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

With no annotations, the description carries the full behavioral burden. It does add genuinely useful context the schema cannot convey: quota consumption ('Uses 1 email of today's allowance') and the transactional-only restriction. However, it says nothing about auth requirements, failure behavior, deliverability limits, or whether sending is reversible/immediately dispatched.

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, and the core scoping constraint plus the alternative route are front-loaded. Every clause earns its place by constraining what the agent may send.

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 and no annotations, the description covers the essentials: scope, quota cost, and the sibling it must not replace. It leaves auth/permission expectations and error conditions unmentioned, which is a modest but real 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 83%, so the schema already documents almost every parameter (including reply_to's fallback behavior and the text/html alternative). The description only indirectly clarifies that 'to' is a single recipient, adding little beyond the structured fields; 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 one transactional email') plus the scope constraint ('to one recipient') and examples of the category (receipt, code, alert). It explicitly distinguishes itself from the sibling send_campaign, so an agent can choose between the two without opening either 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?

It names the alternative route explicitly: 'Marketing mail must go through send_campaign,' which is a clear when-not condition. It gives no positive guidance for edge cases that siblings imply exist (e.g., test sending vs. list_test_messages, or account/auth prerequisites), so it is clear context without full coverage.

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