mcp-repl

mcp-repl is an MCP server that gives an agent a persistent Python or R session, kept alive across tool calls. The agent can load data once, inspect objects, try ideas, read help, make plots, and keep iterating — the way a person would in a REPL.

A shell tool running Rscript -e or python -c keeps forcing the agent to rebuild context. mcp-repl keeps the session open instead: variables, loaded packages, plots, and other state stay available until you or the model reset.

Features

• Sandboxed by default. The backend runs in a sandbox enforced by OS primitives at the process level — not command-specific runtime rules. Network is disabled; writes are constrained to workspace roots and the temp paths the active session needs. On Unix, a memory guardrail kills the worker if it exceeds threshold.
• Curated output. Smart echo (omitted when safe, elided for large multi-expression blocks) and in-band help pages. Plots are returned as inline images through MCP for vision-capable models, so the agent sees the plot directly; non-vision models still get the saved file path. When replies get too large, the tool response stays short and the full output is saved as a structured bundle (transcript + plot files) the model can explore on demand.
• No polling. R and Python run embedded in the worker, not behind a stdio pipe driven by prompt-string heuristics. The server knows precisely when the interpreter is idle and has settled, so each repl call returns the moment the work is done — no fixed waits, no guessing whether more output is on the way.
• Explicit session control. Interrupts and resets are first-class.

Quickstart

1. Install

Install via cargo (needs the Rust toolchain):

cargo install --git https://github.com/posit-dev/mcp-repl --locked
# pin a version with: --tag v0.1.0

Or use a prebuilt binary. Linux/macOS:

curl -fsSL https://raw.githubusercontent.com/posit-dev/mcp-repl/main/scripts/install.sh | sh

Windows PowerShell:

irm https://raw.githubusercontent.com/posit-dev/mcp-repl/main/scripts/install.ps1 | iex
Install-McpRepl

Append --dev (or -Dev) for the rolling dev prerelease. Direct downloads live on the latest release page. Linux x86_64 builds require glibc 2.35+; the glibc build produced on Ubuntu 22.04 supports Ubuntu 22.04+.

Latest release binaries:

• Linux x86_64: https://github.com/posit-dev/mcp-repl/releases/latest/download/mcp-repl-x86_64-unknown-linux-gnu.tar.gz
• macOS arm64: https://github.com/posit-dev/mcp-repl/releases/latest/download/mcp-repl-aarch64-apple-darwin.tar.gz
• Windows x86_64: https://github.com/posit-dev/mcp-repl/releases/latest/download/mcp-repl-x86_64-pc-windows-msvc.zip

Download prebuilt dev binaries:

• Linux x86_64: https://github.com/posit-dev/mcp-repl/releases/download/dev/mcp-repl-x86_64-unknown-linux-gnu.tar.gz
• macOS arm64: https://github.com/posit-dev/mcp-repl/releases/download/dev/mcp-repl-aarch64-apple-darwin.tar.gz
• Windows x86_64: https://github.com/posit-dev/mcp-repl/releases/download/dev/mcp-repl-x86_64-pc-windows-msvc.zip

Prebuilt binaries do not bundle R or Python; install those separately.

2. Wire into your MCP client

Auto-install into agent config files:

mcp-repl install                                 # all supported clients/interpreters
mcp-repl install --client codex                  # only Codex
mcp-repl install --client claude                 # only Claude (~/.claude.json)
mcp-repl install --client codex --interpreter r  # limit to one interpreter

By default this writes entries for both r and python.

install --client codex writes --sandbox inherit --oversized-output files — inherit tells mcp-repl to take sandbox policy from Codex's per-call metadata (it fails closed if the metadata is missing or malformed). install --client claude writes an explicit --sandbox workspace-write because Claude doesn't propagate that metadata. Bare mcp-repl (no install) defaults to --oversized-output pager.

Manual Codex entry:

[mcp_servers.r]
command = "/Users/alice/.cargo/bin/mcp-repl"
tool_timeout_sec = 1800   # outer guard; mcp-repl handles the primary timeout
args = ["--sandbox", "inherit", "--oversized-output", "files", "--interpreter", "r"]

Swap --interpreter r for --interpreter python (and rename the section) for the Python entry.

Manual Claude entry in ~/.claude.json:

{
  "mcpServers": {
    "r": {
      "command": "/Users/alice/.cargo/bin/mcp-repl",
      "args": [
        "--sandbox", "workspace-write",
        "--oversized-output", "files",
        "--interpreter", "r"
      ]
    }
  }
}

3. Pick interpreter (optional)

Resolution order: --interpreter <r|python> → MCP_REPL_INTERPRETER → r.

Runtime discovery

R. Set R_HOME to force a specific installation; otherwise it's discovered from R on PATH (via R RHOME). Verify with R.home() in the session.

Python. The interpreter resolves in this order:

• nearest .venv/bin/python walking upward from cwd
• nearest .venv/bin/python3 walking upward from cwd
• first python3 on PATH
• first python on PATH
• fallback literal python3

.venv search stops at $HOME (inclusive), otherwise at the filesystem root. The selected Python must expose a loadable CPython library via its sysconfig metadata. Runtime-owned stdout/stderr is routed through worker IPC; raw fd writes and child-process output are still captured from the worker's stdout/stderr pipes.

Platform support

• macOS: supported.
• Linux: supported. Dev binaries are glibc builds produced on Ubuntu 22.04.
• Windows: experimental for R. Python is not part of the stable Windows surface yet.

Sandbox

Default policy: workspace-write with network disabled. Write access covers the working area plus worker-required temp paths (exact roots vary by OS/policy). On Windows, the experimental R sandbox uses parent-prepared workspace ACLs plus launch-scoped session-temp ACLs; some environments reject the restricted-token setup.

See docs/sandbox.md for precise behavior.

MCP surface

• repl → { "input": "1+1\n", "timeout_ms": 10000 }
• repl_reset → {}

The exact repl tool description depends on the interpreter and --oversized-output mode. Per-tool guides live in docs/tool-descriptions/.

Session control

• Interrupt: prefix repl input with \u0003 (SIGINT, best-effort). Session continues.
• Reset: call repl_reset, or prefix input with \u0004 (Ctrl-D); any remaining input runs in the fresh session. Reset attempts graceful shutdown, escalates to forceful termination, and on Unix falls back to scanning descendant processes if process-group signaling is unavailable.
• In-band exits: EOF, quit(), etc. also work — output is returned and the next request runs in a fresh worker.

Debugging

Enable JSONL logs per startup:

• CLI: --debug-dir /path/to/debug-root
• Env: MCP_REPL_DEBUG_DIR=/path/to/debug-root

Each startup writes a session directory with events.jsonl, startup logs, and sandbox-state logs. See docs/debugging.md for the full guide, including the external wire-trace proxy.

Docs

• Engineering map: docs/index.md
• Sandbox: docs/sandbox.md
• Worker sideband protocol: docs/worker_sideband_protocol.md
• Tool guides: docs/tool-descriptions/

License

Apache-2.0. See LICENSE.