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# hledit-mcp
`hledit-mcp` is an MCP server exposing [`hledit`](https://github.com/dabito/hledit)'s hash-anchored file edits to standard MCP clients such as Claude Code, Claude Desktop, and Cursor.
Same idea as [`pi-hledit`](https://github.com/dabito/pi-hledit) (the Pi-native integration), but over MCP instead of Pi's extension API, so the same edit primitive can be used from MCP-compatible clients.
Official MCP Registry: [`io.github.dabito/hledit-mcp`](https://registry.modelcontextprotocol.io/?search=hledit).
Instead of asking an agent to reproduce old text exactly, `hledit read` annotates each line with a stable anchor:
```text
5#K7Q:func main() {
6#M9A: fmt.Println("hello")
7#R2C:}
```
Write commands reference anchors such as `6#M9A`. Before changing the file, `hledit` recomputes the hash at that line. If the file changed since it was read, the anchor is rejected and no write happens — the agent gets a `stale` error and a remap hint instead of silently corrupting the wrong line.
## Demo
See [`docs/demo/transcript.md`](docs/demo/transcript.md) for a deterministic MCP stdio transcript showing stale-write rejection over the actual `hledit-mcp` tool surface.
The transcript is generated by [`docs/demo/hledit-mcp-demo.mjs`](docs/demo/hledit-mcp-demo.mjs), which drives `dist/index.js` through `@modelcontextprotocol/sdk`'s stdio client. It does not call `hledit` directly.
## MCP server for stale-write-safe file editing
`hledit-mcp` gives MCP-compatible coding agents one file-editing tool that reads line anchors, validates those anchors against current file contents, and rejects stale writes before touching disk. It is for local workspace edits where you want explicit stale-context failure instead of blind text replacement.
## Why MCP, separately from pi-hledit
`pi-hledit` and `hledit-mcp` share the same tool contract (`core.ts` in this repo — arg-building, batch translation, result formatting) and the same underlying `hledit` CLI. Only the registration/execution glue differs: `pi-hledit` wires that contract into Pi's `registerTool`, this wires it into `@modelcontextprotocol/sdk`'s `McpServer`. MCP has no equivalent of Pi's `renderCall`/`renderResult` terminal rendering, so this package doesn't have one either — that layer is genuinely Pi-specific chrome, not part of the portable tool contract.
Use `hledit-mcp` when you want stale-write rejection over an MCP tool boundary instead of relying on a host's native text replacement behavior.
## Requirements
- [Go 1.21+](https://go.dev/) to install the `hledit` CLI (or a prebuilt binary on `PATH`)
- Node.js 18+
- An MCP-compatible client
## Install
Install the `hledit` CLI first:
```bash
go install github.com/dabito/hledit@latest
```
Then configure your MCP client to run `hledit-mcp`. For Claude Code, verify the current CLI syntax with `claude mcp add --help`; recent versions use:
```bash
claude mcp add hledit -- npx -y hledit-mcp
```
Claude Desktop and other MCP clients can use the same stdio server shape:
```json
{
"mcpServers": {
"hledit": {
"command": "npx",
"args": ["-y", "hledit-mcp"],
"env": {
"HLEDIT_CWD": "/path/to/workspace"
}
}
}
}
```
Set `HLEDIT_CWD` to the workspace the MCP client should allow `hledit` to edit.
### Configuration
| Variable | Default | Description |
| ------------ | --------------------- | ------------------------------------------------------------------- |
| `HLEDIT_BIN` | `hledit` (on `PATH`) | Path to the `hledit` binary, if not on `PATH`. |
| `HLEDIT_CWD` | server's `process.cwd()` | Working directory `hledit` resolves relative paths against. Keep this scoped to the workspace you expect the MCP client to edit. |
## Tool
### `hledit`
One tool, three operations, matching `pi-hledit`'s contract exactly:
| `op` | Purpose |
| ------- | --------------------------------------------------- |
| `edit` | Apply a single replace/insert/delete/replace-range |
| `batch` | Apply multiple anchor-referenced edits in one call |
| Name | Type | Required | Description |
| ------------ | ------- | ------------------ | ---------------------------------------------------------------------- |
| `op` | string | ✓ | `"read"`, `"edit"`, or `"batch"` |
| `path` | string | ✓ | File path |
| `offset` | number | | 1-indexed starting line (`read`) |
| `limit` | number | | Max lines to return (`read`); defaults to `2000` |
| `grep` | string | | Filter lines by substring (`read`) |
| `action` | string | | `replace`, `insert`, `delete`, or `replace-range` (`edit`) |
| `end_anchor` | string | | End anchor for `replace-range`/range delete |
| `content` | string | | Replacement/inserted content; empty = delete |
| `after` | boolean | | For `action:"insert"`, insert after the anchor |
| `edits` | string | for `batch` | JSON array of batch edit ops |
Workflow: `read` to get anchors → `edit` (single change) or `batch` (multiple). If an edit returns `stale`, re-read to get fresh anchors before retrying — the anchor's line moved or changed since it was read.
Default `read` calls are bounded: when `offset`/`limit` are omitted, the server uses `offset=1` and `limit=2000`.
## Development
```bash
npm install
npm test # typecheck + build + unit/e2e tests + lint
npm run build # compile index.ts+core.ts to dist/index.js
npm start # run the server directly from source via tsx (stdio transport)
```
Unit tests in `core.test.ts` cover the same contract as `pi-hledit`'s test suite, minus the Pi-specific rendering assertions (there's no render layer here). `e2e.test.ts` drives the built `dist/index.js` over a real MCP stdio handshake with `@modelcontextprotocol/sdk`'s own `Client`/`StdioClientTransport` — the same artifact `npx hledit-mcp` runs, not just the TypeScript source.
The published `bin`/`main` point at `dist/index.js`, built with esbuild (`build.mjs`) and run via plain `node` — this keeps the `>=18` Node requirement honest. Running `index.ts` directly with `node` (no tsx) only works on Node ≥22.6, which has built-in TypeScript type-stripping; older LTS versions have no such support at all.
## Credits and prior art
The hashline-edit idea comes from Can Bölük / @can1357's coding-agent harness work, especially [“I Improved 15 LLMs at Coding in One Afternoon. Only the Harness Changed.”](https://blog.can.ac/2026/02/12/the-harness-problem/) and [`oh-my-pi`](https://github.com/can1357/oh-my-pi). See [`hledit`'s credits](https://github.com/dabito/hledit#credits-and-prior-art) for more prior art.
## Related packages
- [`hledit`](https://github.com/dabito/hledit) — the standalone CLI both integrations wrap.
- [`pi-hledit`](https://github.com/dabito/pi-hledit) — the Pi-native integration, same tool contract.
