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

Annotations already declare readOnly/idempotent/non-destructive, so the bar is lower, yet the description still adds real behavior: failed messages move neither timestamp, and a cross-inbox id yields not-found despite the person existing. It stops short of describing response shape or rate/permission characteristics, but the semantic edge cases disclosed are substantial.

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?

Purpose is front-loaded in the first clause and the paragraphs are thematically split (timestamps, refresh block, scoping). It runs long and the `refresh`/outcome enumeration overlaps with what an output schema likely already conveys, costing a little tightness.

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 with a full schema and an output schema, this covers everything an agent needs: what is returned, how timestamps behave, what the refresh block means, and the inbox-scoping failure mode. Nothing material 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, but the description adds meaning the schema does not: `number_alias` acts as an inbox scope that makes otherwise-valid ids resolve to not-found. That is genuine semantic value beyond the parameter's own text.

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?

Opens with a specific verb and resource ('Fetch one contact') plus the returned fields, which cleanly separates it from list_contacts and the picture/quota siblings. An agent can tell immediately this is the single-record read.

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?

Clear operational context is given: the id must belong to the inbox named by `number_alias`, and a cross-number id returns not found. It also implicitly routes to `refresh_contact` by explaining what the `refresh` block reports, but it never explicitly says when to prefer this over list_contacts or get_contact_profile_picture.

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