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

Annotations already declare readOnly/closed-world/non-destructive, and the description still adds real value: it discloses the access-control model (readable by issue readers for in-request letters, otherwise only support project full members) and what the returned body actually contains. That is behavior an agent cannot infer from the schema or 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?

Two sentences, front-loaded with the action and the returned content, followed by id provenance and permissions. The first clause ('Read one letter to the support address whole') is slightly compressed/awkward but nothing is wasteful.

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 no output schema, the description carries the burden of describing the return shape and does so (quoted thread, signature, HTML part as text, headers), plus the authorization rule needed to call it successfully. Nothing material is missing for a one-param read tool.

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% for the single integer param, so the baseline is 3. The description goes one step further by naming the two tools that produce a valid letterId, adding provenance guidance the schema does not provide.

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 ('Read one letter to the support address') and immediately enumerates the payload: quoted thread, signature, HTML-as-text, headers. This clearly differentiates it from the list_support_letters sibling and from get_support_request, so an agent can pick it without opening the schema.

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?

Explicitly says where 'letterId' comes from (list_support_letters or get_support_request's messages), which routes the agent to the right predecessor call. It stops short of an explicit when-not statement, but the permission rules make the eligibility boundary clear.

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