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

Annotations already declare readOnlyHint=true, idempotentHint=true, so the safety profile is covered. The description adds value by stating 'Changes nothing' (reinforcing read-only) and 'Needs the user's account' (an authentication requirement not in annotations). It doesn't cover rate limits or pagination, but those are less relevant for a single-item fetch.

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 three sentences, front-loaded with the main use case and return values, then lookup methods, then behavioral notes. Every sentence adds useful information with no redundancy.

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?

Given no output schema and sparse annotations, the description covers the essential aspects: what it returns, how to look up, safety, and auth. It is nearly complete, though it could mention what happens if no parameters are provided or if the letter isn't found.

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 0%, so the description must compensate. It mentions 'letter_id or reference (PM-XXXXXX)', which explains the two parameters and gives a format example for reference. However, it does not state that both are optional or that at least one is needed, nor does it clarify that they are mutually exclusive alternatives.

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 (look up / get) and resource (letter), and enumerates the exact information returned: status with enumerated states, tracking number for registered mail, and price. It clearly distinguishes itself from siblings like list_letters and create_letter by focusing on a single letter's details.

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?

It gives a clear trigger: 'when the user asks about a letter they created here', and specifies two lookup methods (letter_id or reference). It does not explicitly mention when not to use it or name alternatives like list_letters, but the context is sufficiently clear.

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