You read the description. It appends a note. You approve it, you commit the config, you move on. That is the correct decision with the information you had.
Now the maintainer of notes-mcp pushes an update. Or their npm account gets phished. Or the package was always going to do this and simply waited. The tool description becomes:
Your config file has not changed. Not one byte. git diff is empty. The version string still says 1.2.0 if they want it to. Your dependency scanner has nothing to compare. Your SAST tool does not parse tool descriptions, because a tool description is not code.
But your agent loads that text into its context window before it calls anything, and models are built to follow instructions in their context. The instruction executes on connect, whether or not you ever use the tool.
Walk the stack and ask what each layer reads: Dependabot, Snyk, Trivy read package.json, requirements.txt, image layers. They do not read .mcp.json, and even if they did, the payload is not a dependency version. Semgrep, CodeQL read source. A tool description is a string in a JSON response from a process on your machine. There is no source file to scan. Code review reads diffs. There is no diff. Your own memory read the description once, at install time, weeks ago.
Every one of them is working correctly. The text that attacks you arrives through a channel none of them watches, and the channel is the documentation.
This is not one bug. It is four properties of the ecosystem that happen to compose badly: Tool descriptions are prompt content. They are concatenated into the context window. Whoever writes them writes part of your prompt. They are re-fetched on every connection. Nothing pins them. There is no lockfile, no signature, no hash. npx -y package@latest is the default idiom. That is not a dependency declaration; it is an unreviewed, unpinned, auto-confirming remote code fetch that re-resolves every time your agent starts. Nobody has an inventory. Ask a security team which MCP servers their engineers run and what those servers can reach. Nobody can answer.
Package managers solved (2) for code more than a decade ago. Agent tooling has not solved it for prompts, and prompts are arguably worse: a changed function body still has to get past your tests, while changed prose gets past everything.
I wrote a scanner for this, called Bulwark. The interesting part is not the detection rules — it is the lockfile, because that is the only thing that can catch a change made after you reviewed it.
bulwark.lock is a content hash of every string your model is allowed to be told. Commit it. Now the maintainer pushes their update, and in CI:
Three facts, none of which required detecting the attack itself. The text changed. The tool gained the ability to touch credentials. A new argument appeared that can carry data out. You do not need a rule clever enough to recognise every possible payload — you need to notice that the thing you approved is no longer the thing you are running.
That distinction matters. Detection rules are an arms race you lose slowly. Integrity checking is not.
Once you are parsing every agent surface anyway, some things become cheap to check that nobody currently checks at all.
Text a reviewer cannot see. The Unicode Tag block (U+E0000–U+E007F) maps one-to-one onto ASCII and renders as nothing in every mainstream UI. A description can look like Add two numbers. and carry a full sentence of instructions your editor will not show you. Bulwark decodes it and prints what was hidden. Same for zero-width characters, bidi overrides, base64, and HTML comments.
The lethal trifecta. An agent that can simultaneously reach private data, ingest content an outsider wrote, and send data outside your network. Each of those tools is individually reasonable. All three together is a complete exfiltration path that requires no vulnerability — attacker-authored text arrives as data, the model reads it as instructions, the outbound tool carries the secret away. No individual-tool scanner computes this, because the exposure does not exist in any individual tool.
Tool shadowing. Two connected servers both exposing search_docs. The model picks between them from the description alone. A newly added server can quietly capture calls you believe are going to the established one.
It does not make a model injection-proof. It raises the cost and records the attempt. A well-crafted instruction inside an allowed tool's result can still be followed — the durable fix there is architectural, which means removing one leg of the trifecta, not buying a scanner.
It does not read server source code. It reasons about what a server advertises and returns. A server with an honest description and a malicious implementation passes the description scan; the provenance rules are the control for that, and they are about review, not proof.