Screenshot a shared tab

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

Goes well beyond the annotations (which only declare read-only, non-destructive, non-open-world): it discloses output bounds (longest side <= 1600 px, small JPEG), that the image is a short-lived artifact whose artifact_id expires when sharing ends, that the result includes a local file path for hosts that cannot render inline images, and that a user permission grant is required. No contradiction with readOnlyHint=true or destructiveHint=false.

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?

Front-loads the core action and its scope in the first sentence, then adds constraints, fallback behavior and lifecycle in a logical order. Dense but each sentence carries operational information; only 'so you can look at it' is mild padding.

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?

With no output schema and a 7-parameter nested schema, the description carries the return-value burden and does it: what image comes back, how large, where the local file is, how long the artifact lives, and what permission is needed before calling. Nothing an agent needs to interpret the response is missing.

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 baseline is 3; the schema already explains rect coordinates (page-relative CSS pixels), the jpeg-vs-png tradeoff, local_file, include_image and browser_instance_id. The description's mentions of rect and JPEG mostly restate that, and the new detail (1600 px bound) concerns output rather than any parameter.

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?

Specific verb ('Capture') plus resource ('visible viewport (or a CSS-pixel rect) of a shared tab') and the purpose of the output ('return the image so you can look at it'). This distinguishes it from DOM-oriented siblings such as browser_snapshot, which would not return a rendered image.

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?

Gives a clear usage context (visual inspection when inline image is unavailable) and a concrete fallback path ('open that file with your image viewer (for example view_image)'), plus a prerequisite ('Needs the user to have allowed screenshots'). It does not explicitly contrast itself with browser_snapshot or other observation tools, so it stops short of naming when-not-to-use.

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