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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint, so the safety profile is fully covered structurally. The description adds the concrete endpoint (GET /api/customers.json?phone_number=...), which is useful context, but says nothing about not-found behavior, error responses, or what happens with ambiguous/multiple matches.

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 with zero filler, and the core action plus lookup key are front-loaded. The trailing raw HTTP endpoint is mildly redundant against the structured schema but still compact.

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 with full annotations this is close to adequate, but the description omits the not-found/empty-result behavior and does not characterize the returned contact object. With no output schema present, a brief note on the return payload would have been worthwhile.

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 100% — the single phone_number parameter already carries its own description plus a format pattern and maxLength. The description adds no syntax, normalization, or E.164 guidance beyond the schema, so the baseline of 3 applies.

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 (Retrieve), resource (one contact/customer from the address book) and the lookup key (phone number), which is more precise than its title. It does not explicitly distinguish itself from the sibling 'textline_get_customer' (lookup by another key) or 'textline_get_conversation_by_phone', leaving the reader to infer the difference.

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?

The phrase 'by phone number' implies the selection condition (use this when you only have a phone number rather than the customer ID used by textline_get_customer), but there is no explicit when-to-use statement, no exclusions, and no mention of the alternative tools. Usage must be inferred.

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