MAIster manages MCP at three scopes: an installation-wide server catalog, a project’s bindings and local servers, and requirements shipped by Flow packages. A Flow refers to a logical MCP ID; the project decides which concrete server implements it.

​
Prerequisites

  • Global admin access to create or trust platform MCP servers.
  • Project admin or owner access to create bindings and overlays.
  • Secret values available in the supervisor environment.

​
Three scopes

When no explicit binding exists, resolution follows project, platform, then package precedence. An enabled binding overrides that precedence. A disabled binding is an explicit opt-out and makes the logical reference unavailable in that project.

​
Keep secret values out of MAIster data

MCP environment and header entries are name and value pairs. A value is one of two things, and nothing in between:
  • a reference such as env:GITHUB_TOKEN — MAIster stores only the reference, and the execution host substitutes the value when it starts or connects to the MCP server. Set GITHUB_TOKEN in the supervisor service environment on every host that can run the MCP;
  • a literal such as FASTMCP_LOG_LEVEL=ERROR — stored and used exactly as written, which is your statement that it is not a secret.
There is no substitution inside a value: ${HOME} reaches the server as those eight characters. Use a reference for anything secret. Do not put an API key in flow.yaml, a package file, a project form, a URL, or an argument. Client-visible snapshots and logs keep a reference as a reference — the value behind it never appears in them. A literal is visible to anyone who can read the catalog, so the form warns you inline when a literal sits under a name that looks like a credential (*_TOKEN, *_API_KEY, Authorization, …). It is a warning, not a refusal. For a server that requires bearer authentication, use the bearer token env field rather than writing an Authorization header yourself: give it an env:NAME reference and the host composes Authorization: Bearer <value>. Setting both is refused. A project overlay changes the value a project uses for a slot the server declares, and never the name — the name is the server’s own contract. Two projects can therefore point the same slot at different credentials, or one at a plain literal such as a different GH_HOST, without cloning the server or exposing either value.

​
Register a platform MCP server

  1. Open MCPs as a global administrator.
  2. Add the logical ID and supported agent families.
  3. Select stdio, sse, or http and fill in its command or URL fields. sse is legacy — it was deprecated by the MCP specification, and a Codex runner cannot use it at all. Prefer http.
  4. Add the environment rows (for stdio) or header rows (for sse/http), and the bearer token env reference if the server needs one.
  5. Leave the server untrusted until its source and command have been reviewed.
  6. Trust and enable it.
  7. Run Test connection and inspect the initialization result and latency.
An untrusted stdio server is visible but cannot execute or even be probed. A failed probe records an actionable reason and releases any child process it started.

​
Resolve requirements in a project

Open Project → MCPs. The requirements ledger combines attached package requirements, enabled Flow node requirements, and attached platform-agent profiles. Each logical reference is classified as bound, automatically resolved, unbound, misconfigured, or not ready. For every unresolved required reference:
  1. choose a compatible platform, project, or package target;
  2. connect the binding;
  3. add a project overlay when the target’s default values do not fit — the overlay replaces a slot’s value and keeps its name;
  4. test the connection in the project context;
  5. confirm that the requirement becomes ready.
Flow nodes distinguish required MCPs from additional ones. An unresolved required MCP refuses launch. An unavailable additional MCP is omitted and recorded as withheld so the Run explains the degraded capability set. A transport the launching agent cannot use counts as unavailable: a required sse server on a Codex runner refuses the launch before any workspace is created, and an additional one is withheld with the reason shown on the Run.

​
Use MCP from Flow Studio

In a node’s settings, select MCP references from the package and project-aware picker. Package definitions stay portable because they carry logical IDs and environment references — prefilling a template from a platform server converts a literal into a reference, so a shared package never carries one of your values. The project binding and the execution host supply local implementation details. The resolved set is snapshotted into the Run. Later changes to a binding do not change what an existing Run used.

​
Failure signals