diff --git a/cmd/packserver/go.mod b/cmd/packserver/go.mod
index 794e446..958080f 100644
--- a/cmd/packserver/go.mod
+++ b/cmd/packserver/go.mod
@@ -1,15 +1,15 @@
 module src.lwithers.me.uk/go/htpack/cmd/packserver
 
-go 1.22
+go 1.26.0
 
 require (
-	github.com/spf13/cobra v1.8.1
-	src.lwithers.me.uk/go/htpack v1.3.3
+	github.com/spf13/cobra v1.10.2
+	src.lwithers.me.uk/go/htpack v1.5.0
 )
 
 require (
-	github.com/gogo/protobuf v1.3.2 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
-	github.com/spf13/pflag v1.0.5 // indirect
-	golang.org/x/sys v0.22.0 // indirect
+	github.com/spf13/pflag v1.0.10 // indirect
+	golang.org/x/sys v0.47.0 // indirect
+	google.golang.org/protobuf v1.36.12 // indirect
 )
diff --git a/cmd/packserver/go.sum b/cmd/packserver/go.sum
index cda383a..180bbaa 100644
--- a/cmd/packserver/go.sum
+++ b/cmd/packserver/go.sum
@@ -1,45 +1,19 @@
-github.com/cpuguy83/go-md2man/v2 v2.0.4/go.mod h1:tgQtvFlXSQOSOSIRvRPT7W67SCa46tRHOmNcaadrF8o=
-github.com/gogo/protobuf v1.3.2 h1:Ov1cvc58UF3b5XjBnZv7+opcTcQFZebYjWzi34vdm4Q=
-github.com/gogo/protobuf v1.3.2/go.mod h1:P1XiOD3dCwIKUDQYPy72D8LYyHL2YPYrpS2s69NZV8Q=
+github.com/cpuguy83/go-md2man/v2 v2.0.6/go.mod h1:oOW0eioCTA6cOiMLiUPZOpcVxMig6NIQQ7OS05n1F4g=
+github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
+github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
 github.com/inconshreveable/mousetrap v1.1.0 h1:wN+x4NVGpMsO7ErUn/mUI3vEoE6Jt13X2s0bqwp9tc8=
 github.com/inconshreveable/mousetrap v1.1.0/go.mod h1:vpF70FUmC8bwa3OWnCshd2FqLfsEA9PFc4w1p2J65bw=
-github.com/kisielk/errcheck v1.5.0/go.mod h1:pFxgyoBC7bSaBwPgfKdkLd5X25qrDl4LWUI2bnpBCr8=
-github.com/kisielk/gotool v1.0.0/go.mod h1:XhKaO+MFFWcvkIS/tQcRk01m1F5IRFswLeQ+oQHNcck=
 github.com/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
-github.com/spf13/cobra v1.8.1 h1:e5/vxKd/rZsfSJMUX1agtjeTDf+qv1/JdBF8gg5k9ZM=
-github.com/spf13/cobra v1.8.1/go.mod h1:wHxEcudfqmLYa8iTfL+OuZPbBZkmvliBWKIezN3kD9Y=
-github.com/spf13/pflag v1.0.5 h1:iy+VFUOCP1a+8yFto/drg2CJ5u0yRoB7fZw3DKv/JXA=
-github.com/spf13/pflag v1.0.5/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=
-github.com/yuin/goldmark v1.1.27/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
-github.com/yuin/goldmark v1.2.1/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
-golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
-golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
-golang.org/x/crypto v0.0.0-20200622213623-75b288015ac9/go.mod h1:LzIPMQfyMNhhGPhUkYOs5KpL4U8rLKemX1yGLhDgUto=
-golang.org/x/mod v0.2.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
-golang.org/x/mod v0.3.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
-golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
-golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
-golang.org/x/net v0.0.0-20200226121028-0de0cce0169b/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
-golang.org/x/net v0.0.0-20201021035429-f5854403a974/go.mod h1:sp8m0HH+o8qH0wwXwYZr8TS3Oi6o0r6Gce1SSxlDquU=
-golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/sync v0.0.0-20201020160332-67f06af15bc9/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
-golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20200930185726-fdedc70b468f/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.22.0 h1:RI27ohtqKCnwULzJLqkv897zojh5/DwS/ENaMzUOaWI=
-golang.org/x/sys v0.22.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
-golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
-golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
-golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
-golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
-golang.org/x/tools v0.0.0-20200619180055-7c47624df98f/go.mod h1:EkVYQZoAsY45+roYkvgYkIh4xh/qjgUK9TdY2XT94GE=
-golang.org/x/tools v0.0.0-20210106214847-113979e3529a/go.mod h1:emZCQorbCU4vsT4fOWvOPXz4eW1wZW4PmDk9uLelYpA=
-golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
-golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
-golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
-golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+github.com/spf13/cobra v1.10.2 h1:DMTTonx5m65Ic0GOoRY2c16WCbHxOOw6xxezuLaBpcU=
+github.com/spf13/cobra v1.10.2/go.mod h1:7C1pvHqHw5A4vrJfjNwvOdzYu0Gml16OCs2GRiTUUS4=
+github.com/spf13/pflag v1.0.9/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=
+github.com/spf13/pflag v1.0.10 h1:4EBh2KAYBwaONj6b2Ye1GiHfwjqyROoF4RwYO+vPwFk=
+github.com/spf13/pflag v1.0.10/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=
+go.yaml.in/yaml/v3 v3.0.4/go.mod h1:DhzuOOF2ATzADvBadXxruRBLzYTpT36CKvDb3+aBEFg=
+golang.org/x/sys v0.47.0 h1:o7XGOvZQCADBQQ4Y7VNq2dRWQR7JmOUW8Kxx4ZsNgWs=
+golang.org/x/sys v0.47.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
+google.golang.org/protobuf v1.36.12 h1:pJOKDDOyeXErUroCihFAd5LQuwXBSpVnKGrj5o/fwxc=
+google.golang.org/protobuf v1.36.12/go.mod h1:HTf+CrKn2C3g5S8VImy6tdcUvCska2kB7j23XfzDpco=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
-gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
-src.lwithers.me.uk/go/htpack v1.3.3 h1:Xvl6qR9HfSblmCgPyu+ACQ9o3aLQSIy3l8CrMbzj/jc=
-src.lwithers.me.uk/go/htpack v1.3.3/go.mod h1:qKgCBgZ6iiiuYOxZkYOPVpXLBzp6gXEd4A0ksxgR6Nk=
+src.lwithers.me.uk/go/htpack v1.5.0 h1:2UYM4AGIPc/XgktmO6AxUyqFxzmYkqg/jINcDCsZsCk=
+src.lwithers.me.uk/go/htpack v1.5.0/go.mod h1:5dQlPdwV+ZeBAaFBdwiRI6/w5dpSgPsiotO4ZTYau88=
diff --git a/cmd/packserver/main.go b/cmd/packserver/main.go
index 641a100..a374a66 100644
--- a/cmd/packserver/main.go
+++ b/cmd/packserver/main.go
@@ -55,6 +55,10 @@ func main() {
 		"Name of index file (index.html or similar)")
 	rootCmd.Flags().Duration("expiry", 0,
 		"Tell client how long it can cache data for; 0 means no caching")
+	rootCmd.Flags().Duration("expiry404", 0,
+		"Tell client how long it can cache 404 responses for; 0 means default (inherit from --expiry, max. 60s); -1 means no caching")
+	rootCmd.Flags().Bool("immutable", false,
+		"Tell client that cached assets are immutable (will not be revalidated on page reload)")
 	rootCmd.Flags().String("fallback-404", "",
 		"Name of file to return if response would be 404 (spa.html or similar)")
 	rootCmd.Flags().String("frames", "sameorigin",
@@ -135,15 +139,35 @@ func run(c *cobra.Command, args []string) error {
 
 	// parse expiry time
 	//  NB: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control
+	cacheControl200, cacheControl404 := "no-cache", "no-cache"
+	immutable, err := c.Flags().GetBool("immutable")
+	if err != nil {
+		return err
+	}
+	var immutableDirective string
+	if immutable {
+		immutableDirective = ", immutable"
+	}
+
 	expiry, err := c.Flags().GetDuration("expiry")
 	if err != nil {
 		return err
 	}
-	if expiry <= 0 {
-		extraHeaders.Set("Cache-Control", "no-cache")
-	} else {
-		extraHeaders.Set("Cache-Control",
-			fmt.Sprintf("public, max-age=%d", expiry/1e9))
+	if expiry > 0 {
+		cacheControl200 = fmt.Sprintf("public, max-age=%d%s", expiry/1e9, immutableDirective)
+	}
+
+	expiry404, err := c.Flags().GetDuration("expiry404")
+	if err != nil {
+		return err
+	}
+	switch {
+	case expiry404 == 0 && expiry == 0:
+	case expiry404 == 0 && expiry > 0:
+		expiry404 = min(60*time.Second, expiry)
+		fallthrough
+	case expiry404 > 0:
+		cacheControl404 = fmt.Sprintf("public, max-age=%d", expiry404/1e9)
 	}
 
 	// optional index file
@@ -199,6 +223,7 @@ func run(c *cobra.Command, args []string) error {
 		if err != nil {
 			return err
 		}
+		packHandler.SetCacheControl(cacheControl200, cacheControl404)
 		if indexFile != "" {
 			packHandler.SetIndex(indexFile)
 		}
diff --git a/go.mod b/go.mod
index 9f23a57..075ad33 100644
--- a/go.mod
+++ b/go.mod
@@ -1,9 +1,8 @@
 module src.lwithers.me.uk/go/htpack
 
-go 1.25.0
+go 1.26.0
 
 require (
-	github.com/google/go-cmp v0.7.0
-	golang.org/x/sys v0.45.0
-	google.golang.org/protobuf v1.36.11
+	golang.org/x/sys v0.47.0
+	google.golang.org/protobuf v1.36.12
 )
diff --git a/go.sum b/go.sum
index f246438..3bd003e 100644
--- a/go.sum
+++ b/go.sum
@@ -1,6 +1,6 @@
 github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
 github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
-golang.org/x/sys v0.45.0 h1:dO4czNzziLiiXplLQgBCEpCvXQ3dnkn0SdaZSYdQ+FY=
-golang.org/x/sys v0.45.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
-google.golang.org/protobuf v1.36.11 h1:fV6ZwhNocDyBLK0dj+fg8ektcVegBBuEolpbTQyBNVE=
-google.golang.org/protobuf v1.36.11/go.mod h1:HTf+CrKn2C3g5S8VImy6tdcUvCska2kB7j23XfzDpco=
+golang.org/x/sys v0.47.0 h1:o7XGOvZQCADBQQ4Y7VNq2dRWQR7JmOUW8Kxx4ZsNgWs=
+golang.org/x/sys v0.47.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
+google.golang.org/protobuf v1.36.12 h1:pJOKDDOyeXErUroCihFAd5LQuwXBSpVnKGrj5o/fwxc=
+google.golang.org/protobuf v1.36.12/go.mod h1:HTf+CrKn2C3g5S8VImy6tdcUvCska2kB7j23XfzDpco=
diff --git a/handler.go b/handler.go
index a429f19..9f13f42 100644
--- a/handler.go
+++ b/handler.go
@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"net/http"
 	"path"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"time"
@@ -14,7 +15,6 @@ import (
 const (
 	encodingGzip   = "gzip"
 	encodingBrotli = "br"
-	encodingZstd   = "zstd"
 )
 
 // New returns a new handler. Standard security headers are set.
@@ -23,21 +23,14 @@ func New(packfile string) (*Handler, error) {
 	if err != nil {
 		return nil, err
 	}
-	return NewMapped("/", mapped), nil
-}
 
-// NewMapped returns a new handler, given an already-mapped datafile. Standard
-// security headers are set.
-//
-// Files are served under the directory prefix given by path, which may be
-// empty or "/" for the root path. Note the [Handler.ServeMapped] method may specify
-// additional files to be served under different paths, not affected by this
-// path.
-func NewMapped(path string, mapped *packed.Mapped) *Handler {
 	h := &Handler{
-		files:     make(map[string]handlerFile),
-		headers:   make(map[string]string),
-		startTime: time.Now(),
+		mapped:          mapped.Data,
+		dir:             mapped.Directory.Files,
+		headers:         make(map[string]string),
+		cacheControl200: "no-cache",
+		cacheControl404: "no-cache",
+		startTime:       time.Now(),
 	}
 
 	// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options
@@ -46,24 +39,18 @@ func NewMapped(path string, mapped *packed.Mapped) *Handler {
 	// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Content-Type-Options
 	h.SetHeader("X-Content-Type-Options", "nosniff")
 
-	h.addFiles(path, mapped)
-
-	return h
+	return h, nil
 }
 
 // Handler implements http.Handler and allows options to be set.
 type Handler struct {
-	files     map[string]handlerFile
-	headers   map[string]string
-	startTime time.Time
-	notFound  *handlerFile
-}
-
-// AddMapped serves up additional files from the given mapped packfile. The
-// given path is taken as a serving prefix. If a file for a given path already
-// exists, the existing file takes precedence.
-func (h *Handler) AddMapped(path string, mapped *packed.Mapped) {
-	h.addFiles(path, mapped)
+	mapped          []byte
+	dir             map[string]*packed.File
+	headers         map[string]string
+	cacheControl200 string
+	cacheControl404 string
+	startTime       time.Time
+	notFound        *packed.File
 }
 
 // SetHeader allows a custom header to be set on HTTP responses. These are
@@ -92,11 +79,11 @@ func (h *Handler) SetHeader(key, value string) {
 // multiple times with different filenames (noting later calls would not
 // overwrite files matching earlier calls).
 func (h *Handler) SetIndex(filename string) {
-	for k, v := range h.files {
-		if path.Base(k) == filename {
-			routeToAdd := path.Dir(k)
-			if _, exists := h.files[routeToAdd]; !exists {
-				h.files[routeToAdd] = v
+	for k, v := range h.dir {
+		if filepath.Base(k) == filename {
+			routeToAdd := filepath.Dir(k)
+			if _, exists := h.dir[routeToAdd]; !exists {
+				h.dir[routeToAdd] = v
 			}
 		}
 	}
@@ -116,60 +103,25 @@ func (h *Handler) SetNotFound(notFound string) error {
 	}
 
 	notFound = path.Clean(notFound)
-	f, ok := h.files[path.Clean(notFound)]
-	if !ok {
+	dir := h.dir[path.Clean(notFound)]
+	if dir == nil {
 		return fmt.Errorf("no such resource %q", notFound)
 	}
-	h.notFound = &f
+	h.notFound = dir
 	return nil
 }
 
-type handlerFile struct {
-	contentType, etag                string
-	uncompressed, gzip, brotli, zstd []byte
-}
-
-func (h *Handler) addFiles(prefix string, mapped *packed.Mapped) {
-	// our directory contains files like "/index.html". We will add
-	// a prefix, which might be "" or "/foo".
-	prefix = path.Clean(prefix)
-	prefix = strings.TrimSuffix(prefix, "/")
-	switch {
-	case prefix == "":
-	case prefix == ".":
-		prefix = ""
-	case !strings.HasPrefix(prefix, "/"):
-		prefix = "/" + prefix
-	}
-
-	for filePath, file := range mapped.Directory.Files {
-		handlerPath := prefix + filePath
-		if _, exists := h.files[handlerPath]; exists {
-			// don't overwrite existing entries
-			continue
-		}
-		h.files[handlerPath] = buildHandlerFile(mapped.Data, file)
-	}
-}
-
-func buildHandlerFile(data []byte, file *packed.File) handlerFile {
-	byteSlice := func(fileData *packed.FileData) []byte {
-		if fileData == nil {
-			return nil
-		}
-		start := fileData.Offset
-		end := start + fileData.Length
-		return data[start:end]
-	}
-	hf := handlerFile{
-		contentType: file.ContentType,
-		etag:        file.Etag,
-	}
-	hf.uncompressed = byteSlice(file.Uncompressed)
-	hf.gzip = byteSlice(file.Gzip)
-	hf.brotli = byteSlice(file.Brotli)
-	hf.zstd = byteSlice(file.Zstd)
-	return hf
+// SetCacheControl allows control over the cache-control header. If not called,
+// "Cache-Control: no-cache" is the default.
+//
+// The first and second arguments are required, and specify the header to use
+// for 200 responses and 404 responses respectively.
+//
+// TODO: could be expanded to take extra varargs for more specific control over
+// particular assets, like different expiry per content-type.
+func (h *Handler) SetCacheControl(hdr200, hdr404 string) {
+	h.cacheControl200 = hdr200
+	h.cacheControl404 = hdr404
 }
 
 // ServeHTTP handles requests for files. It supports GET and HEAD methods, with
@@ -190,48 +142,46 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 		return
 	}
 
-	info, ok := h.files[path.Clean(req.URL.Path)]
-	if !ok {
+	info := h.dir[path.Clean(req.URL.Path)]
+	if info == nil {
+		w.Header().Set("Cache-Control", h.cacheControl404)
 		if h.notFound == nil {
 			http.NotFound(w, req)
 			return
 		}
-		info = *h.notFound
+		info = h.notFound
 	}
 
 	// set standard headers
+	w.Header().Set("Cache-Control", h.cacheControl200)
 	w.Header().Set("Vary", "Accept-Encoding")
-	w.Header().Set("Etag", info.etag)
-	w.Header().Set("Content-Type", info.contentType)
+	w.Header().Set("Etag", info.Etag)
+	w.Header().Set("Content-Type", info.ContentType)
 	w.Header().Set("Accept-Ranges", "bytes")
 
 	// process etag / modtime
-	if clientHasCachedVersion(info.etag, h.startTime, req) {
+	if clientHasCachedVersion(info.Etag, h.startTime, req) {
 		w.WriteHeader(http.StatusNotModified)
 		return
 	}
 
 	// select compression
-	data := info.uncompressed
-	gzip, brotli, zstd := acceptedEncodings(req)
-	switch {
-	case zstd && info.zstd != nil:
-		data = info.zstd
-		w.Header().Set("Content-Encoding", encodingZstd)
-	case brotli && info.brotli != nil:
-		data = info.brotli
+	data := info.Uncompressed
+	gzip, brotli := acceptedEncodings(req)
+	if brotli && info.Brotli != nil {
+		data = info.Brotli
 		w.Header().Set("Content-Encoding", encodingBrotli)
-	case gzip && info.gzip != nil:
-		data = info.gzip
+	} else if gzip && info.Gzip != nil {
+		data = info.Gzip
 		w.Header().Set("Content-Encoding", encodingGzip)
 	}
 
 	// range support (single-part ranges only)
 	// https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests#Single_part_ranges
-	offset, length, isPartial := getFileRange(uint64(len(data)), req)
+	offset, length, isPartial := getFileRange(data, req)
 	if isPartial {
 		w.Header().Set("Content-Range", fmt.Sprintf("bytes %d-%d/%d",
-			offset, offset+length-1, len(data)))
+			offset, offset+length-1, data.Length))
 	}
 
 	// now we know exactly what we're writing, finalise HTTP header
@@ -247,10 +197,11 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 		return
 	}
 
-	w.Write(data[offset : offset+length])
+	offset += data.Offset
+	w.Write(h.mapped[offset : offset+length])
 }
 
-func acceptedEncodings(req *http.Request) (gzip, brotli, zstd bool) {
+func acceptedEncodings(req *http.Request) (gzip, brotli bool) {
 	encodings := req.Header.Get("Accept-Encoding")
 	for _, enc := range strings.Split(encodings, ",") {
 		switch strings.TrimSpace(enc) {
@@ -293,8 +244,8 @@ func clientHasCachedVersion(etag string, startTime time.Time, req *http.Request,
 
 // getFileRange returns the byte offset and length of the file to serve, along
 // with whether or not it's partial content.
-func getFileRange(dataLength uint64, req *http.Request) (offset, length uint64, isPartial bool) {
-	length = dataLength
+func getFileRange(data *packed.FileData, req *http.Request) (offset, length uint64, isPartial bool) {
+	length = data.Length
 
 	// only accept "Range: bytes=…"
 	r := req.Header.Get("Range")
@@ -321,8 +272,8 @@ func getFileRange(dataLength uint64, req *http.Request) (offset, length uint64,
 	// validate the interval lies within the file
 	switch {
 	case from > to,
-		from >= dataLength,
-		to >= dataLength:
+		from >= data.Length,
+		to >= data.Length:
 		return
 	}
 
diff --git a/handler_test.go b/handler_test.go
deleted file mode 100644
index c0b986f..0000000
--- a/handler_test.go
+++ /dev/null
@@ -1,49 +0,0 @@
-package htpack
-
-import (
-	"bytes"
-	"io"
-	"net/http/httptest"
-	"os"
-	"testing"
-
-	"github.com/google/go-cmp/cmp"
-	"src.lwithers.me.uk/go/htpack/packed"
-)
-
-func TestHandler(t *testing.T) {
-	mapped, err := packed.MapAndLoad("packed/testdata/helloworld.pack")
-	if err != nil {
-		t.Fatal("loading test packfile:", err)
-	}
-	defer mapped.Close()
-
-	orig, err := os.ReadFile("packed/testdata/helloworld.txt")
-	if err != nil {
-		t.Fatal("loading test textfile:", err)
-	}
-
-	h := NewMapped("", mapped)
-	h.AddMapped("/foo", mapped)
-
-	testsv := httptest.NewServer(h)
-	defer testsv.Close()
-	cl := testsv.Client()
-	url := testsv.URL + "/helloworld.txt"
-
-	resp, err := cl.Get(url)
-	if err != nil {
-		t.Fatal("executing GET request:", err)
-	}
-	defer resp.Body.Close()
-
-	body, err := io.ReadAll(resp.Body)
-	if err != nil {
-		t.Fatal("reading response body:", err)
-	}
-
-	if !bytes.Equal(body, orig) {
-		t.Error("difference in response (- got, + expected):")
-		t.Error(cmp.Diff(string(body), string(orig)))
-	}
-}
diff --git a/packed/packed.pb.go b/packed/packed.pb.go
index aa3429f..32fd75a 100644
--- a/packed/packed.pb.go
+++ b/packed/packed.pb.go
@@ -168,8 +168,6 @@ type File struct {
 	Gzip *FileData `protobuf:"bytes,4,opt,name=gzip,proto3" json:"gzip,omitempty"`
 	// Brotli compressed version of the file.
 	Brotli *FileData `protobuf:"bytes,5,opt,name=brotli,proto3" json:"brotli,omitempty"`
-	// Zstd compressed version of the file.
-	Zstd *FileData `protobuf:"bytes,6,opt,name=zstd,proto3" json:"zstd,omitempty"`
 }
 
 func (x *File) Reset() {
@@ -239,13 +237,6 @@ func (x *File) GetBrotli() *FileData {
 	return nil
 }
 
-func (x *File) GetZstd() *FileData {
-	if x != nil {
-		return x.Zstd
-	}
-	return nil
-}
-
 // FileData records the position of the file data within the pack.
 type FileData struct {
 	state         protoimpl.MessageState
@@ -330,7 +321,7 @@ var file_packed_proto_rawDesc = []byte{
 	0x75, 0x65, 0x18, 0x02, 0x20, 0x01, 0x28, 0x0b, 0x32, 0x1c, 0x2e, 0x6c, 0x77, 0x69, 0x74, 0x68,
 	0x65, 0x72, 0x73, 0x2e, 0x68, 0x74, 0x70, 0x61, 0x63, 0x6b, 0x2e, 0x70, 0x61, 0x63, 0x6b, 0x65,
 	0x64, 0x2e, 0x46, 0x69, 0x6c, 0x65, 0x52, 0x05, 0x76, 0x61, 0x6c, 0x75, 0x65, 0x3a, 0x02, 0x38,
-	0x01, 0x22, 0xa9, 0x02, 0x0a, 0x04, 0x46, 0x69, 0x6c, 0x65, 0x12, 0x21, 0x0a, 0x0c, 0x63, 0x6f,
+	0x01, 0x22, 0xf3, 0x01, 0x0a, 0x04, 0x46, 0x69, 0x6c, 0x65, 0x12, 0x21, 0x0a, 0x0c, 0x63, 0x6f,
 	0x6e, 0x74, 0x65, 0x6e, 0x74, 0x5f, 0x74, 0x79, 0x70, 0x65, 0x18, 0x01, 0x20, 0x01, 0x28, 0x09,
 	0x52, 0x0b, 0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, 0x74, 0x54, 0x79, 0x70, 0x65, 0x12, 0x12, 0x0a,
 	0x04, 0x65, 0x74, 0x61, 0x67, 0x18, 0x02, 0x20, 0x01, 0x28, 0x09, 0x52, 0x04, 0x65, 0x74, 0x61,
@@ -345,17 +336,13 @@ var file_packed_proto_rawDesc = []byte{
 	0x06, 0x62, 0x72, 0x6f, 0x74, 0x6c, 0x69, 0x18, 0x05, 0x20, 0x01, 0x28, 0x0b, 0x32, 0x20, 0x2e,
 	0x6c, 0x77, 0x69, 0x74, 0x68, 0x65, 0x72, 0x73, 0x2e, 0x68, 0x74, 0x70, 0x61, 0x63, 0x6b, 0x2e,
 	0x70, 0x61, 0x63, 0x6b, 0x65, 0x64, 0x2e, 0x46, 0x69, 0x6c, 0x65, 0x44, 0x61, 0x74, 0x61, 0x52,
-	0x06, 0x62, 0x72, 0x6f, 0x74, 0x6c, 0x69, 0x12, 0x34, 0x0a, 0x04, 0x7a, 0x73, 0x74, 0x64, 0x18,
-	0x06, 0x20, 0x01, 0x28, 0x0b, 0x32, 0x20, 0x2e, 0x6c, 0x77, 0x69, 0x74, 0x68, 0x65, 0x72, 0x73,
-	0x2e, 0x68, 0x74, 0x70, 0x61, 0x63, 0x6b, 0x2e, 0x70, 0x61, 0x63, 0x6b, 0x65, 0x64, 0x2e, 0x46,
-	0x69, 0x6c, 0x65, 0x44, 0x61, 0x74, 0x61, 0x52, 0x04, 0x7a, 0x73, 0x74, 0x64, 0x22, 0x3a, 0x0a,
-	0x08, 0x46, 0x69, 0x6c, 0x65, 0x44, 0x61, 0x74, 0x61, 0x12, 0x16, 0x0a, 0x06, 0x6f, 0x66, 0x66,
-	0x73, 0x65, 0x74, 0x18, 0x01, 0x20, 0x01, 0x28, 0x06, 0x52, 0x06, 0x6f, 0x66, 0x66, 0x73, 0x65,
-	0x74, 0x12, 0x16, 0x0a, 0x06, 0x6c, 0x65, 0x6e, 0x67, 0x74, 0x68, 0x18, 0x02, 0x20, 0x01, 0x28,
-	0x06, 0x52, 0x06, 0x6c, 0x65, 0x6e, 0x67, 0x74, 0x68, 0x42, 0x22, 0x5a, 0x20, 0x73, 0x72, 0x63,
-	0x2e, 0x6c, 0x77, 0x69, 0x74, 0x68, 0x65, 0x72, 0x73, 0x2e, 0x6d, 0x65, 0x2e, 0x75, 0x6b, 0x2f,
-	0x68, 0x74, 0x70, 0x61, 0x63, 0x6b, 0x2f, 0x70, 0x61, 0x63, 0x6b, 0x65, 0x64, 0x62, 0x06, 0x70,
-	0x72, 0x6f, 0x74, 0x6f, 0x33,
+	0x06, 0x62, 0x72, 0x6f, 0x74, 0x6c, 0x69, 0x22, 0x3a, 0x0a, 0x08, 0x46, 0x69, 0x6c, 0x65, 0x44,
+	0x61, 0x74, 0x61, 0x12, 0x16, 0x0a, 0x06, 0x6f, 0x66, 0x66, 0x73, 0x65, 0x74, 0x18, 0x01, 0x20,
+	0x01, 0x28, 0x06, 0x52, 0x06, 0x6f, 0x66, 0x66, 0x73, 0x65, 0x74, 0x12, 0x16, 0x0a, 0x06, 0x6c,
+	0x65, 0x6e, 0x67, 0x74, 0x68, 0x18, 0x02, 0x20, 0x01, 0x28, 0x06, 0x52, 0x06, 0x6c, 0x65, 0x6e,
+	0x67, 0x74, 0x68, 0x42, 0x22, 0x5a, 0x20, 0x73, 0x72, 0x63, 0x2e, 0x6c, 0x77, 0x69, 0x74, 0x68,
+	0x65, 0x72, 0x73, 0x2e, 0x6d, 0x65, 0x2e, 0x75, 0x6b, 0x2f, 0x68, 0x74, 0x70, 0x61, 0x63, 0x6b,
+	0x2f, 0x70, 0x61, 0x63, 0x6b, 0x65, 0x64, 0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
 }
 
 var (
@@ -383,13 +370,12 @@ var file_packed_proto_depIdxs = []int32{
 	3, // 1: lwithers.htpack.packed.File.uncompressed:type_name -> lwithers.htpack.packed.FileData
 	3, // 2: lwithers.htpack.packed.File.gzip:type_name -> lwithers.htpack.packed.FileData
 	3, // 3: lwithers.htpack.packed.File.brotli:type_name -> lwithers.htpack.packed.FileData
-	3, // 4: lwithers.htpack.packed.File.zstd:type_name -> lwithers.htpack.packed.FileData
-	2, // 5: lwithers.htpack.packed.Directory.FilesEntry.value:type_name -> lwithers.htpack.packed.File
-	6, // [6:6] is the sub-list for method output_type
-	6, // [6:6] is the sub-list for method input_type
-	6, // [6:6] is the sub-list for extension type_name
-	6, // [6:6] is the sub-list for extension extendee
-	0, // [0:6] is the sub-list for field type_name
+	2, // 4: lwithers.htpack.packed.Directory.FilesEntry.value:type_name -> lwithers.htpack.packed.File
+	5, // [5:5] is the sub-list for method output_type
+	5, // [5:5] is the sub-list for method input_type
+	5, // [5:5] is the sub-list for extension type_name
+	5, // [5:5] is the sub-list for extension extendee
+	0, // [0:5] is the sub-list for field type_name
 }
 
 func init() { file_packed_proto_init() }
diff --git a/packed/packed.proto b/packed/packed.proto
index f83ed0e..771e1ec 100644
--- a/packed/packed.proto
+++ b/packed/packed.proto
@@ -47,9 +47,6 @@ message File {
 
 	// Brotli compressed version of the file.
 	FileData brotli = 5;
-
-	// Zstd compressed version of the file.
-	FileData zstd = 6;
 }
 
 // FileData records the position of the file data within the pack.