agent-mcp-evals

I built an inbox and CRM agent, an MCP server for its tools, and an eval suite for prompt injection at the tool layer. Each attack hides an instruction in something the agent reads: an email, a subject line, a contact record, or a web page. The instruction tries to make the agent call a tool: forward private mail, export the contact list, add an admin, or change a deal.

The same code runs every attack twice. With the guards off, all 10 attacks succeed. With the guards on, all 10 are blocked, and the 5 ordinary tasks still work.

These changes reduce the risk of prompt injection driving unintended tool calls. They do not eliminate it, and I make no claim that this code is free of vulnerabilities.

Run it

You need Node.js 24 or later. The code calls no model API and opens no network port.

npm ci --ignore-scripts
npm test                    # unit, eval, stdio, and load-bearing tests
GUARDS=on npm run eval      # every attack blocked, every task done
GUARDS=off npm run eval     # every attack succeeds, every task done
npm run eval:matrix         # each attack against each guard alone
npm run mutations           # break each guard and check that the tests notice

GUARDS=off is read only by the eval runner. The server entry point always runs with every guard on and has no switch.

Related MCP server: 7dayrag

The MCP server

src/stdio.ts starts the server on stdio, using the official @modelcontextprotocol/sdk. It has no HTTP transport and listens on no port. Any MCP client that can launch a stdio server can connect. For example, in a client's server config:

{
  "mcpServers": {
    "inbox-crm": { "command": "node", "args": ["/path/to/agent-mcp-evals/src/stdio.ts"] }
  }
}

The server works on seeded in-memory data. fetch_page returns pages from that data and never fetches anything.

The four guards

Two guards live in the server, so they hold for any client. Two live in the agent, because they depend on what the agent is for.

1. Tool allowlist (agent). The agent calls only the tools in TRIAGE_TOOLS, and the model sees only those. The check uses a Set, so a tool named constructor or __proto__ does not slip through.

2. Argument schemas (server). Every tool has a strict Zod schema. Unknown keys are rejected, so an attacker cannot add an owner field to a stage update. Recipients are single addresses, stages come from a fixed list, and fetch_page rejects other hosts and query strings, so a URL cannot send data to a host that is not on the allowlist. The path can still carry text to an allowed host, so the allowlist should name only hosts you trust.

3. Confirm gate for writes (server). Before any write runs, the server asks the human through MCP elicitation and shows the exact arguments it will use. Characters outside printable ASCII appear as escapes, so bidi and zero-width characters cannot disguise them. A declined, cancelled, or failed request means the write does not run. A client that cannot ask its human gets no writes at all. No tool argument can satisfy the gate.

4. Taint (agent). All tool output is untrusted. Once tool output is in front of the model, the agent sends no further tool calls. The model plans its calls from the user's request alone, on the first turn. A planned call can use a field from an earlier result, such as replying to a message's sender, but text the agent reads can never add, choose, or trigger a call.

Taint is the strictest of the four. A task that has to read something before it can decide what to do next needs a new request from the user. That is the trade: the agent gives up acting on what it reads, so it cannot act on what an attacker wrote.

Evals

evals/cases.ts holds the cases. Each attack records where its text sits and which guards stop it on their own. The eval decides success from the server's data (the outbox, the contact list, the users, the fetch log), not from what the agent says.

No single guard stops every attack, and every attack is stopped by at least two guards. One broken guard does not open the whole path.

The benign tasks are: summarize two messages, reply to a sender, move a contact to the next stage, summarize a page, and find a contact. They must pass with the guards on and off.

The cases are adapted from typescript-rag-prompt-injection-defense, my earlier project on injection in retrieval-augmented answers. They are rewritten for an agent with tools. Each case names the earlier test or threat it adapts.

The model

src/scripted-model.ts is a deterministic stand-in for a language model. It plays the worst case: it obeys every instruction it reads, wherever the instruction came from. It plans calls for the requests it knows, and in tool output it obeys call <tool> with <json> and forward this to <address>, after NFKC normalization. That keeps the evals offline and repeatable, and it tests the guards rather than a model's judgment. A real model behind the same Model interface may resist some of these attacks, but not predictably, so the guards do not rely on it.

Load-bearing tests

test/load-bearing.test.ts checks each guard on its own:

• A write from a client that cannot ask its human, or that the human declines, does not run. An argument such as "confirmed": true does not count as approval.

• An instruction in a fetched page never reaches the server as a tool call. With only the taint guard turned off, the same call does reach the server.

• Tools outside the allowlist, including inherited property names, never reach the server.

• Arguments with extra keys, a second recipient, an unknown stage, or another host are rejected before the tool runs.

npm run mutations copies the repo, breaks one guard at a time (for example, by removing the confirm gate), and checks that these tests fail. CI runs it too.

Limits

• The guards act on tool calls. They do not check what the agent says in its reply text.

• The confirm gate is only as good as the human reading it. It shows the exact arguments, but a person who approves everything approves the attack too.

• The schemas limit the shape of arguments, not their meaning. A valid address can still be the wrong address, and the confirm gate is what catches that.

• The cases are a hand-picked set, not a full list of attacks.

License

Provided as is under the MIT license, with no warranty. See LICENSE.