Thank you for the PR, it looks great especially with the tests.
When I first implemented this, I hesitated between adding those to the source function or not, but I am happy to see this evolving.

I just have a minor feedback to only this templating with the one happening at the beginning

I don't know what happening but I just gave it a test using
And it doesn't seem to work

sources:
  1:
    name: echo 1.2.3
    kind: shell
    dependson:
      - 2
    spec:
      command: echo 1.2.3
  2:
    name: echo 2.3.4
    kind: shell
    spec:
      command: echo 2.3.4

conditions:
  3:
    name: 'echo {{ source "1" }}.x'
    kind: shell
    disablesourceinput: true
    spec:
      command: 'echo {{ source "1" }}'

  4:
    name: 'echo {{ source "1" | replace "." "_" }}'
    kind: shell
    disablesourceinput: true
    spec:
      command: 'echo {{ source "1" | replace "." "_" }}'

I just took another look and unfortunately I am not even sure it's technically possible to achieve what you need.

Updatecli has two kind of templating, one at start time and a second one after each resource execution to leverage dynamic information like source result.

Because the templating engine doesn't know when a resource is executed, we are using this trick
https://github.com/updatecli/updatecli/blame/7e7aaa2e622dc26f9fdfe8b58e228879afcdfda7/pkg/core/config/funcsUpdatecli.go#L86

If a source hasn't been executed yet, then the template returns '{{ source "sourceID"}}', so in the current implementation it would drop any additional filters like replace "." "_"
I don't know if it's possible to return the initial expression instead.

Worth mentioning your tests are passing because it's executed at start time but with source result which never happen in reality

Thanks for the quick review and for looking into this!
I see what you're talking about. I'll try to find a fix when I have more time to tackle this.
Hopefully there's a way around the issue without major changes.