List Inboxes

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

Annotations already declare readOnlyHint=true, destructiveHint=false, so the safety profile is covered. The description adds value beyond that by enumerating the return payload, including the derived 'unread' count and 'last_received', which an agent needs to answer the stated use case. It stops short of mentioning permissions or pagination behavior beyond what the schema already documents.

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 labeled blocks (Purpose, NOT for, Returns, When to use), each one sentence, with purpose and exclusion front-loaded. No filler and no repetition of the schema.

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 explicit Returns list is doing essential work. Combined with purpose, exclusions and trigger conditions, an agent has everything needed to call this against a tool set of ~100 siblings.

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 100% and each of after/limit/before is fully documented in the schema, including the mutual exclusion and 404 behavior. The description adds no parameter-level detail of its own, so the baseline of 3 applies.

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+resource (list Resend inboxes on the account) and immediately distinguishes it from the two most confusable siblings (list-received-emails, list-emails) in the NOT for section. An agent can select this over any other list-* tool without opening a 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 names when to use it ('what inboxes do I have?', need an inbox ID for another inbox tool) and when not to, naming the correct alternatives. Nothing about routing is left to inference.

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