Most attack talk focuses on what the agent reads. This one is about what the agent trusts: the numbers and text its tools hand back. An agent reasons from tool output as if it were ground truth — so if that output is wrong, every downstream decision is wrong, and nothing looks amiss.

How it works

  • A compromised or sloppy third-party tool. If you’ve wired in an MCP server or data tool beyond Robinhood’s own, its output is now part of your agent’s trust boundary. A bad or malicious one can return a spoofed quote, a fake "position," or text laced with instructions.
  • Stale or wrong feeds. Even non-malicious bad data — a lagging quote, a misparsed value — can make an agent act on a price that isn’t real.
  • Injection riding in tool output. Retrieved tool results are just text; an attacker who controls a source can plant instructions there exactly as they would in a news article.

How to bound it

  • Prefer first-party. The official Robinhood Trading MCP is inside your trust boundary by design. Every extra tool or third-party MCP you add widens it — add deliberately, and vet what you connect. (What your agent can access.)
  • Treat tool output as untrusted text, too. The injection-refusal rule isn’t just for news — instructions embedded in a tool result get the same treatment: refuse and flag.
  • Sanity-check before acting. Have the agent confirm a quote looks reasonable (not 10x off, not stale) before sizing a trade on it; require approval on anomalies.
  • The circuit breaker catches the runaway behavior that bad data tends to cause — a burst of trades off a wrong number trips it and halts. (Guardrails checklist.)
  • Caps still hold. Whatever the data says, your per-trade, daily, and concentration limits don’t move.

The takeaway

Your agent is only as trustworthy as the tools you connect to it. Keep the tool set tight, treat tool output with the same suspicion as news, and let the caps and circuit breaker contain anything that slips through. Back to the full attack surface.

Knowing where your agent’s trust boundary really sits is core security thinking — and measurable. Test yours at secprove.com.