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Development guide

Don't use Docker to do development. It's a quick way to get Mastodon running in production, it's really really inconvenient for development. Normally in Rails development environment you get hot reloading of backend code and on-the-fly compilation of assets like JS and CSS, but you lose those benefits by compiling a Docker image. If you want to contribute to Mastodon, it is worth it to simply set up a proper development environment.


In fact, all you need is described in the production guide, with the following exceptions. You don't need:

  • Nginx
  • Systemd
  • An .env.production file. If you need to set any environment variables, you can use an .env file
    • Use LOCAL_HTTPS=false if developing on the same machine
  • To prefix any commands with RAILS_ENV=production since the default environment is "development" anyway
  • Any cronjobs

The command to install Ruby project dependencies is the following:

bundle install

Install JavaScript dependencies with this command:

yarn install --pure-lockfile

By default the development environment wants to connect to a mastodon_development database on localhost using your user/ident to login to Postgres (i.e. not a md5 password)

To setup the mastodon_development database, run:

bundle exec rails db:setup

You can then run Mastodon with:

bundle exec rails server

Since 1.4, we are using Webpack, which in development environment needs to be started as well as the command above:


Another, optional approach to managing the different processes starting (Rails, Webpack, Sidekiq, and the Streaming API) is to use the foreman tool.

gem install foreman
foreman start

Finally, open http://localhost:3000 in your browser.

By default, your development environment will have an admin account created for you to use - the email address will be admin@YOURDOMAIN (e.g. admin@localhost:3000) and the password will be mastodonadmin.

You can run tests with:


You can check localization status with:

i18n-tasks health

And update localization files after adding new strings with:

yarn manage:translations

You can check code quality with:



Follow the Linux setup as described above, but with these considerations:

  • If you use a Ruby version manager (chruby, rbenv, rvm, etc.), you must configure Ruby with CC=clang CXX=clang++. This instructs Ruby to use that compiler when compiling native C gems.
  • Many native C gems need to be told about /usr/local. You can do this by configuring a build.gem_name value using bundle config.
  • Any C gem that uses mkmf.rb's pkg_config method might fail if the linker produces warnings, as happens when a library links with sprintf(3). The cld3 gem uses pkg_config('protobuf'); if you have protobuf installed but it cannot be found while building the gem, this is likely the problem. You will need to directly modify mkmf.rb to get this to install.

The bundle configuration as of Mastodon 2.0's Gemfile:

bundle config build.nokogiri --use-system-libraries --with-xml2-include=/usr/local/include/libxml2/ --with-opt-include=/usr/local/include --with-xslt-include=/usr/local/include/libxslt --with-exslt-include=/usr/local/include/libexslt --with-xml2-lib=/usr/local/lib
bundle config build.charlock_holmes --with-icu-dir=/usr/local --with-opt-dir=/usr/local
bundle config build.idn-ruby --with-idn-dir=/usr/local

Modify mfmk.rb:

@@ -655,7 +655,7 @@
   def try_ldflags(flags, opts = {})
-    try_link(MAIN_DOES_NOTHING, flags, {:werror => true}.update(opts))
+    try_link(MAIN_DOES_NOTHING, flags, {:werror => false}.update(opts))
   def append_ldflags(flags, *opts)


These are self-contained instructions for setting up a development environment on a macOS system. It is assumed that you’ve cloned your fork of Mastodon to a local working directory and that you are in Terminal and in that directory.


  • Get Xcode Command Line Tools:

     xcode-select --install
  • Get Homebrew and use it to install the other dependencies:

     brew install imagemagick ffmpeg yarn postgresql redis rbenv nodejs protobuf libidn
  • Configure Rbenv:

     rbenv init
     rbenv install 2.5.1
  • Install/configure bundler to use your local rbenv:

     gem update --system
     gem install bundler
     rbenv rehash
  • Configure PostgreSQL:

     initdb /usr/local/var/postgres -E utf8
     export PGDATA=/usr/local/var/postgres

    In the prompt:



bundle install --with development
yarn install --pure-lockfile
gem install foreman --no-ri --no-rdoc
bundle exec rails db:setup
bin/rails assets:precompile


In separate Terminal windows/tabs:

  1. Start PostgreSQL: /usr/local/bin/postgres
  2. Start Redis: redis-server
  3. Start Mastodon (from the Mastodon folder): foreman start

Go to http://localhost:3000 to see your development instance.

Admin account is admin@localhost:3000. Password is mastodonadmin.

Development tips

You can use a localhost->world tunneling service like ngrok if you want to test federation, however that should not be your primary mode of operation. If you want to have a permanently federating server, set up a proper instance on a VPS with a domain name, and simply keep it up to date with your own fork of the project while doing development on localhost.

Ngrok and similar services give you a random domain on each start up. This is good enough to test how the code you're working on handles real-world situations. But as soon as your domain changes, for everybody else concerned you're a different instance than before.

Generally, federation bits are tricky to work on for exactly this reason - it's hard to test. And when you are testing with a disposable instance you are polluting the databases of the real servers you're testing against, usually not a big deal but can be annoying. The way I have handled this so far was thus: I have used ngrok for one session, and recorded the exchanges from its web interface to create fixtures and test suites. From then on I've been working with those rather than live servers.

I advise to study the existing code and the RFCs before trying to implement any federation-related changes. It's not that difficult, but I think "here be dragons" applies because it's easy to break.

If your development environment is running remotely (e.g. on a VPS or virtual machine), setting the REMOTE_DEV environment variable will swap your instance from using "letter opener" (which launches a local browser) to "letter opener web" (which collects emails and displays them at /letter_opener ).

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