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

Annotations already cover read-only and idempotency, but the description adds real context: no sign-in, no account read, no side effects, US-only availability, and that values come from the same tables as checkout so they cannot drift from billing. The lack of any note on caching/staleness or rate limits keeps it just short of 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?

The purpose and the 'use before set_up_answering_line' instruction are front-loaded, and each subsequent sentence covers a distinct concern (scope, geography, data provenance, side-effect profile, return shape). The long inline return-field list is dense and slightly repetitive with 'reads no account and changes nothing', but it earns most of its length.

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 no output schema, the description enumerates the returned fields (packs, per_minute_cents, billing_rule, at_zero, outcome packs, countries, owner_must, terms_url, and follow-up tools). Combined with the usage instructions and effect profile, an agent has everything needed to call and interpret 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?

Zero parameters, so the baseline is 4; the description confirms 'Takes no arguments', which is a small but useful reinforcement. There is nothing further to document on this dimension.

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?

Starts with a specific verb and resource ('Read the offer for starting a new After Hours answering line') and immediately defines what the product is. It explicitly scopes itself to After Hours lines and names where other cost data lives (get_m2m_network, get_answering_line), so an agent can distinguish it from all siblings.

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 an explicit trigger: 'Use it before set_up_answering_line, to tell the owner exactly what they would buy.' It also names the alternatives and their coverage boundaries ('Direct credit costs are in get_m2m_network and an existing line's balance is in get_answering_line'), leaving no ambiguity about routing.

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