“Try” is a way to “try out” a proposed change safely before review, without officially landing it. This functionality has been around for a long time in various forms, and can sometimes show its age.

Access to “push to try” is typically available to a much larger group of developers than those who can land changes in integration and release branches. Specifically, try pushes are allowed for anyone with SCM Level 1, while integration branches are at SCM level 3.

Scheduling a Task on Try

There are three methods for scheduling a task on try: legacy try option syntax, try task config, and an empty try.

Try Option Syntax

The first, older method is a command line string called try syntax which is passed into the decision task via the commit message. The resulting commit is then pushed to the repository. An example try syntax might look like:

try: -b o -p linux64 -u mochitest-1 -t none

This gets parsed by taskgraph.try_option_syntax:TryOptionSyntax and returns a list of matching task labels. For more information see the TryServer wiki page.

Try Task Config

The second, more modern method specifies exactly the tasks to run. That list of tasks is usually generated locally with some local tool and attached to the commit pushed to the try repository. This gives finer control over exactly what runs and enables growth of an ecosystem of tooling appropriate to varied circumstances.


This method uses a checked-in file called try_task_config.json which lives at the root of the source dir. The JSON object in this file contains a tasks key giving the labels of the tasks to run. For example, the try_task_config.json file might look like:

  "version": 1,
  "tasks": [

Very simply, this will run any task label that gets passed in as well as their dependencies. While it is possible to manually commit this file and push to try, it is mainly meant to be a generation target for various try server choosers. For example:

$ ./mach try fuzzy

A list of all possible task labels can be obtained by running:

$ ./mach taskgraph tasks

A list of task labels relevant to a tree (defaults to mozilla-central) can be obtained with:

$ ./mach taskgraph target

Modifying Tasks in a Try Push

It’s possible to alter the definition of a task by defining additional configuration in try_task_config.json. For example, to set an environment variable in all tasks, you can add:

  "version": 1,
  "tasks": [...],
  "env": {
    "FOO": "bar"

The allowed configs are defined in taskgraph.decision.try_task_config_schema. The values will be available to all transforms, so how each config applies will vary wildly from one context to the next. Some (such as env) will affect all tasks in the graph. Others might only affect certain kinds of task. The use-artifact-builds config only applies to build tasks for instance.

Empty Try

If there is no try syntax or try_task_config.json, the try_mode parameter is None and no tasks are selected to run. The resulting push will only have a decision task, but one with an “add jobs” action that can be used to add the desired jobs to the try push.

Complex Configuration

If you need more control over the build configuration, (staging releases, for example), you can directly specify parameters to override from the try_task_config.json like this:

    "version": 2,
    "parameters": {
        "optimize_target_tasks": true,
        "release_type": "beta",
        "target_tasks_method": "staging_release_builds"

This format can express a superset of the version 1 format, as the version one configuration is equivalent to the following version 2 config.

    "version": 2,
    "parameters": {
        "try_task_config": {...},
        "try_mode": "try_task_config",