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

Annotations already declare readOnlyHint/idempotentHint/destructiveHint=false, so those claims in the description are redundant. Beyond that, the description adds real value: auth requirement, ordering by 'best matching message', and concrete failure semantics (empty q -> HTTP 400 missing_query, unknown inbox -> inbox_not_found), which annotations cannot express.

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?

Well structured with a when-to-use lead followed by Inputs/Returns/Errors blocks, and no wasted sentences. Minor redundancy in restating 'Read-only' and the parameter docs that already exist in 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?

With no output schema, the description carries the return contract itself ({object:'list', inbox_id, q, data:[thread]}), the ordering rule, parameter defaults, and two named error outcomes. That is everything an agent needs to call and interpret this tool.

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%, so q, limit, and inbox_id semantics are already documented in the schema; the description largely restates them (inbox id/username/email, every word matches, default 20 max 50). It adds only the error conditions attached to bad inputs, so the baseline 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 (search) and resource (threads), and immediately contrasts the granularity with the sibling concept of per-message results: 'the conversations that contain a keyword, sender, recipient, or subject, instead of each matching message.' An agent can distinguish this from cooper_search_messages without opening either 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?

Gives an explicit selection condition ('when you need the conversations ... instead of each matching message'), which functions as both when-to-use and when-not-to-use. The gap is that the alternative tool is described only functionally and never named, so the agent must infer that cooper_search_messages is the counterpart.

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