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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds genuinely new behavior: headers are searched across folders, exact equality is verified after the IMAP search (so near-misses return nothing rather than approximations), and it returns opaque message/thread IDs that feed other tools — useful because there is no output schema.

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?

Two dense sentences, front-loaded with the action and return value, then the mechanics and access requirement. No restatement of the name and no 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?

For a two-parameter read tool with annotations and no output schema, the description covers what it does, how it searches, what it returns, and the access level. The remaining gap is the ambiguous mailbox-vs-folders scope and the unstated not-found behavior, which an agent would want to know before relying on it.

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?

At 50% schema coverage, only rfcMessageId is documented in the schema (with pattern and provenance); accountId has none. The description partially compensates by stating the search scope is limited to the supplied account/mailbox, but it does not explain the accountId format or how it is obtained (list_mail_accounts), leaving a gap the schema does not fill.

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 — find an exact RFC Message-ID and return its opaque message/thread IDs — which separates it from the fuzzy search_emails and search_threads_by_recipient siblings. The scope wording is slightly muddy, though: "one selected mailbox" sits uneasily next to "searches mailbox headers across folders," leaving it unclear whether the search is account-wide or folder-bounded.

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 word "exact" and the schema note that the ID comes from send_email/reply_to_email imply when this tool applies, and "requires read access only" sets a permission precondition. However, no sibling is named and there is no explicit when-not guidance (e.g. use search_emails when you don't have an exact Message-ID), so routing must be inferred.

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