session_preload

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

Annotations only provide a title, so the description carries the burden. It discloses the timing (document-start, before inline scripts), persistence across navigations, and clearing semantics. It doesn't mention error cases or whether scripts are re-run on every navigation, 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 compact and front-loaded with the core action, then explains the timing and use case. It is slightly dense with technical detail (xhs's inline jsvmp) but 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?

For a tool with two parameters and no output schema, the description covers the essential context: what it does, when to set it, and how it behaves across navigations. It could mention whether the preload applies to the current page if set mid-session, but the guidance to set before first navigate is clear.

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 description coverage is 100%, so the schema already documents both parameters. The description adds context about the scripts array's ordering and clearing behavior, but this is largely duplicated in the parameter description. Baseline 3 is appropriate.

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 states a specific verb and resource: 'Replace the session's document-start preload group' and clarifies that an empty array clears it. It also explains the timing ('before each new document's own scripts') and the unique advantage over sibling tools, distinguishing it from session_eval and session_navigate.

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 explicitly says when to use it: 'Set before the first navigate; applies to every navigation from then on.' It also explains the behavioral context (beats pages whose signing layer captures window.fetch/XHR natives at parse time) and the clearing behavior, giving clear guidance on how to use it versus alternatives.

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