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

Annotations already cover safety (readOnlyHint, idempotentHint, openWorldHint=false), and the description adds the response semantics — 'yes/no and the nearest served town' — which matters because there is no output schema. It still omits edge behavior such as invalid/non-US or unserved ZIPs, but the return shape is the key gap and it is filled.

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, framed as the question the tool answers with the return value immediately after. Nothing wasted, purpose front-loaded.

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-param read-only lookup with no output schema, the definition covers what it does, what input it takes, and what it returns. The remaining gap is how unserved or malformed inputs are handled, which is minor given the tool's simplicity.

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 0%, so the description must carry the param's meaning, and it does call out 'US ZIP code'. However, the format constraint is already enforced by the schema pattern ^[0-9]{5}$, and the description adds no further semantics such as leading-zero handling or non-US input behavior.

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?

It states a specific verb+resource (checking a US ZIP code against the El Paso service area) and even gives the scope of the answer (yes/no plus nearest served town). It does not explicitly differentiate itself from the sibling get_service_info, so an agent must infer that this is the boolean membership check while the sibling presumably returns richer detail.

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?

Usage is implied by the question form — you call this when you need to know whether a ZIP is served. There is no explicit when/when-not guidance and no routing to get_service_info for details or request_service for actually ordering service, which would have been easy to add.

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