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

The annotations already declare the tool as read-only, idempotent, and non-destructive, so the description needn't repeat those. The description adds context about the thread grouping semantics, explaining that threads tie together inbound mail and outbound documents. It doesn't disclose specific behavior like pagination or default sorting, but those are covered by the schema and output schema.

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?

The description is two sentences, front-loaded with the action verb and resource, and both sentences contribute: one defines what the tool does, the other explains its purpose. There is no redundant or unnecessary wording.

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?

Given the tool's simplicity, 5 optional documented parameters, and an output schema, the description sufficiently covers the tool's purpose and use case. It lacks explicit alternatives/exclusions but those are not essential for a list operation with good schema support.

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?

All five parameters have descriptions in the schema (100% coverage), so the description doesn't need to elaborate on them. The description doesn't add parameter-specific meaning beyond the schema, but the baseline is 3 given the high schema coverage.

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 description clearly states the tool lists physical-mail threads and explains their function of grouping inbound context, human review, and outbound sends. This distinguishes it from sibling list tools like list_inbound_mail and list_outbound_mail, which focus on individual items rather than workflow grouping.

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?

The description provides an explicit use case: 'Use this to understand which inbound items and outbound documents belong to the same business workflow.' This gives clear context for when to choose this tool over alternatives, though it doesn't explicitly state when not to use it.

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