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mcp · lab mcp-04

The Server That Talks Too Much

· about 25 minutes · runs on ubuntu-26.04-claude · unrated

Read the journal Read its source

An unrated lab. It runs on your machine with no account and no network, and everything about it — the faults, the checks, the hints and the reference solution — is in the repository. An attempt on it is recorded on your profile and never moves a rating: only rated labs do. Rated and unrated labs.

The briefing

~/bin/health-check asks Claude Code, in ~/ops, how loaded this machine is. It gets the numbers from the metrics MCP server, which ~/ops/.mcp.json starts with /usr/local/bin/metrics-mcp. The server's settings are in /etc/metrics-mcp/config.json.

It worked until someone turned on debug logging to chase a slow answer, and tidied the start-up script so the version shows. Since then the job says it has no way to see the machine's load. The server itself runs fine when you start it by hand — it even prints its log for you. The job's JSON result is ~/health.json.

This lab runs in a container, as learner with sudo. claude here is the real Claude Code 2.1.270, talking to a scripted model on this machine; it starts the MCP server for real.

What is expected, and graded:

  1. The job connects to the server and get_load answers it.
  2. Everything the server's command writes to stdout is a protocol message.
  3. The server's debug log of each call is still written somewhere.

What is graded

This lab runs in a container — Docker or Podman, no VM — so there is no boot to survive: the checks run once, against the state you left.

CheckObjective
01_the_server_connectsKeep a stdio MCP server's stdout for the protocol alone
02_stdout_carries_only_the_protocolKeep a stdio MCP server's stdout for the protocol alone
03_the_log_is_still_writtenKeep a server's diagnostics when moving them off stdout

Start it

  1. 2Labs
  2. mcp-04select this one
  3. udownload it
  4. sstart the machine

Reading

Where the lab's hints send you, level by level, as you ask for them (h, then l opens a journal section in the TUI).

Theory for this lab

3 questions on the same topic, in the lab's Theory tab. They never affect the lab's grade. Here they are, to answer in place:

Question 1

A stdio MCP server wants to print its version and debug logs. Where should they go?

Question 2

What does this print — how many lines of the server's stdout a client can parse as JSON, of the three it wrote?

import json
stdout = 'ready {"jsonrpc":"2.0","id":1,"result":{}}\n' \
         '12:00 DEBUG handled initialize\n' \
         '{"jsonrpc":"2.0","id":2,"result":{}}\n'
ok = 0
for line in stdout.splitlines():
    try:
        json.loads(line)
        ok += 1
    except ValueError:
        pass
print(ok)
Question 3

.mcp.json starts a server with `npx some-mcp-server`. Which output can break the protocol?