Nix/direnv dev shell overrides macOS ssh; git pull fails on UseKeychain in ~/.ssh/config

[scott-sixfeetup]

🐛 Describe the bug

When entering the repo with direnv (flake dev shell), PATH resolves ssh to a Nix-provided OpenSSH binary (e.g. /nix/store/.../bin/ssh). That ssh doesn’t accept macOS-specific ~/.ssh/config options like UseKeychain, so git pull fails with “Bad configuration option: usekeychain”. If direnv is disabled (so /usr/bin/ssh is used), git pull succeeds.

💭 Expected Behavior

git pull should work inside the repo dev shell even when a user’s ~/.ssh/config includes macOS options like UseKeychain.

🔁 Steps To Reproduce

1. cd my-scaf-fullstack-app (direnv loads flake shell)
2. Run git pull
3. Observe failure
4. Run direnv disallow
5. Run git pull again and observe it succeeds

🌎 Environments

• Sandbox
• Staging
• Production

📝 Additional context

• In dev shell:
  - which ssh => /nix/store/...-openssh-10.2p1/bin/ssh
  - ssh -V => OpenSSH_10.2p1, OpenSSL 3.6.0 1 Oct 2025
• After direnv disallow:
  - which ssh => /usr/bin/ssh
  - ssh -V => OpenSSH_10.0p2, LibreSSL 3.3.6
  - git pull => Already up to date.

📜 Relevant log output

warning: unhandled Platform key FamilyDisplayName
  /Users/scott/.ssh/config: line 3: Bad configuration option: usekeychain
  /Users/scott/.ssh/config: terminating, 1 bad configuration options
  fatal: Could not read from remote repository.

  Please make sure you have the correct access rights
  and the repository exists.

📸 Screenshots

No response