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

Annotations already declare read-only/idempotent/non-destructive, yet the description goes well beyond them: the 256 KB cap, the text-vs-base64 encoding rule, the delivery-policy refusal of executables/archives, the enumeration of error messages, and a prompt-injection warning about untrusted attachment content.

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?

Dense and front-loaded with the return shape first, then types, then errors and parameters. It is somewhat long, and 'Read-only; it changes nothing' duplicates the readOnlyHint annotation, but nearly every sentence carries operational value.

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 still documents the exact return fields and encodings, plus error cases and the size/type constraints. An agent has everything needed to call and interpret this tool correctly.

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, but the description adds real semantics: email_id's provenance (list_emails/wait_for_email) and the 'a1', 'a2' ordering and lifetime stability of attachment_id. The information is largely reinforcement of the schema's own examples and patterns, so it does not reach a full 5.

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+resource and immediately enumerates the returned shape 'as { id, filename, content_type, kind, size, encoding, content }'. It also scopes the accepted types and size limit, so an agent can distinguish this from read_email or list_emails without opening the 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?

Explicitly states when NOT to fetch: calendar invites are already parsed into the email's calendar field, so fetch the .ics only for the raw file. It also tells the agent that every email lists attachments even unfetchable ones, which sets expectations before a call.

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