mcp-repo-tools

An MCP server that gives AI agents safe, structured access to a local git repository. It is built for code review: read files at any commit, search, diff a branch the way a pull request does, and (only if you opt in) run allowlisted test commands.

It's the foundation of a three-part series on production AI engineering in the code-review domain:

read_file · search_code · list_changed_files · get_diff · run_tests (opt-in)

Tools

Every tool validates its input with Zod and returns structuredContent that matches a published output schema, plus a compact text rendering for chat clients. Refs are resolved to full SHAs and echoed back, so a caller always knows exactly which commit it saw.

mode defaults to merge-base (what a PR shows, i.e. base...head); direct compares the two commits as-is (base..head).

Errors come back as MCP tool errors (isError: true) with text CODE: message, where CODE is one of INVALID_INPUT, PATH_DENIED, NOT_FOUND, TOO_LARGE, INVALID_REF, TOOL_DISABLED, TIMEOUT, GIT_ERROR or INTERNAL. Inputs that fail the JSON schema itself are rejected by the SDK before reaching a tool, with an Input validation error message.

Related MCP server: Git Code Review MCP

Architecture

flowchart LR
  subgraph Clients
    CC[Claude Code / Desktop]
    AG[pr-review-agent]
  end

  CC -- stdio --> T1[StdioServerTransport]
  AG -- Streamable HTTP --> T2["HTTP /mcp<br/>Host/Origin check · bearer token"]

  T1 --> S[McpServer<br/>Zod in/out schemas]
  T2 --> S

  S --> RF[read_file]
  S --> SC[search_code]
  S --> LC[list_changed_files]
  S --> GD[get_diff]
  S -. "--allow-run-tests" .-> RT[run_tests]

  subgraph Policy["Policy layer (src/lib)"]
    P[RepoPaths<br/>containment · allow/deny globs]
    G[Git<br/>hardened · refs → SHAs]
    X[runProcess<br/>no shell · timeout · output caps · scrubbed env]
  end

  RF --> P
  RF --> G
  SC --> G
  SC --> P
  LC --> G
  GD --> G
  GD --> P
  RT --> X
  G --> X

  X --> R[(git repository)]
  P --> R

Tools are plain definitions (inputSchema, outputSchema, run, render) in src/tools/, so they're unit-tested without a server. All enforcement lives in src/lib/: every filesystem read goes through RepoPaths, every subprocess through runProcess.

Quick start

Requires Node ≥ 22.12, git ≥ 2.44 and pnpm. Linux or macOS.

git clone https://github.com/pshashanka/mcp-repo-tools && cd mcp-repo-tools
pnpm install && pnpm build
node dist/index.js --repo /path/to/your/repo   # speaks MCP on stdio

Claude Code

claude mcp add repo-tools -- node /abs/path/to/mcp-repo-tools/dist/index.js --repo /abs/path/to/repo

Or add a project-scoped .mcp.json to the repo you want reviewed (use absolute paths):

{
  "mcpServers": {
    "repo-tools": {
      "command": "node",
      "args": ["/abs/path/to/mcp-repo-tools/dist/index.js", "--repo", "/abs/path/to/repo"]
    }
  }
}

Then ask, for example: "Review the changes on this branch against main. Start with list_changed_files."

Claude Desktop

Add to ~/Library/Application Support/Claude/claude_desktop_config.json (macOS) or %APPDATA%\Claude\claude_desktop_config.json, then restart Claude Desktop:

{
  "mcpServers": {
    "repo-tools": {
      "command": "node",
      "args": ["/abs/path/to/mcp-repo-tools/dist/index.js", "--repo", "/abs/path/to/repo"]
    }
  }
}

Over HTTP (for agents and services)

MCP_REPO_TOOLS_TOKEN=$(openssl rand -hex 32) \
  node dist/index.js --repo /path/to/repo --transport http --port 3333
# → serving /path/to/repo at http://127.0.0.1:3333/mcp

claude mcp add --transport http repo-tools http://127.0.0.1:3333/mcp \
  --header "Authorization: Bearer $MCP_REPO_TOOLS_TOKEN"

Binding beyond loopback (e.g. --host 0.0.0.0) requires MCP_REPO_TOOLS_TOKEN and at least one --allowed-host <name> for every hostname clients will send in their Host header.

Enabling run_tests

run_tests needs both the --allow-run-tests flag and targets in a config file you control:

node dist/index.js --repo /path/to/repo --config ./mcp-repo-tools.example.json --allow-run-tests

See mcp-repo-tools.example.json for the config format. Every key is optional.

CLI reference

--repo <path>        Repository to serve: the work tree root (default: current directory)
--config <file>      JSON config file (path policy, limits, test targets)
--allow-run-tests    Register the run_tests tool (needs targets in --config)
--transport <kind>   stdio (default) or http
--host <host>        HTTP bind address (default: 127.0.0.1)
--port <port>        HTTP port (default: 3333)
--allowed-host <name> Extra hostname accepted in Host/Origin (repeatable)
MCP_REPO_TOOLS_TOKEN If set, HTTP requests must send "Authorization: Bearer <token>"

Security model

The threat model has two untrusted parties: the repository (it may be a stranger's pull request) and the model (it reads that repository, so it can be prompt-injected by it). The server enforces its policy whatever the model asks for. Full details are in SECURITY.md; in summary:

• Read-only by default. Four of the five tools only read. run_tests isn't even registered unless the operator passes --allow-run-tests and configures targets.

• Path containment. Paths must be repo-relative; .., absolute paths and NUL bytes are rejected. Symlinks are followed and the real target is checked again, so a link can't reach outside the repo or reach a denied file.

• Deny list. .git/ (including nested .git directories), .env*, private keys, .npmrc/.netrc, .envrc, .git-credentials, .aws/ and the like are never readable or searchable, in the working tree or at any commit. Matching is case-insensitive. Config can add to the deny list but can't remove the defaults. An optional allow list narrows things further.

• Hardened git. No shell, ever: arguments are passed as an array. Refs that look like options are rejected and refs are resolved to SHAs once. Pathspecs are literal unless a glob is explicitly wanted. System/global git config is ignored; fsmonitor, hooks, external diff and textconv are disabled (a hostile .git/config can name programs for git to run). No optional locks, no lazy network fetches.

• Bounded everything. Per-tool byte limits that a request can lower but never raise; git calls are killed once their output passes the cap; timeouts on every subprocess.

• run_tests runs configured commands, not requested ones. The client chooses a target name from an enum; extra args must fully match an operator-supplied regex. The process gets a scrubbed environment (no tokens or cloud credentials), its own process group (a timeout kills grandchildren too) and a hard timeout. It is not a container: it runs the repository's code with your user's permissions. For untrusted code, run the server inside a container or VM.

• HTTP. Binds to loopback; refuses other addresses without a token; constant-time token comparison; Host/Origin validation against DNS rebinding.

• Config never comes from the served repo, so a PR can't widen its own permissions.

Design decisions

git is the source of truth. search_code is git grep and read_file --ref is git cat-file. That means no ripgrep dependency, .gitignore is respected without extra work, and every tool works at any commit, which matters when an agent needs the old version of a file to judge a change.

Refs in, SHAs out. A ref is validated and resolved once, and only SHAs reach later git commands. Outputs echo the resolved SHAs (base, head, mergeBase, ref), so pr-review-agent can checkpoint and resume against exactly the commits it reviewed even if the branch moves.

Pull-request semantics by default. get_diff and list_changed_files diff from the merge base, because a reviewer wants "what this branch changed", not "how it differs from main today".

A cheap index, then content. list_changed_files (with isTest and line counts) lets an agent plan before it spends tokens on patches. It's also how a test-coverage check asks "did tests change alongside the source?" without parsing diffs.

Budgets degrade per file, not mid-hunk. get_diff runs git once per file (bounded concurrency), which maps patches to files exactly even for odd filenames. It fills a byte budget with whole patches. Files that don't fit, are binary, or are denied stay in the list with patchOmitted set to the reason, so the agent can ask for them specifically. A half-truncated hunk is worse than none.

Names are metadata; contents are policy. A changed .env still appears in list_changed_files, because "this PR touches secrets" is exactly what a reviewer should flag. Its contents are never returned.

Structured output and readable text. Agents consume structuredContent; chat clients see a grep-style or unified-diff rendering rather than escaped JSON. Error codes go in the text (CODE: message) and not in structuredContent, because MCP clients validate structuredContent against the success schema even on errors.

A failing test run is a result, not an error. run_tests returns passed: false with the output. Only "couldn't run" is an error. Output keeps the tail, where failure summaries are. gameable-tests-bench relies on this stable shape to score runs.

One repo per server, stateless HTTP. There's no repo parameter, which keeps the containment model trivial; run one server per repo. HTTP creates a fresh server per request (the SDK's stateless pattern), so there is no session state to leak between callers.

Tools are data. defineTool({ inputSchema, outputSchema, run, render }) keeps the MCP wiring in one small file (src/server.ts), and every tool is tested by calling run directly against a real fixture repo. The fixture is built by a script at test time, so this repo contains no nested .git.

Known limitations

• POSIX only (process groups, /dev/null for git config).

• run_tests runs in the current working tree. Running at an arbitrary ref (in a temporary git worktree) is a planned opt-in.

• search_code covers tracked files only, and its regexes are POSIX ERE, not PCRE.

• TypeScript is pinned to 6.0 until typescript-eslint supports the 7.x native compiler.

Development

pnpm install
pnpm test        # Vitest: unit tests per tool and lib + MCP client integration tests
pnpm lint        # typescript-eslint, strict type-checked
pnpm typecheck
pnpm build

src/
  index.ts            CLI: flags → config → transport
  server.ts           createServer(config): registers tools on an McpServer
  config.ts           Zod config schema and defaults
  tools/              one file per tool, plus the defineTool type
  lib/                paths, git, exec, changes, truncate, errors: all the enforcement
  transports/         stdio, http
test/
  fixtures/           builds a PR-shaped git repo in a temp dir
  unit/               per tool and per lib module
  integration/        SDK Client ↔ server over in-memory, stdio and HTTP

CI (GitHub Actions) runs lint, format check, typecheck, tests and build on Node 22 and 24.

License

MIT