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

Annotations already declare a non-read-only, open-world, non-idempotent operation. The description adds genuinely useful behavior beyond that: the sending identity, a human-confirmation prerequisite, a hard 5-recipient cap, and a transactional-only restriction that rules out bulk mail.

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?

Three short sentences, zero filler. The core action and sending identity come first, followed by the confirmation requirement and the usage constraints, so the most decision-relevant facts are 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 6-parameter mutation tool with no output schema, the description covers identity, safety prerequisite, and scope limits well. Minor gaps remain on delivery behavior, threading semantics, and any rate limits, but nothing essential to correct invocation is missing.

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 83%, so the schema already documents from, html, text, and reply_to_id. The description only restates the recipient limit that maxItems=5 already encodes and gives no guidance on reply_to_id threading or html-vs-text precedence, so it adds little. Baseline 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 and resource ('Send an email') and adds the important scoping detail that mail originates from the agent's @agentline.nz address. It does not explicitly distinguish itself from siblings like send_sms or reply-related tools, so it lands at clear-but-undifferentiated.

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?

'Confirm recipients and content with the user first' plus 'transactional and one-to-one mail only' gives real when-to-use conditions and an implied exclusion (no bulk/marketing blasts). It stops short of naming alternatives such as send_sms for text messaging, so it is context-rich but not fully routing.

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