Resources — Read-Only Data

Resources are the read-only side of MCP. They model anything that has a stable identity and can be fetched: a file on disk, a row in a database, a documentation page, a snapshot of memory state. Each resource has a URI (its name on the wire), a mime type, optional annotations, and a piece of content that is either text or binary.

The protocol exposes three resource methods: resources/list returns the available resources, resources/read returns one resource's content by URI, and resources/subscribe (when the server advertises it) lets the client be notified when a specific resource changes. Subscribers are the foundation for live-data UIs in hosts that want to keep a panel up-to-date.

The right mental model: Resources are what the host can read without asking the user. They are safe by design — no side effects, no state mutations, no irreversible operations. That is why hosts can list and pre-fetch them aggressively without prompting for confirmation.

Common pattern: a Postgres MCP server exposes one resource per table (URI like postgres:///orders), a filesystem server exposes a resource per allowed path, a docs server exposes a resource per page. Tools mutate; Resources just sit there ready to be read.

Code

Defining a resource in the Python SDK·python

from mcp.server.fastmcp import FastMCP

app = FastMCP("docs-server")

@app.resource("docs://README")
def read_readme() -> str:
    return open("/srv/docs/README.md").read()

@app.resource("docs://changelog/{version}")
def read_changelog(version: str) -> str:
    return open(f"/srv/docs/changelog/{version}.md").read()

Reading a resource from the client side·python

async with ClientSession(read, write) as s:
    await s.initialize()
    listing = await s.list_resources()
    for r in listing.resources:
        print(r.uri, r.name, r.mimeType)
    content = await s.read_resource("docs://README")
    print(content.contents[0].text)

Resources — Read-Only Data

Code

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