fetch_favicon

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

The description adds value beyond the annotations by detailing the return structure: 'all discovered icons with their URLs, types, and sizes.' It also uses the qualifier 'discovered,' which hints that the tool scans the website and may not find all icons. However, it does not mention error handling or rate limits, though the readOnlyHint and openWorldHint annotations already cover the general behavior.

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 concise, with two sentences that front-load the purpose and then provide key details about the output. There is no redundant or extraneous information, making it easy for an agent to parse quickly.

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 simple tool with one parameter and no output schema, the description provides sufficient context: it states what the tool returns and the nature of the output. It does not mention error cases, but the schema covers parameter validation, and the annotations cover the read-only and open-world aspects. The description is adequate for an agent to call the tool correctly.

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 schema already fully describes the 'domain' parameter, including its format ('Without https://') and an example. The description does not add any additional information about the parameter, so it adds no value beyond the schema. Given 100% schema coverage, this is acceptable.

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 clearly states the tool's function: finding favicon URLs for a website. It specifies the output: 'all discovered icons with their URLs, types, and sizes,' which distinguishes it from other web tools that might return different data. No sibling tool mentions favicons, so it's uniquely identifiable.

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 implies usage when the agent needs favicon URLs but does not explicitly compare with alternatives or state when not to use it. There is no mention of sibling tools or conditions that would favor another tool, such as extract_meta_tags or take_screenshot. This leaves some ambiguity, but the purpose is clear enough for an agent to infer its use case.

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