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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is covered. The description adds one genuinely useful behavioral fact beyond that – the body is returned as plain text and truncated at 20,000 characters – but says nothing about the not-found case, error behavior, or the other fields returned.

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?

One sentence, front-loaded with the action and resource, with the truncation caveat appended where it is most useful. There is no redundant or filler wording.

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 no output schema, the definition covers the essentials, and the truncation note is a meaningful addition. The minor gap is that "including its plain-text body" implies other fields (sender, subject, timestamp) exist in the response but never names them.

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 coverage is 100% and the single id parameter is already documented in the schema, including its provenance. The description adds nothing beyond it, but with full coverage and only one documented parameter, the baseline is satisfied.

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?

"Return one message by id" is a specific verb plus resource, and "by id" implicitly separates it from get_latest_otp and list_messages, which work off different selection criteria. It stops short of naming those siblings, so the differentiation is inferable rather than stated.

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?

Usage is only implied: the tool takes an id, and the schema notes that the id comes from list_messages or get_latest_otp. There is no explicit statement of when to call this versus those two alternatives, nor any exclusion such as "do not use for the latest OTP."

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