Copier compatibility

deckctl can apply an existing Copier template without converting it. Point it at a template whose root contains a copier.yml and it parses the questions, translates them into its own model, and renders.

deckctl template apply my-copier-template ./out --source work --var project_name=billing

How the format is detected

Under the default --sidecar-format auto:

1. An explicit --sidecar-format deckctl or --sidecar-format copier wins.
2. A --sidecar <filename> override implies the native format.
3. A deckctl.yaml at the template root → native.
4. Otherwise a copier.yml at the template root → Copier.
5. Otherwise native, with built-in defaults.

Native wins when both files are present. Force the other one explicitly:

deckctl template apply my-template ./out --source work --sidecar-format copier

Both template apply and template variables accept --sidecar-format.

What is supported

Question forms

Both the shorthand and the full mapping form parse, and document order is preserved — which matters because computed defaults reference earlier answers.

copier.yml

project_name: myproject          # shorthand: the value is the default

http_port:                       # full form
  type: int
  help: "Port to listen on"
  default: 8080

runtime:
  type: str
  help: "Runtime to target"
  choices: ["go", "python", "node"]
  default: "go"

Types

Copier type	deckctl type
str, or omitted	str
int	int
bool	bool
anything else	str, with a warning

A question with choices becomes a choice variable regardless of its declared type — matching Copier’s own behaviour. List-form choices are supported; dict-form choices are reduced to their values with a warning.

Computed defaults

A default containing the variable-start delimiter is rendered as a Jinja expression against the answers resolved so far:

project_name: myproject
package_name:
  type: str
  default: "{{ project_name|lower|replace('-', '_') }}"
module_path:
  type: str
  default: "gitlab.com/acme/{{ project_name }}"

Rendered values are coerced back to the question’s declared type, so a computed int default reaches the template as an integer and a computed bool as a boolean.

when conditions

use_database:
  type: bool
  default: false

database_url:
  type: str
  when: "{{ use_database }}"

A question whose when evaluates falsy is never prompted for. If it still has a default, that default is recorded so later expressions can reference it. A when that fails to evaluate produces a warning and the question is treated as active.

Truthiness follows Copier/Jinja conventions: "", false, 0, none and no (case-insensitive) are false; everything else is true.

Template settings

Copier key	Mapped to
_subdirectory	Sidecar subdirectory — the template tree is rendered from there
_templates_suffix	Sidecar template_suffixes (replaces the whole list)
_envops	Sidecar delimiters — see Custom delimiters

Any other _-prefixed key produces a warning and is ignored — including _exclude, _skip_if_exists, _tasks, _migrations and _answers_file.

What is not supported

These produce a warning on stderr and are a no-op. Nothing blocks rendering.

Copier feature	Behaviour
validator	Ignored — no validation is performed
secret	Ignored — input is not masked
multiselect	Ignored
placeholder	Ignored
Dict-form choices	Values used, keys dropped
_exclude, _tasks, _migrations, …	Ignored
Answers file / template updates	Not implemented — deckctl has no copier update equivalent

Caution

secret being ignored means a secret question is prompted in cleartext and, if answered, recorded verbatim in the apply history. Pass secrets via --var from an environment variable, or keep them out of the template entirely.

Resolution order

Slightly different from the native path, because Copier defaults can compute:

1. --var key=value — always wins, even over a falsy when
2. The default — evaluated as a Jinja expression if it looks like one
3. Interactive prompt — only if the question is active (when truthy or absent)

A question with a default is therefore never prompted for, even when you wanted to be asked. That is Copier’s behaviour too.

Inspecting a Copier template

deckctl template variables my-copier-template --source work

This shows the translated variables and prints every translation warning to stderr — the fastest way to see what deckctl will and won’t honour before you apply anything.

Worked example

copier.yml

_subdirectory: template
_envops:
  variable_start_string: "[["
  variable_end_string: "]]"

project_name:
  type: str
  help: "Human-readable project name"
  default: "My Project"

package_name:
  type: str
  default: "[[ project_name|lower|replace(' ', '_') ]]"

use_ci:
  type: bool
  default: true

ci_image:
  type: str
  default: "golang:1.26"
  when: "[[ use_ci ]]"

deckctl template apply acme-service ./out --source work --var project_name="Billing Service"

• project_name → "Billing Service" (from --var)
• package_name → "billing_service" (computed)
• use_ci → true (default)
• ci_image → "golang:1.26" (active, default applies)
• Everything under template/ is rendered with [[ ]] delimiters