Configuring Servers

MCP Servers are configured in the fastagent.config.yaml file. Secrets can be kept in fastagent.secrets.yaml, which follows the same format (fast-agent merges the contents of the two files).

Adding a STDIO Server

The below shows an example of configuring an MCP Server named server_one.

fastagent.config.yaml

mcp:
# name used in agent servers array
  server_one:
    # command to run
    command: "npx" 
    # list of arguments for the command
    args: ["@modelcontextprotocol/server-brave-search"]
    # key/value pairs of environment variables
    env:
      BRAVE_API_KEY: your_key
      KEY: value
  server_two:
    # and so on ...

This MCP Server can then be used with an agent as follows:

@fast.agent(name="Search", servers=["server_one"])

Adding an SSE Server

To use SSE Servers, specify the sse transport and specify the endpoint URL and headers:

fastagent.config.yaml

mcp:
# name used in agent servers array
  server_two:
    transport: "sse"
    # url to connect
    url: "http://localhost:8000/sse"
    # timeout in seconds to use for sse sessions (optional)
    read_transport_sse_timeout_seconds: 300
    # request headers for connection
    headers: 
          Authorization: "Bearer <secret>"

Roots

fast-agent supports MCP Roots. Roots are configured on a per-server basis:

fastagent.config.yaml

mcp:
  server_three:
    transport: "sse"
    url: "http://localhost:8000/sse"
    roots:
       uri: "file://...." 
       name: Optional Name
       server_uri_alias: # optional

As per the MCP specification roots MUST be a valid URI starting with file://.

If a server_uri_alias is supplied, fast-agent presents this to the MCP Server. This allows you to present a consistent interface to the MCP Server. An example of this usage would be mounting a local directory to a docker volume, and presenting it as /mnt/data to the MCP Server for consistency.

The data analysis example (fast-agent quickstart data-analysis has a working example of MCP Roots).

Sampling

Sampling is configured by specifying a sampling model for the MCP Server.

fastagent.config.yaml

mcp:
  server_four:
    transport: "sse"
    url: "http://localhost:8000/sse"
    sampling:
      model: "provider.model.<reasoning_effort>"        

Read more about The model string and settings here. Sampling requests support vision - try @llmindset/mcp-webcam for an example.