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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds genuinely new behavior beyond that: a hard cap of 8 results, the empty-query fallback, and the fact that results surface as attachable resource links rather than plain data. It stops short of describing ordering or ranking of the typeahead matches, which keeps it from a 5.

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 short sentences, no filler, with the typeahead purpose front-loaded and the sibling disambiguation placed last as a routing note. Every sentence carries distinct information.

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 single-parameter read-only lookup with no output schema, the description covers what it matches, how many results come back, the empty-query case, and the form of the return (resource links in a composer menu). Nothing an agent needs to call it correctly 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?

With one parameter at 100% schema coverage, the baseline is 3, but the description goes further by explaining what an empty query does semantically ('returns the most recent calls'), which the schema's 'may be empty' does not convey. That is real added meaning for the only parameter.

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?

The description opens with a precise role: 'Typeahead for the ChatGPT composer @-mention menu', then names the resource set (contacts and recent calls as attachable resource links). It also explicitly distinguishes itself from the sibling `search` tool, so an agent can route correctly without opening either schema.

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 names the alternative explicitly ('To look something up for an answer, use search instead') and documents the degenerate case ('An empty query returns the most recent calls'). Both the when-to-use and when-not-to-use conditions are stated outright.

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