modelcontextprotocol/servers · error · BadName

Invalid revision: '{revision}' - cannot start with '-'

Error message

Invalid revision: '{revision}' - cannot start with '-'

What it means

git_show() rejects a revision that starts with '-' to prevent flag injection into `git show`/commit lookup. The check fires before repo.commit(revision). Raises gitdb BadName; propagates raw to the MCP client.

Source

Thrown at src/git/src/mcp_server_git/server.py:229

    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}'"



def git_show(repo: git.Repo, revision: str) -> str:
    # Defense in depth: reject revisions starting with '-' to prevent flag injection,
    # even if a malicious ref with that name exists (e.g. via filesystem manipulation)
    if revision.startswith("-"):
        raise BadName(f"Invalid revision: '{revision}' - cannot start with '-'")
    commit = repo.commit(revision)
    output = [
        f"Commit: {commit.hexsha!r}\n"
        f"Author: {commit.author!r}\n"
        f"Date: {commit.authored_datetime!r}\n"
        f"Message: {commit.message!r}\n"
    ]
    if commit.parents:
        parent = commit.parents[0]
        diff = parent.diff(commit, create_patch=True)
    else:
        diff = commit.diff(git.NULL_TREE, create_patch=True)
    for d in diff:
        output.append(f"\n--- {d.a_path}\n+++ {d.b_path}\n")
        if d.diff is None:
            continue
        if isinstance(d.diff, bytes):
            output.append(d.diff.decode('utf-8'))

View on GitHub (pinned to 76d64c822f)

Solutions

1. Reject revisions starting with '-'.
2. Pass a valid commit SHA, tag, or ref.

Example fix

# before
git_show(repo, revision='-sMalicious')  # -> BadName

# after
if revision.startswith('-'):
    raise ValueError('revision must not start with -')
git_show(repo, revision)

def safe_revision(rev: str) -> str:
    if not rev or rev.startswith('-'):
        raise ValueError('revision must not start with -')
    return rev

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

When it happens

Trigger: Calling git_show with a revision beginning with '-'; malicious or malformed revision input.

Common situations: Adversarial input; malformed SHAs.

Related errors

• Invalid target: '{target}' - cannot start with '-'
• Invalid start_timestamp: '{start_timestamp}' - cannot start
• Invalid end_timestamp: '{end_timestamp}' - cannot start with
• Invalid branch name: '{branch_name}' - cannot start with '-'
• Invalid base branch: '{base_branch}' - cannot start with '-'

AI-assisted analysis of modelcontextprotocol/servers@76d64c822f (2026-08-12). Data as JSON: /api/errors/774cdffb3e1921f3. Report an issue: GitHub.