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

readOnlyHint=true already declares the safety profile, so the description only needs to add context. It does add the shape of the returned record and the underlying endpoint (POST /v1/rex/Contacts/read), which is useful. However the important warning that the full record is large lives only in the schema's `fields` description, not here, and nothing is said about authorization or failure behavior for a missing id.

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 sentences: what comes back, then where the id comes from, followed by the raw endpoint. Zero filler and the core purpose is front-loaded in the first five words.

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 only two well-documented parameters, a read-only annotation, and no output schema, the description is nearly sufficient — it enumerates return contents in place of a missing output schema. The one gap is that the payload-size caveat about omitting `fields` isn't echoed in the prose, where an agent would most likely see it.

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 genuine meaning beyond the schema: the `id` value is not arbitrary — it must be obtained from a search or autocomplete call. That is actionable context not present in the schema's bare 'The contact id.' The optional `fields` projection is left entirely to the schema, hence not a 5.

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 resource ('One contact by id') and enumerates the returned data (names, emails, phones, addresses, tags, relationships, related properties and listings). It implicitly distinguishes itself from rex_search_contacts by returning a single record by id, but never names a sibling tool explicitly.

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 provenance rule — 'The id comes from a search or autocomplete' — which routes the agent to rex_search_contacts or rex_autocomplete_contacts before calling this. It lacks an explicit 'do not use this to find contacts by name' exclusion, so it stops short of full when/when-not guidance.

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