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

Annotations declare readOnlyHint=false and idempotentHint=false but say nothing about failure modes; the description adds concrete refusal codes and clarifies that the new capture is compared against, not written to, the baseline. It stops short of stating explicitly that the baseline is left untouched, which matters for a non-idempotent mutation.

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?

One front-loaded action sentence followed by a compact failure-code clause; every element earns its place. The error enumeration is dense but scannable rather than redundant.

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?

No output schema exists, and the description does not need to describe return values for a compare-and-capture operation whose annotations already convey safety. The main residual gap is whether the comparison result is surfaced or whether the baseline is affected, but for a 3-parameter tool this is largely adequate.

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 coverage is 100%, so id, subPageId, and the DESKTOP/MOBILE viewport enum are already fully documented in the schema. The description adds only the phrase 'one extra page', which loosely maps to subPageId, and no format or default details beyond the schema.

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 specific verb and resource ('Take a fresh screenshot of one extra page now and compare it with its baseline'), which is clearly distinct from the monitor-level vs_visual_capture_now and from vs_visual_accept_baseline. It never names those siblings explicitly, so the differentiation is inferred from 'extra page' rather than stated.

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 three refusal codes (VISUAL_DISABLED, LIMIT_EXCEEDED, SUBPAGE_NOT_ON_PLAN) implicitly tell the agent the preconditions that must hold, which is useful. But there is no explicit when-to-use guidance and no statement about choosing this over vs_visual_capture_now or vs_visual_accept_baseline.

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