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

Annotations already declare readOnly, idempotent, non-destructive, closed-world behavior, so the safety profile is covered. The description adds one genuinely useful behavioral fact: without `since` it returns the latest `limit` messages. It says nothing about ordering, pagination, or what a message record contains.

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 sentences, front-loaded with the trigger condition, with no filler. The middle sentence "Read messages from a room" is near-redundant with the first and slightly dilutes the opening.

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 three-parameter read tool with no output schema, the description covers the trigger, both optional parameters' effects, and the default behavior. The remaining gap is return-value shape and ordering, which is acceptable given annotations carry the safety profile.

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 `room_id`, `limit`, and `since` are already fully documented. The description's restatement of `since` (message id or ISO timestamp) and the default-latest behavior largely duplicates the schema rather than extending it; 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?

"Read messages from a room" states a specific verb and resource, and the leading "Use when you need a room's recent messages" narrows the scope to recent/thread reading. It never names a sibling (e.g. live_feed or read_mentions) to disambiguate, so differentiation is left to inference.

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 opening "Use when you need a room's recent messages" gives explicit trigger context, and the conditional clause explains when to pass `since` versus relying on the default. There are no exclusions or named alternatives, which keeps it short of a 5.

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