precompressor does not specify an Accept-Encoding value

Error message

precompressor does not specify an Accept-Encoding value

What it means

Returned in FileServer.Provision when a precompressor module's AcceptEncoding() returns an empty string. The Accept-Encoding value is the key used to match client requests to the sidecar file, so an empty value makes the precompressor unusable and provisioning aborts.

Source

Thrown at modules/caddyhttp/fileserver/staticfiles.go:233

			if abs, err := caddy.FastAbs(h); err == nil {
				fsrv.Hide[i] = abs
			}
		}
	}

	// support precompressed sidecar files
	mods, err := ctx.LoadModule(fsrv, "PrecompressedRaw")
	if err != nil {
		return fmt.Errorf("loading encoder modules: %v", err)
	}
	for modName, modIface := range mods.(map[string]any) {
		p, ok := modIface.(encode.Precompressed)
		if !ok {
			return fmt.Errorf("module %s is not precompressor", modName)
		}
		ae := p.AcceptEncoding()
		if ae == "" {
			return fmt.Errorf("precompressor does not specify an Accept-Encoding value")
		}
		suffix := p.Suffix()
		if suffix == "" {
			return fmt.Errorf("precompressor does not specify a Suffix value")
		}
		if _, ok := fsrv.precompressors[ae]; ok {
			return fmt.Errorf("precompressor already added: %s", ae)
		}
		if fsrv.precompressors == nil {
			fsrv.precompressors = make(map[string]encode.Precompressed)
		}
		fsrv.precompressors[ae] = p
	}

	if fsrv.Browse != nil {
		// check sort options
		for idx, sortOption := range fsrv.Browse.SortOptions {
			switch idx {

View on GitHub (pinned to 50e54ee279)

Solutions

Make AcceptEncoding() return a non-empty, valid token such as "zstd", "gzip", or "br".
If the value comes from user config, validate it in the module's own Provision/Validate and fail early with a clear message.
Set the missing config field in your Caddyfile/JSON for the plugin.

Example fix

// before
func (Foo) AcceptEncoding() string { return "" }
// after
func (Foo) AcceptEncoding() string { return "foo" }

Defensive patterns

Strategy: validation

Validate before calling

// In your plugin's Validate(): reject empty Accept-Encoding early with a clear message
func (m *MyPrecompressor) Validate() error {
    if m.AcceptEncoding() == "" {
        return fmt.Errorf("accept_encoding must not be empty")
    }
    return nil
}

Prevention

Never leave stub methods returning zero values in shipped plugins.
Add unit tests that instantiate the module and assert non-empty AcceptEncoding().
Fill every plugin config field in examples so users copy complete configs.

When it happens

Trigger: A custom precompressor plugin whose AcceptEncoding() method returns "" (unimplemented stub, fallback value, or a configurable field left empty in config).

Common situations: Bare-bones plugin scaffolding where methods return zero values; a plugin that reads its Accept-Encoding from a config field the user did not set; copy-paste plugin templates with TODO stubs.

Related errors

AI-assisted analysis of caddyserver/caddy@50e54ee279 (2026-08-15). Data as JSON: /api/errors/e7b4dcebfba05b74. Report an issue: GitHub.