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

Annotations already say readOnly, idempotent, openWorld, and non-destructive, so the behavioral bar is lower. The description adds meaningful context beyond annotations: it discloses the auth/plan requirement and explains that items distinguish proved no-reply from unknown. It does not cover pagination, rate limits, or broader operational caveats, so 4 is appropriate.

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?

The description is three tight sentences, front-loading the filtering rule before the output status and auth requirement. Every clause earns its place by adding selection criteria or invocation context. There is no filler or repetition.

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?

Given the output schema exists, the description need not explain return values, and the annotations cover safety. It provides the core filtering semantics, the unknown/no_reply_found caveat, and auth/plan requirements, which is nearly complete for a list-style tool. It leaves within_days semantics and some parameter interactions to the schema, so 4 rather than 5.

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 the schema already documents all five parameters. The description adds real semantic value for older_than_days by defining it as the threshold for the owner's newest message being too old, but it does not clarify within_days, limit, account, or exclude beyond the schema. That is enough for a 4 but not a 5.

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 states a precise verb and resource: it lists threads where the owner wrote last and no reply exists. The criteria, including older_than_days and the no_reply_found/unknown distinction, make the tool's purpose clear. It does not explicitly name a sibling such as list_followups, so it gets a 4 rather than 5.

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?

The description gives the core condition for use: sent mail waiting for a reply based on owner-last-written threads and older_than_days. It does not state when to prefer this over list_followups, search_messages, or triage, nor does it state when not to use it. The prerequisite wording 'Needs read; every plan' helps, but usage guidance remains implied.

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