Execute JavaScript code in a sandboxed environment with access to Math, Date, JSON, and standard built-ins, but no filesystem or network access.
Execute JavaScript code in a sandboxed environment. Has access to Math, Date, JSON, and standard built-ins. No filesystem or network access.
| Name | Required | Description | Default |
|---|---|---|---|
| code | Yes | JavaScript code to execute | |
| timeout_ms | No | Timeout in ms (default 5000) |
Changes observed during successful MCP inspections.
v0.1.0Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations provided, the description carries the full burden of behavioral disclosure. It transparently reveals sandboxing, available built-ins, and denied capabilities (filesystem/network). It stops short of disclosing error handling or the format of the execution result, but the core behavioral traits are well covered.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.
Is the description appropriately sized, front-loaded, and free of redundancy?
The description is extremely concise and well-structured: two sentences, front-loaded with the main action and followed by key constraints. Every word contributes value and there is no redundancy.
Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.
Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
While the description covers the tool's main function and sandbox constraints, it omits crucial information about what the tool returns (e.g., stdout, last expression value, or promise resolution) and how errors are surfaced. Since there is no output schema, the description should have addressed this to be fully complete.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.
Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
The input schema already provides descriptions for both parameters (code and timeout_ms), giving 100% schema coverage. The description adds no parameter-specific meaning beyond what the schema already states, achieving only the baseline score.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.
Does the description clearly state what the tool does and how it differs from similar tools?
The description uses a specific verb 'Execute' with a clear resource 'JavaScript code' and the context 'sandboxed environment'. This unambiguously identifies the tool's function and easily distinguishes it from the sibling notion and web-search tools.
Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.
Does the description explain when to use this tool, when not to, or what alternatives exist?
The description clearly implies when to use the tool (when JavaScript execution is needed) and explicitly states exclusions (no filesystem or network access), helping the agent avoid misuse. However, it does not explicitly name alternative tools or elaborate on when not to use it beyond those constraints.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.