Screenshot a web page

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

Annotations already declare readOnlyHint, openWorldHint, and idempotentHint, so the safety profile is covered. The description adds behavioral context beyond annotations: output artifact formats (PNG vs JPEG) and the non-interactive nature of pixels. It does not contradict the annotations.

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 two tight sentences with no filler. The core action is front-loaded, artifact distinctions follow, and the sibling-routing caveat is placed last. Every sentence earns its place and nothing repeats schema content.

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 100% schema coverage and read-only/idempotent annotations, the description covers the key disambiguation from steel_snapshot and the two capture modes. It does not explain what happens when neither url nor session_id is provided, but the optional parameter schema and mode descriptions give sufficient guidance for a competent agent.

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 descriptions cover all 5 parameters, so the baseline is 3. The description adds mode-level meaning that helps select between url and session_id by specifying output format and audience. It does not re-explain full_page or use_proxy, but those are already well documented in 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?

The description opens with a specific verb and resource: 'Capture a page image.' It further distinguishes URL captures and session captures by artifact type, and differentiates the tool from steel_snapshot by noting that pixels are not action targets. This makes the tool's purpose unambiguous relative to siblings.

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 explicitly tells agents when not to use this tool: 'Pixels are not action targets, so use steel_snapshot to click or type.' It also provides mode-selection guidance by contrasting user-facing PNG URL captures with model-visible JPEG session captures. This gives clear routing versus the main sibling alternative.

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