A StageFlow pipeline is JSON, and a model can write JSON. What it cannot do is know whether the graph it wrote is one this backend will accept: the stages are the backend's, the node types are its core's, and the ceilings are its caller's plan. This is the missing half — a client of the same seven endpoints the editor talks to, handed to an agent as three tools and five resources.

Add it

claude mcp add stageflow -- uvx stageflow-mcp --backend https://stageflow.lazy.su

pipx install stageflow-mcp puts stageflow-mcp on the PATH instead, if you would rather have it installed than fetched. uvx stageflow-mcp@0.1.0 pins a version — worth doing in anything written down, so it still runs the same way next year.

Or let it ask. With no --backend the first tool that needs one asks — through the MCP client's own prompt, not through the conversation:

claude mcp add stageflow -- uvx stageflow-mcp

The answer is remembered in ~/.config/stageflow-mcp/config.json, so it is a question asked once rather than once a session. The address only — a credential is never written there, and there is deliberately no tool that accepts one: a tool argument is written by the model, which puts it in the conversation, the transcript and that client's logs.

Not every client can be asked. At the time of writing the terminal Claude Code can; its VS Code extension declares the capability and declines every request; the Desktop code tab offers none. Where it cannot, the first call says so and names the flag to pass instead — it never guesses an address.

That address is the one you would type on the editor's connection screen, and it is read the same way: localhost:8765, http://localhost:8765/ and .../api all mean one backend. A bare hostname gets https:// unless it is loopback — see below.

A credential, if that backend wants one. It is the same credential the editor carries, in the same header, and it is not this tool's to issue: the core has no idea what a token is, so whoever runs the backend decides what one looks like and hands it over. In the reference backends it is one environment variable (SF_TOKENS="secret:plan") and forty lines of auth.py that a real deployment replaces wholesale. Without one you are whatever that backend calls an anonymous caller — on the public demo above, a narrow plan that works.

stageflow-mcp --backend https://sf.example.org --token "$SF_TOKEN"
stageflow-mcp --backend https://sf.example.org --token "$SF_TOKEN" \
              --auth-header X-Api-Key              # if it is not Authorization

Check the address before wiring an agent to it — the same questions, printed:

$ stageflow-mcp --backend stageflow.lazy.su --check
backend      https://stageflow.lazy.su
credential   none sent
stages       23
core         0.13.0
plan         demo (open)
node types   condition, entry, map, parallel, stage, subpipeline, switch, terminal, try
limits       {"counters": {"seconds": 30, "steps": 300, …}}

What it gives an agent

The bridge to an open editor

stageflow-mcp --backend https://sf.example --bridge

prints a link. Open it, and the editor is looking at the same graph the agent is: show_in_editor puts a pipeline on the canvas, get_editor_graph reads back what the person changed there. "Add a retry to this node" stops being a request to paste anything.

The link is printed to stderr and handed to the agent, because under an MCP client stderr is a log file nobody is looking at — so the assistant can simply tell you where to open it. --editor http://127.0.0.1:8080/ points the link at a copy of the editor you serve yourself.

Off unless asked for — the two tools do not exist without --bridge, because a tool an agent has been told about is a tool it will call, and a socket on somebody's machine is not this process's to open uninvited.

It binds to loopback, requires a token made fresh at every start, and answers only to the editor and to pages served from this machine. The token travels in the fragment of the link, so it never reaches the host serving the editor — not its access log, not the Referer of anything the page fetches.

Your backend needs no changes

There is no /api/validate in the StageFlow contract and this asks for none. POST /api/run with mode: "step" parses a graph, validates it against the caller's policy, and only then admits a run — both reference backends do it in that order, before a thread, a slot or an id exists. So an invalid graph is refused for free, and a valid one comes back parked before its first node, having executed nothing, and is stopped immediately.

Which means this works against a backend that was deployed before it existed, and against yours without asking you for anything.

Nothing is validated or executed here. The semantics live in the core the backend runs, and a second opinion in this process would be a second implementation to drift from it. The one thing read locally is the pipeline JSON Schema — a data file out of stageflow-framework, narrowed by what the live backend says it serves.

Two things worth knowing

A credential travels. An address typed without a scheme becomes https:// unless it is loopback, and a token bound for plain http to something that is not this machine is warned about. The editor fills in http:// instead, and can afford to: a browser will not let it reach one anyway.

A shared backend has slots. A public demo holds a few runs at a time and a couple per caller. Checking costs none of them, which is why the working method is validate, fix, validate, and only then run.

What it does not do

• Inputs. A stage can await input in the core, but the HTTP contract has no channel for it — the editor has none either — so a graph that waits will time out.
• Step debugging. Stepping exists so a person can watch a graph go by; an agent that is not watching would only be parking a slot.
• Serving over HTTP. stdio only. This process carries the credential of whoever started it; making it a service that hands that to whoever connects is a different thing, and one to build deliberately.

Tests

python -m unittest discover -s tests -t . -v

They run against a fake backend on a real socket, so they need no network. The ones that check the contract itself is still what this assumes — that a bad graph is refused before a run is admitted — talk to a live backend and are skipped without one:

STAGEFLOW_BACKEND=https://stageflow.lazy.su python -m unittest tests.test_live

MIT.