C:/Users/svogel/AppData/Local/Programs/Git/plugin marketplace add fails to clone via SSH despite SSH working outside Claude Code (Windows)

Environment

• Claude Code version: 2.1.223
• OS: Windows 11 Pro 10.0.26200
• Git: 2.55.0.windows.3
• Shell tested from: PowerShell and Git Bash (both same result)

What I did

/plugin marketplace add git@gitlab.newtec.zz:newtec/nt-agent-instructions.git

What happened

Error: Marketplace file not found at C:\Users\svogel.claude\plugins\marketplaces\temp_1786015351242.claude-plugin\marketplace.json

Ran it twice (different temp folder name each time, same error both times).

What I expected

The marketplace to be added successfully — the repo has a valid .claude-plugin/marketplace.json at its root (confirmed in a local clone of the same repo).

Troubleshooting already done

• ssh -T git@gitlab.newtec.zz succeeds from Git Bash (/usr/bin/ssh, MinGW-bundled) — returns Welcome to GitLab, @<user>!
• & "C:\Program Files\OpenSSH\ssh.exe" -T git@gitlab.newtec.zz also succeeds — this is the ssh.exe that resolves first on PATH in PowerShell, i.e. what a spawned child process would most likely use
• Cloning the same repo manually via git clone over SSH works fine
• The repo's .claude-plugin/marketplace.json exists and is valid in a manual clone
• The temp directory (~/.claude/plugins/marketplaces/temp_<timestamp>.claude-plugin/) is empty/removed after the failed attempt, so the clone itself appears to not populate it — this looks like the internal git clone invoked by /plugin marketplace add is failing silently (no stderr surfaced to the user) rather than the marketplace.json being genuinely missing from the repo

Secondary/cosmetic bug

The error message path is missing a path separator between the username and .claude:

C:\Users\svogel.claude\plugins\marketplaces\...

should be:

C:\Users\svogel\.claude\plugins\marketplaces\...

This suggests the path is built by concatenating something like %USERPROFILE% + .claude\... without inserting a separator — likely unrelated to the clone failure itself, but worth fixing alongside it since it made initial diagnosis harder (looked like a possibly-invalid path at first).

Suspected root cause

SSH auth to the target host works from every angle testable outside Claude Code, but not from /plugin marketplace add's internal clone. Possible causes:

• The internal git clone doesn't inherit the same environment (HOME, SSH_AUTH_SOCK, PATH) as an interactive shell
• A host-key-checking or agent-forwarding prompt that fails non-interactively without surfacing an error
• GIT_TERMINAL_PROMPT or similar suppressing an auth prompt that would otherwise succeed