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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false. The description adds useful behavior beyond that: it returns delivery status, cost, and any error, so the agent knows the tool is a safe status lookup and what kind of payload to expect. No contradiction with annotations.

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?

The description is compact: three short sentences with the core fetch action and response summary front-loaded. The opening line 'Get message' is mildly redundant with 'Fetch one message by id,' but it does not meaningfully add noise.

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 one fully documented parameter, strong annotations, and an output schema, the description adds the remaining operational context: the id key pattern, the returned fields, and when to call the tool. Nothing required to invoke it correctly is missing.

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?

The only parameter, id, is already fully documented in the schema with a concrete example (msg_qY08hZ2mmDXAazdRTytS). The description adds nothing beyond referring to the same id, so the baseline score of 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?

The description states a specific verb and resource: 'Fetch one message by id (msg_…)' and clarifies the returned data: delivery status, cost, and error. This distinguishes it from list_messages (collection vs single item) and get_report/get_account, making the tool's purpose unambiguous.

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 description explicitly says 'Use after send_sms or list_messages,' which gives clear contextual guidance on when in a workflow this tool is appropriate. It does not name alternative tools to rule out, but the timing guidance is strong enough for an agent to select it correctly.

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