moltline-mcp is a stdio bridge tool for the Moltline Studio hosted MCP server cluster, designed with zero dependencies and relying only on the Python 3.9+ standard library. It is aimed at MCP clients that only support stdio transport, proxying newline-delimited JSON-RPC messages between the client and the hosted server verbatim, and does not collect telemetry data.
Purpose: to provide a bridge for MCP clients that only support stdio transport to connect to the Moltline Studio hosted MCP server cluster. Most modern MCP clients can connect directly to the cluster via Streamable HTTP without installation; this bridge is specifically for clients that only support stdio.
Main capabilities: the cluster contains 22 hosted MCP servers and 160 tools, of which 110 are free. The free tier requires no registration, account, or credentials, and tools can be called immediately after connecting. Premium tools require unlocking via a Moltline license. The bridge supports passing the license via the environment variable MOLTLINE_LICENSE, sent as the X-Moltline-License request header, and it is never written to command-line arguments, URLs, or logs. It supports --timeout to set the per-request timeout (default 300 seconds, or the environment variable MOLTLINE_TIMEOUT), --list to print server slugs and exit, MOLTLINE_BASE_URL to override the cluster base URL, and MOLTLINE_DEBUG=1 to output diagnostic logs to stderr (stdout stays protocol-clean).
Usage: direct connection is recommended; if the client supports Streamable HTTP (such as Claude Desktop, Claude Code, Cursor, and most current clients), simply point it at the hosted endpoint, with no installation required. For clients that only support stdio, you can run the bridge script (a single file, standard library only, ready to use after cloning), or run it via a Docker container (requires docker build -t moltline-mcp . first). The client configuration must point to the absolute path of the script, or run it via Docker. The bridge uses one process per server connection, and each JSON-RPC message read from stdin is POSTed to https://mcp.moltlinestudio.com/<server>, and JSON and SSE frame responses are relayed back to stdout as newline-delimited JSON. The bridge tracks the server-assigned Mcp-Session-Id and echoes the negotiated MCP-Protocol-Version, in compliance with the Streamable HTTP transport specification.
Use cases: scenarios where you need to connect to the Moltline Studio hosted MCP server cluster but the client only supports stdio transport. The cluster tools cover code review, agent governance, business math, regulatory deadlines, crypto tax lots, routing, and optimization, among others.
Limitations: the bridge itself does not inspect, store, or report user traffic. Premium tools require a Moltline license, which must be purchased at moltlinestudio.com (with human involvement). The license is deliberately not a command-line flag, because argv can be read globally via ps; it is also never placed in URLs or logs. A PyPI package is planned; for now, use the methods above. The bridge comes with an offline test suite that requires no network or dependencies, containing 22 tests covering newline-delimited framing, SSE event splitting (multiple events, multi-line data: payloads, trailing events without a blank line, comments, and named events), session ID capture and reuse, protocol version negotiation only at initialize, and error forwarding (JSON-RPC error documents are passed through verbatim, others become well-formed -32603). CI runs on Python 3.9, 3.12, and 3.13, plus a Docker build and container smoke test.
