[BUG] origin remote URL corruption via GIT_COMMON_DIR + rbs/ruby-lsp in devcontainer

Summary

The repository's origin remote was corrupted: the URL changed from
git@github.com:org/myrepo.git to
https://github.com/ruby/gem_rbs_collection.git, and partial clone settings
(promisor = true, partialclonefilter = blob:none) were injected along with
duplicate fetch entries. As a result, git push failed with "Permission denied"
and git fetch was pulling branches from an unrelated repository.

---

Timeline

| Date/Time | Event |
|---|---|
| 2026-04-17 ~16:46 UTC | Claude Code created branch worktree-user+test from origin/main via its isolation: "worktree" feature |
| 2026-04-17 ~17:03 UTC | Commit on worktree-user+test: "dev container compatibility for git worktree" |
| 2026-04-20 12:01:07 UTC | User manually created worktree test: git worktree add -b test ../.claude/worktrees/test user/my-feature |
| 2026-04-20 12:09:31 UTC | .git/config modified: origin URL replaced, partial clone config injected, refspecs duplicated |
| 2026-04-20 12:10:09 UTC | FETCH_HEAD updated with branches from ruby/gem_rbs_collection |

---

Evidence

Resulting .git/config remote section:

[remote "origin"]
    url = https://github.com/ruby/gem_rbs_collection.git
    fetch = +refs/heads/*:refs/remotes/origin/*
    fetch = +refs/heads/*:refs/remotes/origin/*
    promisor = true
    partialclonefilter = blob:none
    fetch = +refs/heads/*:refs/remotes/origin/*
    fetch = +refs/heads/*:refs/remotes/origin/*
    fetch = +refs/heads/*:refs/remotes/origin/*

The config also contains a [branch "worktree-user+test"] entry with
remote = origin and merge = refs/heads/main, auto-generated by Claude Code.

Full corrupted .git/config at time of detection:

[core]
    repositoryformatversion = 0
    filemode = true
    bare = false
    logallrefupdates = true
    ignorecase = true
    precomposeunicode = true
    hooksPath = /Users/user/projects/myrepo/.git/hooks
[remote "origin"]
    url = https://github.com/ruby/gem_rbs_collection.git
    fetch = +refs/heads/*:refs/remotes/origin/*
    fetch = +refs/heads/*:refs/remotes/origin/*
    promisor = true
    partialclonefilter = blob:none
    fetch = +refs/heads/*:refs/remotes/origin/*
    fetch = +refs/heads/*:refs/remotes/origin/*
    fetch = +refs/heads/*:refs/remotes/origin/*
...
[branch "worktree-user+test"]
    remote = origin
    merge = refs/heads/main

---

Root cause (hypothesis)

Evidence strongly points to Claude Code's isolation: "worktree" feature as
the origin of the corruption. A git bug cannot be ruled out entirely, but is
considered unlikely.

Evidence pointing to Claude Code

• The branch worktree-user+test is clearly auto-generated — the user created

-b test, not that name. It was created on April 17 from origin/main and
has a commit authored by Claude Code.

• The EnterWorktree tool is available in Claude Code sessions and is part of the

isolation: "worktree" implementation.

• The promisor = true and partialclonefilter = blob:none settings are

characteristics of a partial clone — a technique Claude Code likely uses to
create lightweight worktrees.

Probable sequence

1. April 17: Claude Code ran an agent with isolation: "worktree", which

invoked EnterWorktree. As part of its fetch mechanism, it configured origin
with the wrong URL (gem_rbs_collection) and partial clone parameters, and
created the worktree-user+test branch.

1. April 20, 12:01: The user manually created the test worktree.

git worktree add itself does not modify remotes.

1. April 20, 12:09–12:10: A subsequent operation (possibly Claude Code

re-invoking EnterWorktree, a fetch triggered by a devcontainer init script,
or an IDE running git operations when opening the devcontainer) ran
git fetch origin against the already-corrupted remote, updating FETCH_HEAD
with gem_rbs_collection branches and consolidating the corruption in the
config.

Why gem_rbs_collection

Unknown. Possible causes:

• Bug in Claude Code when resolving the remote URL during worktree creation.
• Confusion between the project remote and the gem_rbs_collection repository,

which is a Ruby RBS type definitions collection — a dev dependency of the
project's Ruby stack.

Ruled out: git bug

git worktree add is a local operation that does not modify remotes. All
repository hooks are .sample files (inactive). git 2.50.1 (Apple Git-155) has
no known bugs related to this operation.

---

Impact

• git push failed with remote: Permission to ruby/gem_rbs_collection.git denied to username.
• git fetch was pulling branches from an unrelated external repository.
• No commits or project data were lost (local history was not affected).

---

Fix

Restore [remote "origin"] in .git/config:

[remote "origin"]
    url = git@github.com:org/myrepo.git
    fetch = +refs/heads/*:refs/remotes/origin/*

---

Environment

• OS: macOS Darwin 25.4.0
• git: 2.50.1 (Apple Git-155)
• Claude Code: CLI, model claude-sonnet-4-6
• Project structure: monorepo with git worktrees stored under .claude/worktrees/
• Active hooks: none (all .sample)

---

Related issues

• anthropics/claude-code#45201 — isolation: "worktree" also modifies .git/config (sets extensions.worktreeConfig), confirming the feature writes to the shared config as a side effect.
• anthropics/claude-code#47266 — concurrent worktree creation causes .git/config lock collisions, further evidence that the feature performs direct writes to .git/config.