My MCP Server Only Talks to APIs I Trust. That Doesn't Mean the Data Coming Back Is Trustworthy.

I built a small MCP server a while back — developer-presence , seven tools wrapping the GitHub REST API and the DEV.to API so an agent can check my repo stats, list my articles, or draft a new post without me leaving the chat. It's mine, I wrote every line, there's no third-party package doing anything sketchy under the hood. By the usual "vet your MCP servers before installing them" checklist, it passes clean. I've written that checklist article before. What I hadn't thought carefully about until recently is that vetting the server doesn't vet the data. Two of its tools go straight to the point: @mcp.tool () def get_repo_stats ( repo : str ) -> dict : """ Get stars, forks, watchers, open issues for enjoykumawat/<repo>. """ r = _gh ( f " /repos/ { GITHUB_USERNAME } / { repo } " ) return { " name " : r [ " name " ], " stars " : r [ " stargazers_count " ], " forks " : r [ " forks_count " ], " watchers " : r [ " watchers_count " ], " open_issues " : r [ " open_issues_count " ], " language " : r . get ( " language " ), " description " : r . get ( " description " ), } description is free text. Any repo owner can put anything in it. If I ever point this tool at a repo I don't control — someone else's fork, a dependency, anything — that field lands in my agent's context exactly the same way a trusted instruction would: as text in a tool result, with no marker distinguishing "this came from GitHub's database, unfiltered" from "this is something I told the agent to do." The server is safe. The channel is safe. The payload was never vetted at all, because there was nothing to vet — it's just whatever a stranger typed into a form. I only really felt this because of a task I run on a schedule: check dev.to for trending posts in a few tags, score them, and use the highest scorers as source material for what to write about next. Step one of that job is a loop over tag pages: for tag in [ " ai " , " llm " , " mcp " , " claudecode " , " agents " , " productivity " ]: url = f " http