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

Annotations already declare this is a non-destructive write, so the safety profile is covered. The description adds the return payload (id, name, NIF, email), which matters because there is no output schema, but it omits duplicate-handling behavior and any account/permission constraints for a 20-parameter mutation.

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 compact sentences, front-loaded with the action and the required-field constraint, with no filler. The return-value sentence earns its place given the absent output schema.

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 20-parameter tool in a regulated Spanish invoicing domain, the description covers the basic call shape and return values but not the conditional clusters (DIR3 trio requires is_public_admin, email_cc vs no_email_cc interaction) that an agent could get wrong. Adequate, not complete.

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 only 50% across 20 parameters, so the description should compensate more than it does. It usefully explains that nif/email/address are optional and why, but says nothing about recipient_type, dir3_*, no_email_cc, or default_product_group, which are the highest-consequence fields left to 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 ('Create a new customer') and scopes it to the user's contix account, which cleanly separates it from update_customer and get_customer. It does not explicitly name a sibling or state what it does not do, so it falls short of the top band.

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 clarifies the minimal viable call ('Only contact_name is required') and gives one concrete business rule (private individuals without NIF are fine), which is real usage guidance. However, it never says when to prefer this over update_customer, when to look up an existing customer first, or what happens on duplicates.

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