Using Gambit with the Prover

The mutation verifier checks that variants of the original Solidity program do not pass verification against a given specification. You can either use random mutations generated by Gambit or Manual Mutations crafted with a specific error. If a mutated program passes the specification, it may indicate that the specification is vacuous or not rigorous enough.


To use the mutation verifier, first install the Certora Prover and its dependencies. To install it, run

pip install certora-cli

If you already have certora-cli installed and the certoraMutate command is not available, you may need to update to a newer version by running

pip install --upgrade certora-cli


If you are on Linux, you will need pip version 20.3 or above.

Running the Mutation Verifier

The mutation verification script is called certoraMutate. Run it from the command line:

certoraMutate --prover_conf path/to/prover.conf --mutation_conf path/to/mutation.mconf

If you do, the script will generate code mutants, then submit a verification job per mutant to Certora’s server.


You must run certoraMutate from the root of the Solidity project directory. The files prover.conf and mutation.conf can be in their own directories, but must always be within the project directory. All paths in mutation.conf are relative to the parent directory containing mutation.conf. Paths in prover.conf are all relative to the project directory’s root, which is assumed to be the working directory.

When it finishes successfully, you should see the following lines printed:

******************** PROVER END ********************

You will receive an email notification when this mutation test is completed. It may take several hours.
You can follow the test's progress at

Verifying all the mutants is a heavy computational process that may take several hours. The time required to verify all mutants depends on various factors, such as the number of mutants, the complexity of the contracts, and the complexity of the specifications. In some instances, the entire testing process may take several hours.

Once it is completed, you should receive an email that looks like this:

Mutation suceeded email

Following tests with the mutations dashboard

You can track your mutation tests at the mutations dashboard. A test that just started would look like this:

Mutation test running

  • The Mutation ID is a unique identifier for the test.

  • The Message column includes the description given either in the command line via the --msg flag or the conf file’s "msg" key. It aids in identifying and documenting mutation tests. By default, it will show None.

  • The Status column of a test includes two different parts - the status of the test (see below) and a progress counter. The progress counter shows how many of the sent mutant verification jobs have already been executed.

Mutation test statuses

A mutation test can have one of five different statuses:

  • Running, as seen above, indicates that the verification jobs are still being computed.

  • Calculating indicates that all the verification jobs have finished, and the results are now being gathered and processed into a verification report.

Mutation test calculating

  • Executed indicates that all mutant verification jobs were executed correctly and are available in the report, which can be accessed by clicking on the Mutation ID.

Mutation test executed

  • Halted indicates that the mutation test reached a global time limit and was stopped. The partial verification results that were collected before the time limit are available in the verification report. This usually happens when too many mutants are used in a single test.

Mutation test halted

  • Problem indicates the test had errors. A report is usually not generated.

Mutation test problem

Mutation Configuration

The mutation tester expects a configuration file which defines the settings for mutant generation and testing (--mutation_conf). Here is a simple configuration file setup using the example above in mutation.mconf:

  "gambit": [{
    "filename" : "C.sol",
    "num_mutants": 5

Automatic Generation

You can generate a mutation configuration automatically from a Prover configuration with the flag --generate_mutation_conf. The resulting configuration may need some manual path fixing or the addition of other certoraMutate specific options.

Example usage:

certoraRun path/to/prover.conf --generate_mutation_conf path/to/generated/mutation.mconf

Manual Mutations

You have the option to include manually generated mutated files, not produced by Gambit, in your mutation test. We refer to these as ‘manual mutations.’ They can be used for regression tests, or to check mutations that Gambit does not currently support.


It is recommended to limit each manually mutated file to a single mutation for more accurate coverage analysis and better traceability.

To add manual mutations to mutation.mconf, write:

  "gambit": [{
    "filename" : "C.sol",
    "num_mutants": 5
  "manual_mutants": {
     "C.sol": "path/to/dir/with/manual_mutants/for/C.m1.sol"

If you don’t have a gambit object in the mconf file, certoraMutate will run only on the manual mutants, and no other mutants will be generated.


All manual mutations must be named uniquely. For example, if you want to generate manual mutations for C.sol and D.sol, name them C.m1.sol, C.m2.sol, D.m3.sol, D.m4.sol, ... etc.

Original Verification Run

A mutation test requires an original verification job that was completed successfully without halting. All mutant checks will be run with the same verification configuration as the original run, and their results will be compared to the original run. Rules that are not verified on the original run will be ignored.

Prover Configuration

Alternatively, you can provide a verification configuration file for the Certora Prover via --prover_conf. If you do, the Prover will execute the verification job using this file on the code without any mutation, which will be the original run. The original run job runs in parallel to the mutation verification jobs. A mutation testing report will not be produced if the original run job fails or halts.

Here is a simple configuration file setup using the example above in prover.conf:

  "files": [
  "verify": "C:c.spec"

CLI Options

certoraMutate supports the following options; for a comprehensive list, run certoraMutate --help:




Specify the configuration file for mutant generation


Specify the Original Verification Link


Specify the folder where the files will be downloaded from the Original Verification Link


Specify the Prover Configuration file for verifying mutants


Add a message to identify the certoraMutate run


Stop processing after generating mutations


Specify a log file to capture mutant collection failures


Show additional logging information during execution


At the moment, there are a few ways in which certoraMutate can fail. Here are some suggestions on how to troubleshoot when that happens. We are actively working on mitigating them.

  • There are currently no official Gambit binaries for Linux ARM or Windows. That means that Gambit will not be installed with pip install certora-cli, and must be built from source.

  • Sometimes, the problem stems from Gambit’s mutant generation. Try running with --gambit_only and look at the generated mutations.

  • Try running the Prover on your mutants individually using certoraRun. Usually the mutant setup will be in .certora_internal/applied_mutants_dir and can be retried by running the Prover’s .conf file with certoraRun. It is also possible that you are encountering a bug with the underlying version of the Prover.


The mutation verification results are summarized in a user-friendly visualization.

Here is an example summary for the advanced mutation of an ERC20 example. The green outer circles represent the rules, and the gray dots represent the mutants.

Report of the advanced mutation of CertoraInit

Selecting a rule shows which mutants it detected, and selecting a mutant shows which rules caught it.

Selecting a rule

Clicking on a mutant’s patch shows the difference between it and the original program.

Showing a mutant patch


At the top bar of the report there are different coverage metrics.

Mutation metrics

  • The Coverage metric is the ratio of the caught mutants to all mutants tested, also shown under Caught Mutations.

  • The Rules metric shows the ratio of the rules that caught at least one mutation out of all the rules in the tested specification.

  • The Solo Rules metric shows the ratio between the rules that caught a unique mutation and all rules that caught at least one mutation.