Ruby: Psych::BadAlias: Unknown alias: default
Psych 4, the YAML library that ships with Ruby 3.1 onwards, made YAML.load safe by default, and safe loading refuses aliases such as <<: *default unless asked. Files that have always used anchors, database.yml above all, fail when an older gem or Rails version loads them with the plain call. The YAML is valid; the loader got stricter. Newer Psych raises Psych::AliasesNotEnabled for the same reason.
Quick fix
Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.
# Which Psych, and which gem calls YAML.load? (see the backtrace)
ruby -rpsych -e 'puts Psych::VERSION'
bundle outdated rails webpacker
# Best fix: upgrade the gem that loads the file
# In your own code, allow aliases explicitly (trusted files only)
YAML.load_file('config/settings.yml', aliases: true)
YAML.safe_load(text, aliases: true, permitted_classes: [Date, Symbol])
# Stopgap for an old app you cannot upgrade yet (Gemfile)
gem 'psych', '< 4'How to diagnose Ruby errors
Ruby errors are dominated by Bundler and gem resolution, native extension compilation (which needs system headers Ruby cannot install for you), and in Rails by the pending-migration and asset-precompilation guards. Those guards are deliberate safety checks, so the fix is to satisfy them rather than to disable them.
If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.
- Run
bundle installwith verbose output and read which gem fails and why. Native extension failures name the missing header in the mkmf log. - Check the mkmf log path the error prints. It contains the actual compiler error, which the Bundler summary truncates.
- For Rails migration errors, run
rails db:migrate:statusto see exactly which migrations the database has recorded. - For asset errors in production, confirm
rails assets:precompileran during deployment and that the manifest is present inpublic/assets. - Verify the Ruby version matches
.ruby-versionand the Gemfile. Version managers silently switch, producing gem-not-found errors for gems that are installed elsewhere.
Tools worth reaching for
bundle install --verboserails db:migrate:statusgem envrbenv/rvm version checkbundle exec
Authoritative references
Primary documentation for this error, worth reading before applying any fix in production.
Related Ruby errors
- Bundler: Could not find gemBundler cannot locate the specified gem in any of the configured sources (e.g., RubyGems).
- Bundler: Your bundle only supports platforms ["arm64-darwin-23"]Gemfile.lock records the platforms it was resolved for, and it was generated on a Mac, so the…
- Encoding::InvalidByteSequenceErrorA string contains bytes that are invalid for the expected encoding (e.g., invalid UTF-8 bytes).
- LoadError: cannot load such fileRuby cannot find the required file or gem. The gem may not be installed, or the require path…
- Rails: ActionController::InvalidAuthenticityTokenThe CSRF token in the request did not match the one in the session. Rails keeps the session…
- Rails: ActiveRecord::NoDatabaseError: We could not find your databaseRails reached the database server and logged in, but the database it asked for does not exist…
- Rails: Asset not precompiledA referenced asset (CSS, JS, image) was not included in the precompiled asset list for…
- Rails: Blocked hosts: myapp.example.comThis is Rails' own HostAuthorization middleware, added in Rails 6 to stop DNS rebinding…
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- TLS 24Untrusted authorities, expiry, hostname mismatch, chains and cipher negotiation.
Something missing or wrong?
This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.