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

With no annotations, the description carries the full burden and does well: it discloses the headful-session prerequisite and the exact failure behavior (returns "unattended" in headless mode), which an agent needs before calling. It omits edge cases such as what happens with an invalid or unmatched selector, but output_schema exists so return values need not be restated.

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?

Two short sentences, front-loaded with the purpose then the operative constraint. No filler; every clause earns its place.

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 one-parameter visual tool with an output schema already documenting returns, the description covers purpose and the key session constraint adequately. The only real gap is selector semantics, which is left to the schema that does not describe it.

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 0% and the single required parameter "selector" has no schema description, so the description must compensate. It only implies that a selector identifies "an element" without stating the accepted format (CSS, XPath) or matching behavior, leaving the parameter under-specified.

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 ("Outline an element") plus the reason ("so the user can see what you mean"), which makes the visual-emphasis intent distinct from action siblings like click or hover. It does not explicitly name an alternative tool, so it stops short of full sibling differentiation.

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 condition: it requires an attended (headful) session, and in headless mode it is inapplicable. This is an explicit when-not-to-use constraint, though it names no alternative sibling for the headless case.

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