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

Annotations already cover read-only/idempotent safety, but the description adds real behavioral detail: paired-parameter behavior that unlocks crop/letterbox planning, the TikTok null-inset case with its reason, and both error conditions (unknown id whose message lists valid ids, and a single source dimension). That is context the annotations cannot supply.

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 lookup and its return payload, then the optional dimension pair, then the sibling routing, then edge cases. Every sentence carries distinct information — no restatement of the title or schema types.

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 an output schema present, the description needn't detail return shapes, and it still names the returned fields. It covers the optional-input path, the known-degenerate platform (TikTok), and both failure modes — nothing an agent needs to call this correctly is missing.

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 baseline is 3; the description goes further by explaining what the optional pair actually buys you (centre-crop and letterbox plan fitted to your footage), which is outcome semantics rather than a restatement of types. Minor credit withheld because the id enum's camelCase/kebab-case duality is only handled 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?

States a specific verb and resource (look up one placement by id) and enumerates exactly what comes back: frame size, aspect ratio, safe-area insets, usable box, source page. It is immediately distinguishable from list_social_media_placements and check_safe_area, which are named.

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?

Explicit routing: 'Use this when you know the target placement', then names three alternatives with the condition that selects each (browse/compare, test element clearance, compute a non-platform ratio). The when-to-use decision is fully resolved inside the description.

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