hashicorp/vagrant · error · VagrantPlugins::DockerProvider::Errors::SyncedFolderNonDocker

The "docker" synced folder type can't be used because the p

Error message

The "docker" synced folder type can't be used because the  provider
in use is not Docker. This synced folder type only works with the
Docker provider. The provider this machine is using is: %{provider}

What it means

The docker synced folder implementation gates on the machine's provider: usable? returns false (or raises SyncedFolderNonDocker when Vagrant performs a strict check, i.e. the type was pinned explicitly with type: "docker") whenever machine.provider_name is not :docker. The message names the offending provider.

Source

Thrown at plugins/providers/docker/synced_folder.rb:11

# Copyright IBM Corp. 2010, 2025
# SPDX-License-Identifier: BUSL-1.1

module VagrantPlugins
  module DockerProvider
    class SyncedFolder < Vagrant.plugin("2", :synced_folder)
      def usable?(machine, raise_error=false)
        # These synced folders only work if the provider is Docker
        if machine.provider_name != :docker
          if raise_error
            raise Errors::SyncedFolderNonDocker,
              provider: machine.provider_name.to_s
          end

          return false
        end

        true
      end

      def prepare(machine, folders, _opts)
        folders.each do |id, data|
          next if data[:ignore]

          host_path  = data[:hostpath]
          guest_path = data[:guestpath]
          # Append consistency option if it exists, otherwise let it nil out
          consistency = data[:docker_consistency]
          consistency &&= ":" + consistency

View on GitHub (pinned to 35f3160f4a)

Solutions

Remove type: "docker" and let the active provider pick its own synced folder implementation
Use a portable type (rsync, nfs) in shared configs
Scope docker-typed folders to machines that actually use the docker provider

Example fix

# before
config.vm.synced_folder ".", "/vagrant", type: "docker"

# after
config.vm.synced_folder ".", "/vagrant", type: "rsync"
# or omit type and let the provider default apply

Defensive patterns

Strategy: type-guard

Type guard

def docker_synced_folder_applicable?(machine)
  machine.provider_name == :docker
end

# usage
if docker_synced_folder_applicable?(machine)
  machine.config.vm.synced_folder ".", "/vagrant", type: "docker"
end

Prevention

Use provider-conditional blocks when sharing Vagrantfiles across providers
Prefer omitting type: and letting each provider pick its implementation
Reserve type: "docker" for machines pinned to the docker provider via FORCE or provider overrides

When it happens

Trigger: config.vm.synced_folder "src", "/src", type: "docker" on a machine whose provider is virtualbox, hyperv, vmware, etc. - the strict check fires because the type was forced instead of letting Vagrant choose per provider.

Common situations: One Vagrantfile shared across providers; a Docker-machine config copied onto other machines; a shared default that pins one folder type for all machines.

Related errors

AI-assisted analysis of hashicorp/vagrant@35f3160f4a (2026-08-21). Data as JSON: /api/errors/6c707386715b7ff2. Report an issue: GitHub.