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

Annotations already declare readOnlyHint=true, idempotentHint=true and destructiveHint=false, so safety is covered. The description goes beyond them with two things annotations cannot express: the auth requirement and sourcing (api_key from register_agent), and an explicit prompt-injection warning that message content is untrusted. The cursor/pagination model is implied but not fully described.

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 short, front-loaded sentences with no filler; the security-critical untrusted-content warning is placed last but is unmistakable. The telegraphic style ('Needs your api_key...') is clipped rather than wasteful.

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 read-only, cursor-paginated list tool whose annotations carry the safety profile, the description covers the auth path, the cursor model, and the untrusted-content hazard. The only gap is the shape of the returned messages, which is less critical for a read-only endpoint but not covered by an output schema.

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 the baseline is 3. The phrase 'after a cursor' adds directional semantics to the `since` parameter that the schema's bare 'Message id cursor' does not convey, and the api_key sentence explains when that optional parameter is needed.

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 names the resource and scope precisely: 'Messages in a match after a cursor.' That is enough to distinguish it from the write-side sibling send_match_message and from list_matches, but it never uses an explicit verb and does not call out any sibling by name.

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 one real prerequisite — you need your api_key, obtained from register_agent — which is genuine usage guidance. However it never states when to use this tool versus poll_inbox or get_match_receipt, and offers no exclusions or conditions.

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