session_viewport

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

With no readOnlyHint or destructiveHint annotations, the description carries the burden, and it does well by disclosing side effects beyond a simple 'set': scripts observe innerWidth/innerHeight movement, media queries re-evaluate, element rects re-anchor, and mobile=true flips matchMedia pointer/hover answers. It also explains the retain-current-value behavior for omitted dimensions. It doesn't discuss auth or rate limits, but those aren't critical for a viewport setter.

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: two sentences with a front-loaded purpose, followed by a concise list of behavioral effects and a closing usage note. Every phrase adds value, and there is no filler or repetition beyond what the schema already covers.

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 viewport mutation tool with no output schema, the description fully explains what happens when invoked, including the mobile flag's effect and the default for omitted width/height. Combined with the detailed schema, an agent has everything needed to decide whether and how to call the tool.

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 already has 100% parameter descriptions, including 'omit to keep the current width/height' and mobile matchMedia behavior. The description largely restates these details ('Omitted width/height keeps the current value', 'mobile=true flips pointer/hover matchMedia answers to coarse/none'), adding little beyond the schema. 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 opens with a specific verb and resource: 'Set the session's viewport (device emulation).' It goes beyond a vague label by enumerating concrete consequences (innerWidth/innerHeight change, media queries re-evaluate, element rects re-anchor), which clearly distinguishes it from sibling tools like session_navigate or session_scroll.

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 establishes clear context: call this to emulate a device viewport for the session. It also gives a practical usage note ('Omitted width/height keeps the current value'). It doesn't explicitly state when not to use it or mention alternatives, but no sibling tool offers viewport control, so the context is sufficiently clear.

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