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

Annotations already declare destructiveHint=true and idempotentHint=false, so the safety profile is covered; the description adds genuine behavioral detail beyond that — the two-phase destination change (preview + confirmation_token, USD pricing shown to the user) and that non-destination updates apply immediately. It does not, however, describe what a rejected/failed update looks like or any permission requirements.

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?

Front-loaded with the core action and scope, then alternatives, then the gating caveat — a sensible priority order with no filler sentences. The mid-sentence em-dash clause about USD pricing is slightly dense but still earns its place.

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 14-parameter mutation with no output schema, the description covers the critical non-obvious behaviors: partial updates, sibling routing, and the confirmation-gated destination path. It omits any statement of required permissions/account scope and error behavior, which is the remaining gap.

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 all 14 parameters are already documented, including the confirmation_token protocol and destination semantics. The description adds only a deferral ('fields have the same semantics as in create_pstn_trunk') and the USD note, so the baseline 3 is appropriate.

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 and resource ('Update ... PSTN / call-forwarding Voice IN Trunks') and explicitly disambiguates from the two nearest siblings: update_sip_trunk and update_phone_systems_trunk. An agent can pick the right tool without opening any 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?

Gives explicit routing rules ('For SIP trunks use update_sip_trunk; for phone.systems™ trunks use update_phone_systems_trunk'), the partial-update convention ('pass the trunk id and ONLY the fields to change'), and the condition that triggers the confirmation flow versus immediate application.

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