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YAML functions require explicit tags

· 2 min read
Erik Osterman
Erik Osterman
Founder @ Cloud Posse

Starting with the 2026-10-20 edition, Atmos evaluates YAML functions only when they are written as explicit YAML tags. Quoting an entire function expression keeps it as literal text.

Previously, Atmos stored deferred function calls as plain strings to evaluate them after merging and template rendering, which erased the distinction between a real YAML tag and quoted text. That was a bug.

# Reads the environment variable:
foo: !env "SOMETHING"

# Keeps the literal string !env SOMETHING:
literal: "!env SOMETHING"

import:
  - path: configs.d/prod-overrides.yaml
    when: !cel 'env.DEPLOY_ENV == "prod"'

The tag belongs outside the quotes. This applies to all Atmos YAML functions, including !exec, !terraform.state, !store, and !cel. Quoted !unset no longer removes a value. Strings returned by a function are not automatically treated as new function calls.

Condition fields still accept named predicates such as ci and local, and unprefixed expressions such as when: 'env.CI == "true"'. A quoted !cel ... condition is invalid under the new behavior.

Migration​

Move the tag outside the quotes wherever you intend to invoke a function. Leave intentional literal strings unchanged. Explicit evaluation APIs such as atmos.Resolve "!env SOMETHING" continue to accept function strings, and Go-template interpolation remains supported.

Unpinned projects follow the latest behavior. To preserve legacy string-prefix evaluation while migrating, pin an earlier edition:

atmos.yaml
edition: "2026-10-19"

Pins on or after 2026-10-20 use explicit tags. Edition flags and environment variables retain their existing precedence. atmos list editions and atmos describe edition report the yaml_functions.explicit_tags transition.