GitLab CI YAML fails to validate. Why?

I write a GitLab CI YAML.

This:

test:
  script:
    - echo 'hello: world'

Fails with the error: "This GitLab CI configuration is invalid: jobs:test:script config should be a string or a nested array of strings up to 10 levels deep."

I remove the space after the colon:

test:
  script:
    - echo 'hello:world'

And it validates.

I am able to work around this with using a block scalar:

test:
  script:
    - >-
      echo 'hello: world'

But I would like to understand what's happening, and if there is a way to get the echo to work without using a block scalar.

what's happening

I copy the yaml into "online yaml parser" and get:

{
  "test": {
    "script": [
      {
        "echo 'hello": "world'"
      }
    ]
  }
}

It is interpreting echo 'hello as a key, and world' as a value of a dictionary. The quotes ' " are only special if the whole thing is inside quotes. Otherwise, the : starts a dictionary in YAML.

To fix, you can do this:

test:
  script:
    - "echo 'hello: world'"

or this, which is exactly the same thing. YAML is not shell, double '' inside ' are interpreted as a single '.

test:
  script:
    - 'echo ''hello: world'''

But I usually just write it like this. You could use |- or >-, but the trailing newline just does not matter and I find | easy to remember.

test:
  script:
    - |
      echo 'hello: world'

You can test it in one of "online yaml parsers" to see what json happens out of them.

You can check YAML specification https://yaml.org/spec/1.2.2/ for exact parsing rules.