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

Annotations already indicate read-only, idempotent, and non-destructive behavior. The description adds valuable behavioral context: successful screenshots are cached until TTL expiry, while failed or blocked captures are never cached and are retried live on the next call. This goes beyond the annotations and does not contradict them.

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?

Two concise, front-loaded sentences. The first states the action and scope, and the second adds the essential cache and retry behavior. Every sentence earns its place with no filler.

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 read-only capture tool with fully documented parameters and safety annotations, the description covers purpose, device variants, and cache/TTL behavior. The main gap is that with no output schema, it does not explicitly state the return format, such as image URLs or binary image data, so the agent must infer the output shape from the tool name.

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 documents all five parameters with 100% coverage, so the baseline is 3. The description adds meaning by explaining the cache/TTL behavior tied to ttlSeconds and by reinforcing the desktop/mobile/both options, particularly the non-obvious fact that failed captures are never cached.

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: capture desktop, mobile, or both screenshots for a website. It clearly indicates what the tool does, though it does not explicitly differentiate itself from the sibling tool read_page_context.

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?

Usage is implied by the action verb and device variants: use this tool when a visual screenshot of a website is needed. However, there is no explicit guidance about when to prefer this tool over read_page_context or when not to use it.

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