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

Annotations already declare a safe read-only, closed-world, non-destructive operation, so the bar is lower. The description still adds useful context: prices reflect live checkout pricing rather than a static table, and any mutating step is pushed to the external site.

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 sentences, front-loaded with the usage trigger, then the return value, then the boundary. The final sentence about spainvisasubmit.com is somewhat tangential to invoking the tool but still earns its place as a scope guard.

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 two-parameter read tool with no output schema, the description covers trigger, return shape (EUR breakdown) and scope limits. It does not say whether the breakdown includes taxes or per-applicant lines, a minor remaining gap.

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 business meaning by framing the fee choice as 'with or without us paying the government fee Modelo 790-038' and reinforcing that cost is per applicant.

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 ('Returns the EUR breakdown from the current checkout pricing') scoped to filing cost for a number of applicants. It also explicitly distinguishes itself from document-handling siblings by declaring 'It never accepts or reads documents', so an agent knows this is not start_application.

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?

Gives a clear trigger ('Use when the user asks how much filing costs...') and covers both fee modes. It does not name a specific sibling as the alternative, but it does draw the boundary by stating that uploads, payment and power of attorney happen only on spainvisasubmit.com.

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