The cosmic-ray init commands scans a module for all possible mutations, but we don’t always want to execute all of these. For example, we may know that some of these mutations will result in equivalent mutants, so we need a way to prevent these mutations from actually being run.

To account for this, Cosmic Ray includes a number of filters. Filters are nothing more than programs - generally small ones - that modify a session in some way, often by marking certains mutations as “skipped”, thereby preventing them from running. The name “filter” is actually a bit misleading since these programs could modify a session in ways other than simply skipping some mutations. In practice, though, the need to skip certain tests is by far the most common use of these programs.


Filter are what used to be called interceptors. Interceptors were more tightly integrated with the init command, with many of the problems that unnecessarily high coupling often brings with it. Filters are simpler and more flexible than interceptors with no loss in power.

Filters included with Cosmic Ray

Cosmic Ray comes with a number of filters. Remember, though, that they are nothing more than simple programs that modify a session in some way; it should be straightforward to write your own filters should the need arise.


cr-filter-operators allows you to filter out operators according to their names. You provide the filter with a set of regular expressions, and any Cosmic Ray operator who’s name matches a one of these expressions will be skipped entirely.

The configuration is provided through a TOML file such as a standard Cosmic Ray configuration. The expressions must be in a list at the key “cosmic-ray.filters.operators-filter.exclude-operators”. Here’s an example:

exclude-operators = [

For a list of all operators in your Cosmic Ray installation, run cosmic-ray operators.


The cr-filter-pragma filter looks for lines in your source code containing the comment “# pragma: no mutate”. Any mutation in a session that would mutate such a line is skipped.


The cr-filter-git filter looks for edited or new lines from the given git branch. Any mutation in a session that would mutate other lines is skipped.

By default the master branch is used, but you could define another one like this:

[cosmic-ray.filters.git-filter] branch = “rolling”

External filters

Other filters are defined in separate projects.


The cosmic-ray-spor-filter filter modifies a session by skipping mutations which are indicated in a spor anchored metadata repository. In short, spor provides a way to associated arbitrary metadata with ranges of code, and this metadata is stored outside of the code. As your code changes, spor has algorithms to update the metadata (and its association with the code) automatically.

Get more details at the project page.

Using filters

Generally speaking, filters will be run immediately after running cosmic-ray init. It’s up to you to decide which to run, and often they will be run along with init in a batch script or CI configuration.

For example, if you wanted to apply the cr-filter-pragma filter to your session, you could do something like this:

cosmic-ray init cr.conf session.sqlite
cr-filter-pragma session.sqlite

The init would first create a session where all mutation would be run, and then the cr-filter-pragma call would mark as skipped all mutations which are on a line with the pragma comment.