# git-check-assertions

I recently wrote two blog posts arguing that there might be some value in writing verifiable claims, i.e. assertions, inside of our commit messages:

* [Should we start writing verifiable claims in commit messages?](https://sven.memcmp.org/2026-02-19-should-we-start-writing-verifiable-claims-in-commit-messages/)
* [Writing the steps to validate a test that already passes in the commit message](https://sven.memcmp.org/2026-02-20-writing-the-steps-to-validate-a-test-that-already-passes-in-the-commit-message/)

This is a simple verifier for such assertions.

You write assertions in your commit messages, and `git-check-assertions` will then check out every commit (from the point that your branch diverged from `main`), and verify that the assertions in the commit message hold for the version of the code that is in the commit.

For a real-world example, check out the commits in [this pull request](https://codeberg.org/svenvanheugten/git-check-assertions/pulls/8), where `git-check-assertions` is used on itself.

⚠️ Only run this on repositories and branches that you trust, since the commands in the commit messages can do whatever they want.

## Installation

On most systems, clone this repository and add the `bin` directory to your `PATH`.

If you use Nix with flakes, you can simply add it to your program's devshell instead:

<details>
<summary>Instructions for using the flake</summary>

```diff
@@ -2,12 +2,18 @@
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
     flake-utils.url = "github:numtide/flake-utils";
+    git-check-assertions = {
+      url = "git+https://codeberg.org/svenvanheugten/git-check-assertions.git?ref=main";
+      inputs.nixpkgs.follows = "nixpkgs";
+      inputs.flake-utils.follows = "flake-utils";
+    };
   };
   outputs =
     {
       self,
       nixpkgs,
       flake-utils,
+      git-check-assertions,
     }:
     flake-utils.lib.eachDefaultSystem (
       system:
@@ -17,7 +23,7 @@
       {
         packages.default = pkgs.callPackage ./default.nix { };
         devShells.default = pkgs.mkShell {
-          packages = [ ];
+          packages = [ git-check-assertions.packages.${system}.default ];
         };
       }
     )
```
</details>

## What do you put in your commit messages?

Simply add a `git-check-assertions` block to a commit message, e.g.:

~~~
```git-check-assertions
[success] dotnet build
[success] dotnet test --no-build
```
~~~

Each block is parsed line-by-line:

* A line starting with `[success] ` runs the rest of the line as a shell command and asserts that it exits with status `0`.
* A line starting with `[failure] ` runs the rest of the line as a shell command and asserts that it exits with a non-zero status.
* Any following non-empty line belongs to the most recent command and asserts that the combined stdout/stderr from that command contains that string.

Blank lines are ignored. A new `[success]` or `[failure]` line starts a new command block.

## Multiple blocks in one commit message

You can include multiple `git-check-assertions` blocks in a single commit message. When you do, each individual block will run with a 'fresh' version of the commit, in which all changes that you made in the previous block have been undone.

## Cache of successful commits

After a commit's assertions run successfully, `git-check-assertions` appends the commit hash to `.git-check-assertions-cache` in the repo root. On the next run, any commit listed there is skipped.

Be sure to add `.git-check-assertions-cache` to your `.gitignore`.

## Examples of commit messages

Assert that a commit builds:

~~~
```git-check-assertions
[success] dotnet build
```
~~~

Assert that a commit builds and that the tests succeed:

~~~
```git-check-assertions
[success] dotnet build
[success] dotnet test --no-build
```
~~~

Assert that a commit builds, but that the tests do not succeed:

~~~
```git-check-assertions
[success] dotnet build
[failure] dotnet test --no-build
```
~~~

Assert that a commit builds, and that the tests fail with the error that you expect:

~~~
```git-check-assertions
[success] dotnet build
[failure] dotnet test --no-build
Invalid URL
```
~~~

Assert that a specific change breaks the tests (as discussed [here](https://sven.memcmp.org/2026-02-20-writing-the-steps-to-validate-a-test-that-already-passes-in-the-commit-message/)):

~~~
```git-check-assertions
[success] dotnet test

[success] sed -i '/crucial code/d' Main.fs
[failure] dotnet test
```
~~~

Assert that a specific change in a commit is necessary for the tests to succeed:

~~~
```git-check-assertions
[success] dotnet test

[success] git checkout HEAD~ Main.fs
[failure] dotnet test
Invalid URL
```
~~~