@flowerrealm/realmx-shell-tool-mcp

Note: This MCP server is still experimental. When using it with Realmx CLI, ensure the CLI version matches the MCP server version.

@flowerrealm/realmx-shell-tool-mcp is an MCP server that provides a tool named shell that runs a shell command inside a sandboxed instance of Bash. This special instance of Bash intercepts requests to spawn new processes (specifically, execve(2) calls). For each call, it makes a request back to the MCP server to determine whether to allow the proposed command to execute. It also has the option of escalating the command to run unprivileged outside of the sandbox governing the Bash process.

The user can use Realmx .rules files to define how a command should be handled. The action to take is determined by the decision parameter of a matching rule as follows:

• allow: the command will be escalated and run outside the sandbox
• prompt: the command will be subject to human approval via an MCP elicitation (it will run escalated if approved)
• forbidden: the command will fail with exit code 1 and an error message will be written to stderr

Commands that do not match an explicit rule in .rules will be allowed to run as-is, though they will still be subject to the sandbox applied to the parent Bash process.

Motivation

When a software agent asks if it is safe to run a command like ls, without more context, it is unclear whether it will result in executing /bin/ls. Consider:

• There could be another executable named ls that appears before /bin/ls on the $PATH.
• ls could be mapped to a shell alias or function.

Because @flowerrealm/realmx-shell-tool-mcp intercepts execve(2) calls directly, it always knows the full path to the program being executed. In turn, this makes it possible to provide stronger guarantees on how Realmx .rules are enforced.

Usage

First, verify that you can download and run the MCP executable:

npx -y @flowerrealm/realmx-shell-tool-mcp --version

To test out the MCP with a one-off invocation of Realmx CLI, it is important to disable the default shell tool in addition to enabling the MCP so Realmx has exactly one shell-like tool available to it:

realmx --disable shell_tool \
  --config 'mcp_servers.bash={command = "npx", args = ["-y", "@flowerrealm/realmx-shell-tool-mcp"]}'

To configure this permanently so you can use the MCP while running realmx without additional command-line flags, add the following to your ~/.codex/config.toml. The legacy codex command still works:

[features]
shell_tool = false

[mcp_servers.shell-tool]
command = "npx"
args = ["-y", "@flowerrealm/realmx-shell-tool-mcp"]

Note when the @flowerrealm/realmx-shell-tool-mcp launcher runs, it selects the appropriate native binary to run based on the host OS/architecture. For the Bash wrapper, it inspects /etc/os-release on Linux or the Darwin major version on macOS to try to find the best match it has available. See bashSelection.ts for details.

MCP Client Requirements

This MCP server is designed to be used with Realmx, as it declares the following capability that Realmx supports when acting as an MCP client:

{
  "capabilities": {
    "experimental": {
      "codex/sandbox-state": {
        "version": "1.0.0"
      }
    }
  }
}

This capability means the MCP server honors requests like the following to update the sandbox policy the MCP server uses when spawning Bash:

{
  "id": "req-42",
  "method": "codex/sandbox-state/update",
  "params": {
    "sandboxPolicy": {
      "type": "workspace-write",
      "writable_roots": ["/home/user/code/codex"],
      "network_access": false,
      "exclude_tmpdir_env_var": false,
      "exclude_slash_tmp": false
    }
  }
}

Once the server has processed the update, it sends an empty response to acknowledge the request:

{
  "id": "req-42",
  "result": {}
}

The Realmx harness (used by the CLI and the VS Code extension) sends such requests to MCP servers that declare the codex/sandbox-state capability.

Package Contents

This package currently publishes shell binaries only. It bundles:

• A patched Bash that honors EXEC_WRAPPER, built for multiple glibc baselines (Ubuntu 24.04/22.04/20.04, Debian 12/11, CentOS-like 9) and macOS (15/14/13).
• A patched zsh with EXEC_WRAPPER support for the same supported target triples.

It does not currently include the Rust MCP server binaries.

See the README in this repo for details.