Gambit: Mutant Generation for Solidity

Gambit is a state-of-the-art mutation system for Solidity. By applying predefined syntax transformations called mutation operators (for example, convert a + b to a - b) to a Solidity program’s source code, Gambit generates variants of the program called mutants. Mutants can be used to evaluate test suites or specs used for formal verification: each mutant represents a potential bug in the program, and stronger test suites and specifications should detect more mutants.


  1. Gambit is written in Rust. You’ll need to install Rust and Cargo to build Gambit.

  2. Gambit uses solc, the Solidity compiler, to generate mutants. You’ll need to have a solc binary that is compatible with the project you are mutating (see the --solc option in gambit mutate --help)


You can download prebuilt Gambit binaries for Linux x86-64 and Mac from our releases page. For Windows and Linux ARM, you must build Gambit from source.

Building Gambit from source

To build Gambit from source, clone the Gambit repository and run

cargo install --path .

from this repository’s root. This will build Gambit and install it to a globally visible location on your PATH.

You can also build gambit with cargo build --release from the root of this repository. This will create a gambit binary in gambit/target/release/ which you can manually place on your path or invoke directly (e.g., by calling path/to/gambit/target/release/gambit).


Gambit has two main commands: mutate and summary. gambit mutate is responsible for mutating code, and gambit summary is a convenience command for summarizing generated mutants in a human-readable way.

Running gambit mutate will invoke solc, so make sure it is visible on your PATH. Alternatively, you can specify where Gambit can find the Solidity compiler with the option --solc path/to/solc, or specify a solc binary (e.g., solc8.12) with the option --solc solc8.12.


All tests (cargo test) are currently run using solc8.13. Your tests may fail if your solc points at a different version of the compiler.

Running gambit mutate

The gambit mutate command expects either a --filename argument or a --json argument. Using --filename allows you to specify a specific Solidity file to mutate:

gambit mutate --filename file.sol

However, if you want to mutate multiple files or apply a more complex set of parameters, we recommend using a configuration file via the --json option instead:

gambit mutate --json gambit_conf.json

Run gambit --help for more information.


All relative paths specified in a JSON configuration file are interpreted to be relative to the configuration file’s parent directory.

In the following section we provide examples of how to run Gambit using both --filename and --json. We provide more complete documentation in the Configuration Files and CLI Options sections below.


Unless otherwise noted, examples use code from benchmarks/ and are run from the root of the Gambit repository.

Example 1: Mutating a single file

To mutate a single file, use the --filename option (or -f), followed by the file to mutate.

gambit mutate -f benchmarks/BinaryOpMutation/BinaryOpMutation.sol

This will generate:

Generated 34 mutants in 0.69 seconds


The mutated file must be located within your current working directory or one of its subdirectories. If you want to mutate code in an arbitrary directory, use the --sourceroot option.

Example 2: Mutating and downsampling

The above command produced 34 mutants which may be more than you need. Gambit provides a way to randomly downsample the number of mutants with the --num_mutants or -n option:

gambit mutate -f benchmarks/BinaryOpMutation/BinaryOpMutation.sol -n 3

which will generate:

Generated 3 mutants in 0.15 seconds

Example 3: Viewing Gambit results


This example assumes you’ve just completed Example 2.

Gambit outputs all of its results in gambit_out:

tree -L 2 gambit_out

This produces:

├── gambit_results.json
├── input_json
│   ├── BinaryOpMutation.sol_json.ast
│   └── BinaryOpMutation.sol_json.ast.json
├── mutants
│   ├── 1
│   ├── 2
│   └── 3
└── mutants.log

See the Results Directory section for a detailed explanation of this layout. The gambit summary command pretty prints each mutant for easy inspection:

The output of

By default gambit summary prints info on all mutants. If you are interested in particular mutants you can specify a subset of mutant ids with the --mids flag. For instance, gambit summary --mids 3 4 5 will only print info for mutant ids 3 through 5.

Example 4: Specifying solc pass-through arguments

The Solidity compiler (solc) may need some extra information to successfully run on a file or a project. Gambit enables this with pass-through arguments that, as the name suggests, are passed directly through to the solc compiler.

For projects that have complex dependencies and imports, you may need to:

  • Specify base paths: To specify the Solidity --base-path argument, use --solc_base_path:

    gambit mutate --filename path/to/file.sol --solc_base_path base/path/dir
  • Specify remappings: To indicate where Solidity should find libraries, use solc’s import remapping syntax with --solc_remappings:

    gambit mutate --filename path/to/file.sol \
      --solc_remappings @openzeppelin=node_modules/@openzeppelin @foo=node_modules/@foo


    The paths should NOT end with a trailing /

  • Specify allow paths: To include additional allowed paths via solc’s --allow-paths argument, use --solc_allow_paths:

    gambit mutate --filename path/to/file.sol \
      --solc_allow_paths PATH1 --solc_allow_paths PATH2 ...
  • Specify include-path: To make an additional source directory available to the default import callback via solc’s [–include-path][included] argument, use --solc_include_path:

    gambit mutate --filename path/to/file.sol --solc_include_path PATH
  • Use optimization: To run the Solidity compiler with optimizations (solc’s --optimize argument), use --solc_optimize:

    gambit mutate --filename path/to/file.sol --solc_optimize

Example 5: The --sourceroot option

Gambit needs to track the location of source files that it mutates within a project: for instance, imagine there are files foo/Foo.sol and bar/Foo.sol. These are separate files, and their path prefixes are needed to determine this. Gambit addresses this with the --sourceroot option: the source root indicates to Gambit the root of the files that are being mutated, and all source file paths (both original and mutated) are reported relative to this source root.


If Gambit encounters a source file that does not belong to the source root it will print an error message and exit.

When running gambit mutate with the --filename option, source root defaults to the current working directory. When running gambit mutate with the --json option, source root defaults to the directory containing the configuration JSON.

Here are some examples of using the --sourceroot option.

  1. From the root of the Gambit repository, run:

    gambit mutate -f benchmarks/BinaryOpMutation/BinaryOpMutation.sol -n 1
    cat gambit_out/mutants.log
    find gambit_out/mutants -name "*.sol"

    This should output the following:

    Generated 1 mutants in 0.13 seconds
    1,BinaryOpMutation,benchmarks/BinaryOpMutation/BinaryOpMutation.sol,23:10, % ,*

    The first command generates a single mutant, and its source path is relative to ., the default source root. We can see that the reported paths in mutants.log, and the mutant file path in gambit_out/mutants/1, are the relative to this source root: benchmarks/BinaryOpMutation/BinaryOpMutation.sol

  2. Suppose we want our paths to be reported relative to benchmarks/BinaryOpMutation. We can run

    gambit mutate -f benchmarks/BinaryOpMutation/BinaryOpMutation.sol -n 1 --sourceroot benchmarks/BinaryOpMutation
    cat gambit_out/mutants.log
    find gambit_out/mutants -name "*.sol"

    which will output:

    Generated 1 mutants in 0.13 seconds
    1,BinaryOpMutation,BinaryOpMutation.sol,23:10, % ,*

    The reported filenames, and the offset path inside of gambit_out/mutants/1/, are now relative to the source root that we specified.

  3. Finally, suppose we use a source root that doesn’t contain the source file:

    gambit mutate -f benchmarks/BinaryOpMutation/BinaryOpMutation.sol -n 1 --sourceroot scripts

    This will try to find the specified file inside of scripts, and since it doesn’t exist Gambit reports the error:

    [ERROR gambit] [!!] Illegal Configuration: Resolved filename `/Users/USER/Gambit/benchmarks/BinaryOpMutation/BinaryOpMutation.sol` is not prefixed by the derived source root /Users/USER/Gambit/scripts

    Gambit prints an error and exits.

Example 6: Running Gambit using a configuration file

To run gambit with a configuration file, use the --json argument:

gambit mutate --json benchmarks/config-jsons/test1.json

The configuration file is a JSON file containing the command line arguments for gambit and additional configuration options:

    "filename": "../10Power/TenPower.sol",
    "sourceroot": "..",
    "solc_remappings": [

In addition to specifying the command line arguments, you can list the specific mutants that you want to apply, the specific functions you wish to mutate, and more. See the benchmark/config-jsons directory for examples.


Any paths provided by the configuration file are resolved relative to the configuration file’s parent directory.

Configuration Files

Configuration files allow you to save complex configurations and perform multiple mutations at once. Gambit uses a simple JSON object format to store mutation options, where each --option VALUE specified on the CLI is represented as a "option": VALUE key/value pair in the JSON object. Boolean --flags are enabled by storing them as true: "flag": true. For instance, --no_overwrite would be written as "no_overwrite": true.

As an example, consider the command from Example 1:

gambit mutate -f benchmarks/BinaryOpMutation/BinaryOpMutation.sol

To execute this using a configuration file you would write the following to example-1.json to the root of this repository and run gambit mutate --json example-1.json

  "filename": "benchmarks/BinaryOpMutation/BinaryOpMutation.sol"

Gambit also supports using multiple configurations in the same file: instead of a single JSON object, your configuration file should contain an array of objects:

        "filename": "Foo.sol",
        "contract": "C",
        "functions": ["bar", "baz"],
        "solc": "solc8.12",
        "solc_optimize": true
        "filename": "Blip.sol",
        "contract": "D",
        "functions": ["bang"],
        "solc": "solc8.12"
        "mutations": [

This configuration file will perform all mutations on Foo.sol’s functions bar and baz in the contract C, and only binary-op-mutation and swap-arguments-operator-mutation mutations on the function bang in the contract D. Both will compile using the Solidity compiler version solc5.12.

Paths in Configuration Files

Relative paths in a Gambit configuration file are relative to the parent directory of the configuration file. So if the JSON file listed above was moved to the benchmarks/ directory the "filename" would need to be updated to BinaryOpMutation/BinaryOpMutation.sol.

Results Directory

gambit mutate produces all results in an output directory (default: gambit_out). Here is an example:

gambit mutate -f benchmarks/BinaryOpMutation/BinaryOpMutation.sol -n 5
tree gambit_out -L 2

which produces:

Generated 5 mutants in 0.15 seconds

├── gambit_results.json
├── input_json
├── mutants
│   ├── 1
│   ├── 2
│   ├── 3
│   ├── 4
│   └── 5
└── mutants.log

This has the following structure:

  • gambit_results.json: a JSON file with detailed results

  • input_json/: intermediate files produced by solc that are used during mutation

  • mutants/: exported mutants. Each mutant is in its own directory named after its mutant ID (mid) 1, 2, 3, …

  • mutants.log: a log file with all mutant information. This is similar to results.json but in a different format and with different information

CLI Options

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



-o, --outdir

specify Gambit’s output directory (defaults to gambit_out)


do not overwrite an output directory; if the output directory exists, print an error and exit

-n, --num_mutants

randomly downsample to a given number of mutants.

-s, --seed

specify a random seed. For reproducibility, Gambit defaults to using the seed 0. To randomize the seed use --random_seed


use a random seed. Note that this overrides any value specified by --seed


specify a specific contract name to mutate; by default mutate all contracts


specify one or more functions to mutate; by default mutate all functions


specify one or more mutation operators to use; only generates mutants that are created using the specified operators


only generate mutants without validating them by compilation

Gambit also supports pass-through arguments, which are arguments that are passed directly to the Solidity compiler. All pass-through arguments are prefixed with solc_:




passes a value to solc’s --allow-paths argument


passes a value to solc’s --base-path argument


passes a value to solc’s --include-path argument


passes a value to directly to solc: this should be of the form prefix=path.

Mutation Operators

Gambit implements the following mutation operators

Mutation Operator




Replace a binary operator with another

a+b -> a-b


Replace a unary operator with another

~a -> -a


Alter the condition of a require statement

require(some_condition()) -> require(true)


Replaces the right hand side of an assignment

x = foo(); -> x = -1;


Replaces an expression with a no-op (assert(true))

foo(); -> assert(true);


Mutate the conditional of an if statement

if (C) {...} -> if (true) {...}


Swap the order of non-commutative operators

a - b -> b - a


Change a delegatecall() to a call()

_c.delegatecall(...) ->


(Disabled) Changes arguments of a function

add(a, b) -> add(a, a)


(Disabled) Swaps the order of a function’s arguments

add(a, b) -> add(b, a)

For more details on each mutation type, refer to the full documentation.