A mixin that configures git to sign commits and tags using the SSH key forwarded from your host's SSH agent. Works with any agent kit (claude, codex, cursor, etc.).

Sandboxes forward your host's SSH agent automatically — the private key stays on your host. See Signed commits for the underlying mechanism this kit builds on.

On the host, load your SSH key into the agent:

$ ssh-add ~/.ssh/id_ed25519

Then start the sandbox with the kit attached:

$ sbx run claude --kit "git+https://github.com/docker/sbx-kits-contrib.git#dir=git-ssh-sign" ~/my-project

Inside the sandbox, verify that the forwarded agent exposes your key:

$ ssh-add -L
ssh-ed25519 AAAA... you@example.com

If it returns nothing, the key isn't loaded on the host yet — re-run ssh-add there and try again. Git signing will fail with a clear error if no key is available.

$ git log --show-signature -1
commit abc1234...
Good "git" signature for you@example.com with ED25519 key SHA256:...

If signing fails, see Docker's troubleshooting guide.

Git signing requires two things to be available when Git signs the commit: signing config (what format to use and how to resolve a key) and the actual key material from the forwarded SSH agent.

Signing config — written at install time to /etc/gitconfig

The install command writes gpg.format, commit.gpgSign, tag.gpgSign, gpg.ssh.defaultKeyCommand, and gpg.ssh.allowedSignersFile to the system-level git config. This file is read by git at process startup and is never overwritten by the sandbox infrastructure, so the config is always present when git commit begins.

Key material — resolved at signing time

gpg.ssh.defaultKeyCommand points to /home/agent/.config/git/ssh-signing-key-command. When Git needs a signing key, it runs that command. The command reads the first public key from ssh-add -L, writes /home/agent/.config/git/allowed_signers for signature verification, and prints the key in Git's inline key::... format.

This avoids writing key material at install or startup time, when the forwarded SSH agent may not be connected yet. It also avoids relying on Git hooks for signing.

Composing with repo-local hooks

This kit does not set core.hooksPath and does not install a pre-commit hook. Project-level hooks, hook managers, and repo-local core.hooksPath settings can run independently of commit signing.

To also enable SSH push/pull to GitHub from the sandbox, combine this kit with github-ssh:

$ sbx run \
  --kit "git+https://github.com/docker/sbx-kits-contrib.git#dir=git-ssh-sign" \
  --kit "git+https://github.com/docker/sbx-kits-contrib.git#dir=github-ssh" \
  claude