fix(security): workspace sandbox avoid time-of-check/time-of-use (TOCTOU) races (#464)

* chore: Update default host bindings from 0.0.0.0 to 127.0.0.1 for various services and examples.

* config: Update default host bindings to 0.0.0.0 for improved Docker accessibility and add related documentation.

* refactor: reimplement filesystem tools with `os.OpenRoot` for enhanced security and simplified path validation.

* chore: revert other PR content from this branch

* docs: Update Chinese README.

* docs: Update Chinese README.

* docs: Update Chinese README.

* refactor: Reorder filesystem helper functions, extract directory entry formatting logic, and enhance `WriteFileTool`'s result message.

* feat: Enhance `mkdirAllInRoot` to prevent creating directories over existing files and add tests for directory creation functionality.

* Refactor filesystem tools to use a `fileReadWriter` interface for both host and sandboxed I/O, improving atomic writes and error handling.

* refactor: unify filesystem read/write operations with atomic write guarantees and clearer naming.

* refactor: rename `appendFileWithRW` function to `appendFile`

* refactor: unify filesystem access by introducing a `fileSystem` interface and updating tools to use it directly, removing `os.Root` dependency from `sandboxFs`.

* chore: run make fmt

* fix: `validatePath` now returns an error when the workspace is empty.

pkg/tools/edit.go

@@ -2,24 +2,27 @@ package tools
 
 import (
 	"context"
+	"errors"
 	"fmt"
-	"os"
+	"io/fs"
 	"strings"
 )
 
 // EditFileTool edits a file by replacing old_text with new_text.
 // The old_text must exist exactly in the file.
 type EditFileTool struct {
-	allowedDir string
-	restrict   bool
+	fs fileSystem
 }
 
 // NewEditFileTool creates a new EditFileTool with optional directory restriction.
-func NewEditFileTool(allowedDir string, restrict bool) *EditFileTool {
-	return &EditFileTool{
-		allowedDir: allowedDir,
-		restrict:   restrict,
+func NewEditFileTool(workspace string, restrict bool) *EditFileTool {
+	var fs fileSystem
+	if restrict {
+		fs = &sandboxFs{workspace: workspace}
+	} else {
+		fs = &hostFs{}
 	}
+	return &EditFileTool{fs: fs}
 }
 
 func (t *EditFileTool) Name() string {
@@ -67,49 +70,24 @@ func (t *EditFileTool) Execute(ctx context.Context, args map[string]any) *ToolRe
 		return ErrorResult("new_text is required")
 	}
 
-	resolvedPath, err := validatePath(path, t.allowedDir, t.restrict)
-	if err != nil {
+	if err := editFile(t.fs, path, oldText, newText); err != nil {
 		return ErrorResult(err.Error())
 	}
-
-	if _, err = os.Stat(resolvedPath); os.IsNotExist(err) {
-		return ErrorResult(fmt.Sprintf("file not found: %s", path))
-	}
-
-	content, err := os.ReadFile(resolvedPath)
-	if err != nil {
-		return ErrorResult(fmt.Sprintf("failed to read file: %v", err))
-	}
-
-	contentStr := string(content)
-
-	if !strings.Contains(contentStr, oldText) {
-		return ErrorResult("old_text not found in file. Make sure it matches exactly")
-	}
-
-	count := strings.Count(contentStr, oldText)
-	if count > 1 {
-		return ErrorResult(
-			fmt.Sprintf("old_text appears %d times. Please provide more context to make it unique", count),
-		)
-	}
-
-	newContent := strings.Replace(contentStr, oldText, newText, 1)
-
-	if err := os.WriteFile(resolvedPath, []byte(newContent), 0o644); err != nil {
-		return ErrorResult(fmt.Sprintf("failed to write file: %v", err))
-	}
-
 	return SilentResult(fmt.Sprintf("File edited: %s", path))
 }
 
 type AppendFileTool struct {
-	workspace string
-	restrict  bool
+	fs fileSystem
 }
 
 func NewAppendFileTool(workspace string, restrict bool) *AppendFileTool {
-	return &AppendFileTool{workspace: workspace, restrict: restrict}
+	var fs fileSystem
+	if restrict {
+		fs = &sandboxFs{workspace: workspace}
+	} else {
+		fs = &hostFs{}
+	}
+	return &AppendFileTool{fs: fs}
 }
 
 func (t *AppendFileTool) Name() string {
@@ -148,20 +126,52 @@ func (t *AppendFileTool) Execute(ctx context.Context, args map[string]any) *Tool
 		return ErrorResult("content is required")
 	}
 
-	resolvedPath, err := validatePath(path, t.workspace, t.restrict)
-	if err != nil {
+	if err := appendFile(t.fs, path, content); err != nil {
 		return ErrorResult(err.Error())
 	}
-
-	f, err := os.OpenFile(resolvedPath, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o644)
-	if err != nil {
-		return ErrorResult(fmt.Sprintf("failed to open file: %v", err))
-	}
-	defer f.Close()
-
-	if _, err := f.WriteString(content); err != nil {
-		return ErrorResult(fmt.Sprintf("failed to append to file: %v", err))
-	}
-
 	return SilentResult(fmt.Sprintf("Appended to %s", path))
 }
+
+// editFile reads the file via sysFs, performs the replacement, and writes back.
+// It uses a fileSystem interface, allowing the same logic for both restricted and unrestricted modes.
+func editFile(sysFs fileSystem, path, oldText, newText string) error {
+	content, err := sysFs.ReadFile(path)
+	if err != nil {
+		return err
+	}
+
+	newContent, err := replaceEditContent(content, oldText, newText)
+	if err != nil {
+		return err
+	}
+
+	return sysFs.WriteFile(path, newContent)
+}
+
+// appendFile reads the existing content (if any) via sysFs, appends new content, and writes back.
+func appendFile(sysFs fileSystem, path, appendContent string) error {
+	content, err := sysFs.ReadFile(path)
+	if err != nil && !errors.Is(err, fs.ErrNotExist) {
+		return err
+	}
+
+	newContent := append(content, []byte(appendContent)...)
+	return sysFs.WriteFile(path, newContent)
+}
+
+// replaceEditContent handles the core logic of finding and replacing a single occurrence of oldText.
+func replaceEditContent(content []byte, oldText, newText string) ([]byte, error) {
+	contentStr := string(content)
+
+	if !strings.Contains(contentStr, oldText) {
+		return nil, fmt.Errorf("old_text not found in file. Make sure it matches exactly")
+	}
+
+	count := strings.Count(contentStr, oldText)
+	if count > 1 {
+		return nil, fmt.Errorf("old_text appears %d times. Please provide more context to make it unique", count)
+	}
+
+	newContent := strings.Replace(contentStr, oldText, newText, 1)
+	return []byte(newContent), nil
+}

pkg/tools/edit_test.go

@@ -6,6 +6,8 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 // TestEditTool_EditFile_Success verifies successful file editing
@@ -151,14 +153,18 @@ func TestEditTool_EditFile_OutsideAllowedDir(t *testing.T) {
 	result := tool.Execute(ctx, args)
 
 	// Should return error result
-	if !result.IsError {
-		t.Errorf("Expected error when path is outside allowed directory")
-	}
+	assert.True(t, result.IsError, "Expected error when path is outside allowed directory")
 
 	// Should mention outside allowed directory
-	if !strings.Contains(result.ForLLM, "outside") && !strings.Contains(result.ForUser, "outside") {
-		t.Errorf("Expected 'outside allowed' message, got ForLLM: %s", result.ForLLM)
-	}
+	// Note: ErrorResult only sets ForLLM by default, so ForUser might be empty.
+	// We check ForLLM as it's the primary error channel.
+	assert.True(
+		t,
+		strings.Contains(result.ForLLM, "outside") || strings.Contains(result.ForLLM, "access denied") ||
+			strings.Contains(result.ForLLM, "escapes"),
+		"Expected 'outside allowed' or 'access denied' message, got ForLLM: %s",
+		result.ForLLM,
+	)
 }
 
 // TestEditTool_EditFile_MissingPath verifies error handling for missing path
@@ -287,3 +293,145 @@ func TestEditTool_AppendFile_MissingContent(t *testing.T) {
 		t.Errorf("Expected error when content is missing")
 	}
 }
+
+// TestReplaceEditContent verifies the helper function replaceEditContent
+func TestReplaceEditContent(t *testing.T) {
+	tests := []struct {
+		name        string
+		content     []byte
+		oldText     string
+		newText     string
+		expected    []byte
+		expectError bool
+	}{
+		{
+			name:        "successful replacement",
+			content:     []byte("hello world"),
+			oldText:     "world",
+			newText:     "universe",
+			expected:    []byte("hello universe"),
+			expectError: false,
+		},
+		{
+			name:        "old text not found",
+			content:     []byte("hello world"),
+			oldText:     "golang",
+			newText:     "rust",
+			expected:    nil,
+			expectError: true,
+		},
+		{
+			name:        "multiple matches found",
+			content:     []byte("test text test"),
+			oldText:     "test",
+			newText:     "done",
+			expected:    nil,
+			expectError: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result, err := replaceEditContent(tt.content, tt.oldText, tt.newText)
+			if tt.expectError {
+				assert.Error(t, err)
+			} else {
+				assert.NoError(t, err)
+				assert.Equal(t, tt.expected, result)
+			}
+		})
+	}
+}
+
+// TestAppendFileTool_AppendToNonExistent_Restricted verifies that AppendFileTool in restricted mode
+// can append to a file that does not yet exist — it should silently create the file.
+// This exercises the errors.Is(err, fs.ErrNotExist) path in appendFileWithRW + rootRW.
+func TestAppendFileTool_AppendToNonExistent_Restricted(t *testing.T) {
+	workspace := t.TempDir()
+	tool := NewAppendFileTool(workspace, true)
+	ctx := context.Background()
+
+	args := map[string]any{
+		"path":    "brand_new_file.txt",
+		"content": "first content",
+	}
+
+	result := tool.Execute(ctx, args)
+	assert.False(
+		t,
+		result.IsError,
+		"Expected success when appending to non-existent file in restricted mode, got: %s",
+		result.ForLLM,
+	)
+
+	// Verify the file was created with correct content
+	data, err := os.ReadFile(filepath.Join(workspace, "brand_new_file.txt"))
+	assert.NoError(t, err)
+	assert.Equal(t, "first content", string(data))
+}
+
+// TestAppendFileTool_Restricted_Success verifies that AppendFileTool in restricted mode
+// correctly appends to an existing file within the sandbox.
+func TestAppendFileTool_Restricted_Success(t *testing.T) {
+	workspace := t.TempDir()
+	testFile := "existing.txt"
+	err := os.WriteFile(filepath.Join(workspace, testFile), []byte("initial"), 0o644)
+	assert.NoError(t, err)
+
+	tool := NewAppendFileTool(workspace, true)
+	ctx := context.Background()
+	args := map[string]any{
+		"path":    testFile,
+		"content": " appended",
+	}
+
+	result := tool.Execute(ctx, args)
+	assert.False(t, result.IsError, "Expected success, got: %s", result.ForLLM)
+	assert.True(t, result.Silent)
+
+	data, err := os.ReadFile(filepath.Join(workspace, testFile))
+	assert.NoError(t, err)
+	assert.Equal(t, "initial appended", string(data))
+}
+
+// TestEditFileTool_Restricted_InPlaceEdit verifies that EditFileTool in restricted mode
+// correctly edits a file using the single-open editFileInRoot path.
+func TestEditFileTool_Restricted_InPlaceEdit(t *testing.T) {
+	workspace := t.TempDir()
+	testFile := "edit_target.txt"
+	err := os.WriteFile(filepath.Join(workspace, testFile), []byte("Hello World"), 0o644)
+	assert.NoError(t, err)
+
+	tool := NewEditFileTool(workspace, true)
+	ctx := context.Background()
+	args := map[string]any{
+		"path":     testFile,
+		"old_text": "World",
+		"new_text": "Go",
+	}
+
+	result := tool.Execute(ctx, args)
+	assert.False(t, result.IsError, "Expected success, got: %s", result.ForLLM)
+	assert.True(t, result.Silent)
+
+	data, err := os.ReadFile(filepath.Join(workspace, testFile))
+	assert.NoError(t, err)
+	assert.Equal(t, "Hello Go", string(data))
+}
+
+// TestEditFileTool_Restricted_FileNotFound verifies that editFileInRoot returns a proper
+// error message when the target file does not exist.
+func TestEditFileTool_Restricted_FileNotFound(t *testing.T) {
+	workspace := t.TempDir()
+	tool := NewEditFileTool(workspace, true)
+	ctx := context.Background()
+	args := map[string]any{
+		"path":     "no_such_file.txt",
+		"old_text": "old",
+		"new_text": "new",
+	}
+
+	result := tool.Execute(ctx, args)
+	assert.True(t, result.IsError)
+	assert.Contains(t, result.ForLLM, "not found")
+}

pkg/tools/filesystem.go

@@ -3,15 +3,17 @@ package tools
 import (
 	"context"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"strings"
+	"time"
 )
 
 // validatePath ensures the given path is within the workspace if restrict is true.
 func validatePath(path, workspace string, restrict bool) (string, error) {
 	if workspace == "" {
-		return path, nil
+		return path, fmt.Errorf("workspace is not defined")
 	}
 
 	absWorkspace, err := filepath.Abs(workspace)
@@ -76,16 +78,21 @@ func resolveExistingAncestor(path string) (string, error) {
 
 func isWithinWorkspace(candidate, workspace string) bool {
 	rel, err := filepath.Rel(filepath.Clean(workspace), filepath.Clean(candidate))
-	return err == nil && rel != ".." && !strings.HasPrefix(rel, ".."+string(os.PathSeparator))
+	return err == nil && filepath.IsLocal(rel)
 }
 
 type ReadFileTool struct {
-	workspace string
-	restrict  bool
+	fs fileSystem
 }
 
 func NewReadFileTool(workspace string, restrict bool) *ReadFileTool {
-	return &ReadFileTool{workspace: workspace, restrict: restrict}
+	var fs fileSystem
+	if restrict {
+		fs = &sandboxFs{workspace: workspace}
+	} else {
+		fs = &hostFs{}
+	}
+	return &ReadFileTool{fs: fs}
 }
 
 func (t *ReadFileTool) Name() string {
@@ -115,26 +122,25 @@ func (t *ReadFileTool) Execute(ctx context.Context, args map[string]any) *ToolRe
 		return ErrorResult("path is required")
 	}
 
-	resolvedPath, err := validatePath(path, t.workspace, t.restrict)
+	content, err := t.fs.ReadFile(path)
 	if err != nil {
 		return ErrorResult(err.Error())
 	}
-
-	content, err := os.ReadFile(resolvedPath)
-	if err != nil {
-		return ErrorResult(fmt.Sprintf("failed to read file: %v", err))
-	}
-
 	return NewToolResult(string(content))
 }
 
 type WriteFileTool struct {
-	workspace string
-	restrict  bool
+	fs fileSystem
 }
 
 func NewWriteFileTool(workspace string, restrict bool) *WriteFileTool {
-	return &WriteFileTool{workspace: workspace, restrict: restrict}
+	var fs fileSystem
+	if restrict {
+		fs = &sandboxFs{workspace: workspace}
+	} else {
+		fs = &hostFs{}
+	}
+	return &WriteFileTool{fs: fs}
 }
 
 func (t *WriteFileTool) Name() string {
@@ -173,30 +179,25 @@ func (t *WriteFileTool) Execute(ctx context.Context, args map[string]any) *ToolR
 		return ErrorResult("content is required")
 	}
 
-	resolvedPath, err := validatePath(path, t.workspace, t.restrict)
-	if err != nil {
+	if err := t.fs.WriteFile(path, []byte(content)); err != nil {
 		return ErrorResult(err.Error())
 	}
 
-	dir := filepath.Dir(resolvedPath)
-	if err := os.MkdirAll(dir, 0o755); err != nil {
-		return ErrorResult(fmt.Sprintf("failed to create directory: %v", err))
-	}
-
-	if err := os.WriteFile(resolvedPath, []byte(content), 0o644); err != nil {
-		return ErrorResult(fmt.Sprintf("failed to write file: %v", err))
-	}
-
 	return SilentResult(fmt.Sprintf("File written: %s", path))
 }
 
 type ListDirTool struct {
-	workspace string
-	restrict  bool
+	fs fileSystem
 }
 
 func NewListDirTool(workspace string, restrict bool) *ListDirTool {
-	return &ListDirTool{workspace: workspace, restrict: restrict}
+	var fs fileSystem
+	if restrict {
+		fs = &sandboxFs{workspace: workspace}
+	} else {
+		fs = &hostFs{}
+	}
+	return &ListDirTool{fs: fs}
 }
 
 func (t *ListDirTool) Name() string {
@@ -226,24 +227,179 @@ func (t *ListDirTool) Execute(ctx context.Context, args map[string]any) *ToolRes
 		path = "."
 	}
 
-	resolvedPath, err := validatePath(path, t.workspace, t.restrict)
-	if err != nil {
-		return ErrorResult(err.Error())
-	}
-
-	entries, err := os.ReadDir(resolvedPath)
+	entries, err := t.fs.ReadDir(path)
 	if err != nil {
 		return ErrorResult(fmt.Sprintf("failed to read directory: %v", err))
 	}
+	return formatDirEntries(entries)
+}
 
-	result := ""
+func formatDirEntries(entries []os.DirEntry) *ToolResult {
+	var result strings.Builder
 	for _, entry := range entries {
 		if entry.IsDir() {
-			result += "DIR:  " + entry.Name() + "\n"
+			result.WriteString("DIR:  " + entry.Name() + "\n")
 		} else {
-			result += "FILE: " + entry.Name() + "\n"
+			result.WriteString("FILE: " + entry.Name() + "\n")
+		}
+	}
+	return NewToolResult(result.String())
+}
+
+// fileSystem abstracts reading, writing, and listing files, allowing both
+// unrestricted (host filesystem) and sandbox (os.Root) implementations to share the same polymorphic interface.
+type fileSystem interface {
+	ReadFile(path string) ([]byte, error)
+	WriteFile(path string, data []byte) error
+	ReadDir(path string) ([]os.DirEntry, error)
+}
+
+// hostFs is an unrestricted fileReadWriter that operates directly on the host filesystem.
+type hostFs struct{}
+
+func (h *hostFs) ReadFile(path string) ([]byte, error) {
+	content, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, fmt.Errorf("failed to read file: file not found: %w", err)
+		}
+		if os.IsPermission(err) {
+			return nil, fmt.Errorf("failed to read file: access denied: %w", err)
+		}
+		return nil, fmt.Errorf("failed to read file: %w", err)
+	}
+	return content, nil
+}
+
+func (h *hostFs) ReadDir(path string) ([]os.DirEntry, error) {
+	return os.ReadDir(path)
+}
+
+func (h *hostFs) WriteFile(path string, data []byte) error {
+	dir := filepath.Dir(path)
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return fmt.Errorf("failed to create parent directories: %w", err)
+	}
+
+	// We use a "write-then-rename" pattern here to ensure an atomic write.
+	// This prevents the target file from being left in a truncated or partial state
+	// if the operation is interrupted, as the rename operation is atomic on Linux.
+	tmpPath := fmt.Sprintf("%s.%d.tmp", path, time.Now().UnixNano())
+	if err := os.WriteFile(tmpPath, data, 0o644); err != nil {
+		os.Remove(tmpPath) // Ensure cleanup of partial/empty temp file
+		return fmt.Errorf("failed to write temp file: %w", err)
+	}
+
+	if err := os.Rename(tmpPath, path); err != nil {
+		os.Remove(tmpPath)
+		return fmt.Errorf("failed to replace original file: %w", err)
+	}
+	return nil
+}
+
+// sandboxFs is a sandboxed fileSystem that operates within a strictly defined workspace using os.Root.
+type sandboxFs struct {
+	workspace string
+}
+
+func (r *sandboxFs) execute(path string, fn func(root *os.Root, relPath string) error) error {
+	if r.workspace == "" {
+		return fmt.Errorf("workspace is not defined")
+	}
+
+	root, err := os.OpenRoot(r.workspace)
+	if err != nil {
+		return fmt.Errorf("failed to open workspace: %w", err)
+	}
+	defer root.Close()
+
+	relPath, err := getSafeRelPath(r.workspace, path)
+	if err != nil {
+		return err
+	}
+
+	return fn(root, relPath)
+}
+
+func (r *sandboxFs) ReadFile(path string) ([]byte, error) {
+	var content []byte
+	err := r.execute(path, func(root *os.Root, relPath string) error {
+		fileContent, err := root.ReadFile(relPath)
+		if err != nil {
+			if os.IsNotExist(err) {
+				return fmt.Errorf("failed to read file: file not found: %w", err)
+			}
+			// os.Root returns "escapes from parent" for paths outside the root
+			if os.IsPermission(err) || strings.Contains(err.Error(), "escapes from parent") ||
+				strings.Contains(err.Error(), "permission denied") {
+				return fmt.Errorf("failed to read file: access denied: %w", err)
+			}
+			return fmt.Errorf("failed to read file: %w", err)
+		}
+		content = fileContent
+		return nil
+	})
+	return content, err
+}
+
+func (r *sandboxFs) WriteFile(path string, data []byte) error {
+	return r.execute(path, func(root *os.Root, relPath string) error {
+		dir := filepath.Dir(relPath)
+		if dir != "." && dir != "/" {
+			if err := root.MkdirAll(dir, 0o755); err != nil {
+				return fmt.Errorf("failed to create parent directories: %w", err)
+			}
+		}
+
+		// We use a "write-then-rename" pattern here to ensure an atomic write.
+		// This prevents the target file from being left in a truncated or partial state
+		// if the operation is interrupted, as the rename operation is atomic on Linux.
+		tmpRelPath := fmt.Sprintf("%s.%d.tmp", relPath, time.Now().UnixNano())
+
+		if err := root.WriteFile(tmpRelPath, data, 0o644); err != nil {
+			root.Remove(tmpRelPath) // Ensure cleanup of partial/empty temp file
+			return fmt.Errorf("failed to write to temp file: %w", err)
+		}
+
+		if err := root.Rename(tmpRelPath, relPath); err != nil {
+			root.Remove(tmpRelPath)
+			return fmt.Errorf("failed to rename temp file over target: %w", err)
+		}
+		return nil
+	})
+}
+
+func (r *sandboxFs) ReadDir(path string) ([]os.DirEntry, error) {
+	var entries []os.DirEntry
+	err := r.execute(path, func(root *os.Root, relPath string) error {
+		dirEntries, err := fs.ReadDir(root.FS(), relPath)
+		if err != nil {
+			return err
+		}
+		entries = dirEntries
+		return nil
+	})
+	return entries, err
+}
+
+// Helper to get a safe relative path for os.Root usage
+func getSafeRelPath(workspace, path string) (string, error) {
+	if workspace == "" {
+		return "", fmt.Errorf("workspace is not defined")
+	}
+
+	rel := filepath.Clean(path)
+	if filepath.IsAbs(rel) {
+		var err error
+		rel, err = filepath.Rel(workspace, rel)
+		if err != nil {
+			return "", fmt.Errorf("failed to calculate relative path: %w", err)
 		}
 	}
 
-	return NewToolResult(result)
+	if !filepath.IsLocal(rel) {
+		return "", fmt.Errorf("path escapes workspace: %s", path)
+	}
+
+	return rel, nil
 }

pkg/tools/filesystem_test.go

@@ -2,10 +2,13 @@ package tools
 
 import (
 	"context"
+	"io"
 	"os"
 	"path/filepath"
 	"strings"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 // TestFilesystemTool_ReadFile_Success verifies successful file reading
@@ -14,7 +17,7 @@ func TestFilesystemTool_ReadFile_Success(t *testing.T) {
 	testFile := filepath.Join(tmpDir, "test.txt")
 	os.WriteFile(testFile, []byte("test content"), 0o644)
 
-	tool := &ReadFileTool{}
+	tool := NewReadFileTool("", false)
 	ctx := context.Background()
 	args := map[string]any{
 		"path": testFile,
@@ -41,7 +44,7 @@ func TestFilesystemTool_ReadFile_Success(t *testing.T) {
 
 // TestFilesystemTool_ReadFile_NotFound verifies error handling for missing file
 func TestFilesystemTool_ReadFile_NotFound(t *testing.T) {
-	tool := &ReadFileTool{}
+	tool := NewReadFileTool("", false)
 	ctx := context.Background()
 	args := map[string]any{
 		"path": "/nonexistent_file_12345.txt",
@@ -84,7 +87,7 @@ func TestFilesystemTool_WriteFile_Success(t *testing.T) {
 	tmpDir := t.TempDir()
 	testFile := filepath.Join(tmpDir, "newfile.txt")
 
-	tool := &WriteFileTool{}
+	tool := NewWriteFileTool("", false)
 	ctx := context.Background()
 	args := map[string]any{
 		"path":    testFile,
@@ -123,7 +126,7 @@ func TestFilesystemTool_WriteFile_CreateDir(t *testing.T) {
 	tmpDir := t.TempDir()
 	testFile := filepath.Join(tmpDir, "subdir", "newfile.txt")
 
-	tool := &WriteFileTool{}
+	tool := NewWriteFileTool("", false)
 	ctx := context.Background()
 	args := map[string]any{
 		"path":    testFile,
@@ -149,7 +152,7 @@ func TestFilesystemTool_WriteFile_CreateDir(t *testing.T) {
 
 // TestFilesystemTool_WriteFile_MissingPath verifies error handling for missing path
 func TestFilesystemTool_WriteFile_MissingPath(t *testing.T) {
-	tool := &WriteFileTool{}
+	tool := NewWriteFileTool("", false)
 	ctx := context.Background()
 	args := map[string]any{
 		"content": "test",
@@ -165,7 +168,7 @@ func TestFilesystemTool_WriteFile_MissingPath(t *testing.T) {
 
 // TestFilesystemTool_WriteFile_MissingContent verifies error handling for missing content
 func TestFilesystemTool_WriteFile_MissingContent(t *testing.T) {
-	tool := &WriteFileTool{}
+	tool := NewWriteFileTool("", false)
 	ctx := context.Background()
 	args := map[string]any{
 		"path": "/tmp/test.txt",
@@ -192,7 +195,7 @@ func TestFilesystemTool_ListDir_Success(t *testing.T) {
 	os.WriteFile(filepath.Join(tmpDir, "file2.txt"), []byte("content"), 0o644)
 	os.Mkdir(filepath.Join(tmpDir, "subdir"), 0o755)
 
-	tool := &ListDirTool{}
+	tool := NewListDirTool("", false)
 	ctx := context.Background()
 	args := map[string]any{
 		"path": tmpDir,
@@ -216,7 +219,7 @@ func TestFilesystemTool_ListDir_Success(t *testing.T) {
 
 // TestFilesystemTool_ListDir_NotFound verifies error handling for non-existent directory
 func TestFilesystemTool_ListDir_NotFound(t *testing.T) {
-	tool := &ListDirTool{}
+	tool := NewListDirTool("", false)
 	ctx := context.Background()
 	args := map[string]any{
 		"path": "/nonexistent_directory_12345",
@@ -237,7 +240,7 @@ func TestFilesystemTool_ListDir_NotFound(t *testing.T) {
 
 // TestFilesystemTool_ListDir_DefaultPath verifies default to current directory
 func TestFilesystemTool_ListDir_DefaultPath(t *testing.T) {
-	tool := &ListDirTool{}
+	tool := NewListDirTool("", false)
 	ctx := context.Background()
 	args := map[string]any{}
 
@@ -275,7 +278,211 @@ func TestFilesystemTool_ReadFile_RejectsSymlinkEscape(t *testing.T) {
 	if !result.IsError {
 		t.Fatalf("expected symlink escape to be blocked")
 	}
-	if !strings.Contains(result.ForLLM, "symlink resolves outside workspace") {
+	// os.Root might return different errors depending on platform/implementation
+	// but it definitely should error.
+	// Our wrapper returns "access denied or file not found"
+	if !strings.Contains(result.ForLLM, "access denied") && !strings.Contains(result.ForLLM, "file not found") &&
+		!strings.Contains(result.ForLLM, "no such file") {
 		t.Fatalf("expected symlink escape error, got: %s", result.ForLLM)
 	}
 }
+
+func TestFilesystemTool_EmptyWorkspace_AccessDenied(t *testing.T) {
+	tool := NewReadFileTool("", true) // restrict=true but workspace=""
+
+	// Try to read a sensitive file (simulated by a temp file outside workspace)
+	tmpDir := t.TempDir()
+	secretFile := filepath.Join(tmpDir, "shadow")
+	os.WriteFile(secretFile, []byte("secret data"), 0o600)
+
+	result := tool.Execute(context.Background(), map[string]any{
+		"path": secretFile,
+	})
+
+	// We EXPECT IsError=true (access blocked due to empty workspace)
+	assert.True(t, result.IsError, "Security Regression: Empty workspace allowed access! content: %s", result.ForLLM)
+
+	// Verify it failed for the right reason
+	assert.Contains(t, result.ForLLM, "workspace is not defined", "Expected 'workspace is not defined' error")
+}
+
+// TestRootMkdirAll verifies that root.MkdirAll (used by atomicWriteFileInRoot) handles all cases:
+// single dir, deeply nested dirs, already-existing dirs, and a file blocking a directory path.
+func TestRootMkdirAll(t *testing.T) {
+	workspace := t.TempDir()
+	root, err := os.OpenRoot(workspace)
+	if err != nil {
+		t.Fatalf("failed to open root: %v", err)
+	}
+	defer root.Close()
+
+	// Case 1: Single directory
+	err = root.MkdirAll("dir1", 0o755)
+	assert.NoError(t, err)
+	_, err = os.Stat(filepath.Join(workspace, "dir1"))
+	assert.NoError(t, err)
+
+	// Case 2: Deeply nested directory
+	err = root.MkdirAll("a/b/c/d", 0o755)
+	assert.NoError(t, err)
+	_, err = os.Stat(filepath.Join(workspace, "a/b/c/d"))
+	assert.NoError(t, err)
+
+	// Case 3: Already exists — must be idempotent
+	err = root.MkdirAll("a/b/c/d", 0o755)
+	assert.NoError(t, err)
+
+	// Case 4: A regular file blocks directory creation — must error
+	err = os.WriteFile(filepath.Join(workspace, "file_exists"), []byte("data"), 0o644)
+	assert.NoError(t, err)
+	err = root.MkdirAll("file_exists", 0o755)
+	assert.Error(t, err, "expected error when a file exists at the directory path")
+}
+
+func TestFilesystemTool_WriteFile_Restricted_CreateDir(t *testing.T) {
+	workspace := t.TempDir()
+	tool := NewWriteFileTool(workspace, true)
+	ctx := context.Background()
+
+	testFile := "deep/nested/path/to/file.txt"
+	content := "deep content"
+	args := map[string]any{
+		"path":    testFile,
+		"content": content,
+	}
+
+	result := tool.Execute(ctx, args)
+	assert.False(t, result.IsError, "Expected success, got: %s", result.ForLLM)
+
+	// Verify file content
+	actualPath := filepath.Join(workspace, testFile)
+	data, err := os.ReadFile(actualPath)
+	assert.NoError(t, err)
+	assert.Equal(t, content, string(data))
+}
+
+// TestHostRW_Read_PermissionDenied verifies that hostRW.Read surfaces access denied errors.
+func TestHostRW_Read_PermissionDenied(t *testing.T) {
+	if os.Getuid() == 0 {
+		t.Skip("skipping permission test: running as root")
+	}
+	tmpDir := t.TempDir()
+	protected := filepath.Join(tmpDir, "protected.txt")
+	err := os.WriteFile(protected, []byte("secret"), 0o000)
+	assert.NoError(t, err)
+	defer os.Chmod(protected, 0o644) // ensure cleanup
+
+	_, err = (&hostFs{}).ReadFile(protected)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "access denied")
+}
+
+// TestHostRW_Read_Directory verifies that hostRW.Read returns an error when given a directory path.
+func TestHostRW_Read_Directory(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	_, err := (&hostFs{}).ReadFile(tmpDir)
+	assert.Error(t, err, "expected error when reading a directory as a file")
+}
+
+// TestRootRW_Read_Directory verifies that rootRW.Read returns an error when given a directory.
+func TestRootRW_Read_Directory(t *testing.T) {
+	workspace := t.TempDir()
+	root, err := os.OpenRoot(workspace)
+	assert.NoError(t, err)
+	defer root.Close()
+
+	// Create a subdirectory
+	err = root.Mkdir("subdir", 0o755)
+	assert.NoError(t, err)
+
+	_, err = (&sandboxFs{workspace: workspace}).ReadFile("subdir")
+	assert.Error(t, err, "expected error when reading a directory as a file")
+}
+
+// TestHostRW_Write_ParentDirMissing verifies that hostRW.Write creates parent dirs automatically.
+func TestHostRW_Write_ParentDirMissing(t *testing.T) {
+	tmpDir := t.TempDir()
+	target := filepath.Join(tmpDir, "a", "b", "c", "file.txt")
+
+	err := (&hostFs{}).WriteFile(target, []byte("hello"))
+	assert.NoError(t, err)
+
+	data, err := os.ReadFile(target)
+	assert.NoError(t, err)
+	assert.Equal(t, "hello", string(data))
+}
+
+// TestRootRW_Write_ParentDirMissing verifies that rootRW.Write creates
+// nested parent directories automatically within the sandbox.
+func TestRootRW_Write_ParentDirMissing(t *testing.T) {
+	workspace := t.TempDir()
+
+	relPath := "x/y/z/file.txt"
+	err := (&sandboxFs{workspace: workspace}).WriteFile(relPath, []byte("nested"))
+	assert.NoError(t, err)
+
+	data, err := os.ReadFile(filepath.Join(workspace, relPath))
+	assert.NoError(t, err)
+	assert.Equal(t, "nested", string(data))
+}
+
+// TestHostRW_Write verifies the hostRW.Write helper function
+func TestHostRW_Write(t *testing.T) {
+	tmpDir := t.TempDir()
+	testFile := filepath.Join(tmpDir, "atomic_test.txt")
+	testData := []byte("atomic test content")
+
+	err := (&hostFs{}).WriteFile(testFile, testData)
+	assert.NoError(t, err)
+
+	content, err := os.ReadFile(testFile)
+	assert.NoError(t, err)
+	assert.Equal(t, testData, content)
+
+	// Verify it overwrites correctly
+	newData := []byte("new atomic content")
+	err = (&hostFs{}).WriteFile(testFile, newData)
+	assert.NoError(t, err)
+
+	content, err = os.ReadFile(testFile)
+	assert.NoError(t, err)
+	assert.Equal(t, newData, content)
+}
+
+// TestRootRW_Write verifies the rootRW.Write helper function
+func TestRootRW_Write(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	relPath := "atomic_root_test.txt"
+	testData := []byte("atomic root test content")
+
+	erw := &sandboxFs{workspace: tmpDir}
+	err := erw.WriteFile(relPath, testData)
+	assert.NoError(t, err)
+
+	root, err := os.OpenRoot(tmpDir)
+	assert.NoError(t, err)
+	defer root.Close()
+
+	f, err := root.Open(relPath)
+	assert.NoError(t, err)
+	defer f.Close()
+
+	content, err := io.ReadAll(f)
+	assert.NoError(t, err)
+	assert.Equal(t, testData, content)
+
+	// Verify it overwrites correctly
+	newData := []byte("new root atomic content")
+	err = erw.WriteFile(relPath, newData)
+	assert.NoError(t, err)
+
+	f2, err := root.Open(relPath)
+	assert.NoError(t, err)
+	defer f2.Close()
+
+	content, err = io.ReadAll(f2)
+	assert.NoError(t, err)
+	assert.Equal(t, newData, content)
+}