Favicon URL for a page

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

Annotations already declare readOnlyHint=true, and the description is consistent with that ('which favicon URL it has stored'). It adds real value beyond the annotations by defining the empty-result case as 'no icon has been recorded for that page yet' and clarifying that no image is downloaded — exactly the kind of context annotations can't carry.

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?

Three short sentences, each doing distinct work: what it does, what it returns, and how to interpret an empty result. The return-value constraint is front-loaded rather than buried.

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 one required param, no output schema, and readOnlyHint already covering safety, the description fills the remaining gaps an agent needs: return shape ('the URL only'), the non-download caveat, and empty-result semantics.

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% and the single parameter is documented as 'Page URL to look up'. The description implies the lookup is per-page but adds no format guidance (absolute vs relative, internal URLs, etc.) beyond the schema — the 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?

States a specific verb and resource ('Ask Places which favicon URL it has stored for a page') and immediately bounds it ('Returns the URL only — it does not download the image'), so an agent knows exactly what it gets. No sibling tool covers favicons, so differentiation is unambiguous.

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 makes the use case clear (look up the stored favicon URL for a page) and draws the key boundary against a plausible alternative behavior — fetching the icon bytes — which it explicitly disclaims. It stops short of naming a when-not-to-use condition, but no such alternative sibling exists.

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