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

Annotations already declare readOnlyHint=true, openWorldHint=true and destructiveHint=false, so the safety profile is covered. The description adds genuinely new behavioral information — it returns the full contact or null on no match (so a miss is not an error), and names the backing GraphQL query contactByIdentifier. No auth or rate-limit context, but that is a minor gap here.

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?

Front-loaded with the action and keys in one sentence, followed by return behavior and then a trailing GraphQL name fragment that is marginally useful. No filler, though the query-name detail is the weakest element.

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 no output schema, the description covers keys, format, and the null-on-no-match return, which is what an agent needs to branch on results. It does not say what happens when both email and mobile are supplied, leaving one small behavioral question open.

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 would be 3. The description exceeds that by resolving the ambiguity of zero required parameters — it states at least one identifier must be supplied — and reinforces the E.164 format expectation for mobile.

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 plus the exact lookup keys ('Find one contact by exact mobile number or email address'). The emphasis on 'exact' and 'one' implicitly separates it from list_contacts and from fuzzy lookups like search_numbers, so an agent can pick it without opening sibling schemas.

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 useful invocation context ('give at least one') and format guidance (E.164), but never names an alternative such as get_contact when an ID is already known, nor says when this tool is the wrong choice. Usage is implied rather than explicitly routed.

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