SMELT

Custom MCP

Custom MCP lets a workflow call tools exposed by your own MCP-compatible server. Connect a server, discover the tools it offers, then call one from a Custom MCP block.

What it is

MCP (Model Context Protocol) is an open standard for exposing tools to an agent. A Custom MCP connection points Studio at a server you run or control, separate from the built-in connectors covered in Integrations. Studio validates the server address, discovers the tools it publishes, and lets a workflow call them like any other tool.

Create a connection

Open a Custom MCP block’s configuration panel and choose Saved connection, then New connection. Enter:

  • The server URL
  • An optional auth header, for servers that require one
  • An optional display name

The server URL is checked before the connection is saved. The auth header, if you provide one, is stored encrypted with the connection and is never written into the workflow definition. A saved connection can be reused across any number of blocks and workflows.

For a server that does not need an auth header, choose Public URL instead and enter the address directly in the block. This is quicker for a quick test but is not saved for reuse.

Add the block

Press the + handle on the block you want it to follow and choose Custom MCP from the palette. Pick its source: a saved connection or a public URL, as above.

Discover a tool

With a source selected, click Discover tools. Studio queries the server and lists the tools it offers in a dropdown. Select the tool the block should call, then set its parameters as JSON. Wire the block’s output into the rest of the workflow as normal.

Pro and Business only

Custom MCP is available on the Pro and Business tiers. A Free workspace gets a 402 response with code TIER_LIMIT_CUSTOM_MCP when it tries to save a connection. See Billing & Limits to upgrade.

Limits

  • The server URL must be a public address; Studio blocks addresses that resolve to private or internal networks.
  • A block uses exactly one source: a saved connection or a public URL, never both at once.
  • Tool discovery and every tool call run within your workflow’s usual execution limits. See Billing & Limits.

Next: Integrations or Workflow Builder