Ruby "Psych::DisallowedClass" loading YAML in CI

Newer Psych makes YAML.load safe by default, refusing to instantiate arbitrary classes. Loading a YAML document that contains a tagged object (a Symbol, Time, or custom class) raises Psych::DisallowedClass unless that class is explicitly permitted.

What this error means

Code or a fixture load fails with "Psych::DisallowedClass: Tried to load unspecified class: X", often after a Ruby/Psych upgrade where YAML.load became strict.

Psych::DisallowedClass: Tried to load unspecified class: Symbol
  from /usr/local/lib/ruby/3.3.0/psych/class_loader.rb:...

Common causes

Psych made YAML.load safe by default

A newer Psych version routes YAML.load through safe loading, so previously-loadable tagged classes are now rejected.

The YAML contains non-permitted types

The document embeds Symbols, Time, or custom objects that are not in the default allowlist.

How to fix it

Permit the classes you actually need

1. Switch to safe_load with an explicit permitted_classes list.
2. Add only the classes the document legitimately contains.
3. Enable aliases only if the YAML uses anchors.

YAML.safe_load(text, permitted_classes: [Symbol, Time], aliases: true)

Use unsafe_load only for fully trusted input

If the source is entirely trusted (your own fixtures), unsafe_load restores the old behavior, but never use it on external input.

YAML.unsafe_load(File.read('config/trusted.yml'))

How to prevent it

• Prefer safe_load with an explicit permitted_classes list.
• Avoid embedding arbitrary Ruby objects in YAML.
• Reserve unsafe_load for input you fully control.

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