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

Annotations only declare readOnly/openWorld/non-destructive; the description adds real behavioral context beyond them: the inbox_token requirement means only the creator can read, the call returns immediately rather than blocking, mail typically lands within seconds, and an empty result is safe to retry shortly. That auth + latency/retry disclosure is exactly the kind of value annotations cannot carry.

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 compact sentences with the core purpose and ordering front-loaded, then auth, then timing, then the format option. Every sentence carries information; it runs slightly long but no clause is wasted.

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?

There is no output schema, so the description must convey returns, and it does: newest-first messages, plain-text flattening with link targets by default, or raw HTML when format="html". Retry behavior on empty results is covered; pagination or message-count limits are not mentioned, keeping it just under fully complete.

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 three parameters including the format enum are already documented in the schema. The description's format="html" note largely restates the schema wording, adding no syntax or format detail the schema lacks — 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 ("Read mail delivered to an inbox"), names the originating sibling (create_inbox), and specifies ordering ("newest first"). An agent can distinguish this from create_inbox or the review tools 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 a concrete motivating scenario (verification code or confirmation link from a signup email) and the format="html" case for design review, so the agent knows the typical context. It never states an explicit when-not-to-use or an alternative to pick instead, so it stops short of a 5.

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