Thread overview

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

The annotations already declare readOnlyHint=true, and the description adds valuable behavior: output structure, layout semantics ('tree' vs 'flat'), and token efficiency compared to mail_read_thread. It does not contradict annotations, and adds meaningful context beyond the safety hints.

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 appropriately sized and well-structured: it starts with the output format, then gives usage guidance, then details arguments. Every sentence earns its place with no redundant or vague filler.

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?

The description is highly complete for a read-only overview tool: it specifies output fields, layout options, usage context, and parameter origins. Minor gaps like pagination or max message limits are not addressed, but the description is sufficient for the tool's purpose.

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?

With 0% schema description coverage, the description fully compensates. thread_id is explained as a value from mail_search with/without 'thread:' prefix, and layout is thoroughly described with tree/flat semantics and its relation to mail_read_thread's numbering. This adds substantial meaning beyond the schema's bare types.

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 clearly states the tool's output format ('One line per message: index, date, size, from, and a marker when the subject changes') and identifies the resource as a mail thread overview. It distinguishes itself from sibling mail_read_thread by emphasizing its lightweight, token-efficient nature.

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?

Explicit guidance is provided: 'Use this before mail_read_thread on any thread you have not looked at yet' and 'Do not use it in place of mail_read_thread when you already know you need the full content.' This clearly states when to use and when not to use, and names the alternative tool.

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