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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds only the REST endpoint (GET /channels/{channel}/messages) and the newness ordering, which is already stated in the schema's direction field. It says nothing about pagination behavior, permissions, or returned message shape.

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?

Two tightly written sentences with the core action front-loaded and zero filler. Nothing could be removed without losing information.

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 five-parameter retrieval tool with no output schema, the description leaves return-value shape and pagination semantics unexplained. The schema covers the inputs well, but the absence of an output schema means the description could reasonably carry more weight than it does.

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 all five parameters (including the direction enum and start/end/limit bounds) are fully documented in the schema. The description adds no syntax or format detail beyond what the schema already provides, making the baseline 3 appropriate.

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 and resource ('Retrieve historical messages published to a channel') with a behavioral default ('newest-first'). This is clearly distinguishable from sibling tools like ably_get_presence_history (different resource) and ably_publish_message (opposite action), though it never names an alternative explicitly.

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 the default ordering but no when-to-use guidance, no exclusions, and no comparison to sibling tools such as ably_get_presence_history or ably_list_channels. The agent must infer context from the resource name alone.

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