fix(git): create a branch from a detached HEAD

src/git/src/mcp_server_git/server.py

@@ -188,11 +188,17 @@ def git_create_branch(repo: git.Repo, branch_name: str, base_branch: str | None
         raise BadName(f"Invalid base branch: '{base_branch}' - cannot start with '-'")
     if base_branch:
         base = repo.references[base_branch]
+        base_name = base.name
+    elif repo.head.is_detached:
+        # No active branch to start from; branch from the current commit.
+        base = repo.head.commit.hexsha
+        base_name = base
     else:
         base = repo.active_branch
+        base_name = base.name
 
     repo.create_head(branch_name, base)
-    return f"Created branch '{branch_name}' from '{base.name}'"
+    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,

src/git/tests/test_server.py

@@ -255,6 +255,17 @@ def test_git_create_branch_from_base(test_repository):
 
     assert "Created branch 'derived-branch' from 'base-branch'" in result
 
+def test_git_create_branch_detached_head(test_repository):
+    # `git switch -c <name>` from a detached HEAD branches from the current commit.
+    detached_sha = test_repository.head.commit.hexsha
+    test_repository.git.checkout(detached_sha)
+    assert test_repository.head.is_detached
+
+    result = git_create_branch(test_repository, "rescued-branch")
+
+    assert "Created branch 'rescued-branch'" in result
+    assert test_repository.heads["rescued-branch"].commit.hexsha == detached_sha
+
 def test_git_show(test_repository):
     file_path = Path(test_repository.working_dir) / "show_test.txt"
     file_path.write_text("show content")