Replaces the broken write_file placeholder (no real implementation, only
_FakeTool stubs in cli/benchmark.py) with a structured editor offering four
commands: create, str_replace, insert_at_line, view.
Security model (file-system analog of the 6-layer terminal security paradigm,
reject-by-default + prefix match):
1. Reject absolute paths (force relative interpretation vs workspace root).
2. Reject any .. path component (path traversal).
3. Path.resolve() follows symlinks, then relative_to(workspace_root)
rejects symlink escape and residual traversal.
Data-loss guard: create refuses to overwrite existing files. str_replace
requires a unique anchor (0 or >1 matches error). insert_at_line is 1-based
(0 = prepend, > EOF = append). All FS I/O wrapped in asyncio.to_thread.
Registers str_replace_editor in _DEFAULT_CORE_TOOLS (replacing write_file)
so its full description is always injected into the LLM prompt. Updates
test_tool_search.py which used write_file as a sample core tool.
Tests: 34 cases in test_str_replace_editor.py cover happy path, edge cases
(empty file, multi-match, insert at 0/beyond EOF, view range), error paths
(overwrite refusal, anchor not found, path traversal, absolute path, symlink
escape, unknown command, missing args), and integration contract (in
_DEFAULT_CORE_TOOLS, exported from agentkit.tools, schema enum, prompt
injection via _build_tool_use_prompt).
Verification: ruff check clean; targeted regression suite 412 passed
(the single failure in test_calendar_tool.py is a pre-existing date-sensitive
bug in an untouched file, today 2026-07-03 Friday makes the next-Wednesday
assertion fail).
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