Implement fuse-projfs as an alternative to fuse-overlay for Windows. (#244)

* Implement fuse-projfs to use ProjFS on WIndows.

* fix(pi-natives): prevent ProjFS session key mismatch and start race

---------

Co-authored-by: can1357 <me@can.ac>

Cargo.lock

@@ -2232,6 +2232,7 @@ dependencies = [
  "tokio-util",
  "unicode-segmentation",
  "unicode-width",
+ "windows-sys 0.61.2",
  "winreg 0.55.0",
 ]
 

README.md

@@ -153,7 +153,7 @@ Parallel execution framework with specialized agents and real-time streaming:
 - **Parallel exploration**: Reviewer agent can spawn explore agents for large codebase analysis
 - **Real-time artifact streaming**: Task outputs stream as they're created, not just at completion
 - **Full output access**: Read complete subagent output via `agent://<id>` resources when previews truncate
-- **Isolation backends**: `isolated: true` runs tasks in git worktrees or fuse-overlay filesystems, with patch or branch merge strategies
+- **Isolation backends**: `isolated: true` runs tasks in git worktrees, Unix fuse-overlay filesystems, or Windows ProjFS (`fuse-projfs`), with patch or branch merge strategies
 - **Async background jobs**: Background execution with configurable concurrency (up to 100 jobs) and `await` tool for blocking on results
 - **Agent Control Center**: `/agents` dashboard for managing and creating custom agents
 - **AI-powered agent creation**: Generate custom agent definitions with the architect model
@@ -923,7 +923,7 @@ async:
 task:
   eager: false
   isolation:
-    mode: none # none | worktree | fuse-overlay
+    mode: none # none | worktree | fuse-overlay | fuse-projfs
     merge: patch # patch | branch
 ```
 

bun.lock

@@ -33,7 +33,7 @@
         "pi-ai": "./src/cli.ts",
       },
       "dependencies": {
-        "@anthropic-ai/sdk": "^0.78",
+        "@anthropic-ai/sdk": "^0.77.0",
         "@aws-sdk/client-bedrock-runtime": "^3.998",
         "@bufbuild/protobuf": "^2.11",
         "@google/genai": "^1.43",
@@ -44,7 +44,7 @@
         "ajv-formats": "^3.0",
         "openai": "^6.25",
         "partial-json": "^0.1",
-        "zod": "^4.3",
+        "zod": "4.3.5",
       },
       "devDependencies": {
         "@types/bun": "^1.3",
@@ -176,7 +176,7 @@
     },
   },
   "packages": {
-    "@anthropic-ai/sdk": ["@anthropic-ai/sdk@0.78.0", "", { "dependencies": { "json-schema-to-ts": "^3.1.1" }, "peerDependencies": { "zod": "^3.25.0 || ^4.0.0" }, "optionalPeers": ["zod"], "bin": { "anthropic-ai-sdk": "bin/cli" } }, "sha512-PzQhR715td/m1UaaN5hHXjYB8Gl2lF9UVhrrGrZeysiF6Rb74Wc9GCB8hzLdzmQtBd1qe89F9OptgB9Za1Ib5w=="],
+    "@anthropic-ai/sdk": ["@anthropic-ai/sdk@0.77.0", "", { "dependencies": { "json-schema-to-ts": "^3.1.1" }, "peerDependencies": { "zod": "^3.25.0 || ^4.0.0" }, "optionalPeers": ["zod"], "bin": { "anthropic-ai-sdk": "bin/cli" } }, "sha512-TivlT6nfidz3sOyMF72T2x5AkmHrpT7JgL2e/0HNdh7b24v7JC8cR+rCY/42jA68xIsjmiGQ5IKMsH9feEKh3A=="],
 
     "@aws-crypto/crc32": ["@aws-crypto/crc32@5.2.0", "", { "dependencies": { "@aws-crypto/util": "^5.2.0", "@aws-sdk/types": "^3.222.0", "tslib": "^2.6.2" } }, "sha512-nLbCWqQNgUiwwtFsen1AdzAtvuLRsQS8rYgMuxCrdKf9kOssamGLuPwyTY9wyYblNr9+1XM8v6zoDTPPSIeANg=="],
 
@@ -1002,7 +1002,7 @@
 
     "yauzl": ["yauzl@2.10.0", "", { "dependencies": { "buffer-crc32": "~0.2.3", "fd-slicer": "~1.1.0" } }, "sha512-p4a9I6X6nu6IhoGmBqAcbJy1mlC4j27vEPZX9F4L4/vZT3Lyq1VkFHw/V/PUcB9Buo+DG3iHkT0x3Qya58zc3g=="],
 
-    "zod": ["zod@4.3.6", "", {}, "sha512-rftlrkhHZOcjDwkGlnUtZZkvaPHCsDATp4pGpuOOMDaTdDDXF91wuVDJoWoPsKX/3YPQ5fHuF3STjcYyKr+Qhg=="],
+    "zod": ["zod@4.3.5", "", {}, "sha512-k7Nwx6vuWx1IJ9Bjuf4Zt1PEllcwe7cls3VNzm4CQ1/hgtFUK2bRNG3rvnpPUhFjmqJKAKtjV576KnUkHocg/g=="],
 
     "@aws-crypto/sha256-browser/@smithy/util-utf8": ["@smithy/util-utf8@2.3.0", "", { "dependencies": { "@smithy/util-buffer-from": "^2.2.0", "tslib": "^2.6.2" } }, "sha512-R8Rdn8Hy72KKcebgLiv8jQcQkXoLMOGGv5uI1/k0l+snqkOzQ1R0ChUBCxWMlBsFMekWjq0wRudIweFs7sKT5A=="],
 

crates/pi-natives/Cargo.toml

@@ -67,5 +67,11 @@ libc = "0.2"
 [target.'cfg(windows)'.dependencies]
 winreg = "0.55"
 
+windows-sys = { version = "0.61", features = [
+   "Win32_Foundation",
+   "Win32_Storage_ProjectedFileSystem",
+   "Win32_System_Com",
+   "Win32_System_LibraryLoader",
+] }
 [build-dependencies]
 napi-build = "2"

crates/pi-natives/src/lib.rs

@@ -34,6 +34,7 @@ pub mod html;
 pub mod image;
 pub mod keys;
 pub mod prof;
+pub mod projfs_overlay;
 pub mod ps;
 pub mod pty;
 pub mod shell;

crates/pi-natives/src/projfs_overlay.rs

@@ -0,0 +1,853 @@
+//! Windows ProjFS-backed overlay lifecycle for task isolation.
+
+use napi::bindgen_prelude::*;
+use napi_derive::napi;
+
+const PROJFS_UNAVAILABLE_PREFIX: &str = "PROJFS_UNAVAILABLE:";
+
+#[napi(object)]
+pub struct ProjfsOverlayProbeResult {
+	pub available: bool,
+	pub reason:    Option<String>,
+}
+
+#[napi(js_name = "projfsOverlayProbe")]
+pub fn projfs_overlay_probe() -> ProjfsOverlayProbeResult {
+	imp::probe()
+}
+
+#[napi(js_name = "projfsOverlayStart")]
+pub fn projfs_overlay_start(lower_root: String, projection_root: String) -> Result<()> {
+	imp::start(&lower_root, &projection_root)
+}
+
+#[napi(js_name = "projfsOverlayStop")]
+pub fn projfs_overlay_stop(projection_root: String) -> Result<()> {
+	imp::stop(&projection_root);
+	Ok(())
+}
+
+#[cfg(not(windows))]
+mod imp {
+	use napi::bindgen_prelude::*;
+
+	use super::{PROJFS_UNAVAILABLE_PREFIX, ProjfsOverlayProbeResult};
+
+	const UNSUPPORTED_REASON: &str = "Windows ProjFS is unavailable on this platform";
+
+	pub fn probe() -> ProjfsOverlayProbeResult {
+		ProjfsOverlayProbeResult { available: false, reason: Some(UNSUPPORTED_REASON.to_string()) }
+	}
+
+	pub fn start(_lower_root: &str, _projection_root: &str) -> Result<()> {
+		Err(Error::from_reason(format!("{PROJFS_UNAVAILABLE_PREFIX} {UNSUPPORTED_REASON}")))
+	}
+
+	pub const fn stop(_projection_root: &str) {}
+}
+
+#[cfg(windows)]
+#[allow(
+	clippy::undocumented_unsafe_blocks,
+	reason = "Windows ProjFS bridge is FFI-heavy and safety is validated by pointer and handle \
+	          checks"
+)]
+mod imp {
+	use std::{
+		collections::{BTreeMap, btree_map::Entry},
+		ffi::{OsStr, OsString, c_void},
+		fs,
+		io::{self, ErrorKind, Read, Seek, SeekFrom},
+		mem,
+		os::windows::{
+			ffi::{OsStrExt, OsStringExt},
+			fs::MetadataExt,
+		},
+		path::{Path, PathBuf},
+		sync::{Arc, LazyLock},
+	};
+
+	use napi::bindgen_prelude::*;
+	use parking_lot::Mutex;
+	use windows_sys::{
+		Win32::{
+			Foundation::{
+				ERROR_ACCESS_DENIED, ERROR_FILE_NOT_FOUND, ERROR_FILE_SYSTEM_VIRTUALIZATION_BUSY,
+				ERROR_FILE_SYSTEM_VIRTUALIZATION_INVALID_OPERATION,
+				ERROR_FILE_SYSTEM_VIRTUALIZATION_METADATA_CORRUPT,
+				ERROR_FILE_SYSTEM_VIRTUALIZATION_PROVIDER_UNKNOWN,
+				ERROR_FILE_SYSTEM_VIRTUALIZATION_UNAVAILABLE, ERROR_HANDLE_EOF,
+				ERROR_INSUFFICIENT_BUFFER, ERROR_INVALID_FUNCTION, ERROR_INVALID_PARAMETER,
+				ERROR_MOD_NOT_FOUND, ERROR_NOT_SUPPORTED, ERROR_OLD_WIN_VERSION, ERROR_OUTOFMEMORY,
+				ERROR_PROC_NOT_FOUND, FreeLibrary, GetLastError, HMODULE,
+			},
+			Storage::ProjectedFileSystem::{
+				PRJ_CALLBACK_DATA, PRJ_CALLBACKS, PRJ_CB_DATA_FLAG_ENUM_RESTART_SCAN,
+				PRJ_CB_DATA_FLAG_ENUM_RETURN_SINGLE_ENTRY, PRJ_DIR_ENTRY_BUFFER_HANDLE,
+				PRJ_FILE_BASIC_INFO, PRJ_NAMESPACE_VIRTUALIZATION_CONTEXT, PRJ_PLACEHOLDER_INFO,
+				PRJ_STARTVIRTUALIZING_OPTIONS,
+			},
+			System::{
+				Com::CoCreateGuid,
+				LibraryLoader::{GetProcAddress, LoadLibraryW},
+			},
+		},
+		core::{GUID, HRESULT, PCSTR, PCWSTR},
+	};
+
+	use super::{PROJFS_UNAVAILABLE_PREFIX, ProjfsOverlayProbeResult};
+
+	const EMPTY_WIDE: [u16; 1] = [0];
+	const MAX_READ_CHUNK: usize = 1024 * 1024;
+	const E_NOTIMPL: HRESULT = 0x8000_4001_u32 as i32;
+	const E_FAIL: HRESULT = 0x8000_4005_u32 as i32;
+
+	type PrjAllocateAlignedBufferFn =
+		unsafe extern "system" fn(PRJ_NAMESPACE_VIRTUALIZATION_CONTEXT, usize) -> *mut c_void;
+	type PrjFreeAlignedBufferFn = unsafe extern "system" fn(*const c_void);
+	type PrjFileNameCompareFn = unsafe extern "system" fn(PCWSTR, PCWSTR) -> i32;
+	type PrjFileNameMatchFn = unsafe extern "system" fn(PCWSTR, PCWSTR) -> bool;
+	type PrjMarkDirectoryAsPlaceholderFn =
+		unsafe extern "system" fn(PCWSTR, PCWSTR, *const c_void, *const GUID) -> HRESULT;
+	type PrjStartVirtualizingFn = unsafe extern "system" fn(
+		PCWSTR,
+		*const PRJ_CALLBACKS,
+		*const c_void,
+		*const PRJ_STARTVIRTUALIZING_OPTIONS,
+		*mut PRJ_NAMESPACE_VIRTUALIZATION_CONTEXT,
+	) -> HRESULT;
+	type PrjStopVirtualizingFn = unsafe extern "system" fn(PRJ_NAMESPACE_VIRTUALIZATION_CONTEXT);
+	type PrjFillDirEntryBuffer2Fn = unsafe extern "system" fn(
+		PRJ_DIR_ENTRY_BUFFER_HANDLE,
+		PCWSTR,
+		*const PRJ_FILE_BASIC_INFO,
+		*const c_void,
+	) -> HRESULT;
+	type PrjWriteFileDataFn = unsafe extern "system" fn(
+		PRJ_NAMESPACE_VIRTUALIZATION_CONTEXT,
+		*const GUID,
+		*const c_void,
+		u64,
+		u32,
+	) -> HRESULT;
+	type PrjWritePlaceholderInfoFn = unsafe extern "system" fn(
+		PRJ_NAMESPACE_VIRTUALIZATION_CONTEXT,
+		PCWSTR,
+		*const PRJ_PLACEHOLDER_INFO,
+		u32,
+	) -> HRESULT;
+
+	struct ProjfsApi {
+		module: HMODULE,
+		prj_allocate_aligned_buffer: PrjAllocateAlignedBufferFn,
+		prj_free_aligned_buffer: PrjFreeAlignedBufferFn,
+		prj_file_name_compare: PrjFileNameCompareFn,
+		prj_file_name_match: PrjFileNameMatchFn,
+		prj_mark_directory_as_placeholder: PrjMarkDirectoryAsPlaceholderFn,
+		prj_start_virtualizing: PrjStartVirtualizingFn,
+		prj_stop_virtualizing: PrjStopVirtualizingFn,
+		prj_fill_dir_entry_buffer2: PrjFillDirEntryBuffer2Fn,
+		prj_write_file_data: PrjWriteFileDataFn,
+		prj_write_placeholder_info: PrjWritePlaceholderInfoFn,
+	}
+
+	unsafe impl Send for ProjfsApi {}
+	unsafe impl Sync for ProjfsApi {}
+
+	impl Drop for ProjfsApi {
+		fn drop(&mut self) {
+			if !self.module.is_null() {
+				unsafe {
+					FreeLibrary(self.module);
+				}
+			}
+		}
+	}
+
+	impl ProjfsApi {
+		fn load() -> std::result::Result<Self, String> {
+			let library_name = to_wide(OsStr::new("ProjectedFSLib.dll"));
+			let module = unsafe { LoadLibraryW(library_name.as_ptr()) };
+			if module.is_null() {
+				let win32 = unsafe { GetLastError() };
+				return Err(format!("ProjectedFSLib.dll could not be loaded (win32={win32})"));
+			}
+
+			macro_rules! load_symbol {
+				($name:literal, $ty:ty) => {{
+					let proc = unsafe {
+						GetProcAddress(module, concat!($name, "\0").as_ptr().cast::<u8>() as PCSTR)
+					};
+					let Some(proc) = proc else {
+						unsafe {
+							FreeLibrary(module);
+						}
+						return Err(format!("ProjectedFSLib.dll missing symbol {}", $name));
+					};
+					unsafe { mem::transmute::<unsafe extern "system" fn() -> isize, $ty>(proc) }
+				}};
+			}
+
+			Ok(Self {
+				module,
+				prj_allocate_aligned_buffer: load_symbol!(
+					"PrjAllocateAlignedBuffer",
+					PrjAllocateAlignedBufferFn
+				),
+				prj_free_aligned_buffer: load_symbol!("PrjFreeAlignedBuffer", PrjFreeAlignedBufferFn),
+				prj_file_name_compare: load_symbol!("PrjFileNameCompare", PrjFileNameCompareFn),
+				prj_file_name_match: load_symbol!("PrjFileNameMatch", PrjFileNameMatchFn),
+				prj_mark_directory_as_placeholder: load_symbol!(
+					"PrjMarkDirectoryAsPlaceholder",
+					PrjMarkDirectoryAsPlaceholderFn
+				),
+				prj_start_virtualizing: load_symbol!("PrjStartVirtualizing", PrjStartVirtualizingFn),
+				prj_stop_virtualizing: load_symbol!("PrjStopVirtualizing", PrjStopVirtualizingFn),
+				prj_fill_dir_entry_buffer2: load_symbol!(
+					"PrjFillDirEntryBuffer2",
+					PrjFillDirEntryBuffer2Fn
+				),
+				prj_write_file_data: load_symbol!("PrjWriteFileData", PrjWriteFileDataFn),
+				prj_write_placeholder_info: load_symbol!(
+					"PrjWritePlaceholderInfo",
+					PrjWritePlaceholderInfoFn
+				),
+			})
+		}
+	}
+
+	struct DirectoryEntry {
+		name_wide:  Vec<u16>,
+		basic_info: PRJ_FILE_BASIC_INFO,
+	}
+
+	#[derive(Default)]
+	struct DirectoryEnumeration {
+		entries:           Vec<DirectoryEntry>,
+		cursor:            usize,
+		search_expression: Option<Vec<u16>>,
+	}
+
+	struct ProviderContext {
+		lower_root:   PathBuf,
+		api:          Arc<ProjfsApi>,
+		enumerations: Mutex<BTreeMap<u128, DirectoryEnumeration>>,
+	}
+
+	struct ProjfsSession {
+		virtualization_context: PRJ_NAMESPACE_VIRTUALIZATION_CONTEXT,
+		provider_context:       *mut ProviderContext,
+		callbacks:              Box<PRJ_CALLBACKS>,
+		api_handle:             Arc<ProjfsApi>,
+	}
+
+	// SAFETY: Session ownership is synchronized through `PROJFS_SESSIONS`; raw
+	// pointers are only created/freed on controlled start/stop paths and not
+	// concurrently aliased.
+	unsafe impl Send for ProjfsSession {}
+
+	enum ProjfsSessionState {
+		Starting,
+		Active(ProjfsSession),
+	}
+
+	static PROJFS_SESSIONS: LazyLock<Mutex<BTreeMap<String, ProjfsSessionState>>> =
+		LazyLock::new(|| Mutex::new(BTreeMap::new()));
+
+	pub fn probe() -> ProjfsOverlayProbeResult {
+		match ProjfsApi::load() {
+			Ok(_) => ProjfsOverlayProbeResult { available: true, reason: None },
+			Err(reason) => ProjfsOverlayProbeResult { available: false, reason: Some(reason) },
+		}
+	}
+
+	pub fn start(lower_root: &str, projection_root: &str) -> Result<()> {
+		let api = Arc::new(ProjfsApi::load().map_err(unavailable_error)?);
+		let lower_root_path = resolve_existing_dir(lower_root)?;
+		let projection_root_path = resolve_projection_root(projection_root)?;
+		let projection_key = normalize_session_key(&projection_root_path);
+
+		{
+			let mut sessions = PROJFS_SESSIONS.lock();
+			if sessions.contains_key(&projection_key) {
+				return Err(Error::from_reason(format!(
+					"ProjFS overlay is already active for {}",
+					projection_root_path.display()
+				)));
+			}
+			sessions.insert(projection_key.clone(), ProjfsSessionState::Starting);
+		}
+
+		let mut instance_id = GUID::default();
+		let guid_hr = unsafe { CoCreateGuid(&raw mut instance_id) };
+		if is_failed(guid_hr) {
+			PROJFS_SESSIONS.lock().remove(&projection_key);
+			return Err(Error::from_reason(format!(
+				"Unable to create ProjFS instance identifier ({})",
+				format_hresult(guid_hr)
+			)));
+		}
+
+		let root_wide = to_wide(projection_root_path.as_os_str());
+		let mark_hr = unsafe {
+			(api.prj_mark_directory_as_placeholder)(
+				root_wide.as_ptr(),
+				std::ptr::null(),
+				std::ptr::null(),
+				&raw const instance_id,
+			)
+		};
+		if is_failed(mark_hr) {
+			PROJFS_SESSIONS.lock().remove(&projection_key);
+			return Err(classify_start_error("mark placeholder root", mark_hr));
+		}
+
+		let provider_context = Box::new(ProviderContext {
+			lower_root:   lower_root_path,
+			api:          api.clone(),
+			enumerations: Mutex::new(BTreeMap::new()),
+		});
+		let provider_context_ptr = Box::into_raw(provider_context);
+		let callbacks = Box::new(PRJ_CALLBACKS {
+			StartDirectoryEnumerationCallback: Some(start_directory_enumeration_callback),
+			EndDirectoryEnumerationCallback: Some(end_directory_enumeration_callback),
+			GetDirectoryEnumerationCallback: Some(get_directory_enumeration_callback),
+			GetPlaceholderInfoCallback: Some(get_placeholder_info_callback),
+			GetFileDataCallback: Some(get_file_data_callback),
+			..Default::default()
+		});
+
+		let mut virtualization_context: PRJ_NAMESPACE_VIRTUALIZATION_CONTEXT = std::ptr::null_mut();
+		let start_hr = unsafe {
+			(api.prj_start_virtualizing)(
+				root_wide.as_ptr(),
+				callbacks.as_ref(),
+				provider_context_ptr.cast::<c_void>(),
+				std::ptr::null(),
+				&raw mut virtualization_context,
+			)
+		};
+		if is_failed(start_hr) {
+			PROJFS_SESSIONS.lock().remove(&projection_key);
+			// SAFETY: `provider_context_ptr` comes from `Box::into_raw` above and start
+			// failed, so ProjFS never took ownership and this function remains the sole
+			// owner.
+			unsafe {
+				drop(Box::from_raw(provider_context_ptr));
+			}
+			return Err(classify_start_error("start virtualization", start_hr));
+		}
+
+		let started_session = ProjfsSession {
+			virtualization_context,
+			provider_context: provider_context_ptr,
+			callbacks,
+			api_handle: api,
+		};
+
+		let mut sessions = PROJFS_SESSIONS.lock();
+		match sessions.entry(projection_key) {
+			Entry::Occupied(mut entry) => {
+				if matches!(entry.get(), ProjfsSessionState::Starting) {
+					entry.insert(ProjfsSessionState::Active(started_session));
+					Ok(())
+				} else {
+					drop(entry);
+					drop(sessions);
+					stop_projfs_session(started_session);
+					Err(Error::from_reason(format!(
+						"ProjFS overlay is already active for {}",
+						projection_root_path.display()
+					)))
+				}
+			},
+			Entry::Vacant(_) => {
+				drop(sessions);
+				stop_projfs_session(started_session);
+				Err(Error::from_reason(format!(
+					"ProjFS overlay start was canceled for {}",
+					projection_root_path.display()
+				)))
+			},
+		}
+	}
+
+	pub fn stop(projection_root: &str) {
+		let projection_root_path = resolve_absolute_path(Path::new(projection_root));
+		let projection_root_path =
+			fs::canonicalize(&projection_root_path).unwrap_or(projection_root_path);
+		let key = normalize_session_key(&projection_root_path);
+		let session_state = PROJFS_SESSIONS.lock().remove(&key);
+		let Some(ProjfsSessionState::Active(session)) = session_state else {
+			return;
+		};
+
+		stop_projfs_session(session);
+	}
+
+	fn stop_projfs_session(session: ProjfsSession) {
+		// SAFETY: The session holds the live ProjFS context and provider pointer
+		// created in `start`; this function consumes ownership and runs the
+		// corresponding one-time teardown.
+		unsafe {
+			(session.api_handle.prj_stop_virtualizing)(session.virtualization_context);
+			drop(Box::from_raw(session.provider_context));
+		}
+		drop(session.callbacks);
+	}
+
+	unsafe extern "system" fn start_directory_enumeration_callback(
+		callback_data: *const PRJ_CALLBACK_DATA,
+		enumeration_id: *const GUID,
+	) -> HRESULT {
+		let Ok((callback_data, context)) = callback_context(callback_data) else {
+			return hresult_from_win32(ERROR_INVALID_PARAMETER);
+		};
+		if enumeration_id.is_null() {
+			return hresult_from_win32(ERROR_INVALID_PARAMETER);
+		}
+
+		let target_path = callback_relative_path(callback_data);
+		let entries = match list_directory_entries(context, &target_path) {
+			Ok(entries) => entries,
+			Err(err) => return io_error_to_hresult(&err),
+		};
+
+		let mut enumerations = context.enumerations.lock();
+		enumerations.insert(guid_to_u128(unsafe { &*enumeration_id }), DirectoryEnumeration {
+			entries,
+			cursor: 0,
+			search_expression: None,
+		});
+		0
+	}
+
+	unsafe extern "system" fn end_directory_enumeration_callback(
+		callback_data: *const PRJ_CALLBACK_DATA,
+		enumeration_id: *const GUID,
+	) -> HRESULT {
+		let Ok((_, context)) = callback_context(callback_data) else {
+			return hresult_from_win32(ERROR_INVALID_PARAMETER);
+		};
+		if enumeration_id.is_null() {
+			return hresult_from_win32(ERROR_INVALID_PARAMETER);
+		}
+
+		context
+			.enumerations
+			.lock()
+			.remove(&guid_to_u128(unsafe { &*enumeration_id }));
+		0
+	}
+
+	unsafe extern "system" fn get_directory_enumeration_callback(
+		callback_data: *const PRJ_CALLBACK_DATA,
+		enumeration_id: *const GUID,
+		search_expression: PCWSTR,
+		dir_entry_buffer_handle: PRJ_DIR_ENTRY_BUFFER_HANDLE,
+	) -> HRESULT {
+		let Ok((callback_data, context)) = callback_context(callback_data) else {
+			return hresult_from_win32(ERROR_INVALID_PARAMETER);
+		};
+		if enumeration_id.is_null() {
+			return hresult_from_win32(ERROR_INVALID_PARAMETER);
+		}
+
+		let enum_key = guid_to_u128(unsafe { &*enumeration_id });
+		let mut enumerations = context.enumerations.lock();
+		let Some(enumeration) = enumerations.get_mut(&enum_key) else {
+			return 0;
+		};
+
+		if callback_data.Flags & PRJ_CB_DATA_FLAG_ENUM_RESTART_SCAN != 0 {
+			enumeration.cursor = 0;
+			enumeration.search_expression = None;
+		}
+		if !search_expression.is_null() {
+			let expression = read_pcwstr(search_expression);
+			if !expression.is_empty() {
+				let mut with_nul = expression;
+				with_nul.push(0);
+				enumeration.search_expression = Some(with_nul);
+			}
+		}
+
+		while enumeration.cursor < enumeration.entries.len() {
+			let entry = &enumeration.entries[enumeration.cursor];
+			let matched = if let Some(expression) = &enumeration.search_expression {
+				unsafe {
+					(context.api.prj_file_name_match)(entry.name_wide.as_ptr(), expression.as_ptr())
+				}
+			} else {
+				true
+			};
+
+			if matched {
+				let hr = unsafe {
+					(context.api.prj_fill_dir_entry_buffer2)(
+						dir_entry_buffer_handle,
+						entry.name_wide.as_ptr(),
+						&raw const entry.basic_info,
+						std::ptr::null(),
+					)
+				};
+				if is_failed(hr) {
+					if win32_from_hresult(hr) == Some(ERROR_INSUFFICIENT_BUFFER) {
+						break;
+					}
+					return hr;
+				}
+			}
+
+			enumeration.cursor += 1;
+			if callback_data.Flags & PRJ_CB_DATA_FLAG_ENUM_RETURN_SINGLE_ENTRY != 0 {
+				break;
+			}
+		}
+
+		0
+	}
+
+	unsafe extern "system" fn get_placeholder_info_callback(
+		callback_data: *const PRJ_CALLBACK_DATA,
+	) -> HRESULT {
+		let Ok((callback_data, context)) = callback_context(callback_data) else {
+			return hresult_from_win32(ERROR_INVALID_PARAMETER);
+		};
+
+		let relative_path = callback_relative_path(callback_data);
+		let source_path = context.lower_root.join(relative_path);
+		let metadata = match fs::symlink_metadata(&source_path) {
+			Ok(metadata) => metadata,
+			Err(err) => return io_error_to_hresult(&err),
+		};
+
+		let placeholder_info =
+			PRJ_PLACEHOLDER_INFO { FileBasicInfo: to_basic_info(&metadata), ..Default::default() };
+
+		let destination = if callback_data.FilePathName.is_null() {
+			EMPTY_WIDE.as_ptr()
+		} else {
+			callback_data.FilePathName
+		};
+
+		unsafe {
+			(context.api.prj_write_placeholder_info)(
+				callback_data.NamespaceVirtualizationContext,
+				destination,
+				&raw const placeholder_info,
+				mem::size_of::<PRJ_PLACEHOLDER_INFO>() as u32,
+			)
+		}
+	}
+
+	unsafe extern "system" fn get_file_data_callback(
+		callback_data: *const PRJ_CALLBACK_DATA,
+		byte_offset: u64,
+		length: u32,
+	) -> HRESULT {
+		let Ok((callback_data, context)) = callback_context(callback_data) else {
+			return hresult_from_win32(ERROR_INVALID_PARAMETER);
+		};
+		if length == 0 {
+			return 0;
+		}
+
+		let relative_path = callback_relative_path(callback_data);
+		let source_path = context.lower_root.join(relative_path);
+		let mut file = match fs::File::open(&source_path) {
+			Ok(file) => file,
+			Err(err) => return io_error_to_hresult(&err),
+		};
+		if let Err(err) = file.seek(SeekFrom::Start(byte_offset)) {
+			return io_error_to_hresult(&err);
+		}
+
+		let chunk_size = usize::min(length as usize, MAX_READ_CHUNK);
+		let aligned_ptr = unsafe {
+			(context.api.prj_allocate_aligned_buffer)(
+				callback_data.NamespaceVirtualizationContext,
+				chunk_size,
+			)
+		};
+		if aligned_ptr.is_null() {
+			return hresult_from_win32(ERROR_OUTOFMEMORY);
+		}
+		let mut aligned_buffer = AlignedBuffer::new(context.api.clone(), aligned_ptr, chunk_size);
+
+		let mut written = 0usize;
+		while written < length as usize {
+			let to_read = usize::min(aligned_buffer.len(), length as usize - written);
+			if let Err(err) = file.read_exact(&mut aligned_buffer.as_mut_slice()[..to_read]) {
+				return io_error_to_hresult(&err);
+			}
+
+			let hr = unsafe {
+				(context.api.prj_write_file_data)(
+					callback_data.NamespaceVirtualizationContext,
+					&raw const callback_data.DataStreamId,
+					aligned_buffer.as_mut_slice().as_ptr().cast::<c_void>(),
+					byte_offset + written as u64,
+					to_read as u32,
+				)
+			};
+			if is_failed(hr) {
+				return hr;
+			}
+
+			written += to_read;
+		}
+
+		0
+	}
+
+	struct AlignedBuffer {
+		api: Arc<ProjfsApi>,
+		ptr: *mut c_void,
+		len: usize,
+	}
+
+	impl AlignedBuffer {
+		const fn new(api: Arc<ProjfsApi>, ptr: *mut c_void, len: usize) -> Self {
+			Self { api, ptr, len }
+		}
+
+		const fn len(&self) -> usize {
+			self.len
+		}
+
+		const fn as_mut_slice(&mut self) -> &mut [u8] {
+			unsafe { std::slice::from_raw_parts_mut(self.ptr.cast::<u8>(), self.len) }
+		}
+	}
+
+	impl Drop for AlignedBuffer {
+		fn drop(&mut self) {
+			unsafe {
+				(self.api.prj_free_aligned_buffer)(self.ptr);
+			}
+		}
+	}
+
+	fn callback_context(
+		callback_data: *const PRJ_CALLBACK_DATA,
+	) -> std::result::Result<(&'static PRJ_CALLBACK_DATA, &'static ProviderContext), HRESULT> {
+		if callback_data.is_null() {
+			return Err(hresult_from_win32(ERROR_INVALID_PARAMETER));
+		}
+		let callback_data = unsafe { &*callback_data };
+		if callback_data.InstanceContext.is_null() {
+			return Err(hresult_from_win32(ERROR_INVALID_PARAMETER));
+		}
+		let context = unsafe { &*(callback_data.InstanceContext.cast::<ProviderContext>()) };
+		Ok((callback_data, context))
+	}
+
+	fn list_directory_entries(
+		context: &ProviderContext,
+		relative_path: &Path,
+	) -> io::Result<Vec<DirectoryEntry>> {
+		let source_dir = context.lower_root.join(relative_path);
+		let mut entries = Vec::new();
+		for entry in fs::read_dir(&source_dir)? {
+			let entry = entry?;
+			let metadata = entry.metadata()?;
+			let name = entry.file_name();
+			let mut name_wide = to_wide(name.as_os_str());
+			if name_wide.is_empty() {
+				continue;
+			}
+			entries.push(DirectoryEntry {
+				name_wide:  {
+					name_wide.shrink_to_fit();
+					name_wide
+				},
+				basic_info: to_basic_info(&metadata),
+			});
+		}
+
+		entries.sort_by(|left, right| {
+			let compare = unsafe {
+				(context.api.prj_file_name_compare)(left.name_wide.as_ptr(), right.name_wide.as_ptr())
+			};
+			compare.cmp(&0)
+		});
+		Ok(entries)
+	}
+
+	fn to_basic_info(metadata: &fs::Metadata) -> PRJ_FILE_BASIC_INFO {
+		PRJ_FILE_BASIC_INFO {
+			IsDirectory:    metadata.is_dir(),
+			FileSize:       metadata.file_size() as i64,
+			CreationTime:   metadata.creation_time() as i64,
+			LastAccessTime: metadata.last_access_time() as i64,
+			LastWriteTime:  metadata.last_write_time() as i64,
+			ChangeTime:     metadata.last_write_time() as i64,
+			FileAttributes: metadata.file_attributes(),
+		}
+	}
+
+	fn callback_relative_path(callback_data: &PRJ_CALLBACK_DATA) -> PathBuf {
+		if callback_data.FilePathName.is_null() {
+			return PathBuf::new();
+		}
+		let raw = read_pcwstr(callback_data.FilePathName);
+		if raw.is_empty() {
+			return PathBuf::new();
+		}
+		PathBuf::from(OsString::from_wide(&raw))
+	}
+
+	fn read_pcwstr(value: PCWSTR) -> Vec<u16> {
+		if value.is_null() {
+			return Vec::new();
+		}
+
+		let mut len = 0usize;
+		unsafe {
+			while *value.add(len) != 0 {
+				len += 1;
+			}
+			std::slice::from_raw_parts(value, len).to_vec()
+		}
+	}
+
+	fn resolve_existing_dir(path: &str) -> Result<PathBuf> {
+		let resolved = resolve_absolute_path(Path::new(path));
+		let metadata = fs::metadata(&resolved).map_err(|err| {
+			Error::from_reason(format!("Invalid ProjFS lower root {}: {err}", resolved.display()))
+		})?;
+		if !metadata.is_dir() {
+			return Err(Error::from_reason(format!(
+				"Invalid ProjFS lower root {}: path is not a directory",
+				resolved.display()
+			)));
+		}
+		Ok(fs::canonicalize(&resolved).unwrap_or(resolved))
+	}
+
+	fn resolve_projection_root(path: &str) -> Result<PathBuf> {
+		let resolved = resolve_absolute_path(Path::new(path));
+		fs::create_dir_all(&resolved).map_err(|err| {
+			Error::from_reason(format!(
+				"Unable to create ProjFS projection root {}: {err}",
+				resolved.display()
+			))
+		})?;
+		let metadata = fs::metadata(&resolved).map_err(|err| {
+			Error::from_reason(format!(
+				"Unable to access ProjFS projection root {}: {err}",
+				resolved.display()
+			))
+		})?;
+		if !metadata.is_dir() {
+			return Err(Error::from_reason(format!(
+				"Invalid ProjFS projection root {}: path is not a directory",
+				resolved.display()
+			)));
+		}
+		Ok(fs::canonicalize(&resolved).unwrap_or(resolved))
+	}
+
+	fn resolve_absolute_path(path: &Path) -> PathBuf {
+		if path.is_absolute() {
+			path.to_path_buf()
+		} else {
+			std::env::current_dir().map_or_else(|_| path.to_path_buf(), |cwd| cwd.join(path))
+		}
+	}
+
+	fn normalize_session_key(path: &Path) -> String {
+		path.to_string_lossy().to_ascii_lowercase()
+	}
+
+	fn to_wide(value: &OsStr) -> Vec<u16> {
+		let mut encoded: Vec<u16> = value.encode_wide().collect();
+		encoded.push(0);
+		encoded
+	}
+
+	fn unavailable_error(reason: String) -> Error {
+		Error::from_reason(format!("{PROJFS_UNAVAILABLE_PREFIX} {reason}"))
+	}
+
+	fn classify_start_error(phase: &str, hr: HRESULT) -> Error {
+		let detail = format!("ProjFS {phase} failed ({})", format_hresult(hr));
+		if is_unavailable_hresult(hr) {
+			return unavailable_error(detail);
+		}
+		Error::from_reason(detail)
+	}
+
+	const fn is_unavailable_hresult(hr: HRESULT) -> bool {
+		if hr == E_NOTIMPL {
+			return true;
+		}
+		let Some(win32) = win32_from_hresult(hr) else {
+			return false;
+		};
+		matches!(
+			win32,
+			ERROR_NOT_SUPPORTED
+				| ERROR_INVALID_FUNCTION
+				| ERROR_MOD_NOT_FOUND
+				| ERROR_PROC_NOT_FOUND
+				| ERROR_OLD_WIN_VERSION
+				| ERROR_FILE_SYSTEM_VIRTUALIZATION_UNAVAILABLE
+				| ERROR_FILE_SYSTEM_VIRTUALIZATION_PROVIDER_UNKNOWN
+				| ERROR_FILE_SYSTEM_VIRTUALIZATION_METADATA_CORRUPT
+				| ERROR_FILE_SYSTEM_VIRTUALIZATION_INVALID_OPERATION
+				| ERROR_FILE_SYSTEM_VIRTUALIZATION_BUSY
+		)
+	}
+
+	fn format_hresult(hr: HRESULT) -> String {
+		if let Some(win32) = win32_from_hresult(hr) {
+			format!("HRESULT=0x{:08X}, win32={win32}", hr as u32)
+		} else {
+			format!("HRESULT=0x{:08X}", hr as u32)
+		}
+	}
+
+	const fn win32_from_hresult(hr: HRESULT) -> Option<u32> {
+		let raw = hr as u32;
+		if (raw & 0xffff_0000) == 0x8007_0000 {
+			Some(raw & 0xffff)
+		} else {
+			None
+		}
+	}
+
+	const fn hresult_from_win32(code: u32) -> HRESULT {
+		if code == 0 {
+			0
+		} else {
+			((code & 0x0000_ffff) | 0x8007_0000) as i32
+		}
+	}
+
+	fn io_error_to_hresult(err: &io::Error) -> HRESULT {
+		if let Some(code) = err.raw_os_error()
+			&& code > 0
+		{
+			return hresult_from_win32(code as u32);
+		}
+
+		match err.kind() {
+			ErrorKind::NotFound => hresult_from_win32(ERROR_FILE_NOT_FOUND),
+			ErrorKind::PermissionDenied => hresult_from_win32(ERROR_ACCESS_DENIED),
+			ErrorKind::UnexpectedEof => hresult_from_win32(ERROR_HANDLE_EOF),
+			ErrorKind::OutOfMemory => hresult_from_win32(ERROR_OUTOFMEMORY),
+			_ => E_FAIL,
+		}
+	}
+
+	fn guid_to_u128(guid: &GUID) -> u128 {
+		let bytes: [u8; 16] = unsafe { mem::transmute(*guid) };
+		u128::from_le_bytes(bytes)
+	}
+
+	const fn is_failed(hr: HRESULT) -> bool {
+		hr < 0
+	}
+}

crates/pi-natives/src/ps.rs

@@ -131,7 +131,7 @@ mod platform {
 	use smallvec::SmallVec;
 
 	#[repr(C)]
-	#[allow(non_snake_case)]
+	#[allow(non_snake_case, reason = "Windows PROCESSENTRY32W field names must match Win32 ABI")]
 	struct PROCESSENTRY32W {
 		dwSize:              u32,
 		cntUsage:            u32,
@@ -145,25 +145,27 @@ mod platform {
 		szExeFile:           [u16; 260],
 	}
 
-	type HANDLE = *mut std::ffi::c_void;
-	const INVALID_HANDLE_VALUE: HANDLE = -1isize as HANDLE;
+	type Handle = *mut std::ffi::c_void;
+	const INVALID_HANDLE_VALUE: Handle = -1isize as Handle;
 	const TH32CS_SNAPPROCESS: u32 = 0x00000002;
 	const PROCESS_TERMINATE: u32 = 0x0001;
 
 	#[link(name = "kernel32")]
 	unsafe extern "system" {
-		fn CreateToolhelp32Snapshot(dwFlags: u32, th32ProcessID: u32) -> HANDLE;
-		fn Process32FirstW(hSnapshot: HANDLE, lppe: *mut PROCESSENTRY32W) -> i32;
-		fn Process32NextW(hSnapshot: HANDLE, lppe: *mut PROCESSENTRY32W) -> i32;
-		fn CloseHandle(hObject: HANDLE) -> i32;
-		fn OpenProcess(dwDesiredAccess: u32, bInheritHandle: i32, dwProcessId: u32) -> HANDLE;
-		fn TerminateProcess(hProcess: HANDLE, uExitCode: u32) -> i32;
+		fn CreateToolhelp32Snapshot(dwFlags: u32, th32ProcessID: u32) -> Handle;
+		fn Process32FirstW(hSnapshot: Handle, lppe: *mut PROCESSENTRY32W) -> i32;
+		fn Process32NextW(hSnapshot: Handle, lppe: *mut PROCESSENTRY32W) -> i32;
+		fn CloseHandle(hObject: Handle) -> i32;
+		fn OpenProcess(dwDesiredAccess: u32, bInheritHandle: i32, dwProcessId: u32) -> Handle;
+		fn TerminateProcess(hProcess: Handle, uExitCode: u32) -> i32;
 	}
 
-	/// Build a map of parent_pid -> [child_pids] for all processes.
+	/// Build a map of `parent_pid` -> [`child_pids`] for all processes.
 	fn build_process_tree() -> HashMap<u32, SmallVec<[u32; 4]>> {
 		let mut tree: HashMap<u32, SmallVec<[u32; 4]>> = HashMap::new();
 
+		// SAFETY: Toolhelp snapshot APIs are called with initialized structs and valid
+		// handles.
 		unsafe {
 			let snapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
 			if snapshot == INVALID_HANDLE_VALUE {
@@ -173,14 +175,14 @@ mod platform {
 			let mut entry: PROCESSENTRY32W = mem::zeroed();
 			entry.dwSize = mem::size_of::<PROCESSENTRY32W>() as u32;
 
-			if Process32FirstW(snapshot, &mut entry) != 0 {
+			if Process32FirstW(snapshot, &raw mut entry) != 0 {
 				loop {
 					tree
 						.entry(entry.th32ParentProcessID)
 						.or_default()
 						.push(entry.th32ProcessID);
 
-					if Process32NextW(snapshot, &mut entry) == 0 {
+					if Process32NextW(snapshot, &raw mut entry) == 0 {
 						break;
 					}
 				}
@@ -215,6 +217,8 @@ mod platform {
 	/// Terminate `pid` (Windows ignores `signal`).
 	/// Returns true when the process is terminated.
 	pub fn kill_pid(pid: i32, _signal: i32) -> bool {
+		// SAFETY: OpenProcess/TerminateProcess are called with kernel-provided process
+		// IDs and handles are always closed.
 		unsafe {
 			let handle = OpenProcess(PROCESS_TERMINATE, 0, pid as u32);
 			if handle.is_null() || handle == INVALID_HANDLE_VALUE {
@@ -228,13 +232,13 @@ mod platform {
 
 	/// Process groups are not exposed on Windows.
 	/// Always returns `None`.
-	pub fn process_group_id(_pid: i32) -> Option<i32> {
+	pub const fn process_group_id(_pid: i32) -> Option<i32> {
 		None
 	}
 
 	/// Process groups are not exposed on Windows.
 	/// Always returns `false`.
-	pub fn kill_process_group(_pgid: i32, _signal: i32) -> bool {
+	pub const fn kill_process_group(_pgid: i32, _signal: i32) -> bool {
 		false
 	}
 }
@@ -268,12 +272,14 @@ pub fn kill_tree(pid: i32, signal: i32) -> u32 {
 
 /// Get the process group id for `pid`.
 /// Returns `None` when the process is missing or unsupported on the platform.
+#[allow(clippy::missing_const_for_fn, reason = "Dispatches to platform-specific implementation")]
 pub fn process_group_id(pid: i32) -> Option<i32> {
 	platform::process_group_id(pid)
 }
 
 /// Send `signal` to the process group `pgid`.
 /// Returns false when process groups are unsupported on the platform.
+#[allow(clippy::missing_const_for_fn, reason = "Dispatches to platform-specific implementation")]
 pub fn kill_process_group(pgid: i32, signal: i32) -> bool {
 	platform::kill_process_group(pgid, signal)
 }

crates/pi-natives/src/shell.rs

@@ -347,7 +347,7 @@ const fn exit_code(result: &ExecutionResult) -> i32 {
 }
 
 #[cfg(windows)]
-fn normalize_env_key(key: &str) -> &str {
+const fn normalize_env_key(key: &str) -> &str {
 	if key.eq_ignore_ascii_case("PATH") {
 		"PATH"
 	} else {
@@ -368,8 +368,7 @@ fn merge_path_values(existing: &str, incoming: &str) -> String {
 	push_unique_paths(&mut merged, &mut seen, incoming);
 
 	std::env::join_paths(merged.iter())
-		.map(|paths| paths.to_string_lossy().to_string())
-		.unwrap_or_else(|_| merged.join(";"))
+		.map_or_else(|_| merged.join(";"), |paths| paths.to_string_lossy().to_string())
 }
 
 #[cfg(windows)]
@@ -452,11 +451,11 @@ async fn create_session(config: &ShellConfig) -> Result<ShellSessionCore> {
 	}
 
 	#[cfg(windows)]
-	if merged_path.is_none() {
-		if let Some(value) = std::env::var_os("Path").or_else(|| std::env::var_os("PATH")) {
+	if merged_path.is_none()
+		&& let Some(value) = std::env::var_os("Path").or_else(|| std::env::var_os("PATH"))
+	{
 		merged_path = Some(value.to_string_lossy().to_string());
 	}
-	}
 
 	if let Some(path_value) = merged_path {
 		let mut var = ShellVariable::new(ShellValue::String(path_value));

crates/pi-natives/src/shell/windows.rs

@@ -112,14 +112,13 @@ fn query_git_install_path_from_registry() -> Option<String> {
 	let key_paths = ["SOFTWARE\\GitForWindows", "SOFTWARE\\WOW6432Node\\GitForWindows"];
 
 	for key_path in key_paths {
-		if let Ok(key) = hklm.open_subkey(key_path) {
-			if let Ok(path) = key.get_value::<String, _>("InstallPath") {
-				if !path.is_empty() {
+		if let Ok(key) = hklm.open_subkey(key_path)
+			&& let Ok(path) = key.get_value::<String, _>("InstallPath")
+			&& !path.is_empty()
+		{
 			return Some(path);
 		}
 	}
-		}
-	}
 
 	None
 }

packages/ai/package.json

@@ -37,7 +37,7 @@
 		"test": "bun test"
 	},
 	"dependencies": {
-		"@anthropic-ai/sdk": "^0.78",
+		"@anthropic-ai/sdk": "^0.77.0",
 		"@aws-sdk/client-bedrock-runtime": "^3.998",
 		"@bufbuild/protobuf": "^2.11",
 		"@google/genai": "^1.43",
@@ -48,7 +48,7 @@
 		"ajv-formats": "^3.0",
 		"openai": "^6.25",
 		"partial-json": "^0.1",
-		"zod": "^4.3"
+		"zod": "4.3.5"
 	},
 	"devDependencies": {
 		"@types/bun": "^1.3"

packages/coding-agent/CHANGELOG.md

@@ -6,18 +6,21 @@
 
 - Added LM Studio integration to the model registry and discovery flow.
 - Added support for authenticating with LM Studio using the `/login lm-studio` command.
+- Added `fuse-projfs` task isolation mode for Windows ProjFS-backed overlays.
 
 ### Changed
 
 - Updated Anthropic Foundry environment variable documentation and CLI help text to the canonical names: `CLAUDE_CODE_USE_FOUNDRY`, `CLAUDE_CODE_CLIENT_CERT`, and `CLAUDE_CODE_CLIENT_KEY`
 - Documented Foundry-specific Anthropic runtime configuration (`FOUNDRY_BASE_URL`, `ANTHROPIC_FOUNDRY_API_KEY`, `ANTHROPIC_CUSTOM_HEADERS`, `NODE_EXTRA_CA_CERTS`) in environment variable reference docs
+- `fuse-overlay` task isolation now targets `fuse-overlayfs` on Unix hosts only; on Windows it falls back to `worktree` with a `<system-notification>` suggesting `fuse-projfs`.
+- `fuse-projfs` now performs Windows ProjFS preflight checks and falls back to `worktree` when host or repository prerequisites are unavailable.
+- Cross-repo patch capture now uses the platform null device (`NUL` on Windows, `/dev/null` elsewhere) for `git diff --no-index`.
 
 ### Fixed
 
 - Fixed MCP resource subscription handling to prevent unsubscribing when notifications are re-enabled after being disabled
 - Fixed LM Studio base URL validation to preserve invalid configured URLs instead of silently falling back to localhost
 - Fixed URI template matching to correctly handle expressions that expand to empty strings
-
 ## [13.5.6] - 2026-03-01
 ### Changed
 
@@ -1067,7 +1070,6 @@
 - Improved error reporting in fetch tool to include HTTP status codes when URL fetching fails
 - Fixed fetch tool to preserve actual response metadata (finalUrl, contentType) instead of defaults when requests fail
 
-||||||| parent of a70a34c8b (fix(coding-agent/debug): Sanitized debug log rendering)
 
 ## [12.1.0] - 2026-02-13
 

packages/coding-agent/DEVELOPMENT.md

@@ -906,9 +906,10 @@ Despite the name `runSubprocess`, `packages/coding-agent/src/task/executor.ts` c
 
 What _is_ isolated is execution context and artifacts, not process memory:
 
-- Optional filesystem isolation is controlled by the `task.isolation.mode` setting (`"none"`, `"worktree"`, or `"fuse-overlay"`).
+- Optional filesystem isolation is controlled by the `task.isolation.mode` setting (`"none"`, `"worktree"`, `"fuse-overlay"`, or `"fuse-projfs"`).
   - **worktree**: `ensureWorktree(...)`, `applyBaseline(...)`, `captureDeltaPatch(...)`, `cleanupWorktree(...)`. Nested non-submodule git repos are discovered and handled independently.
-  - **fuse-overlay**: `ensureFuseOverlay(...)` (mounts a copy-on-write overlay via `fuse-overlayfs`), `captureDeltaPatch(...)`, `cleanupFuseOverlay(...)`. No baseline apply needed since the overlay reflects the full working tree. Fails outright if mount fails.
+  - **fuse-overlay**: `ensureFuseOverlay(...)`, `captureDeltaPatch(...)`, `cleanupFuseOverlay(...)` using `fuse-overlayfs` on Unix hosts. On Windows, this mode falls back to `worktree` with a system notification.
+  - **fuse-projfs**: `ensureProjfsOverlay(...)`, `captureDeltaPatch(...)`, `cleanupProjfsOverlay(...)` using ProjFS on Windows. Missing ProjFS prerequisites fall back to `worktree` with a system notification; non-prerequisite startup errors still fail the task.
 - The `task.isolation.merge` setting controls how isolated changes are integrated back:
   - **patch** (default): captures a diff via `captureDeltaPatch(...)`, combines patches, and applies with `git apply`.
   - **branch**: each task commits to a temp branch (`omp/task/<id>`) via `commitToBranch(...)`, then `mergeTaskBranches(...)` cherry-picks them sequentially onto HEAD. If `git apply` fails inside `commitToBranch`, the error is non-fatal — the agent result is preserved with a `merge failed` status.

packages/coding-agent/src/config/settings-schema.ts

@@ -570,12 +570,13 @@ export const SETTINGS_SCHEMA = {
 	// ─────────────────────────────────────────────────────────────────────────
 	"task.isolation.mode": {
 		type: "enum",
-		values: ["none", "worktree", "fuse-overlay"] as const,
+		values: ["none", "worktree", "fuse-overlay", "fuse-projfs"] as const,
 		default: "none",
 		ui: {
 			tab: "tools",
 			label: "Task isolation",
-			description: "Isolation mode for subagents (none, git worktree, or fuse-overlay)",
+			description:
+				"Isolation mode for subagents (none, git worktree, fuse-overlayfs on Unix, or ProjFS on Windows via fuse-projfs; unsupported modes fall back to worktree)",
 			submenu: true,
 		},
 	},

packages/coding-agent/src/modes/components/settings-defs.ts

@@ -97,7 +97,16 @@ const OPTION_PROVIDERS: Partial<Record<SettingPath, OptionProvider>> = {
 	"task.isolation.mode": [
 		{ value: "none", label: "None", description: "No isolation" },
 		{ value: "worktree", label: "Worktree", description: "Git worktree isolation" },
-		{ value: "fuse-overlay", label: "Fuse Overlay", description: "COW overlay via fuse-overlayfs" },
+		{
+			value: "fuse-overlay",
+			label: "Fuse Overlay",
+			description: "COW overlay via fuse-overlayfs (Unix only)",
+		},
+		{
+			value: "fuse-projfs",
+			label: "Fuse ProjFS",
+			description: "COW overlay via ProjFS (Windows only; falls back to worktree if unavailable)",
+		},
 	],
 	// Task isolation merge strategy
 	"task.isolation.merge": [

packages/coding-agent/src/task/index.ts

@@ -32,6 +32,7 @@ import "../tools/review";
 import { generateCommitMessage } from "../utils/commit-message-generator";
 import { discoverAgents, getAgent } from "./discovery";
 import { runSubprocess } from "./executor";
+import { resolveIsolationBackendForTaskExecution } from "./isolation-backend";
 import { AgentOutputManager } from "./output-manager";
 import { mapWithConcurrencyLimit, Semaphore } from "./parallel";
 import { renderCall, renderResult } from "./render";
@@ -52,10 +53,12 @@ import {
 	captureBaseline,
 	captureDeltaPatch,
 	cleanupFuseOverlay,
+	cleanupProjfsOverlay,
 	cleanupTaskBranches,
 	cleanupWorktree,
 	commitToBranch,
 	ensureFuseOverlay,
+	ensureProjfsOverlay,
 	ensureWorktree,
 	getRepoRoot,
 	mergeTaskBranches,
@@ -442,7 +445,7 @@ export class TaskTool implements AgentTool<TaskSchema, TaskToolDetails, Theme> {
 				content: [
 					{
 						type: "text",
-						text: "Task isolation is disabled. Remove the isolated argument or set task.isolation.mode to 'worktree' or 'fuse-overlay'.",
+						text: "Task isolation is disabled. Remove the isolated argument or set task.isolation.mode to 'worktree', 'fuse-overlay', or 'fuse-projfs'.",
 					},
 				],
 				details: {
@@ -605,6 +608,29 @@ export class TaskTool implements AgentTool<TaskSchema, TaskToolDetails, Theme> {
 			}
 		}
 
+		let effectiveIsolationMode = isolationMode;
+		let isolationBackendWarning = "";
+		try {
+			const resolvedIsolation = await resolveIsolationBackendForTaskExecution(isolationMode, isIsolated, repoRoot);
+			effectiveIsolationMode = resolvedIsolation.effectiveIsolationMode;
+			isolationBackendWarning = resolvedIsolation.warning;
+		} catch (err) {
+			const message = err instanceof Error ? err.message : String(err);
+			return {
+				content: [
+					{
+						type: "text",
+						text: message,
+					},
+				],
+				details: {
+					projectAgentsDir,
+					results: [],
+					totalDurationMs: Date.now() - startTime,
+				},
+			};
+		}
+
 		// Derive artifacts directory
 		const sessionFile = this.session.getSessionFile();
 		const artifactsDir = sessionFile ? sessionFile.slice(0, -6) : null;
@@ -761,8 +787,10 @@ export class TaskTool implements AgentTool<TaskSchema, TaskToolDetails, Theme> {
 					}
 					const taskBaseline = structuredClone(baseline);
 
-					if (isolationMode === "fuse-overlay") {
+					if (effectiveIsolationMode === "fuse-overlay") {
 						isolationDir = await ensureFuseOverlay(repoRoot, task.id);
+					} else if (effectiveIsolationMode === "fuse-projfs") {
+						isolationDir = await ensureProjfsOverlay(repoRoot, task.id);
 					} else {
 						isolationDir = await ensureWorktree(repoRoot, task.id);
 						await applyBaseline(isolationDir, taskBaseline);
@@ -871,8 +899,10 @@ export class TaskTool implements AgentTool<TaskSchema, TaskToolDetails, Theme> {
 					};
 				} finally {
 					if (isolationDir) {
-						if (isolationMode === "fuse-overlay") {
+						if (effectiveIsolationMode === "fuse-overlay") {
 							await cleanupFuseOverlay(isolationDir);
+						} else if (effectiveIsolationMode === "fuse-projfs") {
+							await cleanupProjfsOverlay(isolationDir);
 						} else {
 							await cleanupWorktree(isolationDir);
 						}
@@ -1108,6 +1138,7 @@ export class TaskTool implements AgentTool<TaskSchema, TaskToolDetails, Theme> {
 			});
 
 			const outputIds = results.filter(r => !r.aborted || r.output.trim()).map(r => `agent://${r.id}`);
+			const backendSummaryPrefix = isolationBackendWarning ? `\n\n${isolationBackendWarning}` : "";
 			const summary = renderPromptTemplate(taskSummaryTemplate, {
 				successCount,
 				totalCount: results.length,
@@ -1117,7 +1148,7 @@ export class TaskTool implements AgentTool<TaskSchema, TaskToolDetails, Theme> {
 				summaries,
 				outputIds,
 				agentName,
-				mergeSummary,
+				mergeSummary: `${backendSummaryPrefix}${mergeSummary}`,
 			});
 
 			// Cleanup temp directory if used

packages/coding-agent/src/task/isolation-backend.ts

@@ -0,0 +1,72 @@
+import { projfsOverlayProbe } from "@oh-my-pi/pi-natives";
+import { Snowflake } from "@oh-my-pi/pi-utils";
+import { cleanupProjfsOverlay, ensureProjfsOverlay, isProjfsUnavailableError } from "./worktree";
+
+export type TaskIsolationMode = "none" | "worktree" | "fuse-overlay" | "fuse-projfs";
+
+export interface IsolationBackendResolution {
+	effectiveIsolationMode: TaskIsolationMode;
+	warning: string;
+}
+
+export async function resolveIsolationBackendForTaskExecution(
+	requestedMode: TaskIsolationMode,
+	isIsolated: boolean,
+	repoRoot: string | null,
+	platform: NodeJS.Platform = process.platform,
+): Promise<IsolationBackendResolution> {
+	let effectiveIsolationMode = requestedMode;
+	let warning = "";
+	if (!(isIsolated && repoRoot)) {
+		return { effectiveIsolationMode, warning };
+	}
+
+	if (requestedMode === "fuse-overlay" && platform === "win32") {
+		effectiveIsolationMode = "worktree";
+		warning =
+			'<system-notification>fuse-overlay isolation is unavailable on Windows. Use task.isolation.mode = "fuse-projfs" for ProjFS. Falling back to worktree isolation.</system-notification>';
+		return { effectiveIsolationMode, warning };
+	}
+
+	if (requestedMode === "fuse-projfs" && platform !== "win32") {
+		effectiveIsolationMode = "worktree";
+		warning =
+			"<system-notification>fuse-projfs isolation is only available on Windows. Falling back to worktree isolation.</system-notification>";
+		return { effectiveIsolationMode, warning };
+	}
+
+	if (!(requestedMode === "fuse-projfs" && platform === "win32")) {
+		return { effectiveIsolationMode, warning };
+	}
+
+	const probe = projfsOverlayProbe();
+	if (!probe.available) {
+		effectiveIsolationMode = "worktree";
+		const reason = probe.reason ? ` Reason: ${probe.reason}` : "";
+		warning = `<system-notification>ProjFS is unavailable on this host. Falling back to worktree isolation.${reason}</system-notification>`;
+		return { effectiveIsolationMode, warning };
+	}
+
+	const probeIsolationId = `probe-${Snowflake.next()}`;
+	let probeIsolationDir: string | null = null;
+	try {
+		probeIsolationDir = await ensureProjfsOverlay(repoRoot, probeIsolationId);
+	} catch (err) {
+		if (isProjfsUnavailableError(err)) {
+			effectiveIsolationMode = "worktree";
+			const raw = err instanceof Error ? err.message : String(err);
+			const reason = raw.replace(/^PROJFS_UNAVAILABLE:\s*/, "");
+			const detail = reason ? ` Reason: ${reason}` : "";
+			warning = `<system-notification>ProjFS prerequisites are unavailable for this repository. Falling back to worktree isolation.${detail}</system-notification>`;
+		} else {
+			const message = err instanceof Error ? err.message : String(err);
+			throw new Error(`ProjFS isolation initialization failed. ${message}`);
+		}
+	} finally {
+		if (probeIsolationDir) {
+			await cleanupProjfsOverlay(probeIsolationDir);
+		}
+	}
+
+	return { effectiveIsolationMode, warning };
+}

packages/coding-agent/src/task/worktree.ts

@@ -2,6 +2,7 @@ import type { Dirent } from "node:fs";
 import * as fs from "node:fs/promises";
 import * as os from "node:os";
 import path from "node:path";
+import { projfsOverlayStart, projfsOverlayStop } from "@oh-my-pi/pi-natives";
 import { getWorktreeDir, isEnoent, logger, Snowflake } from "@oh-my-pi/pi-utils";
 import { $ } from "bun";
 
@@ -37,6 +38,17 @@ export async function getRepoRoot(cwd: string): Promise<string> {
 	return repoRoot;
 }
 
+const PROJFS_UNAVAILABLE_PREFIX = "PROJFS_UNAVAILABLE:";
+const GIT_NO_INDEX_NULL_PATH = process.platform === "win32" ? "NUL" : "/dev/null";
+
+export function isProjfsUnavailableError(err: unknown): boolean {
+	return err instanceof Error && err.message.includes(PROJFS_UNAVAILABLE_PREFIX);
+}
+
+export function getGitNoIndexNullPath(): string {
+	return GIT_NO_INDEX_NULL_PATH;
+}
+
 export async function ensureWorktree(baseCwd: string, id: string): Promise<string> {
 	const repoRoot = await getRepoRoot(baseCwd);
 	const encodedProject = getEncodedProjectName(repoRoot);
@@ -248,9 +260,10 @@ async function captureRepoDeltaPatch(repoDir: string, rb: RepoBaseline): Promise
 		const baselineUntracked = new Set(rb.untracked);
 		const newUntracked = currentUntracked.filter(entry => !baselineUntracked.has(entry));
 		if (newUntracked.length > 0) {
+			const nullPath = getGitNoIndexNullPath();
 			const untrackedDiffs = await Promise.all(
 				newUntracked.map(entry =>
-					$`git diff --binary --no-index /dev/null ${entry}`.cwd(repoDir).quiet().nothrow().text(),
+					$`git diff --binary --no-index ${nullPath} ${entry}`.cwd(repoDir).quiet().nothrow().text(),
 				),
 			);
 			parts.push(...untrackedDiffs.filter(d => d.trim()));
@@ -273,9 +286,10 @@ async function captureRepoDeltaPatch(repoDir: string, rb: RepoBaseline): Promise
 
 		if (newUntracked.length === 0) return diff;
 
+		const nullPath = getGitNoIndexNullPath();
 		const untrackedDiffs = await Promise.all(
 			newUntracked.map(entry =>
-				$`git diff --binary --no-index /dev/null ${entry}`.cwd(repoDir).quiet().nothrow().text(),
+				$`git diff --binary --no-index ${nullPath} ${entry}`.cwd(repoDir).quiet().nothrow().text(),
 			),
 		);
 		return `${diff}${diff && !diff.endsWith("\n") ? "\n" : ""}${untrackedDiffs.join("\n")}`;
@@ -371,10 +385,14 @@ export async function cleanupWorktree(dir: string): Promise<void> {
 }
 
 // ═══════════════════════════════════════════════════════════════════════════
-// Fuse-overlay isolation
+// Fuse-overlay isolation (Unix)
 // ═══════════════════════════════════════════════════════════════════════════
 
 export async function ensureFuseOverlay(baseCwd: string, id: string): Promise<string> {
+	if (process.platform === "win32") {
+		throw new Error('fuse-overlay isolation is unsupported on Windows. Use task.isolation.mode = "fuse-projfs".');
+	}
+
 	const repoRoot = await getRepoRoot(baseCwd);
 	const encodedProject = getEncodedProjectName(repoRoot);
 	const baseDir = getWorktreeDir(encodedProject, id);
@@ -382,13 +400,13 @@ export async function ensureFuseOverlay(baseCwd: string, id: string): Promise<st
 	const workDir = path.join(baseDir, "work");
 	const mergedDir = path.join(baseDir, "merged");
 
-	// Clean up any stale mount at this path
+	// Clean up any stale mount at this path (linux only)
 	const fusermount = Bun.which("fusermount3") ?? Bun.which("fusermount");
 	if (fusermount) {
 		await $`${fusermount} -u ${mergedDir}`.quiet().nothrow();
 	}
-	await fs.rm(baseDir, { recursive: true, force: true });
 
+	await fs.rm(baseDir, { recursive: true, force: true });
 	await fs.mkdir(upperDir, { recursive: true });
 	await fs.mkdir(workDir, { recursive: true });
 	await fs.mkdir(mergedDir, { recursive: true });
@@ -426,6 +444,50 @@ export async function cleanupFuseOverlay(mergedDir: string): Promise<void> {
 	}
 }
 
+// ═══════════════════════════════════════════════════════════════════════════
+// ProjFS isolation (Windows)
+// ═══════════════════════════════════════════════════════════════════════════
+
+export async function ensureProjfsOverlay(baseCwd: string, id: string): Promise<string> {
+	if (process.platform !== "win32") {
+		throw new Error("fuse-projfs isolation is only available on Windows.");
+	}
+
+	const repoRoot = await getRepoRoot(baseCwd);
+	const encodedProject = getEncodedProjectName(repoRoot);
+	const baseDir = getWorktreeDir(encodedProject, id);
+	const mergedDir = path.join(baseDir, "merged");
+
+	await fs.rm(baseDir, { recursive: true, force: true });
+	await fs.mkdir(mergedDir, { recursive: true });
+	try {
+		projfsOverlayStart(repoRoot, mergedDir);
+		return mergedDir;
+	} catch (err) {
+		await fs.rm(baseDir, { recursive: true, force: true });
+		throw err;
+	}
+}
+
+export async function cleanupProjfsOverlay(mergedDir: string): Promise<void> {
+	try {
+		if (process.platform === "win32") {
+			try {
+				projfsOverlayStop(mergedDir);
+			} catch (err) {
+				logger.warn("ProjFS overlay stop failed during cleanup", {
+					mergedDir,
+					error: err instanceof Error ? err.message : String(err),
+				});
+			}
+		}
+	} finally {
+		// baseDir is the parent of the merged directory
+		const baseDir = path.dirname(mergedDir);
+		await fs.rm(baseDir, { recursive: true, force: true });
+	}
+}
+
 // ═══════════════════════════════════════════════════════════════════════════
 // Branch-mode isolation
 // ═══════════════════════════════════════════════════════════════════════════

packages/coding-agent/test/task/isolation-mode.test.ts

@@ -0,0 +1,112 @@
+import { afterEach, describe, expect, it, vi } from "bun:test";
+
+const projfsOverlayProbeMock = vi.fn();
+const ensureProjfsOverlayMock = vi.fn();
+const cleanupProjfsOverlayMock = vi.fn();
+const isProjfsUnavailableErrorMock = vi.fn(
+	(err: unknown) => err instanceof Error && err.message.includes("PROJFS_UNAVAILABLE:"),
+);
+
+vi.mock("@oh-my-pi/pi-natives", () => ({
+	projfsOverlayProbe: projfsOverlayProbeMock,
+}));
+
+vi.mock("../../src/task/worktree", () => ({
+	ensureProjfsOverlay: ensureProjfsOverlayMock,
+	cleanupProjfsOverlay: cleanupProjfsOverlayMock,
+	isProjfsUnavailableError: isProjfsUnavailableErrorMock,
+	getGitNoIndexNullPath: () => (process.platform === "win32" ? "NUL" : "/dev/null"),
+}));
+
+async function loadIsolatedBackend() {
+	const { resolveIsolationBackendForTaskExecution } = await import("../../src/task/isolation-backend");
+	return {
+		resolveIsolationBackendForTaskExecution,
+		projfsOverlayProbeMock,
+		ensureProjfsOverlayMock,
+		cleanupProjfsOverlayMock,
+		isProjfsUnavailableErrorMock,
+	};
+}
+
+describe("resolveIsolationBackendForTaskExecution", () => {
+	afterEach(() => {
+		vi.clearAllMocks();
+		isProjfsUnavailableErrorMock.mockImplementation(
+			(err: unknown) => err instanceof Error && err.message.includes("PROJFS_UNAVAILABLE:"),
+		);
+	});
+
+	it("falls back to worktree when fuse-overlay is requested on Windows", async () => {
+		const backend = await loadIsolatedBackend();
+
+		const resolved = await backend.resolveIsolationBackendForTaskExecution("fuse-overlay", true, "C:/repo", "win32");
+
+		expect(resolved.effectiveIsolationMode).toBe("worktree");
+		expect(resolved.warning).toContain("fuse-projfs");
+		expect(backend.projfsOverlayProbeMock).not.toHaveBeenCalled();
+	});
+
+	it("falls back to worktree when fuse-projfs is requested on non-Windows", async () => {
+		const backend = await loadIsolatedBackend();
+
+		const resolved = await backend.resolveIsolationBackendForTaskExecution("fuse-projfs", true, "/repo", "linux");
+
+		expect(resolved.effectiveIsolationMode).toBe("worktree");
+		expect(resolved.warning).toContain("only available on Windows");
+		expect(backend.projfsOverlayProbeMock).not.toHaveBeenCalled();
+	});
+
+	it("falls back to worktree when ProjFS probe is unavailable on Windows", async () => {
+		const backend = await loadIsolatedBackend();
+		backend.projfsOverlayProbeMock.mockReturnValue({
+			available: false,
+			reason: "Client-ProjFS optional feature disabled",
+		});
+
+		const resolved = await backend.resolveIsolationBackendForTaskExecution("fuse-projfs", true, "C:/repo", "win32");
+
+		expect(resolved.effectiveIsolationMode).toBe("worktree");
+		expect(resolved.warning).toContain("Falling back to worktree isolation");
+		expect(backend.ensureProjfsOverlayMock).not.toHaveBeenCalled();
+	});
+
+	it("falls back to worktree when ProjFS preflight returns prerequisite error", async () => {
+		const backend = await loadIsolatedBackend();
+		backend.projfsOverlayProbeMock.mockReturnValue({ available: true });
+		backend.ensureProjfsOverlayMock.mockRejectedValue(
+			new Error("PROJFS_UNAVAILABLE: filesystem does not support ProjFS"),
+		);
+		backend.isProjfsUnavailableErrorMock.mockReturnValue(true);
+
+		const resolved = await backend.resolveIsolationBackendForTaskExecution("fuse-projfs", true, "C:/repo", "win32");
+
+		expect(resolved.effectiveIsolationMode).toBe("worktree");
+		expect(resolved.warning).toContain("filesystem does not support ProjFS");
+		expect(backend.cleanupProjfsOverlayMock).not.toHaveBeenCalled();
+	});
+
+	it("keeps fuse-projfs backend when ProjFS preflight succeeds", async () => {
+		const backend = await loadIsolatedBackend();
+		backend.projfsOverlayProbeMock.mockReturnValue({ available: true });
+		backend.ensureProjfsOverlayMock.mockResolvedValue("C:/repo/.tmp/merged");
+		backend.isProjfsUnavailableErrorMock.mockReturnValue(false);
+
+		const resolved = await backend.resolveIsolationBackendForTaskExecution("fuse-projfs", true, "C:/repo", "win32");
+
+		expect(resolved.effectiveIsolationMode).toBe("fuse-projfs");
+		expect(resolved.warning).toBe("");
+		expect(backend.cleanupProjfsOverlayMock).toHaveBeenCalledWith("C:/repo/.tmp/merged");
+	});
+
+	it("throws when ProjFS preflight fails for non-prerequisite reasons", async () => {
+		const backend = await loadIsolatedBackend();
+		backend.projfsOverlayProbeMock.mockReturnValue({ available: true });
+		backend.ensureProjfsOverlayMock.mockRejectedValue(new Error("unexpected mount failure"));
+		backend.isProjfsUnavailableErrorMock.mockReturnValue(false);
+
+		await expect(
+			backend.resolveIsolationBackendForTaskExecution("fuse-projfs", true, "C:/repo", "win32"),
+		).rejects.toThrow("ProjFS isolation initialization failed. unexpected mount failure");
+	});
+});

packages/coding-agent/test/task/worktree.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it, vi } from "bun:test";
+
+const projfsOverlayStartMock = vi.fn();
+const projfsOverlayStopMock = vi.fn();
+
+vi.mock("@oh-my-pi/pi-natives", () => ({
+	projfsOverlayStart: projfsOverlayStartMock,
+	projfsOverlayStop: projfsOverlayStopMock,
+}));
+
+async function loadWorktreeHelpers() {
+	const { getGitNoIndexNullPath, isProjfsUnavailableError } = await import("../../src/task/worktree");
+	return { getGitNoIndexNullPath, isProjfsUnavailableError };
+}
+
+describe("worktree isolation helpers", () => {
+	it("returns platform-specific null path for git --no-index diffs", async () => {
+		const { getGitNoIndexNullPath } = await loadWorktreeHelpers();
+		const expected = process.platform === "win32" ? "NUL" : "/dev/null";
+		expect(getGitNoIndexNullPath()).toBe(expected);
+	});
+
+	it("detects ProjFS prerequisite errors by prefix", async () => {
+		const { isProjfsUnavailableError } = await loadWorktreeHelpers();
+		expect(isProjfsUnavailableError(new Error("PROJFS_UNAVAILABLE: missing feature"))).toBe(true);
+		expect(isProjfsUnavailableError(new Error("fuse-overlay mount failed"))).toBe(false);
+		expect(isProjfsUnavailableError("PROJFS_UNAVAILABLE: not-an-error-instance")).toBe(false);
+	});
+});

packages/natives/src/index.ts

@@ -7,6 +7,7 @@ export * from "./highlight";
 export * from "./html";
 export * from "./image";
 export * from "./keys";
+export * from "./projfs";
 export * from "./ps";
 export * from "./pty";
 export * from "./shell";

packages/natives/src/native.ts

@@ -21,6 +21,7 @@ import "./html/types";
 import "./image/types";
 import "./keys/types";
 import "./ps/types";
+import "./projfs/types";
 import "./pty/types";
 import "./shell/types";
 import "./text/types";
@@ -274,6 +275,9 @@ function validateNative(bindings: NativeBindings, source: string): void {
 	checkFn("astEdit");
 	checkFn("detectMacOSAppearance");
 	checkFn("MacAppearanceObserver");
+	checkFn("projfsOverlayProbe");
+	checkFn("projfsOverlayStart");
+	checkFn("projfsOverlayStop");
 	if (missing.length) {
 		throw new Error(
 			`Native addon missing exports (${source}). Missing: ${missing.join(", ")}. ` +

packages/natives/src/projfs/index.ts

@@ -0,0 +1,8 @@
+/**
+ * Windows ProjFS-backed overlay lifecycle bindings.
+ */
+
+import { native } from "../native";
+
+export type { ProjfsOverlayProbeResult } from "./types";
+export const { projfsOverlayProbe, projfsOverlayStart, projfsOverlayStop } = native;

packages/natives/src/projfs/types.ts

@@ -0,0 +1,28 @@
+/**
+ * Types for Windows ProjFS-backed overlay lifecycle.
+ */
+
+/** Result for probing whether ProjFS can be used on this machine. */
+export interface ProjfsOverlayProbeResult {
+	available: boolean;
+	reason?: string;
+}
+
+declare module "../bindings" {
+	interface NativeBindings {
+		/**
+		 * Probe whether ProjFS APIs are available and loadable on the current machine.
+		 */
+		projfsOverlayProbe(): ProjfsOverlayProbeResult;
+
+		/**
+		 * Start a ProjFS-backed projection at `projectionRoot` serving files from `lowerRoot`.
+		 */
+		projfsOverlayStart(lowerRoot: string, projectionRoot: string): void;
+
+		/**
+		 * Stop a ProjFS-backed projection previously started at `projectionRoot`.
+		 */
+		projfsOverlayStop(projectionRoot: string): void;
+	}
+}