performance_timeline

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

With no annotations, the description carries the full burden, and it does disclose useful behavioral details: the exact metric set and the fact that CLS is unavailable in Firefox because layout-shift is not implemented. However, it does not state whether the tool is read-only, what the response structure looks like, or any potential side effects, leaving some behavioral context unstated.

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 two sentences with no fluff. The metric list is front-loaded, and the important Firefox/CLS limitation is placed at the end for context. Every sentence earns its place.

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?

The tool is a no-param, no-output-schema utility with a straightforward purpose. The description covers what metrics are included and one key environment limitation, which is sufficient for a tool of this simplicity. It could explicitly say that it returns perormance data for the current page, but the wording already implies this strongly.

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 tool has zero parameters and the input schema is empty, so the schema fully covers paramater semantics. The description correctly focuses on behavior rather than params, and the no-parameter baseline of 4 applies.

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 clearly states the tool provides navigation, paint, and resource timing metrics for the current page, and enumerates exact values such as TTFB, DOMContentLoaded, FCP, LCP, and slowest resources. It lacks an explicit verb phrase like 'retrieves' or 'returns', and does not distinguish itself from sibling tools like network_get or page_stats, so it falls short of a 5.

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 gives no guidance on when to use this tool instead of related performance/network/page-stat tools. It only notes a Firefox limitation about CLS, which is a compatibility caveat, not a usage instruction. No alternatives or exclusion conditions are mentioned.

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