modelcontextprotocol/servers · error · BadName

Invalid branch name: '{branch_name}' - cannot start with '-'

Error message

Invalid branch name: '{branch_name}' - cannot start with '-'

What it means

git_create_branch() rejects a branch_name that starts with '-' to prevent flag injection into `git branch`. Raises gitdb BadName before repo.create_head is invoked; the exception propagates raw to the MCP client (server runs raise_exceptions=True).

Source

Thrown at src/git/src/mcp_server_git/server.py:203

                )
        return log
    else:
        # Use existing logic for simple log without date filtering
        commits = list(repo.iter_commits(max_count=max_count))
        log = []
        for commit in commits:
            log.append(
                f"Commit: {commit.hexsha!r}\n"
                f"Author: {commit.author!r}\n"
                f"Date: {commit.authored_datetime}\n"
                f"Message: {commit.message!r}\n"
            )
        return log

def git_create_branch(repo: git.Repo, branch_name: str, base_branch: str | None = None) -> str:
    # Defense in depth: reject names starting with '-' to prevent flag injection
    if branch_name.startswith("-"):
        raise BadName(f"Invalid branch name: '{branch_name}' - cannot start with '-'")
    if base_branch and base_branch.startswith("-"):
        raise BadName(f"Invalid base branch: '{base_branch}' - cannot start with '-'")
    if base_branch:
        base = repo.references[base_branch]
    else:
        base = repo.active_branch

    repo.create_head(branch_name, base)
    return f"Created branch '{branch_name}' from '{base.name}'"

def git_checkout(repo: git.Repo, branch_name: str) -> str:
    # Defense in depth: reject branch names starting with '-' to prevent flag injection,
    # even if a malicious ref with that name exists (e.g. via filesystem manipulation)
    if branch_name.startswith("-"):
        raise BadName(f"Invalid branch name: '{branch_name}' - cannot start with '-'")
    repo.rev_parse(branch_name)  # Validates branch_name is a real git ref, throws BadName if not
    repo.git.checkout(branch_name)
    return f"Switched to branch '{branch_name}'"

View on GitHub (pinned to 76d64c822f)

Solutions

  1. Reject branch names starting with '-' (git also forbids refnames beginning with '-').
  2. Validate the name against git's ref naming rules before calling.

Example fix

# before
git_create_branch(repo, branch_name='-bMalicious')  # -> BadName

# after
if branch_name.startswith('-'):
    raise ValueError('branch_name must not start with -')
git_create_branch(repo, branch_name)
Defensive patterns

Strategy: validation

Validate before calling

def safe_branch(name: str) -> str:
    if not name or name.startswith('-'):
        raise ValueError('branch_name must not start with -')
    return name

Try / catch

from gitdb.exc import BadName
try:
    git_create_branch(repo, branch_name)
except BadName as e:
    if 'cannot start with' in str(e):
        # sanitize name and retry
    raise

Prevention

When it happens

Trigger: Calling git_create_branch with branch_name beginning with '-'; malicious ref-name input.

Common situations: Adversarial input; UI allowing arbitrary branch names.

Related errors

AI-assisted analysis of modelcontextprotocol/servers@76d64c822f (2026-08-12). Data as JSON: /api/errors/562db5bae2c57b39. Report an issue: GitHub.