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 the underlying endpoint mapping (GET /v1/sms/messages/{message_id}), which is mildly useful context, but it says nothing about error behavior for a missing/invalid id or what the payload 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?

Two short sentences, front-loaded with the purpose, with no filler. The endpoint reference is arguably redundant with the tool name but is still compact.

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 single-parameter read tool with annotations covering safety and a fully documented schema, this is adequate. With no output schema, the agent gets no hint about the returned message fields, though that is a minor gap for a simple getter.

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?

Only one parameter, message_id, and schema coverage is 100%, so the schema fully documents it. Baseline 3 applies; the description adds no format or sourcing detail beyond the schema's 'SMS message id.'

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 ('Fetch a single SMS message by id'), which cleanly separates it from the sibling bird_list_sms_messages. It does not, however, explicitly name the list sibling or note that it returns one record vs many, so sibling differentiation is only implied by 'single ... by id'.

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?

There is no guidance on when to use this versus bird_list_sms_messages or the other get_* siblings. It says what it fetches but never conditions, prerequisites, or alternatives.

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