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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), and the description adds genuinely non-obvious behavior: HTML is converted to text rather than returned as markup, attachments are listed as metadata only, and bodies are capped at 20000 characters with a 'truncated' flag signaling more content. These are exactly the traits an agent cannot infer from structured fields.

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 short sentences, each carrying distinct information (purpose, id provenance, body/attachment behavior, truncation). The core capability is front-loaded and nothing repeats.

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 single-parameter read tool with an output schema and full annotation coverage, the description covers everything an agent needs: how to obtain the id, what the body looks like, what is excluded (attachment content), and how to detect truncation.

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 coverage is 100% and the sole parameter's schema description already says 'The message id from a search_messages row,' so the description's 'give it the id of a row from search_messages' is essentially a restatement. Baseline 3 applies since the schema does the work.

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?

Opens with a specific verb and resource ('Read one message: its headers and body') and immediately scopes what is and isn't returned. It clearly distinguishes itself from search_messages (which supplies the id) and get_attachment (which supplies attachment content).

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?

Explicitly states the id must come from a search_messages row, giving the required upstream workflow, and explicitly routes attachment-content requests to get_attachment. An agent knows both when to call this and when to call a sibling instead.

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