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

The description adds genuinely useful behavior beyond annotations: it discloses billing (prepaid UK credits, one per distinct valid number), the fixture behavior of test API keys, and the possible result values. The credit-charging detail is especially valuable since annotations declare readOnlyHint=false without explaining why a screening call is a non-read operation.

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?

Four tightly packed sentences, front-loaded with purpose, then result values, then cost behavior, then API mapping. Every sentence carries distinct information with no repetition of the name or title.

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 tool with full schema coverage and no output schema, the description supplies everything an agent needs: what it does, when to use it, what the outputs mean, and the cost implication of each call including the zero-cost test path.

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%, so both parameters (campaign_ref, phone_numbers) are fully documented in structured fields. The description adds only an indirect hint ('one per distinct valid number') about billing/deduplication, but nothing about parameter formats or limits beyond what the schema already states.

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 gives a specific verb and resource (screen UK phone numbers against TPS and CTPS) plus the timing context ('before calling'). It clearly distinguishes itself from siblings like check_dead_numbers (dead numbers, not regulatory screening) and wash_au_numbers (Australian numbers), so an agent can select it without opening a 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?

'...before calling' gives a clear usage context, and the TPS/CTPS register framing implies the compliance use case. However, it never explicitly names the sibling alternatives (check_dead_numbers, wash_au_numbers) or states when NOT to use this tool, so it stops short of full routing guidance.

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