exec-mcp

A dependency-free Node.js implementation for a bounded remote exec MCP gateway.

It intentionally exposes one capability: run a single bounded remote shell command and return a capped result with stdout/stderr tails plus an authoritative exec summary. It is not a GitOps API, not a Kubernetes API, and not a file-management API. Higher-level tools such as kubectl, git, docker, helm, argocd, and flux are left to the remote shell.

Current interface

• GET /healthz
• GET /metrics
• POST /exec with Accept: text/event-stream for SSE event streaming
• POST /mcp for MCP Streamable HTTP / JSON-RPC tool calls

The MCP server exposes exactly one tool:

exec

The tool executes one command on the configured remote execution host. In the current dev deployment, the ssh-mcp connector runs commands on dev-debian through SSH.

MCP tool contract

Request arguments:

{
  "command": "kubectl get pods -A",
  "cwd": "/root/config-git",
  "timeout_seconds": 120,
  "max_output_bytes": 5242880,
  "env": {
    "NO_COLOR": "1"
  }
}

Field semantics:

• command: command string run through /bin/sh -c on the configured remote execution host. Use explicit quoting for pipelines, redirection, &&, and variable expansion. Avoid interactive or unbounded long-running commands.
• cwd: working directory on the remote execution host. It must be under the configured allowlist. If omitted, DEFAULT_CWD is used.
• timeout_seconds: maximum runtime. Values above MAX_TIMEOUT_SECONDS are rejected. On timeout, the server aborts the exec and sends SIGTERM, then SIGKILL after the configured kill grace period.
• max_output_bytes: maximum combined stdout/stderr bytes forwarded before truncation. The process is still drained until exit. Final byte counts and bounded tails are included in the summary.
• env: additional environment variables. Invalid variable names are ignored. ENV and BASH_ENV are removed before spawning.

Output semantics:

• MCP tools/call returns final text, not live SSE events.
• The final text contains bounded stdout/stderr tail content followed by [exec summary].
• [exec summary] is authoritative for exit, signal, duration_ms, byte counts, truncated, and timed_out.
• stderr output alone is not failure. Non-zero exit code, signal, or timed_out=true is failure.
• Output may be truncated. Do not assume the returned text is the full command output.

Concurrency semantics:

• Active execs are bounded by MAX_CONCURRENT_EXECS.
• When full, the tool returns too_many_active_execs with active, max, oldest_age_seconds, and states.
• too_many_active_execs usually means real concurrency pressure, not necessarily a service fault.
• v11 tracks active execs through ExecRegistry and releases slots through finally, timeout abort, client-close abort, and a reaper fallback.

Cancellation boundary:

• The current connected request can be aborted by client close/disconnect.
• There is no cancel-by-exec-id API in v11.
• v11 protects local active-slot bookkeeping; it does not provide full remote process lifecycle governance.

Commands

npm test
npm run build
npm run validate
npm start

Validation status

Current local validation:

node --test: 32 tests, 32 pass
npm run build: pass
npm run validate: pass

Memory smoke writes 5 MB from a child process while forwarding only 1 KiB plus summary/tail metadata. The gateway drains child output and keeps only bounded tails in memory.