Labels in Cromwell are a way to group together workflows that are related or associated to each other. For example, if you ran workflows to analyze data from a diabetes study, you can assign the label project:diabetes-cohort.

Custom Labels JSON

In order to assign labels to a workflow, the first step is to create a JSON file with key-value pairs that define a label. For the example above, the labels JSON should look like:


When choosing key-value pairs, it's important to make sure you're adhering to Cromwell supported label syntax below.

There are two ways to add labels to a workflow:
1. Upon workflow submission set the labels parameter of the Submit endpoint, or
2. Setting the -l argument when running in Command Line mode.

Labels can be added to existing workflows by using the Labels patch endpoint.

After adding labels to your workflows, you can take advantage of features like Query to filter tagged workflows. The Google backend supports labelling cloud resources and you can learn more about that here.

Label Format

When labels are supplied to Cromwell, it will fail any request containing invalid label strings. Below are the requirements for a valid label key/value pair in Cromwell:

  • Label keys may not be empty but label values may be empty.
  • Label key and values have a max char limit of 255.

For default labels applied by the Google backend, Cromwell will modify workflow/task/call names to fit the schema, according to the following rules:

  • Any capital letters are converted to lowercase.
  • Any character which is not one of [a-z], [0-9] or - will be replaced with -.
  • If the start character does not match [a-z] then prefix with x--
  • If the final character does not match [a-z0-9] then suffix with --x
  • If the string is too long, only take the first 30 and last 30 characters and add --- between them.