diff --git a/internal/checker/checker.go b/internal/checker/checker.go
index a113e2b..f822fa2 100644
--- a/internal/checker/checker.go
+++ b/internal/checker/checker.go
@@ -17,9 +17,9 @@ import (
 
 // Result represents the outcome of checking a single file.
 type Result struct {
-	Path    string // Relative path from manifest
-	Status  Status // Verification result status
-	Message string // Human-readable description of the result
+	Path    mfer.RelFilePath // Relative path from manifest
+	Status  Status           // Verification result status
+	Message string           // Human-readable description of the result
 }
 
 // Status represents the verification status of a file.
@@ -55,22 +55,22 @@ func (s Status) String() string {
 
 // CheckStatus contains progress information for the check operation.
 type CheckStatus struct {
-	TotalFiles   int64         // Total number of files in manifest
-	CheckedFiles int64         // Number of files checked so far
-	TotalBytes   int64         // Total bytes to verify (sum of all file sizes)
-	CheckedBytes int64         // Bytes verified so far
-	BytesPerSec  float64       // Current throughput rate
-	ETA          time.Duration // Estimated time to completion
-	Failures     int64         // Number of verification failures encountered
+	TotalFiles   mfer.FileCount // Total number of files in manifest
+	CheckedFiles mfer.FileCount // Number of files checked so far
+	TotalBytes   mfer.FileSize  // Total bytes to verify (sum of all file sizes)
+	CheckedBytes mfer.FileSize  // Bytes verified so far
+	BytesPerSec  float64        // Current throughput rate
+	ETA          time.Duration  // Estimated time to completion
+	Failures     mfer.FileCount // Number of verification failures encountered
 }
 
 // Checker verifies files against a manifest.
 type Checker struct {
-	basePath string
+	basePath mfer.AbsFilePath
 	files    []*mfer.MFFilePath
 	fs       afero.Fs
 	// manifestPaths is a set of paths in the manifest for quick lookup
-	manifestPaths map[string]struct{}
+	manifestPaths map[mfer.RelFilePath]struct{}
 }
 
 // NewChecker creates a new Checker for the given manifest, base path, and filesystem.
@@ -92,13 +92,13 @@ func NewChecker(manifestPath string, basePath string, fs afero.Fs) (*Checker, er
 	}
 
 	files := m.Files()
-	manifestPaths := make(map[string]struct{}, len(files))
+	manifestPaths := make(map[mfer.RelFilePath]struct{}, len(files))
 	for _, f := range files {
-		manifestPaths[f.Path] = struct{}{}
+		manifestPaths[mfer.RelFilePath(f.Path)] = struct{}{}
 	}
 
 	return &Checker{
-		basePath:      abs,
+		basePath:      mfer.AbsFilePath(abs),
 		files:         files,
 		fs:            fs,
 		manifestPaths: manifestPaths,
@@ -106,15 +106,15 @@ func NewChecker(manifestPath string, basePath string, fs afero.Fs) (*Checker, er
 }
 
 // FileCount returns the number of files in the manifest.
-func (c *Checker) FileCount() int64 {
-	return int64(len(c.files))
+func (c *Checker) FileCount() mfer.FileCount {
+	return mfer.FileCount(len(c.files))
 }
 
 // TotalBytes returns the total size of all files in the manifest.
-func (c *Checker) TotalBytes() int64 {
-	var total int64
+func (c *Checker) TotalBytes() mfer.FileSize {
+	var total mfer.FileSize
 	for _, f := range c.files {
-		total += f.Size
+		total += mfer.FileSize(f.Size)
 	}
 	return total
 }
@@ -131,12 +131,12 @@ func (c *Checker) Check(ctx context.Context, results chan<- Result, progress cha
 		defer close(progress)
 	}
 
-	totalFiles := int64(len(c.files))
+	totalFiles := mfer.FileCount(len(c.files))
 	totalBytes := c.TotalBytes()
 
-	var checkedFiles int64
-	var checkedBytes int64
-	var failures int64
+	var checkedFiles mfer.FileCount
+	var checkedBytes mfer.FileSize
+	var failures mfer.FileCount
 
 	startTime := time.Now()
 
@@ -186,28 +186,29 @@ func (c *Checker) Check(ctx context.Context, results chan<- Result, progress cha
 	return nil
 }
 
-func (c *Checker) checkFile(entry *mfer.MFFilePath, checkedBytes *int64) Result {
-	absPath := filepath.Join(c.basePath, entry.Path)
+func (c *Checker) checkFile(entry *mfer.MFFilePath, checkedBytes *mfer.FileSize) Result {
+	absPath := filepath.Join(string(c.basePath), entry.Path)
+	relPath := mfer.RelFilePath(entry.Path)
 
 	// Check if file exists
 	info, err := c.fs.Stat(absPath)
 	if err != nil {
 		if errors.Is(err, afero.ErrFileNotFound) || errors.Is(err, errors.New("file does not exist")) {
-			return Result{Path: entry.Path, Status: StatusMissing, Message: "file not found"}
+			return Result{Path: relPath, Status: StatusMissing, Message: "file not found"}
 		}
 		// Check for "file does not exist" style errors
 		exists, _ := afero.Exists(c.fs, absPath)
 		if !exists {
-			return Result{Path: entry.Path, Status: StatusMissing, Message: "file not found"}
+			return Result{Path: relPath, Status: StatusMissing, Message: "file not found"}
 		}
-		return Result{Path: entry.Path, Status: StatusError, Message: err.Error()}
+		return Result{Path: relPath, Status: StatusError, Message: err.Error()}
 	}
 
 	// Check size
 	if info.Size() != entry.Size {
-		*checkedBytes += info.Size()
+		*checkedBytes += mfer.FileSize(info.Size())
 		return Result{
-			Path:    entry.Path,
+			Path:    relPath,
 			Status:  StatusSizeMismatch,
 			Message: "size mismatch",
 		}
@@ -216,31 +217,31 @@ func (c *Checker) checkFile(entry *mfer.MFFilePath, checkedBytes *int64) Result
 	// Open and hash file
 	f, err := c.fs.Open(absPath)
 	if err != nil {
-		return Result{Path: entry.Path, Status: StatusError, Message: err.Error()}
+		return Result{Path: relPath, Status: StatusError, Message: err.Error()}
 	}
 	defer func() { _ = f.Close() }()
 
 	h := sha256.New()
 	n, err := io.Copy(h, f)
 	if err != nil {
-		return Result{Path: entry.Path, Status: StatusError, Message: err.Error()}
+		return Result{Path: relPath, Status: StatusError, Message: err.Error()}
 	}
-	*checkedBytes += n
+	*checkedBytes += mfer.FileSize(n)
 
 	// Encode as multihash and compare
 	computed, err := multihash.Encode(h.Sum(nil), multihash.SHA2_256)
 	if err != nil {
-		return Result{Path: entry.Path, Status: StatusError, Message: err.Error()}
+		return Result{Path: relPath, Status: StatusError, Message: err.Error()}
 	}
 
 	// Check against all hashes in manifest (at least one must match)
 	for _, hash := range entry.Hashes {
 		if bytes.Equal(computed, hash.MultiHash) {
-			return Result{Path: entry.Path, Status: StatusOK}
+			return Result{Path: relPath, Status: StatusOK}
 		}
 	}
 
-	return Result{Path: entry.Path, Status: StatusHashMismatch, Message: "hash mismatch"}
+	return Result{Path: relPath, Status: StatusHashMismatch, Message: "hash mismatch"}
 }
 
 // FindExtraFiles walks the filesystem and reports files not in the manifest.
@@ -250,7 +251,7 @@ func (c *Checker) FindExtraFiles(ctx context.Context, results chan<- Result) err
 		defer close(results)
 	}
 
-	return afero.Walk(c.fs, c.basePath, func(path string, info os.FileInfo, err error) error {
+	return afero.Walk(c.fs, string(c.basePath), func(path string, info os.FileInfo, err error) error {
 		if err != nil {
 			return err
 		}
@@ -267,10 +268,11 @@ func (c *Checker) FindExtraFiles(ctx context.Context, results chan<- Result) err
 		}
 
 		// Get relative path
-		relPath, err := filepath.Rel(c.basePath, path)
+		rel, err := filepath.Rel(string(c.basePath), path)
 		if err != nil {
 			return err
 		}
+		relPath := mfer.RelFilePath(rel)
 
 		// Check if path is in manifest
 		if _, exists := c.manifestPaths[relPath]; !exists {
diff --git a/internal/checker/checker_test.go b/internal/checker/checker_test.go
index d6bc462..cba9cde 100644
--- a/internal/checker/checker_test.go
+++ b/internal/checker/checker_test.go
@@ -40,7 +40,7 @@ func createTestManifest(t *testing.T, fs afero.Fs, manifestPath string, files ma
 	builder := mfer.NewBuilder()
 	for path, content := range files {
 		reader := bytes.NewReader(content)
-		_, err := builder.AddFile(path, int64(len(content)), time.Now(), reader, nil)
+		_, err := builder.AddFile(mfer.RelFilePath(path), mfer.FileSize(len(content)), mfer.ModTime(time.Now()), reader, nil)
 		require.NoError(t, err)
 	}
 
@@ -72,7 +72,7 @@ func TestNewChecker(t *testing.T) {
 		chk, err := NewChecker("/manifest.mf", "/", fs)
 		require.NoError(t, err)
 		assert.NotNil(t, chk)
-		assert.Equal(t, int64(2), chk.FileCount())
+		assert.Equal(t, mfer.FileCount(2), chk.FileCount())
 	})
 
 	t.Run("missing manifest", func(t *testing.T) {
@@ -101,8 +101,8 @@ func TestCheckerFileCountAndTotalBytes(t *testing.T) {
 	chk, err := NewChecker("/manifest.mf", "/", fs)
 	require.NoError(t, err)
 
-	assert.Equal(t, int64(3), chk.FileCount())
-	assert.Equal(t, int64(2+11+1000), chk.TotalBytes())
+	assert.Equal(t, mfer.FileCount(3), chk.FileCount())
+	assert.Equal(t, mfer.FileSize(2+11+1000), chk.TotalBytes())
 }
 
 func TestCheckAllFilesOK(t *testing.T) {
@@ -158,7 +158,7 @@ func TestCheckMissingFile(t *testing.T) {
 			okCount++
 		case StatusMissing:
 			missingCount++
-			assert.Equal(t, "missing.txt", r.Path)
+			assert.Equal(t, mfer.RelFilePath("missing.txt"), r.Path)
 		}
 	}
 
@@ -186,7 +186,7 @@ func TestCheckSizeMismatch(t *testing.T) {
 
 	r := <-results
 	assert.Equal(t, StatusSizeMismatch, r.Status)
-	assert.Equal(t, "file.txt", r.Path)
+	assert.Equal(t, mfer.RelFilePath("file.txt"), r.Path)
 }
 
 func TestCheckHashMismatch(t *testing.T) {
@@ -212,7 +212,7 @@ func TestCheckHashMismatch(t *testing.T) {
 
 	r := <-results
 	assert.Equal(t, StatusHashMismatch, r.Status)
-	assert.Equal(t, "file.txt", r.Path)
+	assert.Equal(t, mfer.RelFilePath("file.txt"), r.Path)
 }
 
 func TestCheckWithProgress(t *testing.T) {
@@ -246,11 +246,11 @@ func TestCheckWithProgress(t *testing.T) {
 	assert.NotEmpty(t, progressUpdates)
 	// Final progress should show all files checked
 	final := progressUpdates[len(progressUpdates)-1]
-	assert.Equal(t, int64(2), final.TotalFiles)
-	assert.Equal(t, int64(2), final.CheckedFiles)
-	assert.Equal(t, int64(300), final.TotalBytes)
-	assert.Equal(t, int64(300), final.CheckedBytes)
-	assert.Equal(t, int64(0), final.Failures)
+	assert.Equal(t, mfer.FileCount(2), final.TotalFiles)
+	assert.Equal(t, mfer.FileCount(2), final.CheckedFiles)
+	assert.Equal(t, mfer.FileSize(300), final.TotalBytes)
+	assert.Equal(t, mfer.FileSize(300), final.CheckedBytes)
+	assert.Equal(t, mfer.FileCount(0), final.Failures)
 }
 
 func TestCheckContextCancellation(t *testing.T) {
@@ -301,7 +301,7 @@ func TestFindExtraFiles(t *testing.T) {
 	}
 
 	assert.Len(t, extras, 1)
-	assert.Equal(t, "file2.txt", extras[0].Path)
+	assert.Equal(t, mfer.RelFilePath("file2.txt"), extras[0].Path)
 	assert.Equal(t, StatusExtra, extras[0].Status)
 	assert.Equal(t, "not in manifest", extras[0].Message)
 }
@@ -390,8 +390,8 @@ func TestCheckEmptyManifest(t *testing.T) {
 	chk, err := NewChecker("/manifest.mf", "/data", fs)
 	require.NoError(t, err)
 
-	assert.Equal(t, int64(0), chk.FileCount())
-	assert.Equal(t, int64(0), chk.TotalBytes())
+	assert.Equal(t, mfer.FileCount(0), chk.FileCount())
+	assert.Equal(t, mfer.FileSize(0), chk.TotalBytes())
 
 	results := make(chan Result, 10)
 	err = chk.Check(context.Background(), results, nil)
diff --git a/internal/cli/freshen.go b/internal/cli/freshen.go
index 1de2713..ac4c96b 100644
--- a/internal/cli/freshen.go
+++ b/internal/cli/freshen.go
@@ -113,7 +113,7 @@ func (mfa *CLIApp) freshenManifestOperation(ctx *cli.Context) error {
 		}
 
 		// Handle dotfiles
-		if !includeDotfiles && pathIsHidden(relPath) {
+		if !includeDotfiles && mfer.IsHiddenPath(filepath.ToSlash(relPath)) {
 			if info.IsDir() {
 				return filepath.SkipDir
 			}
@@ -274,10 +274,14 @@ func (mfa *CLIApp) freshenManifestOperation(ctx *cli.Context) error {
 			hashedFiles++
 
 			// Add to builder with computed hash
-			addFileToBuilder(builder, e.path, e.size, e.mtime, hash)
+			if err := addFileToBuilder(builder, e.path, e.size, e.mtime, hash); err != nil {
+				return fmt.Errorf("failed to add %s: %w", e.path, err)
+			}
 		} else {
 			// Use existing entry
-			addExistingToBuilder(builder, e.existing)
+			if err := addExistingToBuilder(builder, e.existing); err != nil {
+				return fmt.Errorf("failed to add %s: %w", e.path, err)
+			}
 		}
 	}
 
@@ -360,38 +364,15 @@ func hashFile(r io.Reader, size int64, progress func(int64)) ([]byte, int64, err
 }
 
 // addFileToBuilder adds a new file entry to the builder
-func addFileToBuilder(b *mfer.Builder, path string, size int64, mtime time.Time, hash []byte) {
-	// Use the builder's internal method indirectly by creating an entry
-	// Since Builder.AddFile reads from a reader, we need to use a different approach
-	// We'll access the builder's files directly through a custom method
-	b.AddFileWithHash(path, size, mtime, hash)
+func addFileToBuilder(b *mfer.Builder, path string, size int64, mtime time.Time, hash []byte) error {
+	return b.AddFileWithHash(mfer.RelFilePath(path), mfer.FileSize(size), mfer.ModTime(mtime), hash)
 }
 
 // addExistingToBuilder adds an existing manifest entry to the builder
-func addExistingToBuilder(b *mfer.Builder, entry *mfer.MFFilePath) {
+func addExistingToBuilder(b *mfer.Builder, entry *mfer.MFFilePath) error {
 	mtime := time.Unix(entry.Mtime.Seconds, int64(entry.Mtime.Nanos))
-	if len(entry.Hashes) > 0 {
-		b.AddFileWithHash(entry.Path, entry.Size, mtime, entry.Hashes[0].MultiHash)
+	if len(entry.Hashes) == 0 {
+		return nil
 	}
-}
-
-// pathIsHidden checks if a path contains hidden components
-func pathIsHidden(p string) bool {
-	// "." is not hidden, it's the current directory
-	if p == "." {
-		return false
-	}
-	// Check each path component
-	for p != "" && p != "." && p != "/" {
-		base := filepath.Base(p)
-		if len(base) > 0 && base[0] == '.' {
-			return true
-		}
-		parent := filepath.Dir(p)
-		if parent == p {
-			break
-		}
-		p = parent
-	}
-	return false
+	return b.AddFileWithHash(mfer.RelFilePath(entry.Path), mfer.FileSize(entry.Size), mfer.ModTime(mtime), entry.Hashes[0].MultiHash)
 }
diff --git a/internal/cli/gen.go b/internal/cli/gen.go
index f1d87f0..15d8633 100644
--- a/internal/cli/gen.go
+++ b/internal/cli/gen.go
@@ -13,29 +13,29 @@ import (
 	"github.com/spf13/afero"
 	"github.com/urfave/cli/v2"
 	"sneak.berlin/go/mfer/internal/log"
-	"sneak.berlin/go/mfer/internal/scanner"
+	"sneak.berlin/go/mfer/mfer"
 )
 
 func (mfa *CLIApp) generateManifestOperation(ctx *cli.Context) error {
 	log.Debug("generateManifestOperation()")
 
-	opts := &scanner.Options{
+	opts := &mfer.ScannerOptions{
 		IncludeDotfiles: ctx.Bool("IncludeDotfiles"),
 		FollowSymLinks:  ctx.Bool("FollowSymLinks"),
 		Fs:              mfa.Fs,
 	}
 
-	s := scanner.NewWithOptions(opts)
+	s := mfer.NewScannerWithOptions(opts)
 
 	// Phase 1: Enumeration - collect paths and stat files
 	args := ctx.Args()
 	showProgress := ctx.Bool("progress")
 
 	// Set up enumeration progress reporting
-	var enumProgress chan scanner.EnumerateStatus
+	var enumProgress chan mfer.EnumerateStatus
 	var enumWg sync.WaitGroup
 	if showProgress {
-		enumProgress = make(chan scanner.EnumerateStatus, 1)
+		enumProgress = make(chan mfer.EnumerateStatus, 1)
 		enumWg.Add(1)
 		go func() {
 			defer enumWg.Done()
@@ -117,10 +117,10 @@ func (mfa *CLIApp) generateManifestOperation(ctx *cli.Context) error {
 	}()
 
 	// Phase 2: Scan - read file contents and generate manifest
-	var scanProgress chan scanner.ScanStatus
+	var scanProgress chan mfer.ScanStatus
 	var scanWg sync.WaitGroup
 	if showProgress {
-		scanProgress = make(chan scanner.ScanStatus, 1)
+		scanProgress = make(chan mfer.ScanStatus, 1)
 		scanWg.Add(1)
 		go func() {
 			defer scanWg.Done()
diff --git a/mfer/builder.go b/mfer/builder.go
index 585abc5..df5eca2 100644
--- a/mfer/builder.go
+++ b/mfer/builder.go
@@ -2,6 +2,7 @@ package mfer
 
 import (
 	"crypto/sha256"
+	"errors"
 	"io"
 	"sync"
 	"time"
@@ -9,9 +10,42 @@ import (
 	"github.com/multiformats/go-multihash"
 )
 
+// RelFilePath represents a relative file path within a manifest.
+type RelFilePath string
+
+// AbsFilePath represents an absolute file path on the filesystem.
+type AbsFilePath string
+
+// FileSize represents the size of a file in bytes.
+type FileSize int64
+
+// FileCount represents a count of files.
+type FileCount int64
+
+// ModTime represents a file's modification time.
+type ModTime time.Time
+
+// UnixSeconds represents seconds since Unix epoch.
+type UnixSeconds int64
+
+// UnixNanos represents the nanosecond component of a timestamp (0-999999999).
+type UnixNanos int32
+
+// Timestamp converts ModTime to a protobuf Timestamp.
+func (m ModTime) Timestamp() *Timestamp {
+	t := time.Time(m)
+	return &Timestamp{
+		Seconds: t.Unix(),
+		Nanos:   int32(t.Nanosecond()),
+	}
+}
+
+// Multihash represents a multihash-encoded file hash (typically SHA2-256).
+type Multihash []byte
+
 // FileHashProgress reports progress during file hashing.
 type FileHashProgress struct {
-	BytesRead int64 // Total bytes read so far for the current file
+	BytesRead FileSize // Total bytes read so far for the current file
 }
 
 // Builder constructs a manifest by adding files one at a time.
@@ -33,24 +67,24 @@ func NewBuilder() *Builder {
 // Progress updates are sent to the progress channel (if non-nil) without blocking.
 // Returns the number of bytes read.
 func (b *Builder) AddFile(
-	path string,
-	size int64,
-	mtime time.Time,
+	path RelFilePath,
+	size FileSize,
+	mtime ModTime,
 	reader io.Reader,
 	progress chan<- FileHashProgress,
-) (int64, error) {
+) (FileSize, error) {
 	// Create hash writer
 	h := sha256.New()
 
 	// Read file in chunks, updating hash and progress
-	var totalRead int64
+	var totalRead FileSize
 	buf := make([]byte, 64*1024) // 64KB chunks
 
 	for {
 		n, err := reader.Read(buf)
 		if n > 0 {
 			h.Write(buf[:n])
-			totalRead += int64(n)
+			totalRead += FileSize(n)
 			sendFileHashProgress(progress, FileHashProgress{BytesRead: totalRead})
 		}
 		if err == io.EOF {
@@ -69,12 +103,12 @@ func (b *Builder) AddFile(
 
 	// Create file entry
 	entry := &MFFilePath{
-		Path: path,
-		Size: size,
+		Path: string(path),
+		Size: int64(size),
 		Hashes: []*MFFileChecksum{
 			{MultiHash: mh},
 		},
-		Mtime: newTimestampFromTime(mtime),
+		Mtime: mtime.Timestamp(),
 	}
 
 	b.mu.Lock()
@@ -104,19 +138,31 @@ func (b *Builder) FileCount() int {
 
 // AddFileWithHash adds a file entry with a pre-computed hash.
 // This is useful when the hash is already known (e.g., from an existing manifest).
-func (b *Builder) AddFileWithHash(path string, size int64, mtime time.Time, hash []byte) {
+// Returns an error if path is empty, size is negative, or hash is nil/empty.
+func (b *Builder) AddFileWithHash(path RelFilePath, size FileSize, mtime ModTime, hash Multihash) error {
+	if path == "" {
+		return errors.New("path cannot be empty")
+	}
+	if size < 0 {
+		return errors.New("size cannot be negative")
+	}
+	if len(hash) == 0 {
+		return errors.New("hash cannot be nil or empty")
+	}
+
 	entry := &MFFilePath{
-		Path: path,
-		Size: size,
+		Path: string(path),
+		Size: int64(size),
 		Hashes: []*MFFileChecksum{
 			{MultiHash: hash},
 		},
-		Mtime: newTimestampFromTime(mtime),
+		Mtime: mtime.Timestamp(),
 	}
 
 	b.mu.Lock()
 	b.files = append(b.files, entry)
 	b.mu.Unlock()
+	return nil
 }
 
 // Build finalizes the manifest and writes it to the writer.
diff --git a/mfer/builder_test.go b/mfer/builder_test.go
new file mode 100644
index 0000000..a92994f
--- /dev/null
+++ b/mfer/builder_test.go
@@ -0,0 +1,104 @@
+package mfer
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNewBuilder(t *testing.T) {
+	b := NewBuilder()
+	assert.NotNil(t, b)
+	assert.Equal(t, 0, b.FileCount())
+}
+
+func TestBuilderAddFile(t *testing.T) {
+	b := NewBuilder()
+	content := []byte("test content")
+	reader := bytes.NewReader(content)
+
+	bytesRead, err := b.AddFile("test.txt", FileSize(len(content)), ModTime(time.Now()), reader, nil)
+	require.NoError(t, err)
+	assert.Equal(t, FileSize(len(content)), bytesRead)
+	assert.Equal(t, 1, b.FileCount())
+}
+
+func TestBuilderAddFileWithHash(t *testing.T) {
+	b := NewBuilder()
+	hash := make([]byte, 34) // SHA256 multihash is 34 bytes
+
+	err := b.AddFileWithHash("test.txt", 100, ModTime(time.Now()), hash)
+	require.NoError(t, err)
+	assert.Equal(t, 1, b.FileCount())
+}
+
+func TestBuilderAddFileWithHashValidation(t *testing.T) {
+	t.Run("empty path", func(t *testing.T) {
+		b := NewBuilder()
+		hash := make([]byte, 34)
+		err := b.AddFileWithHash("", 100, ModTime(time.Now()), hash)
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "path")
+	})
+
+	t.Run("negative size", func(t *testing.T) {
+		b := NewBuilder()
+		hash := make([]byte, 34)
+		err := b.AddFileWithHash("test.txt", -1, ModTime(time.Now()), hash)
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "size")
+	})
+
+	t.Run("nil hash", func(t *testing.T) {
+		b := NewBuilder()
+		err := b.AddFileWithHash("test.txt", 100, ModTime(time.Now()), nil)
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "hash")
+	})
+
+	t.Run("empty hash", func(t *testing.T) {
+		b := NewBuilder()
+		err := b.AddFileWithHash("test.txt", 100, ModTime(time.Now()), []byte{})
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "hash")
+	})
+
+	t.Run("valid inputs", func(t *testing.T) {
+		b := NewBuilder()
+		hash := make([]byte, 34)
+		err := b.AddFileWithHash("test.txt", 100, ModTime(time.Now()), hash)
+		assert.NoError(t, err)
+		assert.Equal(t, 1, b.FileCount())
+	})
+}
+
+func TestBuilderBuild(t *testing.T) {
+	b := NewBuilder()
+	content := []byte("test content")
+	reader := bytes.NewReader(content)
+
+	_, err := b.AddFile("test.txt", FileSize(len(content)), ModTime(time.Now()), reader, nil)
+	require.NoError(t, err)
+
+	var buf bytes.Buffer
+	err = b.Build(&buf)
+	require.NoError(t, err)
+
+	// Should have magic bytes
+	assert.True(t, strings.HasPrefix(buf.String(), MAGIC))
+}
+
+func TestBuilderBuildEmpty(t *testing.T) {
+	b := NewBuilder()
+
+	var buf bytes.Buffer
+	err := b.Build(&buf)
+	require.NoError(t, err)
+
+	// Should still produce valid manifest with 0 files
+	assert.True(t, strings.HasPrefix(buf.String(), MAGIC))
+}
diff --git a/internal/scanner/scanner.go b/mfer/scanner.go
similarity index 82%
rename from internal/scanner/scanner.go
rename to mfer/scanner.go
index f286ebd..645b685 100644
--- a/internal/scanner/scanner.go
+++ b/mfer/scanner.go
@@ -1,4 +1,4 @@
-package scanner
+package mfer
 
 import (
 	"context"
@@ -13,7 +13,6 @@ import (
 	"github.com/dustin/go-humanize"
 	"github.com/spf13/afero"
 	"sneak.berlin/go/mfer/internal/log"
-	"sneak.berlin/go/mfer/mfer"
 )
 
 // Phase 1: Enumeration
@@ -23,8 +22,8 @@ import (
 
 // EnumerateStatus contains progress information for the enumeration phase.
 type EnumerateStatus struct {
-	FilesFound int64 // Number of files discovered so far
-	BytesFound int64 // Total size of discovered files (from stat)
+	FilesFound FileCount // Number of files discovered so far
+	BytesFound FileSize  // Total size of discovered files (from stat)
 }
 
 // Phase 2: Scan (ToManifest)
@@ -34,16 +33,16 @@ type EnumerateStatus struct {
 
 // ScanStatus contains progress information for the scan phase.
 type ScanStatus struct {
-	TotalFiles   int64         // Total number of files to scan
-	ScannedFiles int64         // Number of files scanned so far
-	TotalBytes   int64         // Total bytes to read (sum of all file sizes)
-	ScannedBytes int64         // Bytes read so far
+	TotalFiles   FileCount     // Total number of files to scan
+	ScannedFiles FileCount     // Number of files scanned so far
+	TotalBytes   FileSize      // Total bytes to read (sum of all file sizes)
+	ScannedBytes FileSize      // Bytes read so far
 	BytesPerSec  float64       // Current throughput rate
 	ETA          time.Duration // Estimated time to completion
 }
 
-// Options configures scanner behavior.
-type Options struct {
+// ScannerOptions configures scanner behavior.
+type ScannerOptions struct {
 	IncludeDotfiles bool     // Include files and directories starting with a dot (default: exclude)
 	FollowSymLinks  bool     // Resolve symlinks instead of skipping them
 	Fs              afero.Fs // Filesystem to use, defaults to OsFs if nil
@@ -51,30 +50,31 @@ type Options struct {
 
 // FileEntry represents a file that has been enumerated.
 type FileEntry struct {
-	Path    string    // Relative path (used in manifest)
-	AbsPath string    // Absolute path (used for reading file content)
-	Size    int64     // File size in bytes
-	Mtime   time.Time // Last modification time
-	Ctime   time.Time // Creation time (platform-dependent)
+	Path    RelFilePath // Relative path (used in manifest)
+	AbsPath AbsFilePath // Absolute path (used for reading file content)
+	Size    FileSize    // File size in bytes
+	Mtime   ModTime     // Last modification time
+	Ctime   time.Time   // Creation time (platform-dependent)
 }
 
 // Scanner accumulates files and generates manifests from them.
 type Scanner struct {
-	mu      sync.RWMutex
-	files   []*FileEntry
-	options *Options
-	fs      afero.Fs
+	mu         sync.RWMutex
+	files      []*FileEntry
+	totalBytes FileSize // cached sum of all file sizes
+	options    *ScannerOptions
+	fs         afero.Fs
 }
 
-// New creates a new Scanner with default options.
-func New() *Scanner {
-	return NewWithOptions(nil)
+// NewScanner creates a new Scanner with default options.
+func NewScanner() *Scanner {
+	return NewScannerWithOptions(nil)
 }
 
-// NewWithOptions creates a new Scanner with the given options.
-func NewWithOptions(opts *Options) *Scanner {
+// NewScannerWithOptions creates a new Scanner with the given options.
+func NewScannerWithOptions(opts *ScannerOptions) *Scanner {
 	if opts == nil {
-		opts = &Options{}
+		opts = &ScannerOptions{}
 	}
 	fs := opts.Fs
 	if fs == nil {
@@ -154,7 +154,7 @@ func (s *Scanner) enumerateFS(afs afero.Fs, basePath string, progress chan<- Enu
 		if err != nil {
 			return err
 		}
-		if !s.options.IncludeDotfiles && pathIsHidden(p) {
+		if !s.options.IncludeDotfiles && IsHiddenPath(p) {
 			if info.IsDir() {
 				return filepath.SkipDir
 			}
@@ -206,21 +206,19 @@ func (s *Scanner) enumerateFileWithInfo(filePath string, basePath string, info f
 	}
 
 	entry := &FileEntry{
-		Path:    cleanPath,
-		AbsPath: absPath,
-		Size:    info.Size(),
-		Mtime:   info.ModTime(),
+		Path:    RelFilePath(cleanPath),
+		AbsPath: AbsFilePath(absPath),
+		Size:    FileSize(info.Size()),
+		Mtime:   ModTime(info.ModTime()),
 		// Note: Ctime not available from fs.FileInfo on all platforms
 		// Will need platform-specific code to extract it
 	}
 
 	s.mu.Lock()
 	s.files = append(s.files, entry)
-	filesFound := int64(len(s.files))
-	var bytesFound int64
-	for _, f := range s.files {
-		bytesFound += f.Size
-	}
+	s.totalBytes += entry.Size
+	filesFound := FileCount(len(s.files))
+	bytesFound := s.totalBytes
 	s.mu.Unlock()
 
 	sendEnumerateStatus(progress, EnumerateStatus{
@@ -241,21 +239,17 @@ func (s *Scanner) Files() []*FileEntry {
 }
 
 // FileCount returns the number of files in the scanner.
-func (s *Scanner) FileCount() int64 {
+func (s *Scanner) FileCount() FileCount {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
-	return int64(len(s.files))
+	return FileCount(len(s.files))
 }
 
 // TotalBytes returns the total size of all files in the scanner.
-func (s *Scanner) TotalBytes() int64 {
+func (s *Scanner) TotalBytes() FileSize {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
-	var total int64
-	for _, f := range s.files {
-		total += f.Size
-	}
-	return total
+	return s.totalBytes
 }
 
 // ToManifest reads all file contents, computes hashes, and generates a manifest.
@@ -270,17 +264,17 @@ func (s *Scanner) ToManifest(ctx context.Context, w io.Writer, progress chan<- S
 	s.mu.RLock()
 	files := make([]*FileEntry, len(s.files))
 	copy(files, s.files)
-	totalFiles := int64(len(files))
-	var totalBytes int64
+	totalFiles := FileCount(len(files))
+	var totalBytes FileSize
 	for _, f := range files {
 		totalBytes += f.Size
 	}
 	s.mu.RUnlock()
 
-	builder := mfer.NewBuilder()
+	builder := NewBuilder()
 
-	var scannedFiles int64
-	var scannedBytes int64
+	var scannedFiles FileCount
+	var scannedBytes FileSize
 	lastProgressTime := time.Now()
 	startTime := time.Now()
 
@@ -293,18 +287,18 @@ func (s *Scanner) ToManifest(ctx context.Context, w io.Writer, progress chan<- S
 		}
 
 		// Open file
-		f, err := s.fs.Open(entry.AbsPath)
+		f, err := s.fs.Open(string(entry.AbsPath))
 		if err != nil {
 			return err
 		}
 
 		// Create progress channel for this file
-		var fileProgress chan mfer.FileHashProgress
+		var fileProgress chan FileHashProgress
 		var wg sync.WaitGroup
 		if progress != nil {
-			fileProgress = make(chan mfer.FileHashProgress, 1)
+			fileProgress = make(chan FileHashProgress, 1)
 			wg.Add(1)
-			go func(baseScannedBytes int64) {
+			go func(baseScannedBytes FileSize) {
 				defer wg.Done()
 				for p := range fileProgress {
 					// Send progress at most once per second
@@ -382,9 +376,10 @@ func (s *Scanner) ToManifest(ctx context.Context, w io.Writer, progress chan<- S
 	return builder.Build(w)
 }
 
-// pathIsHidden returns true if the path or any of its parent directories
+// IsHiddenPath returns true if the path or any of its parent directories
 // start with a dot (hidden files/directories).
-func pathIsHidden(p string) bool {
+// The path should use forward slashes.
+func IsHiddenPath(p string) bool {
 	tp := path.Clean(p)
 	if strings.HasPrefix(tp, ".") {
 		return true
diff --git a/internal/scanner/scanner_test.go b/mfer/scanner_test.go
similarity index 69%
rename from internal/scanner/scanner_test.go
rename to mfer/scanner_test.go
index 67edc98..f30c67b 100644
--- a/internal/scanner/scanner_test.go
+++ b/mfer/scanner_test.go
@@ -1,4 +1,4 @@
-package scanner
+package mfer
 
 import (
 	"bytes"
@@ -11,77 +11,77 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func TestNew(t *testing.T) {
-	s := New()
+func TestNewScanner(t *testing.T) {
+	s := NewScanner()
 	assert.NotNil(t, s)
-	assert.Equal(t, int64(0), s.FileCount())
-	assert.Equal(t, int64(0), s.TotalBytes())
+	assert.Equal(t, FileCount(0), s.FileCount())
+	assert.Equal(t, FileSize(0), s.TotalBytes())
 }
 
-func TestNewWithOptions(t *testing.T) {
+func TestNewScannerWithOptions(t *testing.T) {
 	t.Run("nil options", func(t *testing.T) {
-		s := NewWithOptions(nil)
+		s := NewScannerWithOptions(nil)
 		assert.NotNil(t, s)
 	})
 
 	t.Run("with options", func(t *testing.T) {
 		fs := afero.NewMemMapFs()
-		opts := &Options{
+		opts := &ScannerOptions{
 			IncludeDotfiles: true,
 			FollowSymLinks:  true,
 			Fs:              fs,
 		}
-		s := NewWithOptions(opts)
+		s := NewScannerWithOptions(opts)
 		assert.NotNil(t, s)
 	})
 }
 
-func TestEnumerateFile(t *testing.T) {
+func TestScannerEnumerateFile(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	require.NoError(t, afero.WriteFile(fs, "/test.txt", []byte("hello world"), 0644))
 
-	s := NewWithOptions(&Options{Fs: fs})
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
 	err := s.EnumerateFile("/test.txt")
 	require.NoError(t, err)
 
-	assert.Equal(t, int64(1), s.FileCount())
-	assert.Equal(t, int64(11), s.TotalBytes())
+	assert.Equal(t, FileCount(1), s.FileCount())
+	assert.Equal(t, FileSize(11), s.TotalBytes())
 
 	files := s.Files()
 	require.Len(t, files, 1)
-	assert.Equal(t, "test.txt", files[0].Path)
-	assert.Equal(t, int64(11), files[0].Size)
+	assert.Equal(t, RelFilePath("test.txt"), files[0].Path)
+	assert.Equal(t, FileSize(11), files[0].Size)
 }
 
-func TestEnumerateFileMissing(t *testing.T) {
+func TestScannerEnumerateFileMissing(t *testing.T) {
 	fs := afero.NewMemMapFs()
-	s := NewWithOptions(&Options{Fs: fs})
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
 	err := s.EnumerateFile("/nonexistent.txt")
 	assert.Error(t, err)
 }
 
-func TestEnumeratePath(t *testing.T) {
+func TestScannerEnumeratePath(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	require.NoError(t, fs.MkdirAll("/testdir/subdir", 0755))
 	require.NoError(t, afero.WriteFile(fs, "/testdir/file1.txt", []byte("one"), 0644))
 	require.NoError(t, afero.WriteFile(fs, "/testdir/file2.txt", []byte("two"), 0644))
 	require.NoError(t, afero.WriteFile(fs, "/testdir/subdir/file3.txt", []byte("three"), 0644))
 
-	s := NewWithOptions(&Options{Fs: fs})
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
 	err := s.EnumeratePath("/testdir", nil)
 	require.NoError(t, err)
 
-	assert.Equal(t, int64(3), s.FileCount())
-	assert.Equal(t, int64(3+3+5), s.TotalBytes())
+	assert.Equal(t, FileCount(3), s.FileCount())
+	assert.Equal(t, FileSize(3+3+5), s.TotalBytes())
 }
 
-func TestEnumeratePathWithProgress(t *testing.T) {
+func TestScannerEnumeratePathWithProgress(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	require.NoError(t, fs.MkdirAll("/testdir", 0755))
 	require.NoError(t, afero.WriteFile(fs, "/testdir/file1.txt", []byte("one"), 0644))
 	require.NoError(t, afero.WriteFile(fs, "/testdir/file2.txt", []byte("two"), 0644))
 
-	s := NewWithOptions(&Options{Fs: fs})
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
 	progress := make(chan EnumerateStatus, 10)
 
 	err := s.EnumeratePath("/testdir", progress)
@@ -95,25 +95,25 @@ func TestEnumeratePathWithProgress(t *testing.T) {
 	assert.NotEmpty(t, updates)
 	// Final update should show all files
 	final := updates[len(updates)-1]
-	assert.Equal(t, int64(2), final.FilesFound)
-	assert.Equal(t, int64(6), final.BytesFound)
+	assert.Equal(t, FileCount(2), final.FilesFound)
+	assert.Equal(t, FileSize(6), final.BytesFound)
 }
 
-func TestEnumeratePaths(t *testing.T) {
+func TestScannerEnumeratePaths(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	require.NoError(t, fs.MkdirAll("/dir1", 0755))
 	require.NoError(t, fs.MkdirAll("/dir2", 0755))
 	require.NoError(t, afero.WriteFile(fs, "/dir1/a.txt", []byte("aaa"), 0644))
 	require.NoError(t, afero.WriteFile(fs, "/dir2/b.txt", []byte("bbb"), 0644))
 
-	s := NewWithOptions(&Options{Fs: fs})
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
 	err := s.EnumeratePaths(nil, "/dir1", "/dir2")
 	require.NoError(t, err)
 
-	assert.Equal(t, int64(2), s.FileCount())
+	assert.Equal(t, FileCount(2), s.FileCount())
 }
 
-func TestExcludeDotfiles(t *testing.T) {
+func TestScannerExcludeDotfiles(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	require.NoError(t, fs.MkdirAll("/testdir/.hidden", 0755))
 	require.NoError(t, afero.WriteFile(fs, "/testdir/visible.txt", []byte("visible"), 0644))
@@ -121,54 +121,31 @@ func TestExcludeDotfiles(t *testing.T) {
 	require.NoError(t, afero.WriteFile(fs, "/testdir/.hidden/inside.txt", []byte("inside"), 0644))
 
 	t.Run("exclude by default", func(t *testing.T) {
-		s := NewWithOptions(&Options{Fs: fs, IncludeDotfiles: false})
+		s := NewScannerWithOptions(&ScannerOptions{Fs: fs, IncludeDotfiles: false})
 		err := s.EnumeratePath("/testdir", nil)
 		require.NoError(t, err)
 
-		assert.Equal(t, int64(1), s.FileCount())
+		assert.Equal(t, FileCount(1), s.FileCount())
 		files := s.Files()
-		assert.Equal(t, "visible.txt", files[0].Path)
+		assert.Equal(t, RelFilePath("visible.txt"), files[0].Path)
 	})
 
 	t.Run("include when enabled", func(t *testing.T) {
-		s := NewWithOptions(&Options{Fs: fs, IncludeDotfiles: true})
+		s := NewScannerWithOptions(&ScannerOptions{Fs: fs, IncludeDotfiles: true})
 		err := s.EnumeratePath("/testdir", nil)
 		require.NoError(t, err)
 
-		assert.Equal(t, int64(3), s.FileCount())
+		assert.Equal(t, FileCount(3), s.FileCount())
 	})
 }
 
-func TestPathIsHidden(t *testing.T) {
-	tests := []struct {
-		path   string
-		hidden bool
-	}{
-		{"file.txt", false},
-		{".hidden", true},
-		{"dir/file.txt", false},
-		{"dir/.hidden", true},
-		{".dir/file.txt", true},
-		{"/absolute/path", false},
-		{"/absolute/.hidden", true},
-		{"./relative", false}, // path.Clean removes leading ./
-		{"a/b/c/.d/e", true},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.path, func(t *testing.T) {
-			assert.Equal(t, tt.hidden, pathIsHidden(tt.path), "pathIsHidden(%q)", tt.path)
-		})
-	}
-}
-
-func TestToManifest(t *testing.T) {
+func TestScannerToManifest(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	require.NoError(t, fs.MkdirAll("/testdir", 0755))
 	require.NoError(t, afero.WriteFile(fs, "/testdir/file1.txt", []byte("content one"), 0644))
 	require.NoError(t, afero.WriteFile(fs, "/testdir/file2.txt", []byte("content two"), 0644))
 
-	s := NewWithOptions(&Options{Fs: fs})
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
 	err := s.EnumeratePath("/testdir", nil)
 	require.NoError(t, err)
 
@@ -178,15 +155,15 @@ func TestToManifest(t *testing.T) {
 
 	// Manifest should have magic bytes
 	assert.True(t, buf.Len() > 0)
-	assert.Equal(t, "ZNAVSRFG", string(buf.Bytes()[:8]))
+	assert.Equal(t, MAGIC, string(buf.Bytes()[:8]))
 }
 
-func TestToManifestWithProgress(t *testing.T) {
+func TestScannerToManifestWithProgress(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	require.NoError(t, fs.MkdirAll("/testdir", 0755))
 	require.NoError(t, afero.WriteFile(fs, "/testdir/file.txt", bytes.Repeat([]byte("x"), 1000), 0644))
 
-	s := NewWithOptions(&Options{Fs: fs})
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
 	err := s.EnumeratePath("/testdir", nil)
 	require.NoError(t, err)
 
@@ -204,13 +181,13 @@ func TestToManifestWithProgress(t *testing.T) {
 	assert.NotEmpty(t, updates)
 	// Final update should show completion
 	final := updates[len(updates)-1]
-	assert.Equal(t, int64(1), final.TotalFiles)
-	assert.Equal(t, int64(1), final.ScannedFiles)
-	assert.Equal(t, int64(1000), final.TotalBytes)
-	assert.Equal(t, int64(1000), final.ScannedBytes)
+	assert.Equal(t, FileCount(1), final.TotalFiles)
+	assert.Equal(t, FileCount(1), final.ScannedFiles)
+	assert.Equal(t, FileSize(1000), final.TotalBytes)
+	assert.Equal(t, FileSize(1000), final.ScannedBytes)
 }
 
-func TestToManifestContextCancellation(t *testing.T) {
+func TestScannerToManifestContextCancellation(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	require.NoError(t, fs.MkdirAll("/testdir", 0755))
 	// Create many files to ensure we have time to cancel
@@ -219,7 +196,7 @@ func TestToManifestContextCancellation(t *testing.T) {
 		require.NoError(t, afero.WriteFile(fs, "/testdir/"+name, bytes.Repeat([]byte("x"), 100), 0644))
 	}
 
-	s := NewWithOptions(&Options{Fs: fs})
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
 	err := s.EnumeratePath("/testdir", nil)
 	require.NoError(t, err)
 
@@ -231,9 +208,9 @@ func TestToManifestContextCancellation(t *testing.T) {
 	assert.ErrorIs(t, err, context.Canceled)
 }
 
-func TestToManifestEmptyScanner(t *testing.T) {
+func TestScannerToManifestEmptyScanner(t *testing.T) {
 	fs := afero.NewMemMapFs()
-	s := NewWithOptions(&Options{Fs: fs})
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
 
 	var buf bytes.Buffer
 	err := s.ToManifest(context.Background(), &buf, nil)
@@ -241,14 +218,14 @@ func TestToManifestEmptyScanner(t *testing.T) {
 
 	// Should still produce a valid manifest
 	assert.True(t, buf.Len() > 0)
-	assert.Equal(t, "ZNAVSRFG", string(buf.Bytes()[:8]))
+	assert.Equal(t, MAGIC, string(buf.Bytes()[:8]))
 }
 
-func TestFilesCopiesSlice(t *testing.T) {
+func TestScannerFilesCopiesSlice(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	require.NoError(t, afero.WriteFile(fs, "/test.txt", []byte("hello"), 0644))
 
-	s := NewWithOptions(&Options{Fs: fs})
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
 	require.NoError(t, s.EnumerateFile("/test.txt"))
 
 	files1 := s.Files()
@@ -258,7 +235,7 @@ func TestFilesCopiesSlice(t *testing.T) {
 	assert.NotSame(t, &files1[0], &files2[0])
 }
 
-func TestEnumerateFS(t *testing.T) {
+func TestScannerEnumerateFS(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	require.NoError(t, fs.MkdirAll("/testdir/sub", 0755))
 	require.NoError(t, afero.WriteFile(fs, "/testdir/file.txt", []byte("hello"), 0644))
@@ -267,11 +244,11 @@ func TestEnumerateFS(t *testing.T) {
 	// Create a basepath filesystem
 	baseFs := afero.NewBasePathFs(fs, "/testdir")
 
-	s := NewWithOptions(&Options{Fs: fs})
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
 	err := s.EnumerateFS(baseFs, "/testdir", nil)
 	require.NoError(t, err)
 
-	assert.Equal(t, int64(2), s.FileCount())
+	assert.Equal(t, FileCount(2), s.FileCount())
 }
 
 func TestSendEnumerateStatusNonBlocking(t *testing.T) {
@@ -317,27 +294,27 @@ func TestSendStatusNilChannel(t *testing.T) {
 	sendScanStatus(nil, ScanStatus{})
 }
 
-func TestFileEntryFields(t *testing.T) {
+func TestScannerFileEntryFields(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	now := time.Now().Truncate(time.Second)
 	require.NoError(t, afero.WriteFile(fs, "/test.txt", []byte("content"), 0644))
 	require.NoError(t, fs.Chtimes("/test.txt", now, now))
 
-	s := NewWithOptions(&Options{Fs: fs})
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
 	require.NoError(t, s.EnumerateFile("/test.txt"))
 
 	files := s.Files()
 	require.Len(t, files, 1)
 
 	entry := files[0]
-	assert.Equal(t, "test.txt", entry.Path)
-	assert.Contains(t, entry.AbsPath, "test.txt")
-	assert.Equal(t, int64(7), entry.Size)
+	assert.Equal(t, RelFilePath("test.txt"), entry.Path)
+	assert.Contains(t, string(entry.AbsPath), "test.txt")
+	assert.Equal(t, FileSize(7), entry.Size)
 	// Mtime should be set (within a second of now)
-	assert.WithinDuration(t, now, entry.Mtime, 2*time.Second)
+	assert.WithinDuration(t, now, time.Time(entry.Mtime), 2*time.Second)
 }
 
-func TestLargeFileEnumeration(t *testing.T) {
+func TestScannerLargeFileEnumeration(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	require.NoError(t, fs.MkdirAll("/testdir", 0755))
 
@@ -347,7 +324,7 @@ func TestLargeFileEnumeration(t *testing.T) {
 		require.NoError(t, afero.WriteFile(fs, name, []byte("data"), 0644))
 	}
 
-	s := NewWithOptions(&Options{Fs: fs})
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
 	progress := make(chan EnumerateStatus, 200)
 
 	err := s.EnumeratePath("/testdir", progress)
@@ -357,6 +334,29 @@ func TestLargeFileEnumeration(t *testing.T) {
 	for range progress {
 	}
 
-	assert.Equal(t, int64(100), s.FileCount())
-	assert.Equal(t, int64(400), s.TotalBytes()) // 100 * 4 bytes
+	assert.Equal(t, FileCount(100), s.FileCount())
+	assert.Equal(t, FileSize(400), s.TotalBytes()) // 100 * 4 bytes
+}
+
+func TestIsHiddenPath(t *testing.T) {
+	tests := []struct {
+		path   string
+		hidden bool
+	}{
+		{"file.txt", false},
+		{".hidden", true},
+		{"dir/file.txt", false},
+		{"dir/.hidden", true},
+		{".dir/file.txt", true},
+		{"/absolute/path", false},
+		{"/absolute/.hidden", true},
+		{"./relative", false}, // path.Clean removes leading ./
+		{"a/b/c/.d/e", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.path, func(t *testing.T) {
+			assert.Equal(t, tt.hidden, IsHiddenPath(tt.path), "IsHiddenPath(%q)", tt.path)
+		})
+	}
 }
diff --git a/mfer/url.go b/mfer/url.go
new file mode 100644
index 0000000..fb1da96
--- /dev/null
+++ b/mfer/url.go
@@ -0,0 +1,53 @@
+package mfer
+
+import (
+	"net/url"
+	"strings"
+)
+
+// ManifestURL represents a URL pointing to a manifest file.
+type ManifestURL string
+
+// FileURL represents a URL pointing to a file to be fetched.
+type FileURL string
+
+// BaseURL represents a base URL for constructing file URLs.
+type BaseURL string
+
+// JoinPath safely joins a relative file path to a base URL.
+// The path is properly URL-encoded to prevent path traversal.
+func (b BaseURL) JoinPath(path RelFilePath) (FileURL, error) {
+	base, err := url.Parse(string(b))
+	if err != nil {
+		return "", err
+	}
+
+	// Ensure base path ends with /
+	if !strings.HasSuffix(base.Path, "/") {
+		base.Path += "/"
+	}
+
+	// Parse and encode the relative path
+	ref, err := url.Parse(url.PathEscape(string(path)))
+	if err != nil {
+		return "", err
+	}
+
+	resolved := base.ResolveReference(ref)
+	return FileURL(resolved.String()), nil
+}
+
+// String returns the URL as a string.
+func (b BaseURL) String() string {
+	return string(b)
+}
+
+// String returns the URL as a string.
+func (f FileURL) String() string {
+	return string(f)
+}
+
+// String returns the URL as a string.
+func (m ManifestURL) String() string {
+	return string(m)
+}