browser_evaluate
Execute JavaScript in the browser and return the result. Use for complex interactions, reading localStorage/cookies, or extracting data that CSS selectors can't reach.
This record as markdown: /tools/io-github-kivanccakmak-yaver/browser-evaluate.md
What browser_evaluate does on Yaver
AI agents invoke browser_evaluate to trigger actions in Yaver. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call: builds kicked off, notifications sent, workflows started.
| Parameter | Type | Required | Description |
|---|---|---|---|
javascript | string | Yes | JavaScript code to execute. Return value is sent back as JSON. |
session_id | string | Yes | Session ID |
Parameters from the server's own tool schema.
Why browser_evaluate is rated High
This tool allows execution of arbitrary JavaScript code in a browser environment, which can perform side effects including DOM manipulation, network requests, local storage modification, and other dynamic actions. While it also retrieves data (localStorage/cookies), the primary and most dangerous capability is code execution.
From the tool's definition Tool description states 'Execute JavaScript in the browser and return the result' and mentions use for 'complex interactions, reading localStorage/cookies, or extracting data.' The word 'Execute' and the capability to run arbitrary JavaScript code in a…
Risk signalsAccepts freeform code/query input (javascript)
Attacks that exploit this kind of access
The rule that runs browser_evaluate safely
PolicyLayer is an MCP gateway: it sits between your AI agents and Yaver, and checks every tool call against a rule you set before the call runs. Nothing changes on the server itself. For browser_evaluate, this is the rule to start with:
browser_evaluate stays usable, but rate-capped: a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.
The button opens the PolicyLayer dashboard: create your workspace, connect Yaver, apply this rule, and every browser_evaluate call is checked against it from then on.
Questions about browser_evaluate
Execute JavaScript in the browser and return the result. Use for complex interactions, reading localStorage/cookies, or extracting data that CSS selectors can't reach. It is categorised as a Execute tool in the Yaver MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
browser_evaluate accepts 2 parameters: javascript, session_id. Required: javascript, session_id. The full parameter table on this page comes from the server's own tool schema.
Register the Yaver MCP server in PolicyLayer and add a rule for browser_evaluate: 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 Yaver. Nothing to install.
browser_evaluate is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the browser_evaluate 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 browser_evaluate. 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.
browser_evaluate is provided by the Yaver MCP server (yaver-cli). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
More on Yaver, and thousands of servers like it.
Across the catalogue