MCP::Server::Tool::Shell

A command runner for MCP::Server.

Six tools — run, start, poll, output, kill and allowed — let an LLM invoke local programs. Which programs is your decision: either an exact allow list, or allow-any for deployments that ask a human instead. Arguments are handed to the program as an argv vector (there is no shell anywhere in the code path), output is captured into capped buffers so a runaway command cannot exhaust the server's memory, foreground runs are bounded by a timeout, and work that will not fit in one tool call can be started as a background job and polled.

Synopsis

Plug it into a server you are already building:

use MCP::Server;
use MCP::Server::Tool::Shell;

my $server = MCP::Server.new(:name<dev-tools>, :version<1.0>);

$server.plug: MCP::Server::Tool::Shell.new(
    allow   => <git rg /usr/bin/jq>,   # exact argv0 strings
    cwd     => '/home/me/project',     # optional default working directory
    timeout => 15,                     # seconds, default 30
);

$server.run;   # registers sh_run, sh_start, sh_poll, sh_output, sh_kill, sh_allowed

Or assemble a server out of toolkit packs in one shot, with no glue code:

use MCP::Server;

MCP::Server.new(
    :name<dev-tools>,
    :tools[
        'Shell' => { allow => <git rg>, cwd => '/srv/app', prefix => 'git' },
    ],
).run;   # registers git_run, git_start, ...

Or run it straight off the raku-mcp command line that ships with MCP::Server:

raku-mcp --tool=Shell='{"allow":["git","rg"],"timeout":10}'

raku-mcp --config=mcp.json

{
  "name": "dev-tools",
  "instructions": "Read-only git and ripgrep access to the checkout.",
  "tools": {
    "Shell#git": { "allow": ["git"], "cwd": "/srv/app", "prefix": "git" },
    "Shell#search": { "allow": ["rg"], "cwd": "/srv/app", "prefix": "rg", "timeout": 10 }
  }
}

The #alias suffix lets the same pack be loaded twice with different settings — here one instance can only run git and the other only rg, each under its own prefix.

Wiring that into Claude Code:

{
  "mcpServers": {
    "dev-tools": {
      "command": "raku-mcp",
      "args": ["--config=/home/me/mcp.json"]
    }
  }
}

Configuration

• allow must contain at least one non-empty string unless allow-any is set. Entries are matched with eq — see Security.

• allow-any and allow are mutually exclusive, and one of them is required: a runner that can run everything and a runner that can run nothing are both configurations somebody meant, but "I forgot to say" is not. See [Running anything: allow-any and MCP::Client::Policy](#Running anything: allow-any and MCP::Client::Policy).

• cwd must be an existing directory at construction time; the constructor dies otherwise. A per-call cwd argument overrides it.

• timeout must be greater than zero. Fractional values are fine (0.5 is half a second). A per-call timeout argument overrides it for one run; start takes one too, but has no default.

• max-output-bytes (floor 1024) and max-output-lines (floor 10) apply per stream, so a run may hold twice them at most. See Truncation.

• max-jobs, max-finished-jobs (both at least 1) and job-ttl (greater than zero) govern background jobs. See [Background jobs](#Background jobs).

• prefix is not a setting of this pack: it is reserved by the toolkit system. Pass it to .plug($kit, :prefix<git>), or beside the entry in :tools[...] / the JSON config, to namespace the tool names.

Every setting is validated when the toolkit is constructed, so a typo or a missing directory fails at startup rather than on the first tool call:

MCP::Server::Tool::Shell.new(allow => []);
# Shell toolkit requires a non-empty allow list (or allow-any => True when
# consent is enforced elsewhere, e.g. by an MCP::Client::Policy)

MCP::Server::Tool::Shell.new(allow => <git>, allow-any => True);
# Shell toolkit allow and allow-any are mutually exclusive: ...

MCP::Server::Tool::Shell.new(allow => <git>, cwd => '/nope');
# Shell toolkit cwd is not an existing directory: /nope

MCP::Server::Tool::Shell.from-config({ allow => <git>, timeut => 5 });
# Unknown config key(s) for MCP::Server::Tool::Shell: timeut.
# Valid keys: allow, allow-any, cwd, job-ttl, max-finished-jobs, max-jobs,
# max-output-bytes, max-output-lines, timeout

The default prefix is sh, so the tools below are registered as sh_run, sh_start, sh_poll, sh_output, sh_kill and sh_allowed unless you pass a prefix of your own.

Tools

run

Runs one command in the foreground and waits for it.

The result is a JSON object as text content:

{"exit":0,"stderr":"","stdout":"main\n","timed-out":false}

exit reports:

• the command's own exit status for a normal exit (0 on success);

• 128 + signal when the command was terminated by a signal, following the POSIX shell convention — a timeout kill on Unix therefore shows up as 137 (128 + SIGKILL), while on Windows TerminateProcess yields 1;

• -1 when the command could not be spawned at all (not on PATH, not executable, and so on). The reason is appended to stderr;

• null in the one case where there is no honest answer: the command was killed but its status never arrived (a grandchild is still holding the pipes, say), so the run is reported with timed-out: true and an unknown status.

A missing binary is a runtime condition, not a caller mistake, so it is reported in the payload:

{"exit":-1,"stderr":"Failed to spawn process nosuchtool: no such file or directory (error code -2)","stdout":"","timed-out":false}

Anything the caller got wrong — a command that is not allowlisted, an args value that is not an array of strings, a cwd that does not exist, a timeout that is not a positive number — is thrown instead, which MCP::Server turns into an isError tool result:

Command 'rm' is not allowlisted. Allowed: git, rg

Examples of the call shape (arguments as an MCP client would send them):

{ "command": "git", "args": ["rev-parse", "--abbrev-ref", "HEAD"] }
{ "command": "rg", "args": ["--json", "TODO|FIXME", "lib"], "cwd": "/srv/app" }
{ "command": "git", "args": ["log", "--grep", "fix the thing"] }
{ "command": "jq", "args": ["-r", ".name"], "stdin": "{\"name\":\"widget\"}" }

That third one is the point of an argv vector: fix the thing is a single argument, quoted by nobody, split by nobody.

Standard input

stdin is written to the command and the pipe is then closed, so a filter sees a normal end of input. Three states, and they are all different:

• omitted — standard input is left unconnected, exactly as it was before this feature existed. Nothing is written to the command and it is not handed an end of file we invented for it.

• empty string — the pipe is opened and closed immediately, so the command reads an immediate EOF. This is what you want for a filter you have nothing to say to.

• a string — written and then closed.

The write is detached from the run: a command that never reads its input cannot stall the server, however much you send it, and a command that exits before reading everything just makes the write fail (EPIPE), which is a runtime condition rather than an error. NUL bytes are fine here, unlike in args: this is a byte stream, not a C string.

Truncation

Output is captured into a bounded buffer per stream, so a command that decides to print a gigabyte does not make the server hold a gigabyte. Each buffer keeps a head — the first quarter of the budget, which is where a compiler puts the error that matters — and a rolling tail — the remaining three quarters, which is where a test runner puts the summary. What falls between them is dropped, and the payload says so twice: in the text, and in a report.

{
  "exit": 0,
  "stdout": "001 ...\n002 ...\n[... 7176 bytes / 175 lines dropped ...]\n199 ...\n",
  "stderr": "",
  "timed-out": false,
  "truncated": { "stdout": { "bytes": 7176, "lines": 175 } }
}

• The truncated key is only present when something was actually dropped, and only names the streams that dropped something. A run inside its budget has exactly the four keys it has always had, byte for byte.

• The numbers in the marker and the numbers in truncated are the same numbers: both come out of one atomic read of the buffer.

• Cuts land on character boundaries, so a multi-byte character is never sliced in half.

• max-output-lines is enforced alongside max-output-bytes; whichever runs out first decides.

The buffer is MCP::Server::Tool::Shell::Buffer and is usable on its own if you want the same behaviour elsewhere.

Streaming a run in progress

A 2026-07-28 (modern era) request that opts into logging — _meta io.modelcontextprotocol/logLevel of info or lower — gets the command's output as notifications/message on that request's own channel while the command is still running:

{
  "jsonrpc": "2.0",
  "method": "notifications/message",
  "params": {
    "level": "info",
    "logger": "sh_run",
    "data": { "stream": "stdout", "text": "compiling lib/Thing.rakumod\n" }
  }
}

• logger is the prefixed tool name, so a host that plugged several instances can tell them apart.

• Batches are flushed at least every 0.25s and are never larger than 32 KiB; a bigger burst becomes several notifications, in order.

• For a run that finished on its own, concatenating every stdout batch gives exactly the stdout of the final payload (when nothing was truncated): the last flush happens once the command's streams have closed. A run that was killed may have written a little more than it managed to stream.

• Nothing is streamed for a request that did not opt in, and nothing at all is streamed on the legacy era, where notifications travel on one channel shared by the whole session and a run's output would surface inside somebody else's request. Use a background job for progress on legacy clients.

If the caller hangs up mid-run (an HTTP client closing its response stream), the run notices within 0.25s, kills the command and unwinds. The answer it produces is a dead letter nobody reads, and is not marked timed-out: the budget was never reached.

Background jobs

run has to answer inside one tool call, which puts a ceiling on what it can be asked to do. start escapes that ceiling: it spawns the command, returns a handle, and answers immediately.

{ "command": "make", "args": ["-j8", "test"] }
→ {"job":"j1-1x9k2w","state":"running"}

start

Takes exactly the same arguments as run (command, args, cwd, stdin, timeout) and returns { job, state } . Its timeout is optional and has no default: escaping the foreground budget is the point. Starting a job when max-jobs are already running is a caller error, and the message names the jobs in the way.

poll

Takes job and reports how it is getting on, without returning the output:

{"exit":null,"job":"j1-1x9k2w","killed":false,"runtime":12.004,
 "started-at":"2026-08-09T09:15:03Z","state":"running",
 "stderr-bytes":0,"stdout-bytes":81920,"timed-out":false}

state is running, exited or killed; exit is null until there is a status; stdout-bytes and stderr-bytes count everything the command has produced, including whatever the buffers have since dropped.

output

Takes job and an optional cursor, and returns { job, state, stdout, stderr, cursor, eof } plus truncated when something was dropped.

• Without a cursor you get a capped snapshot of the whole run so far — head, marker, tail — exactly as run would have rendered it.

• With the cursor from a previous call you get only what has been written since, so a long job can be followed a piece at a time. Concatenating the instalments reconstructs the output, with a [... N bytes / M lines dropped ...] marker wherever the reader fell so far behind that the buffer rolled past it.

• The cursor is opaque: it carries a position in both streams, in bytes and in lines. Pass it back exactly as it was given; anything else is a caller error.

• eof is true once the job has finished and you have read to the end.

kill

Takes job, kills it with SIGKILL and returns { job, state, killed } immediately — collecting the status is the watcher's business, so poll for the final state. Killing a job that has already finished changes nothing and is not an error.

Job lifetime

• At most max-jobs run at once.

• A finished job stays pollable for job-ttl seconds, and only the max-finished-jobs most recently finished are kept. After that its handle is refused with a message saying why — start a new job rather than hoping.

• A job does not observe the cancellation of the request that started it: that request was answered the moment the handle was minted. Stopping a job is explicit, or its own timeout.

• Jobs die with the server process. Nothing here survives a restart.

• Job ids (j3-1x9k2w) are handles, not secrets: anyone who can call these tools can already start jobs of their own.

Push notifications, honestly

While a job runs, its output and lifecycle events are also pushed as notifications/message — output batches at debug (so a legacy client can silence them with logging/setLevel) and started / exited / killed lines at info, all labelled with the prefixed start tool name. Whether they arrive depends on the transport, and this is worth knowing before you build a UI on it:

So: poll and output are the reliable way to follow a job, on every transport. Push is a bonus where the transport happens to support it, never a substitute for polling — and a push channel that refuses or fails never affects the job itself, whose output is safe in its buffers either way.

allowed

Takes no arguments and returns the allow list, sorted, one entry per line:

git
rg
/usr/bin/jq

Give the LLM this tool and it can discover what it may run instead of guessing and collecting errors. In allow-any mode it says so in one line instead.

Security

This pack is a gate, not a sandbox. What it guarantees:

• Exact-string allowlist (unless you turn it off). The command argument must be eq one of the allow entries. There is no globbing, no basename matching, no PATH rewriting and no path normalisation: if you allow /usr/bin/jq then jq is refused, and if you allow git then /usr/bin/git is refused. Prefer absolute paths when you care which binary runs; prefer bare names when you want PATH resolution (including .exe resolution on Windows).

• No shell, ever. Commands are spawned with Proc::Async, which execs the program directly with the argv vector you supplied. No shell is involved at any point, so quoting, word splitting, globbing, redirection, pipelines, command substitution and environment-variable expansion simply do not happen. args => ['$HOME; rm -rf /'] is one argument containing punctuation.

• Argument hygiene. args must be an array; entries must be strings (or numbers, which are stringified) and may not contain NUL bytes, since a NUL would silently truncate the argument at the exec boundary. The same goes for command itself.

• Bounded runtime. A foreground run is killed once timeout seconds have elapsed and reported with timed-out: true. The kill is SIGKILL — the uncatchable signal — rather than a polite SIGTERM first: Rakudo's Proc::Async ignores every .kill after the first on a given process, so a catchable signal would leave no way to escalate against a child that traps it. On Windows SIGKILL is one of the three signals libuv maps onto TerminateProcess, which is equally uncatchable. A killed command therefore gets no chance to clean up after itself.

• Bounded memory. Output is capped per stream per run, and a background job's buffers are capped the same way however long it runs.

What it deliberately does not do:

• It does not constrain what an allowed command can do. Allowlisting an interpreter or a shell (sh, bash, pwsh, raku, python) hands over arbitrary code execution, because those programs take code as an argument. Allowlist leaf tools, not launchers.

• It does not filter arguments. git can push, rg can read any file the server user can read, and cwd is unrestricted unless you leave it unset and only allow commands that ignore it. Run the server as a user whose privileges you are comfortable handing to the model.

• It does not scrub the environment. Children inherit the server process's environment, including any credentials in it.

• It only kills the command itself. Grandchildren the command spawned may outlive a kill, and can hold the pipes open long enough for the exit status to be lost (reported as exit: null).

Running anything: allow-any and MCP::Client::Policy

allow-any => True removes the allow list entirely. Everything else stays: no shell, argv vectors, NUL rejection, capped output, timeouts, jobs. What is gone is the decision about what may run — this pack will run anything the server's user can run, including a shell, including rm.

That is only a sane configuration if the decision is being made somewhere else. The intended pairing is a policy layer on the client side, such as MCP::Client::Policy, whose rules are name-based and whose default for an unmatched tool is to ask a human:

# Server: no allow list, because the client is where consent lives.
$server.plug: MCP::Server::Tool::Shell.new(:allow-any);

With a policy in front, sh_run and sh_start match no allow rule, so every call is put to the user before it happens — which is a better gate than a static list for an interactive coding agent, and a much worse one for an unattended server. If nothing is asking a human, use allow.

Windows notes

• Bare names resolve the way CreateProcess resolves them, so allow => <git> runs git.exe off PATH. Allowlist the string you want the caller to pass (git), not the file that ends up being executed (git.exe) — the match happens before any resolution.

• Arguments are still a vector: Rakudo quotes each entry for CreateProcess, so an argument containing spaces stays one argument.

• A kill becomes TerminateProcess, which the command cannot trap or delay. There are no signals, so there is no 128 + signal exit status: a killed run or job reports whatever TerminateProcess left behind (1, in practice). Test for exit != 0 rather than for 137.

• Everything else — stdin, capped output, streaming, jobs — behaves the same as on Unix.

Examples

• examples/shell-server.raku — a git + ripgrep server, with the invocation line in the header.

See Also

• MCP::Server — the server framework, the MCP::Server::Toolkit role this pack implements, and the raku-mcp command.

Author

Matt Doughty

License

Artistic-2.0