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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint, and destructiveHint=false, so the safety profile is covered. The description adds the permission requirement ("Needs contacts.read"), which is real value beyond the structured data, but says nothing about not-found/error behavior or that embeds may silently return null without extra scopes (only the schema hints at this).

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?

Two short sentences, zero filler, with the primary action and its identifier requirement front-loaded ahead of the permission note. Nothing is padded or repeated.

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 3-parameter read tool with no output schema, the description covers what it does and what auth it needs. It omits the shape of the returned contact, behavior on an unknown id, and the fact that requesting embeds can yield nulls — gaps the schema only partially mitigates.

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 description coverage is 100%, and the schema itself documents all three parameters including embed scope caveats and field projection examples, so the baseline of 3 applies. The description adds nothing beyond "by id", contributing no semantics the schema lacks.

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?

"Get one contact by id" gives a specific verb (get), resource (contact), and scope (single, by id), which cleanly separates it from sellsy_list_contacts and sellsy_search_contacts. It stops short of naming those siblings explicitly, so differentiation is inferred from the singular/by-id framing rather than stated.

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?

The phrase "by id" implicitly tells the agent to use this when a contact id is already known, and the sibling set (list_contacts, search_contacts) makes the alternative paths guessable. However, no explicit when-to-use/when-not or named alternative is provided, leaving routing to inference.

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