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

Goes well beyond the annotations (which already declare readOnly/idempotent/non-destructive) by disclosing that results are metadata-only, capped at 50 threads, that Drafts/Trash/Junk are skipped, and how the complete:false flag signals partial coverage. That partial-coverage semantics is exactly the kind of behavior an agent cannot infer from schema or annotations.

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 tight sentences, front-loaded with what it finds and where, followed by output shape and the caveat. Nearly every clause earns its place; the 'across selectable folders' phrasing is the one piece that adds ambiguity rather than information.

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?

With no output schema, the description carries the return-value burden and does so partially: it states metadata-only, 50-thread cap, and the complete flag. It doesn't indicate what metadata fields come back or why folders are mentioned without a folder input, leaving minor gaps for a 3-parameter search tool.

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 0%, so the schema names parameters without explaining them. The description clarifies recipientEmail must be an exact participant email and that results are capped at 50 (matching the limit maximum), but it never explains accountId directly and mentions 'selectable folders' despite no folder parameter existing in the schema, which is more confusing than clarifying.

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?

Specific verb (find), specific resource (thread candidates in one selected mailbox), and explicit scope constraints (exact participant email, selectable folders, excluding Drafts/Trash/Junk). It is clearly distinguishable from search_emails (recipient-exact, thread-level) and get_thread (verification, not discovery).

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 gives a clear workflow instruction: verify each candidate with get_thread before importing, and warns about the complete:false case. It doesn't compare itself against search_emails or find_message_by_rfc_message_id for the same lookup need, so the alternative selection is only partially covered.

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