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

The readOnlyHint annotation already covers the safety profile, so the bar is lower. The description adds incremental behavior: the mutual exclusivity of the anchor parameters, the underlying endpoint shape, and that the result is a { messages[] } payload. It does not explain pagination behavior (how next_limit/prev_limit interact), ordering defaults, or the include/reverse interactions.

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 purpose, then the anchor rule, then the endpoint/return shape. No filler and nothing buried.

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 11 parameters fully documented in the schema and a read-only annotation, the description only needs to orient the agent, which it does. The main residual gap is that no output schema exists and the return description is minimal ('{ messages[] }'), so nothing explains the shape of individual message entries.

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 schema already documents all 11 parameters including the anchor alternatives and enum meanings. The description restates the message_ts/message_id exclusivity and unit (unix ms) but adds nothing about the pagination limits or filters. Baseline 3 is appropriate when the schema carries the parameter burden.

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 gives a specific verb+resource+scope ('List messages in a channel') and adds the anchoring concept, so an agent immediately knows what the tool does. It never names or contrasts a sibling tool, but the resource is distinct enough from send_message/list_users that confusion is unlikely.

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 states one important usage rule clearly: 'Provide EITHER message_ts (unix ms) OR message_id.' That is a real constraint on invocation, but there is no guidance on when to use this tool versus other ways of retrieving messages, and no mention of prerequisites such as channel membership or permissions.

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