AI agents call web_fetch to retrieve information from Onion without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
This tool retrieves and queries content from a URL with no side effects. It performs a read-only fetch operation that returns data as plain text. There is no modification, deletion, code execution, or financial impact. The automatic HTML stripping is a presentation transformation of retrieved data, not a destructive or write operation.
From the tool's definition web_fetch: 抓取指定 URL 的网页内容，返回纯文本（自动去除 HTML 标签）—fetches webpage content from a specified URL and returns plain text with HTML tags automatically removed.
Attacks that exploit this kind of access
抓取指定 URL 的网页内容，返回纯文本（自动去除 HTML 标签）。. It is categorised as a Read tool in the Onion MCP Server, which means it retrieves data without modifying state.
Register the Onion MCP server in PolicyLayer and add a rule for web_fetch: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Onion. Nothing to install.
web_fetch is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the web_fetch rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for web_fetch. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
web_fetch is provided by the Onion MCP server (onion-ai/mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Every MCP server has a record like this.
Type a name, get the same breakdown: verified identity, auth posture, risk grade, capabilities, recommended policy.
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