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

Annotations already mark this read-only and non-destructive, and the description goes well beyond them: one admitted API call consumes one shared account request (including admitted errors) and there are no automatic retries. These quota/retry semantics are exactly the behavioral context an agent needs and cannot get from structured fields.

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?

Four tight sentences, front-loaded with the action and result, then constraints. Dense but every sentence carries distinct information; nothing is filler, though the packing is close to terse.

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 need not explain return values, and it still adds quota, retry, ID-sourcing, and face/catalogue semantics. For an image-fetch tool with catalogue variants and shared request accounting, nothing an agent needs to call it 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 75% and the schema already documents face and catalogue_card_id. The description adds real value by explaining that face selects a Magic face and that catv_ variants require their parent cat_ ID, tying parameters to the catalogue workflow rather than just restating them.

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 states a specific verb and resource: return verified WebP artwork as an MCP image block. It is clearly distinct from data-returning siblings like get_card or search_cards. It stops short of explicitly naming which sibling to prefer when the agent wants card data rather than artwork.

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?

It explains where valid IDs come from (the images.small/large query fields), the prerequisite for catv_ IDs (their parent catalogue_card_id), and an explicit prohibition (never pass an arbitrary image URL). No explicit when-to-use-vs-alternative routing against siblings, but the invocation conditions are concrete.

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