#schema #json-schema #json #tar-archive

app cargo-concordium

A tool to build and test smart contracts on the Concordium blockchain

5 stable releases

3.1.4 Oct 27, 2023
3.0.0 Aug 21, 2023
2.9.0 Jun 16, 2023
2.8.1 May 22, 2023
2.8.0 May 11, 2023
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Cargo Concordium

cargo-concordium is a tool for building and testing smart contracts on the Concordium blockchain.

See developer documentation for guides on how to use the tool in detail.

This crate is a binary, and its versioning applies to the command-line API. There are no guarantees about internal crate API.

Creating a new Concordium smart contract project

To start a new Concordium smart contract project, run the command:

cargo concordium init

This command will generate a new project from the templates in the template folder.

Compiling smart contracts

cargo concordium build -e --out contract.wasm.v1

will build a contract, embed the schema, and output the artifact to contract.wasm.v1. This can be deployed to the chain or tested locally.

Compilation options

Since a contract running on the chain will typically not be able to recover from panics, and error traces are not reported, it is useful not to bloat code size with them. Setting panic=abort will make it so that the compiler will generate simple Wasm traps on any panic that occurs. This option can be specified either in .cargo/config or in the Cargo.toml file as

# Don't unwind on panics, just trap.
panic = "abort"

The latter will only set this option in release builds, for debug builds use

# Don't unwind on panics, just trap.
panic = "abort"


Note that currently this is the default already for wasm32-unknown-unknown target.

An additional option that might be useful to minimize code size at the cost of some performance in some cases is

# Tell `rustc` to optimize for small code size.
opt-level = "s"

or even opt-level = "z".

In some cases using opt-level=3 actually leads to smaller code sizes, presumably due to more inlining and dead code removal as a result.

Reproducible and verifiable builds.

A normal build with cargo concordium build is generally not reproducible since some host information is embedded into the resulting binary. For this reason cargo concordium supports so-called verifiable or reproducible builds that always build the contract in a fixed environment specified as a Docker image. The list of available images can be found on DockerHub

Note that the image used to verify a build is part of the chain of trust. When verifying a build you must only use images you trust.

Both cargo concordium build and cargo concordium test support verifiable builds, which can be requested by adding the option --verifiable to the build command. The value of this option should be a docker image listed above. For example

cargo concordium build --verifiable docker.io/concordium/verifiable-sc:1.70.0 -o contract.wasm.v1 -e

This will build the smart contract, embed the schema (-e) and output it to a contract.wasm.v1 file. In addition to this cargo concordium will also produce a file contract.wasm.v1.tar that contains the exact sources that were used to build the contract.

When constructing the tar archive cargo concordium will include the contents of the package root subject to the following

  • files listed in .gitignore are ignored (both .gitignore in the package directory and parent directories)
  • the package build directory (typically target) will be ignored
  • additional files listed in any .ignore files will be ignored. The format of this file should be the same as a .gitignore file.
  • hidden files are ignored.

Information about the sources and the build will be embedded into contract.wasm.v1 file. This information includes

  • SHA2-256 hash of the tar file
  • the docker image used in the build (docker.io/concordium/verifiable-sc:1.70.0 in the example above)
  • the exact build command executed inside the image
  • optionally the link to the sources if the --source flag is provided. If this is not provided the link can be embedded later. The source link should point either to the tar file directly, or to a gzipped version of the file.

Publishing the sources

The tar archive is meant to be published and its link embedded in the source that is put to the chain. Before registering the module on the chain the tar file should be uploaded somewhere that is accessible via http GET request. Then the link should be embedded into the wasm.v1 file using the edit-build-info command.

cargo concordium edit-build-info --module contract.wasm.v1 --source-link https://domain.com/contract.wasm.v1.tar --verify

The --verify flag is optional, and if it is set cargo concordium will download the contents at the supplied link and verify that it matches the build metadata that is already embedded in the module. If it does, the link is added to the metadata.

Verifying a build

To verify that a deployed module was built from given sources there is a cargo concordium verify-build command. This takes a path to the module and optionally a path to the source tar archive. If the tar archive is not provided then it will be downloaded from a link embedded in the module, if available. The source link may also point to a gzipped tar archive.

For example

cargo concordium verify-build --module contract.wasm.v1

Printing build information

cargo concordium print-build-info --module contract.wasm.v1

Will print any embedded build information.


  • The Cargo.lock file must be up to date for reproducible builds.
  • The contract sources must be either available remotely on a package repository such as crates.io or entirely under the package root directory.
  • The crypto-primitives feature is not supported. This feature is only needed for running tests and can be disabled for a build.

Locally executing contracts

The following are some example invocations of the cargo concordium binary's subcommand run.

cargo concordium run init --context init-context.json --parameter parameter.bin --source ./simple_game.wasm --out state.bin --amount 123

with input files

    "metadata": {
        "slotNumber": 1,
        "blockHeight": 1,
        "finalizedHeight": 1,
        "slotTime": "2021-01-01T00:00:01Z"
    "initOrigin": "3uxeCZwa3SxbksPWHwXWxCsaPucZdzNaXsRbkztqUUYRo1MnvF"

and parameter.bin as


(as a text file without a newline).

cargo concordium run receive --context receive-context.json --parameter parameter-receive.bin --source ./simple_game.wasm --state state-in.bin --amount 0 --name "receive_help_yourself" --balance 13 --out state-out.bin

where an example receive context is

    "metadata": {
        "slotNumber": 1,
        "blockHeight": 1,
        "finalizedHeight": 1,
        "slotTime": "2021-01-01T00:00:01Z"
    "invoker": "3uxeCZwa3SxbksPWHwXWxCsaPucZdzNaXsRbkztqUUYRo1MnvF",
    "selfAddress": {"index": 0, "subindex": 0},
    "selfBalance": 0,
    "sender": {
        "type": "Account",
        "address": "3uxeCZwa3SxbksPWHwXWxCsaPucZdzNaXsRbkztqUUYRo1MnvF"
    "owner": "3uxeCZwa3SxbksPWHwXWxCsaPucZdzNaXsRbkztqUUYRo1MnvF"

See --help or help option to cargo concordium run for an explanation of the options.

Removing Host Information from Binary

By default the compiled binary from a rust crate contains some information from the host machine, namely rust-related paths such as the path to .cargo. This can be seen by inspecting the produced binary:

Lets assume your username is tom and you have a smart contract foo located in your home folder, which you compiled in release-mode to WASM32. By running the following command inside the foo folder, you will be able to see the paths included in the binary: strings target/wasm32-unknown-unknown/release/foo.wasm | grep tom

To remove the host information, the path prefixes can be remapped using a flag given to the compiler. RUSTFLAGS=--remap-path-prefix=/home/tom=secret cargo build --release --target wasm32-unknown-unknown, where /home/tom is the prefix you want to change into secret. The flag can be specified multiple times to remap multiple prefixes.

The flags can also be set permanently in the .cargo/config file in your crate, under the build section:

rustflags = ["--remap-path-prefix=/home/tom=secret"]


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