Blocking prompt injection in tool output
When an AI agent calls a tool, the response can carry hidden instructions that try to hijack the agent — a technique called indirect, or response, prompt injection. G8KEPR's response-poisoning detection inspects tool and MCP server output before it reaches your model, so poisoned content can be flagged or blocked instead of being followed.
What response poisoning is
Unlike a direct prompt injection typed by a user, an indirect injection hides inside data your agent retrieves — a web page, a document, a database row, an API payload, or the output of an MCP tool. The text looks like normal content but contains commands aimed at your model, such as "ignore previous instructions," "exfiltrate the API key," or "email this data externally."
Because the agent tends to treat tool output as trusted context, these instructions can quietly redirect its behavior. Scanning the output side of a tool call is what helps close this gap.
How G8KEPR detects it
G8KEPR inspects tool responses as they return through your MCP proxy or gateway and looks for indicators of injection, including:
- ▸Instruction-style phrasing directed at the model (role overrides, "ignore," "disregard," "you are now")
- ▸Attempts to reveal system prompts, credentials, or hidden context
- ▸Requests to perform unrequested actions such as sending data to an external destination
- ▸Obfuscation tricks like hidden characters, encoded payloads, or invisible text
Each response gets a risk assessment, and content that crosses your policy threshold can be flagged or blocked before the agent reads it.
Turning it on
- 1.Route your agent's MCP or tool traffic through the G8KEPR gateway.
- 2.In the dashboard, open MCP Security and enable response-poisoning detection.
- 3.Choose an enforcement mode — monitor to log detections only, or block to strip or reject poisoned responses.
- 4.Set the sensitivity threshold that matches your risk tolerance.
Start in monitor mode for a day or two to review detections against your real tool traffic, then switch to block once you've confirmed the signal is clean.
Reviewing what was blocked
Detections are recorded with the offending tool, a snippet of the flagged content, and the action taken. Filter by tool or server in the dashboard to spot a specific MCP source that is repeatedly returning poisoned output.
A single compromised or malicious MCP server can poison downstream agent calls. Investigate any tool that produces repeated detections before trusting its output again.
For related protections, see Securing MCP tool calls and Detecting direct prompt injection.