Read a message

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

The annotations already declare readOnlyHint=true and openWorldHint=false, but the description goes well beyond them: HTML-to-text conversion, body truncation with the exact marker and truncated flag, attachment contents excluded, unknown message_id producing a 'not found' error, and the fact that reading does not mark the message as read. Those are the non-obvious behaviors an agent would otherwise have to discover by trial.

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?

It is front-loaded with what is returned, then routing, then edge cases (truncation, missing attachments, bad id, side effects) — a logical order with no filler sentences. Every clause carries operational information; the long opening list is a payload description, not verbosity.

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?

There is no output schema, so the description correctly compensates by enumerating the returned fields and flagging the truncation indicator. For a 3-parameter read tool it fully covers routing, limits, failure mode, and side-effect profile — nothing an agent needs to call it correctly is missing.

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%, so the baseline is 3. The description nonetheless adds behavioral meaning to max_chars by explaining that the body is cut at that limit, marked '[truncated]', and flagged with truncated=true, which tells the agent how to react to the value rather than just what it is. message_id semantics (unknown id returns a 'not found' error) are also surfaced, though account is left entirely to the schema.

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 first sentence gives a specific verb and resource ('Read one message in full') and enumerates exactly what is returned: from, to, cc, reply-to, date, subject, labels, body as plain text, and attachments with part_id/name/type/size. It explicitly distinguishes itself from read_thread (whole conversation) and get_attachment (attachment contents), so an agent can route without opening any schema.

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 states the trigger clearly ('after search_emails when you need what a single message says') and names the two alternatives with the conditions that select them: read_thread for a whole conversation, get_attachment for attachment contents. Both a when-to-use and a when-to-use-something-else are present.

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