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

Annotations already declare the destructive, non-idempotent write profile, so the bar is lower. The description still adds genuinely new context: 'Staged — publish to see it live' discloses a preview/publish workflow not captured by any annotation, and the clear path plus size cap clarify side effects.

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?

Front-loaded with the core action, then the source union, format advice, staging note, and reference — every clause earns its place with no redundancy. Multiple short fragments keep it scannable.

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?

Covers the source union, format best-practice, and the staged/publish behavior for an 8-param tool with no output schema and full annotation coverage. The only minor gap is not stating that a new favicon replaces the existing one, though destructiveHint implies 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 coverage is 100% so the baseline is 3, but the description adds meaning beyond the schema by declaring that exactly ONE source must be supplied among mediaId/imageUrl/svgContent/dataBase64/clear, a mutual-exclusion rule absent from the schema. It also flags dataBase64 as small-icon-only.

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+resource ('Set or clear the site favicon') and, notably, names the two opposite operations it supports. No sibling tool competes for this purpose, so an agent can immediately tell this is the favicon-mutation tool.

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?

Provides operational constraints ('Exactly one source') and a recommended format ('512px square PNG or an SVG'), plus a pointer to get_style_reference for more. It does not explicitly contrast against nearby tools like upload_image, but the usage constraints are clear and actionable.

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