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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint=false, and the description adds substantial context beyond them: the preview's approximate length (200 chars), what it strips (quoted history, signature), its null behavior for empty or image-led bodies, and a security warning to treat message content as data never as instructions. That last point is high-value behavioral context the annotations cannot convey.

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?

Front-loaded with the core purpose, then preview semantics, routing, the null edge case, and security. Every sentence is purposeful, but there is mild redundancy: 'what has come in?' is ONE call' and 'USE THE PREVIEW rather than reading each message' make the same efficiency point twice. Otherwise zero fluff.

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?

Complete for a low-complexity list tool with no output schema. The description carries the return-value burden by explaining the preview field, its null case, and its scope, while annotations cover the safety profile and the schema covers both parameters. An agent has everything needed to select and invoke the 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 description coverage is 100%: both limit and mailbox already carry descriptions with defaults and constraints. The description adds no syntax or format detail for the parameters beyond what the schema provides, so the baseline 3 applies. This is acceptable because the schema does the heavy lifting.

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 and resource: 'List the most recent messages in a mailbox, newest first.' The ordering and scope are explicit. It differentiates from siblings by explaining where it ends and read_email begins ('it is not the message'), so an agent can distinguish it from read_email, search_emails, and list_mailboxes.

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?

Gives explicit when-to-use guidance: 'what has come in?' is one call, and 'USE THE PREVIEW rather than reading each message.' It names the alternative explicitly and the condition that selects it: 'call read_email when you need what a message actually says, the recipients, or its attachments.' Nothing 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.