open_instance

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations, the description carries full burden. It discloses side effects (separate Chrome process, own profile directory, remote-debugging port), a prerequisite, and the return JSON structure. It does not mention cleanup via close_instance, but siblings list that tool; still, the behavioral disclosure is substantial. A 4 is justified because it omits any mention of resource persistence or lifecycle beyond creation.

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 slightly long but each sentence carries information: side effects, prerequisites, return format, usage, alternatives, and parameter details. It is front-loaded with critical side-effect and prerequisite info. A 5 would require trimming redundancy; the phrase 'webmcp_list_tools' isn't repeated but the 'features' explanation could be more succinct. A 4 is appropriate.

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?

For a 4-parameter tool with no required parameters and no output schema, the description covers everything an agent needs: side effects, return JSON with instance_id guidance, usage context, alternative routing, and parameter semantics. Nothing critical is missing for correct invocation.

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?

Schema already covers all 4 parameters with descriptions, so baseline is 3. The description adds meaningful context: explains the 'features' closed set (WEB_MCP and WEBGL_SOFTWARE) and recommends headless=false for visibility. This elevates value beyond the schema, scoring a 4.

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?

States a clear verb+resource: 'Opens a new independent Chrome instance'. Explicitly distinguishes from open_tab (additional tabs within existing instance) and remote-debugging port context, making it easy to route correctly.

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?

Explicitly states when to use (isolate browsing sessions, cookies, proxies) and lists the alternative open_tab for additional tabs. Also notes a prerequisite (rejected in --user-profile mode), which is beyond typical guidance.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.