op-injection-scanner

An MCP server for prompt injection boundary enforcement. AI agents call scan_url(url) instead of fetching URLs directly. The server fetches the URL (with SSRF protection), strips HTML to plain text, and scans the content for prompt injection using a three-tier model escalation strategy (Haiku → Sonnet → Opus). Results are cached by URL and TTL bucket so repeated calls within the cache window incur no additional LLM cost.

Installation

git clone https://github.com/ethereum-optimism/op-injection-scanner
cd op-injection-scanner
bun install

Claude Code MCP config

Add to ~/.claude.json or your project's .claude/settings.json:

{
  "mcpServers": {
    "op-injection-scanner": {
      "command": "bun",
      "args": ["run", "/absolute/path/to/op-injection-scanner/src/server.ts"]
    }
  }
}

The server requires ANTHROPIC_API_KEY in the environment.

Usage

Agents call scan_url with a URL instead of fetching it directly:

scan_url({ url: "https://some-external-docs.example.com/page" })

Clean response — status 200, tool returns: ``json { "status": "clean", "content": "The stripped page text...", "url": "https://some-external-docs.example.com/page", "scanned_at": "2026-03-08T12:00:00.000Z", "model_used": "claude-haiku-4-5", "escalated": false } ``

Blocked response — tool returns isError: true so the calling agent receives a tool error: ``json { "status": "blocked", "reason": "Content contains instructions attempting to override agent behaviour", "url": "https://malicious.example.com/page", "scanned_at": "2026-03-08T12:00:00.000Z", "model_used": "claude-haiku-4-5", "escalated": false } ``

When the tool returns isError: true, the calling agent should not process the URL further.

Environment variables

| Variable | Default | Purpose | |----------|---------|---------| | ANTHROPIC_API_KEY | (required) | Anthropic API key — read automatically by the SDK | | CACHE_MAX_ENTRIES | 500 | Maximum number of cached scan results | | CACHE_TTL_SECONDS | 3600 | Cache entry lifetime in seconds (1 hour) | | CONFIDENCE_THRESHOLD | 0.8 | Minimum confidence for a verdict — below this, escalate to next tier | | FETCH_TIMEOUT_MS | 10000 | HTTP request timeout in milliseconds | | MAX_RESPONSE_BYTES | 2097152 | Maximum response body size (2 MB) | | MAX_REDIRECTS | 5 | Maximum number of HTTP redirects to follow | | MAX_URL_LENGTH | 2048 | Maximum URL length in characters |

Design decisions

Why MCP, not a hook

A Claude Code hook fires automatically on every tool call, but it cannot be selectively invoked or tested in isolation. An MCP server is explicit — agents opt in to scanning, the interface is testable, and it can be extended (new tools, new scan types) without touching hook infrastructure.

v1 scope

This version detects prompt injection only. Malicious intent detection (e.g. phishing, credential harvesting) is a future iteration. The clean verdict means "no injection attempt detected" — not "content is safe in all respects."

Tiered model strategy

Haiku — fast, cheap, handles the majority of clear-cut cases
Sonnet — escalated when Haiku returns uncertain or confidence < 0.8
Opus — final arbiter, called only when Sonnet also cannot reach a confident verdict; schema enforces a definitive clean or blocked verdict (no uncertain allowed); parse failure defaults to blocked

No auth or rate limiting

This is internal dev tooling. Network isolation is the access control. Auth and per-session rate limiting are noted as future work in the source.

DNS rebinding (known v1 limitation)

The SSRF guard resolves DNS once before the request and checks all returned IPs. A sophisticated attacker controlling a low-TTL DNS record could return a public IP on the initial check and re-resolve to a private IP at TCP connection time, bypassing the guard. Full mitigation requires a custom DNS-pinning HTTP client. This is acceptable for internal tooling but must be addressed before any public or shared deployment.

op-injection-scanner