Can an Agent Use It?

Can an agent build and call an MCP server with FastMCP 4.1? Yes, on the first try

Yes, on the first try: Claude Code (Sonnet 5.5, sandbox tools only) had FastMCP 4.1.4's quickstart server answering a greet call from a FastMCP client in 15 seconds, with no outside reference material in the log.

The task

Using fastmcp 4.1.4, follow the FastMCP quickstart: build the quickstart's MCP server with its greet tool, then call that tool from a FastMCP client. Call greet with the name "Ford" and print what the tool returns.

Docs: https://gofastmcp.com/getting-started/quickstart

When you're done, leave a script at /work/run.sh that runs the result end to end from a fresh shell and exits 0.

What happened

How we checked

The hidden check ran bash /work/run.sh in the box after killing everything the agent left running, and required exit 0 and Hello, Ford! in the output. Tail:

CallToolResult(content=[TextContent(type='text', text='Hello, Ford!', annotations=None, meta=None)], structured_content={'result': 'Hello, Ford!'}, meta={'fastmcp': {'wrap_result': True}, 'io.modelcontextprotocol/serverInfo': {'name': 'My MCP Server', 'version': '4.1.4'}}, data='Hello, Ford!', is_error=False)

The version probe saw 4.1.4 and fastmcp-4.1.4.dist-info in the venv. The server's own serverInfo also reports 4.1.4. On an untouched box the same check failed (exit 127, bash: /work/run.sh: No such file or directory), as it should. Method: SPEC.md.

Transcript

Long outputs are clipped below. The complete redacted logs are at https://github.com/Simbastack-hq/canagentuseit/tree/main/results/2026-10-30T0511Z-fastmcp-4-1-4 (added when this post goes live).

Transcript (attempt 1, Claude Code, claude-sonnet-5-5)

Claude Code 2.1.314, model claude-sonnet-5-5

[00:01] fetch_url

https://gofastmcp.com/getting-started/quickstart

output:

Quickstart - FastMCP

    Documentation Index
Fetch the complete documentation index at: /llms.txt
Use this file to discover all available pages before exploring further.
Skip to main content
Assistant

Responses are generated using AI and may contain mistakes.

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On this page
Back to topCreate a FastMCP Server
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This guide builds a working MCP server from scratch: a tool, a way to run it, a client that calls it, and a visual UI for the result. It ends with the server deployed and reachable over the internet.
If you haven’t alrea
… (18556 more chars)

[00:06] run

mkdir -p /work && cd /work && cat > my_server.py <<'EOF'
from fastmcp import FastMCP

mcp = FastMCP("My MCP Server")

@mcp.tool
def greet(name: str) -> str:
    return f"Hello, {name}!"

if __name__ == "__main__":
    mcp.run(transport="http", port=8000)
EOF
cat > my_client.py <<'EOF'
import asyncio
from fastmcp import Client

client = Client("http://localhost:8000/mcp")

async def call_tool(name: str):
    async with client:
        result = await client.call_tool("greet", {"name": name})
        print(result)

asyncio.run(call_tool("Ford"))
EOF
cat > run.sh <<'EOF'
#!/usr/bin/env bash
set -euo pipefail
cd "$(dirname "$0")"
[ -d .venv ] || uv venv -q .venv
uv pip install -q --python .venv/bin/python "fastmcp==4.1.4"
.venv/bin/python my_server.py >server.log 2>&1 &
SERVER=$!
trap 'kill $SERVER 2>/dev/null || true' EXIT
for _ in $(seq 60); do
  (echo >/dev/tcp/127.0.0.1/8000) 2>/dev/null && break
  sleep 0.5
done
.venv/bin/python my_client.py
EOF
chmod +x run.sh && env -i HOME=$HOME PATH=$PATH bash -lc ./run.sh; echo exit=$?

output:

CallToolResult(content=[TextContent(type='text', text='Hello, Ford!', annotations=None, meta=None)], structured_content={'result': 'Hello, Ford!'}, meta={'fastmcp': {'wrap_result': True}, 'io.modelcontextprotocol/serverInfo': {'name': 'My MCP Server', 'version': '4.1.4'}}, data='Hello, Ford!', is_error=False)
exit=0

[exit code 0]

[00:13] Agent: I built the quickstart server with its greet tool and called it from a FastMCP client with the name "Ford". /work/run.sh ran from a clean shell and exited 0.

The tool returned Hello, Ford!. The client prints the whole result object, as the quickstart does, so the output looks like this:

CallToolResult(content=[TextContent(type='text', text='Hello, Ford!', ...)], structured_content={'result': 'Hello, Ford!'}, ..., data='Hello, Ford!', is_error=False)

The files are all in /work:

I skipped the quickstart's optional "Give Your Tool a UI" and deploy sections, since you only asked for the server and the client call.

[00:13] Stopped: done, 3 turns, $0.06645809999999999

Written by an agent from the run log. Verdicts are about this agent, this version, on this date. Spot an error? Request a correction.

Reviewed by a human on 2026-10-30 before publishing.

#works#sdk#Prefect