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

Annotations already declare readOnlyHint=true, so safety is covered. The description adds real behavioral context beyond that: the result set is limited to consenting shared-project collaborators and is never a global directory, and it discloses a workflow shortcut (no need to open the chat first). It omits return shape/pagination, which keeps it from a 5.

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?

Three tight sentences, front-loaded with the scope constraint, then the exclusions, then the next-step guidance. No filler or restatement of the title.

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?

For a zero-parameter, read-only list tool with no output schema, the description covers what the list contains and how to act on results, which is sufficient to call it correctly. Only minor gaps remain, such as whether the list can be empty or how results are ordered.

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?

Zero parameters, so the baseline is 4; there is nothing in the input schema needing elaboration. The mention of email/name relates to send_chat_message's inputs rather than this tool's, so it adds no parameter meaning here but costs nothing.

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 states a specific verb+resource (list chat contacts) and narrows the scope to 'active consenting shared project collaborators eligible for an AccountHub one-to-one chat', which separates it from a directory lookup. It also explicitly negates a global directory search ('Never searches a global directory'), implicitly distinguishing it from search_contacts/list_contacts, though it does not name those siblings outright.

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?

It gives a clear use case (finding eligible chat partners) and a follow-on action: 'To message one of them, call send_chat_message with their name or email (no need to open the chat first).' It also excludes the global-directory use case. It stops short of explicitly naming which sibling to use instead (e.g., search_contacts) for non-collaborators.

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