public-mcp

A public MCP server on AWS Lambda that exposes deterministic checking tools.

Status: Plan 01 implemented: the three spec.* tools run locally over stateless Streamable HTTP. No AWS deployment yet (Plan 03).

Principle: no server-side LLM: the client brings the model, the server checks, validates, seals.

See Roadmap.md and .claude/plans/Plan-01.md.

Tools

All inputs are text; caller code is parsed, never executed. Each text field may be at most 32 KiB (UTF-8) and an HTTP body at most 80 KiB. Invalid input (too large, not valid Python, too deeply nested) is an MCP tool error (isError: true) whose message names the field.

Related MCP server: ai-guardrail-mcp

Run locally

Requires Python 3.12 or newer.

python -m venv .venv

.venv/bin/pip install -e ".[dev]"

.venv/bin/pytest

.venv/bin/ruff check .

Start the server (endpoint http://127.0.0.1:8000/mcp; --host/--port or PUBLIC_MCP_HOST/PUBLIC_MCP_PORT override the defaults):

.venv/bin/python -m public_mcp.http

Inspect it with the MCP Inspector (UI), or list the tools from the command line:

npx @modelcontextprotocol/inspector@0.17.2 --cli http://127.0.0.1:8000/mcp --transport http --method tools/list

Add it to Claude Code:

claude mcp add --transport http public-mcp http://127.0.0.1:8000/mcp

Layout

src/public_mcp/
  server.py      build_server(): registers the tools, transport-agnostic
  http.py        local stateless Streamable HTTP entry (python -m public_mcp.http)
  limits.py      MAX_FIELD_BYTES, MAX_BODY_BYTES, check_field()
  errors.py      tool error messages
  spec/          the checking core: pure, standard library only
  tools/         one adapter per tool: limits -> core -> structured result
tests/

Provenance and licence

src/public_mcp/spec/ is derived from Agora v0.1.0 (Copyright 2026 Agora contributors, Apache License 2.0) and maintained here; each module's docstring names its source file, the functions taken and the changes made. This project is licensed under the Apache License 2.0 (see LICENSE).