__.gitkeep.snap

@@ -0,0 +1,6 @@
+---
+source: tests/tests/syncback.rs
+assertion_line: 31
+expression: src/ChildWithDuplicates/DuplicateChild/.gitkeep
+---
+

src/cli/syncback.rs

@@ -54,6 +54,11 @@ 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, forces syncback to use JSON model files instead of binary
+    /// .rbxm files for instances that would otherwise serialize as binary.
+    #[clap(long)]
+    pub dangerously_force_json: bool,
 }
 
 impl SyncbackCommand {
@@ -104,6 +109,7 @@ impl SyncbackCommand {
             &mut dom_old,
             dom_new,
             session_old.root_project(),
+            self.dangerously_force_json,
         )?;
         log::debug!(
             "Syncback finished in {:.02}s!",

src/snapshot_middleware/csv.rs

@@ -109,8 +109,14 @@ pub fn syncback_csv<'sync>(
 
         if !meta.is_empty() {
             let parent = snapshot.path.parent_err()?;
+            let file_name = snapshot
+                .path
+                .file_name()
+                .and_then(|n| n.to_str())
+                .unwrap_or("");
+            let meta_stem = file_name.strip_suffix(".csv").unwrap_or(file_name);
             fs_snapshot.add_file(
-                parent.join(format!("{}.meta.json", new_inst.name)),
+                parent.join(format!("{meta_stem}.meta.json")),
                 serde_json::to_vec_pretty(&meta).context("cannot serialize metadata")?,
             )
         }

src/snapshot_middleware/dir.rs

@@ -8,10 +8,13 @@ use memofs::{DirEntry, Vfs};
 
 use crate::{
     snapshot::{InstanceContext, InstanceMetadata, InstanceSnapshot, InstigatingSource},
-    syncback::{hash_instance, FsSnapshot, SyncbackReturn, SyncbackSnapshot},
+    syncback::{
+        extension_for_middleware, hash_instance, FsSnapshot, SyncbackReturn,
+        SyncbackSnapshot,
+    },
 };
 
-use super::{meta_file::DirectoryMetadata, snapshot_from_vfs};
+use super::{meta_file::DirectoryMetadata, snapshot_from_vfs, Middleware};
 
 const EMPTY_DIR_KEEP_NAME: &str = ".gitkeep";
 
@@ -91,6 +94,22 @@ pub fn snapshot_dir_no_meta(
     Ok(Some(snapshot))
 }
 
+/// Splits a filesystem name into (stem, extension) based on middleware type.
+/// For directory middleware, the extension is empty. For file middleware,
+/// the extension comes from `extension_for_middleware`.
+fn split_name_and_ext(name: &str, middleware: Middleware) -> (&str, &str) {
+    if middleware.is_dir() {
+        (name, "")
+    } else {
+        let ext = extension_for_middleware(middleware);
+        if let Some(stem) = name.strip_suffix(&format!(".{ext}")) {
+            (stem, ext)
+        } else {
+            (name, "")
+        }
+    }
+}
+
 pub fn syncback_dir<'sync>(
     snapshot: &SyncbackSnapshot<'sync>,
 ) -> anyhow::Result<SyncbackReturn<'sync>> {
@@ -134,65 +153,128 @@ pub fn syncback_dir_no_meta<'sync>(
     let mut children = Vec::new();
     let mut removed_children = Vec::new();
 
-    // We have to enforce unique child names for the file system.
-    let mut child_names = HashSet::with_capacity(new_inst.children().len());
-    let mut duplicate_set = HashSet::new();
-    for child_ref in new_inst.children() {
-        let child = snapshot.get_new_instance(*child_ref).unwrap();
-        if !child_names.insert(child.name.to_lowercase()) {
-            duplicate_set.insert(child.name.as_str());
-        }
-    }
-    if !duplicate_set.is_empty() {
-        if duplicate_set.len() <= 25 {
-            anyhow::bail!(
-                "Instance has children with duplicate name (case may not exactly match):\n {}",
-                duplicate_set.into_iter().collect::<Vec<&str>>().join(", ")
-            );
-        }
-        anyhow::bail!("Instance has more than 25 children with duplicate names");
-    }
-
+    // Build the old child map early so it can be used for deduplication below.
+    let mut old_child_map = HashMap::new();
     if let Some(old_inst) = snapshot.old_inst() {
-        let mut old_child_map = HashMap::with_capacity(old_inst.children().len());
         for child in old_inst.children() {
             let inst = snapshot.get_old_instance(*child).unwrap();
             old_child_map.insert(inst.name(), inst);
         }
+    }
 
-        for new_child_ref in new_inst.children() {
-            let new_child = snapshot.get_new_instance(*new_child_ref).unwrap();
-            if let Some(old_child) = old_child_map.remove(new_child.name.as_str()) {
-                if old_child.metadata().relevant_paths.is_empty() {
-                    log::debug!(
-                        "Skipping instance {} because it doesn't exist on the disk",
-                        old_child.name()
-                    );
-                    continue;
-                } else if matches!(
-                    old_child.metadata().instigating_source,
-                    Some(InstigatingSource::ProjectNode { .. })
-                ) {
-                    log::debug!(
-                        "Skipping instance {} because it originates in a project file",
-                        old_child.name()
-                    );
-                    continue;
-                }
-                // This child exists in both doms. Pass it on.
-                children.push(snapshot.with_joined_path(*new_child_ref, Some(old_child.id()))?);
-            } else {
-                // The child only exists in the the new dom
-                children.push(snapshot.with_joined_path(*new_child_ref, None)?);
+    // --- Two-pass collision resolution ---
+    //
+    // Pass 1: Collect each child's base filesystem name and old ref, applying
+    // skip conditions. Track which names are used (lowercased) so we can
+    // detect collisions.
+    struct ChildEntry {
+        new_ref: rbx_dom_weak::types::Ref,
+        old_ref: Option<rbx_dom_weak::types::Ref>,
+        base_name: String,
+        middleware: Middleware,
+        skip: bool,
+    }
+
+    let mut entries = Vec::with_capacity(new_inst.children().len());
+    let mut used_names: HashSet<String> = HashSet::with_capacity(new_inst.children().len());
+    let mut collision_indices: Vec<usize> = Vec::new();
+
+    for new_child_ref in new_inst.children() {
+        let new_child = snapshot.get_new_instance(*new_child_ref).unwrap();
+
+        // Determine old_ref and apply skip conditions.
+        let old_child = if snapshot.old_inst().is_some() {
+            old_child_map.remove(new_child.name.as_str())
+        } else {
+            None
+        };
+
+        let mut skip = false;
+        if let Some(ref old) = old_child {
+            if old.metadata().relevant_paths.is_empty() {
+                log::debug!(
+                    "Skipping instance {} because it doesn't exist on the disk",
+                    old.name()
+                );
+                skip = true;
+            } else if matches!(
+                old.metadata().instigating_source,
+                Some(InstigatingSource::ProjectNode { .. })
+            ) {
+                log::debug!(
+                    "Skipping instance {} because it originates in a project file",
+                    old.name()
+                );
+                skip = true;
             }
         }
-        // Any children that are in the old dom but not the new one are removed.
-        removed_children.extend(old_child_map.into_values());
-    } else {
-        // There is no old instance. Just add every child.
-        for new_child_ref in new_inst.children() {
-            children.push(snapshot.with_joined_path(*new_child_ref, None)?);
+
+        let old_ref = old_child.as_ref().map(|o| o.id());
+
+        if skip {
+            entries.push(ChildEntry {
+                new_ref: *new_child_ref,
+                old_ref,
+                base_name: String::new(),
+                middleware: Middleware::Dir,
+                skip: true,
+            });
+            continue;
         }
+
+        let (middleware, base_name) =
+            snapshot.child_middleware_and_name(*new_child_ref, old_ref)?;
+
+        let idx = entries.len();
+        let lower = base_name.to_lowercase();
+        if !used_names.insert(lower) {
+            // Name already claimed — needs resolution.
+            collision_indices.push(idx);
+        }
+
+        entries.push(ChildEntry {
+            new_ref: *new_child_ref,
+            old_ref,
+            base_name,
+            middleware,
+            skip: false,
+        });
+    }
+
+    // Pass 2: Resolve collisions by appending incrementing suffixes.
+    for idx in collision_indices {
+        let entry = &entries[idx];
+        let (stem, ext) = split_name_and_ext(&entry.base_name, entry.middleware);
+        let mut counter = 1u32;
+        loop {
+            let candidate = if ext.is_empty() {
+                format!("{stem}{counter}")
+            } else {
+                format!("{stem}{counter}.{ext}")
+            };
+            let lower = candidate.to_lowercase();
+            if used_names.insert(lower) {
+                // Safe to mutate — we only visit each collision index once.
+                let entry = &mut entries[idx];
+                entry.base_name = candidate;
+                break;
+            }
+            counter += 1;
+        }
+    }
+
+    // Create snapshots from resolved entries.
+    for entry in &entries {
+        if entry.skip {
+            continue;
+        }
+        let resolved_path = snapshot.path.join(&entry.base_name);
+        children.push(snapshot.with_new_path(resolved_path, entry.new_ref, entry.old_ref));
+    }
+
+    // Any children that are in the old dom but not the new one are removed.
+    if snapshot.old_inst().is_some() {
+        removed_children.extend(old_child_map.into_values());
     }
     let mut fs_snapshot = FsSnapshot::new();
 
@@ -225,6 +307,12 @@ pub fn syncback_dir_no_meta<'sync>(
 mod test {
     use super::*;
 
+    use std::path::PathBuf;
+
+    use crate::{
+        snapshot::{InstanceMetadata, InstanceSnapshot},
+        Project, RojoTree, SyncbackData, SyncbackSnapshot,
+    };
     use memofs::{InMemoryFs, VfsSnapshot};
 
     #[test]
@@ -261,4 +349,302 @@ mod test {
 
         insta::assert_yaml_snapshot!(instance_snapshot);
     }
+
+    fn make_project() -> Project {
+        serde_json::from_str(r#"{"tree": {"$className": "DataModel"}}"#).unwrap()
+    }
+
+    fn make_vfs() -> Vfs {
+        let mut imfs = InMemoryFs::new();
+        imfs.load_snapshot("/root", VfsSnapshot::empty_dir()).unwrap();
+        Vfs::new(imfs)
+    }
+
+    /// Two children whose Roblox names are identical when lowercased ("Alpha"
+    /// and "alpha") but live at different filesystem paths because of the
+    /// `name` property ("Beta/" and "Alpha/" respectively). The dedup check
+    /// must use the actual filesystem paths, not the raw Roblox names, to
+    /// avoid a false-positive duplicate error.
+    #[test]
+    fn syncback_no_false_duplicate_with_name_prop() {
+        use rbx_dom_weak::{InstanceBuilder, WeakDom};
+
+        // Old child A: Roblox name "Alpha", on disk at "/root/Beta"
+        //   (name property maps "Alpha" → "Beta" on the filesystem)
+        let old_child_a = InstanceSnapshot::new()
+            .name("Alpha")
+            .class_name("Folder")
+            .metadata(
+                InstanceMetadata::new()
+                    .instigating_source(PathBuf::from("/root/Beta"))
+                    .relevant_paths(vec![PathBuf::from("/root/Beta")]),
+            );
+        // Old child B: Roblox name "alpha", on disk at "/root/Alpha"
+        let old_child_b = InstanceSnapshot::new()
+            .name("alpha")
+            .class_name("Folder")
+            .metadata(
+                InstanceMetadata::new()
+                    .instigating_source(PathBuf::from("/root/Alpha"))
+                    .relevant_paths(vec![PathBuf::from("/root/Alpha")]),
+            );
+        let old_parent = InstanceSnapshot::new()
+            .name("Parent")
+            .class_name("Folder")
+            .children(vec![old_child_a, old_child_b])
+            .metadata(
+                InstanceMetadata::new()
+                    .instigating_source(PathBuf::from("/root"))
+                    .relevant_paths(vec![PathBuf::from("/root")]),
+            );
+        let old_tree = RojoTree::new(old_parent);
+
+        // New state: same two children in Roblox.
+        let mut new_tree = WeakDom::new(InstanceBuilder::new("ROOT"));
+        let new_parent = new_tree.insert(
+            new_tree.root_ref(),
+            InstanceBuilder::new("Folder").with_name("Parent"),
+        );
+        new_tree.insert(new_parent, InstanceBuilder::new("Folder").with_name("Alpha"));
+        new_tree.insert(new_parent, InstanceBuilder::new("Folder").with_name("alpha"));
+
+        let vfs = make_vfs();
+        let project = make_project();
+        let data = SyncbackData::for_test(&vfs, &old_tree, &new_tree, &project);
+        let snapshot = SyncbackSnapshot {
+            data,
+            old: Some(old_tree.get_root_id()),
+            new: new_parent,
+            path: PathBuf::from("/root"),
+            middleware: None,
+        };
+
+        let result = syncback_dir_no_meta(&snapshot);
+        assert!(
+            result.is_ok(),
+            "should not error when two children have the same lowercased Roblox \
+             name but map to distinct filesystem paths: {:?}",
+            result.as_ref().err(),
+        );
+    }
+
+    /// Two completely new children with the same name get resolved via
+    /// incrementing suffixes instead of erroring.
+    #[test]
+    fn syncback_resolves_sibling_duplicate_names() {
+        use rbx_dom_weak::{InstanceBuilder, WeakDom};
+
+        let old_parent = InstanceSnapshot::new()
+            .name("Parent")
+            .class_name("Folder")
+            .metadata(
+                InstanceMetadata::new()
+                    .instigating_source(PathBuf::from("/root"))
+                    .relevant_paths(vec![PathBuf::from("/root")]),
+            );
+        let old_tree = RojoTree::new(old_parent);
+
+        let mut new_tree = WeakDom::new(InstanceBuilder::new("ROOT"));
+        let new_parent = new_tree.insert(
+            new_tree.root_ref(),
+            InstanceBuilder::new("Folder").with_name("Parent"),
+        );
+        new_tree.insert(new_parent, InstanceBuilder::new("Folder").with_name("Foo"));
+        new_tree.insert(new_parent, InstanceBuilder::new("Folder").with_name("Foo"));
+
+        let vfs = make_vfs();
+        let project = make_project();
+        let data = SyncbackData::for_test(&vfs, &old_tree, &new_tree, &project);
+        let snapshot = SyncbackSnapshot {
+            data,
+            old: Some(old_tree.get_root_id()),
+            new: new_parent,
+            path: PathBuf::from("/root"),
+            middleware: None,
+        };
+
+        let result = syncback_dir_no_meta(&snapshot);
+        assert!(
+            result.is_ok(),
+            "should resolve duplicate names with suffixes, not error: {:?}",
+            result.as_ref().err(),
+        );
+        let children = result.unwrap().children;
+        let mut names: Vec<String> = children
+            .iter()
+            .map(|c| c.path.file_name().unwrap().to_string_lossy().into_owned())
+            .collect();
+        names.sort();
+        assert_eq!(names, vec!["Foo", "Foo1"]);
+    }
+
+    /// A new child named "Init" (as a ModuleScript) would naively become
+    /// "Init.luau", which case-insensitively matches the parent's reserved
+    /// "init.luau". Syncback must resolve this automatically by prefixing the
+    /// filesystem name with '_' (→ "_Init.luau") rather than erroring.
+    #[test]
+    fn syncback_resolves_init_name_conflict() {
+        use rbx_dom_weak::{InstanceBuilder, WeakDom};
+
+        let old_parent = InstanceSnapshot::new()
+            .name("Parent")
+            .class_name("Folder")
+            .metadata(
+                InstanceMetadata::new()
+                    .instigating_source(PathBuf::from("/root"))
+                    .relevant_paths(vec![PathBuf::from("/root")]),
+            );
+        let old_tree = RojoTree::new(old_parent);
+
+        let mut new_tree = WeakDom::new(InstanceBuilder::new("ROOT"));
+        let new_parent = new_tree.insert(
+            new_tree.root_ref(),
+            InstanceBuilder::new("Folder").with_name("Parent"),
+        );
+        new_tree.insert(
+            new_parent,
+            InstanceBuilder::new("ModuleScript").with_name("Init"),
+        );
+
+        let vfs = make_vfs();
+        let project = make_project();
+        let data = SyncbackData::for_test(&vfs, &old_tree, &new_tree, &project);
+        let snapshot = SyncbackSnapshot {
+            data,
+            old: Some(old_tree.get_root_id()),
+            new: new_parent,
+            path: PathBuf::from("/root"),
+            middleware: None,
+        };
+
+        let result = syncback_dir_no_meta(&snapshot);
+        assert!(
+            result.is_ok(),
+            "should resolve init-name conflict by prefixing '_', not error: {:?}",
+            result.as_ref().err(),
+        );
+        // The child should have been placed at "_Init.luau", not "Init.luau".
+        let child_file_name = result
+            .unwrap()
+            .children
+            .into_iter()
+            .next()
+            .and_then(|c| c.path.file_name().map(|n| n.to_string_lossy().into_owned()))
+            .unwrap_or_default();
+        assert!(
+            child_file_name.starts_with('_'),
+            "child filesystem name should start with '_' to avoid init collision, \
+             got: {child_file_name}",
+        );
+    }
+
+    /// A child whose filesystem name is stored with a slugified prefix (e.g.
+    /// "_Init") must NOT be blocked — only the bare "init" stem is reserved.
+    #[test]
+    fn syncback_allows_slugified_init_name() {
+        use rbx_dom_weak::{InstanceBuilder, WeakDom};
+
+        // Existing child: on disk as "_Init" (slugified from a name with an
+        // illegal character), its stem is "_init" which is not reserved.
+        let old_child = InstanceSnapshot::new()
+            .name("Init")
+            .class_name("Folder")
+            .metadata(
+                InstanceMetadata::new()
+                    .instigating_source(PathBuf::from("/root/_Init"))
+                    .relevant_paths(vec![PathBuf::from("/root/_Init")]),
+            );
+        let old_parent = InstanceSnapshot::new()
+            .name("Parent")
+            .class_name("Folder")
+            .children(vec![old_child])
+            .metadata(
+                InstanceMetadata::new()
+                    .instigating_source(PathBuf::from("/root"))
+                    .relevant_paths(vec![PathBuf::from("/root")]),
+            );
+        let old_tree = RojoTree::new(old_parent);
+
+        let mut new_tree = WeakDom::new(InstanceBuilder::new("ROOT"));
+        let new_parent = new_tree.insert(
+            new_tree.root_ref(),
+            InstanceBuilder::new("Folder").with_name("Parent"),
+        );
+        new_tree.insert(new_parent, InstanceBuilder::new("Folder").with_name("Init"));
+
+        let vfs = make_vfs();
+        let project = make_project();
+        let data = SyncbackData::for_test(&vfs, &old_tree, &new_tree, &project);
+        let snapshot = SyncbackSnapshot {
+            data,
+            old: Some(old_tree.get_root_id()),
+            new: new_parent,
+            path: PathBuf::from("/root"),
+            middleware: None,
+        };
+
+        let result = syncback_dir_no_meta(&snapshot);
+        assert!(
+            result.is_ok(),
+            "should allow a child whose filesystem name is slugified away from \
+             the reserved 'init' stem: {:?}",
+            result.as_ref().err(),
+        );
+    }
+
+    /// Two new children both named "Init" (ModuleScripts) should get
+    /// "_Init.luau" and "_Init1.luau" respectively.
+    #[test]
+    fn syncback_resolves_multiple_init_conflicts() {
+        use rbx_dom_weak::{InstanceBuilder, WeakDom};
+
+        let old_parent = InstanceSnapshot::new()
+            .name("Parent")
+            .class_name("Folder")
+            .metadata(
+                InstanceMetadata::new()
+                    .instigating_source(PathBuf::from("/root"))
+                    .relevant_paths(vec![PathBuf::from("/root")]),
+            );
+        let old_tree = RojoTree::new(old_parent);
+
+        let mut new_tree = WeakDom::new(InstanceBuilder::new("ROOT"));
+        let new_parent = new_tree.insert(
+            new_tree.root_ref(),
+            InstanceBuilder::new("Folder").with_name("Parent"),
+        );
+        new_tree.insert(
+            new_parent,
+            InstanceBuilder::new("ModuleScript").with_name("Init"),
+        );
+        new_tree.insert(
+            new_parent,
+            InstanceBuilder::new("ModuleScript").with_name("Init"),
+        );
+
+        let vfs = make_vfs();
+        let project = make_project();
+        let data = SyncbackData::for_test(&vfs, &old_tree, &new_tree, &project);
+        let snapshot = SyncbackSnapshot {
+            data,
+            old: Some(old_tree.get_root_id()),
+            new: new_parent,
+            path: PathBuf::from("/root"),
+            middleware: None,
+        };
+
+        let result = syncback_dir_no_meta(&snapshot);
+        assert!(
+            result.is_ok(),
+            "should resolve multiple init conflicts with suffixes: {:?}",
+            result.as_ref().err(),
+        );
+        let children = result.unwrap().children;
+        let mut names: Vec<String> = children
+            .iter()
+            .map(|c| c.path.file_name().unwrap().to_string_lossy().into_owned())
+            .collect();
+        names.sort();
+        assert_eq!(names, vec!["_Init.luau", "_Init1.luau"]);
+    }
 }

src/snapshot_middleware/lua.rs

@@ -158,16 +158,23 @@ pub fn syncback_lua<'sync>(
 
         if !meta.is_empty() {
             let parent_location = snapshot.path.parent_err()?;
-            let instance_name = &snapshot.new_inst().name;
-            let slugified;
-            let meta_name = if crate::syncback::validate_file_name(instance_name).is_err() {
-                slugified = crate::syncback::slugify_name(instance_name);
-                &slugified
-            } else {
-                instance_name
-            };
+            let file_name = snapshot
+                .path
+                .file_name()
+                .and_then(|n| n.to_str())
+                .unwrap_or("");
+            let meta_stem = file_name
+                .strip_suffix(".server.luau")
+                .or_else(|| file_name.strip_suffix(".server.lua"))
+                .or_else(|| file_name.strip_suffix(".client.luau"))
+                .or_else(|| file_name.strip_suffix(".client.lua"))
+                .or_else(|| file_name.strip_suffix(".plugin.luau"))
+                .or_else(|| file_name.strip_suffix(".plugin.lua"))
+                .or_else(|| file_name.strip_suffix(".luau"))
+                .or_else(|| file_name.strip_suffix(".lua"))
+                .unwrap_or(file_name);
             fs_snapshot.add_file(
-                parent_location.join(format!("{}.meta.json", meta_name)),
+                parent_location.join(format!("{meta_stem}.meta.json")),
                 serde_json::to_vec_pretty(&meta).context("cannot serialize metadata")?,
             );
         }

src/snapshot_middleware/meta_file.rs

@@ -154,11 +154,13 @@ impl AdjacentMetadata {
             .old_inst()
             .and_then(|inst| inst.metadata().specified_name.clone())
             .or_else(|| {
-                // If this is a new instance and its name is invalid for the filesystem,
-                // we need to specify the name in meta.json so it can be preserved
+                // Write name when name_for_inst would produce a different
+                // filesystem stem (slugification or init-prefix).
                 if snapshot.old_inst().is_none() {
                     let instance_name = &snapshot.new_inst().name;
-                    if validate_file_name(instance_name).is_err() {
+                    if validate_file_name(instance_name).is_err()
+                        || instance_name.to_lowercase() == "init"
+                    {
                         Some(instance_name.clone())
                     } else {
                         None
@@ -421,11 +423,13 @@ impl DirectoryMetadata {
             .old_inst()
             .and_then(|inst| inst.metadata().specified_name.clone())
             .or_else(|| {
-                // If this is a new instance and its name is invalid for the filesystem,
-                // we need to specify the name in meta.json so it can be preserved
+                // Write name when name_for_inst would produce a different
+                // directory name (slugification or init-prefix).
                 if snapshot.old_inst().is_none() {
                     let instance_name = &snapshot.new_inst().name;
-                    if validate_file_name(instance_name).is_err() {
+                    if validate_file_name(instance_name).is_err()
+                        || instance_name.to_lowercase() == "init"
+                    {
                         Some(instance_name.clone())
                     } else {
                         None

src/snapshot_middleware/txt.rs

@@ -58,8 +58,14 @@ pub fn syncback_txt<'sync>(
 
         if !meta.is_empty() {
             let parent = snapshot.path.parent_err()?;
+            let file_name = snapshot
+                .path
+                .file_name()
+                .and_then(|n| n.to_str())
+                .unwrap_or("");
+            let meta_stem = file_name.strip_suffix(".txt").unwrap_or(file_name);
             fs_snapshot.add_file(
-                parent.join(format!("{}.meta.json", new_inst.name)),
+                parent.join(format!("{meta_stem}.meta.json")),
                 serde_json::to_vec_pretty(&meta).context("could not serialize metadata")?,
             );
         }

src/syncback/file_names.rs

@@ -36,23 +36,33 @@ pub fn name_for_inst<'a>(
             | Middleware::ServerScriptDir
             | Middleware::ClientScriptDir
             | Middleware::ModuleScriptDir => {
-                if validate_file_name(&new_inst.name).is_err() {
+                let name = if validate_file_name(&new_inst.name).is_err() {
                     Cow::Owned(slugify_name(&new_inst.name))
                 } else {
-                    Cow::Borrowed(&new_inst.name)
+                    Cow::Borrowed(new_inst.name.as_str())
+                };
+                // Prefix "init" to avoid colliding with reserved init files.
+                if name.to_lowercase() == "init" {
+                    Cow::Owned(format!("_{name}"))
+                } else {
+                    name
                 }
             }
             _ => {
                 let extension = extension_for_middleware(middleware);
                 let slugified;
-                let final_name = if validate_file_name(&new_inst.name).is_err() {
+                let stem: &str = if validate_file_name(&new_inst.name).is_err() {
                     slugified = slugify_name(&new_inst.name);
                     &slugified
                 } else {
                     &new_inst.name
                 };
-
-                Cow::Owned(format!("{final_name}.{extension}"))
+                // Prefix "init" stems to avoid colliding with reserved init files.
+                if stem.to_lowercase() == "init" {
+                    Cow::Owned(format!("_{stem}.{extension}"))
+                } else {
+                    Cow::Owned(format!("{stem}.{extension}"))
+                }
             }
         })
     }

src/syncback/mod.rs

@@ -28,7 +28,7 @@ use crate::{
     Project,
 };
 
-pub use file_names::{extension_for_middleware, name_for_inst, slugify_name, validate_file_name};
+pub use file_names::{extension_for_middleware, name_for_inst, validate_file_name};
 pub use fs_snapshot::FsSnapshot;
 pub use hash::*;
 pub use property_filter::{filter_properties, filter_properties_preallocated};
@@ -52,6 +52,7 @@ pub fn syncback_loop(
     old_tree: &mut RojoTree,
     mut new_tree: WeakDom,
     project: &Project,
+    force_json: bool,
 ) -> anyhow::Result<FsSnapshot> {
     let ignore_patterns = project
         .syncback_rules
@@ -153,6 +154,7 @@ pub fn syncback_loop(
         old_tree,
         new_tree: &new_tree,
         project,
+        force_json,
     };
 
     let mut snapshots = vec![SyncbackSnapshot {
@@ -197,7 +199,7 @@ pub fn syncback_loop(
             }
         }
 
-        let middleware = get_best_middleware(&snapshot);
+        let middleware = get_best_middleware(&snapshot, force_json);
 
         log::trace!(
             "Middleware for {inst_path} is {:?} (path is {})",
@@ -213,10 +215,14 @@ pub fn syncback_loop(
         let syncback = match middleware.syncback(&snapshot) {
             Ok(syncback) => syncback,
             Err(err) if middleware == Middleware::Dir => {
-                let new_middleware = match env::var(DEBUG_MODEL_FORMAT_VAR) {
-                    Ok(value) if value == "1" => Middleware::Rbxmx,
-                    Ok(value) if value == "2" => Middleware::JsonModel,
-                    _ => Middleware::Rbxm,
+                let new_middleware = if force_json {
+                    Middleware::JsonModel
+                } else {
+                    match env::var(DEBUG_MODEL_FORMAT_VAR) {
+                        Ok(value) if value == "1" => Middleware::Rbxmx,
+                        Ok(value) if value == "2" => Middleware::JsonModel,
+                        _ => Middleware::Rbxm,
+                    }
                 };
                 let file_name = snapshot
                     .path
@@ -295,7 +301,7 @@ pub struct SyncbackReturn<'sync> {
     pub removed_children: Vec<InstanceWithMeta<'sync>>,
 }
 
-pub fn get_best_middleware(snapshot: &SyncbackSnapshot) -> Middleware {
+pub fn get_best_middleware(snapshot: &SyncbackSnapshot, force_json: bool) -> Middleware {
     // At some point, we're better off using an O(1) method for checking
     // equality for classes like this.
     static JSON_MODEL_CLASSES: OnceLock<HashSet<&str>> = OnceLock::new();
@@ -367,10 +373,18 @@ pub fn get_best_middleware(snapshot: &SyncbackSnapshot) -> Middleware {
     }
 
     if middleware == Middleware::Rbxm {
-        middleware = match env::var(DEBUG_MODEL_FORMAT_VAR) {
-            Ok(value) if value == "1" => Middleware::Rbxmx,
-            Ok(value) if value == "2" => Middleware::JsonModel,
-            _ => Middleware::Rbxm,
+        middleware = if force_json {
+            if !inst.children().is_empty() {
+                Middleware::Dir
+            } else {
+                Middleware::JsonModel
+            }
+        } else {
+            match env::var(DEBUG_MODEL_FORMAT_VAR) {
+                Ok(value) if value == "1" => Middleware::Rbxmx,
+                Ok(value) if value == "2" => Middleware::JsonModel,
+                _ => Middleware::Rbxm,
+            }
         }
     }
 

src/syncback/snapshot.rs

@@ -20,6 +20,7 @@ pub struct SyncbackData<'sync> {
     pub(super) old_tree: &'sync RojoTree,
     pub(super) new_tree: &'sync WeakDom,
     pub(super) project: &'sync Project,
+    pub(super) force_json: bool,
 }
 
 pub struct SyncbackSnapshot<'sync> {
@@ -31,6 +32,25 @@ pub struct SyncbackSnapshot<'sync> {
 }
 
 impl<'sync> SyncbackSnapshot<'sync> {
+    /// Computes the middleware and filesystem name for a child without
+    /// creating a full snapshot. Uses the same logic as `with_joined_path`.
+    pub fn child_middleware_and_name(
+        &self,
+        new_ref: Ref,
+        old_ref: Option<Ref>,
+    ) -> anyhow::Result<(Middleware, String)> {
+        let temp = Self {
+            data: self.data,
+            old: old_ref,
+            new: new_ref,
+            path: PathBuf::new(),
+            middleware: None,
+        };
+        let middleware = get_best_middleware(&temp, self.data.force_json);
+        let name = name_for_inst(middleware, temp.new_inst(), temp.old_inst())?;
+        Ok((middleware, name.into_owned()))
+    }
+
     /// Constructs a SyncbackSnapshot from the provided refs
     /// while inheriting this snapshot's path and data. This should be used for
     /// directories.
@@ -43,7 +63,7 @@ impl<'sync> SyncbackSnapshot<'sync> {
             path: PathBuf::new(),
             middleware: None,
         };
-        let middleware = get_best_middleware(&snapshot);
+        let middleware = get_best_middleware(&snapshot, self.data.force_json);
         let name = name_for_inst(middleware, snapshot.new_inst(), snapshot.old_inst())?;
         snapshot.path = self.path.join(name.as_ref());
 
@@ -69,7 +89,7 @@ impl<'sync> SyncbackSnapshot<'sync> {
             path: PathBuf::new(),
             middleware: None,
         };
-        let middleware = get_best_middleware(&snapshot);
+        let middleware = get_best_middleware(&snapshot, self.data.force_json);
         let name = name_for_inst(middleware, snapshot.new_inst(), snapshot.old_inst())?;
         snapshot.path = base_path.join(name.as_ref());
 
@@ -237,6 +257,25 @@ pub fn inst_path(dom: &WeakDom, referent: Ref) -> String {
     path.join("/")
 }
 
+impl<'sync> SyncbackData<'sync> {
+    /// Constructs a `SyncbackData` for use in unit tests.
+    #[cfg(test)]
+    pub fn for_test(
+        vfs: &'sync Vfs,
+        old_tree: &'sync RojoTree,
+        new_tree: &'sync WeakDom,
+        project: &'sync Project,
+    ) -> Self {
+        Self {
+            vfs,
+            old_tree,
+            new_tree,
+            project,
+            force_json: false,
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use rbx_dom_weak::{InstanceBuilder, WeakDom};

tests/tests/syncback.rs

@@ -60,8 +60,8 @@ syncback_tests! {
     // Ensures that projects can be reserialized by syncback and that
     // default.project.json doesn't change unexpectedly.
     project_reserialize => ["attribute_mismatch.luau", "property_mismatch.project.json"],
-    // Confirms that Instances that cannot serialize as directories serialize as rbxms
-    rbxm_fallback => ["src/ChildWithDuplicates.rbxm"],
+    // Confirms that duplicate children are resolved with incrementing suffixes
+    rbxm_fallback => ["src/ChildWithDuplicates/DuplicateChild/.gitkeep", "src/ChildWithDuplicates/DuplicateChild1/.gitkeep"],
     // Ensures that ref properties are linked properly on the file system
     ref_properties => ["src/pointer.model.json", "src/target.model.json"],
     // Ensures that ref properties are linked when no attributes are manually