zoom

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

Annotations carry readOnlyHint, and the descrption's observe/capture framing is consistent with it — no contradiction. It adds genuinely useful behavioral traits beyond the annotation: never-scaled full-resolution capture, and the refusal error when the region exceeds half the desktop, which an agent must know before invoking. It stops short of disclosing what the tool returns or how the PNG result is consumed, which would have made it fully transparent.

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?

Four compact sentences and front-loaded: the core purpose and sibling contrast land in sentence one, with use cases and the size cap following in order of importance. The final clause 'zoom exists to spend tokens on FEW pixels' is mildly redundant with the refusal statement, but costs little and reinforces intent.

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 full schema param coverage, the descrption covers purpose, alternative, use cases, and the key failure mode. The one substantive gap is the return value — with no output schema, the agent is left to infer from the path parameter that a PNG path is what comes back.

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 the schema documents all four parameters and the baseline is 3. The descrption adds one meaningful param-level constraint the schema lacks — 'Refuses more than half the desktop' caps the region size — and the 'small area' framing ties the region parameter to the tool's intended use. That value above baseline justifies 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 specific action and resource — 'Look closer at a small area at FULL resolution' — and immediately differentiates itself from the screenshot sibling by the scaling contrast. Names concrete use cases (tiny glyph, hairline border, icon) and a precise size constraint. An agent can select this over screenshot without opening either schema.

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 names the alternative (screenshot) and the decisive differentiator (the model's 1568px ceiling vs full-resolution capture). The 'For a tiny glyph, a hairline border, an icon' list gives concrete when-to-use guidance, and 'Refuses more than half the desktop' supplies the when-not boundary.

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