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

Goes well beyond the annotations by disclosing the refund-to-credit side effect, the irreversibility boundary for printed/posted letters, the account requirement, and a confirmation expectation. These are exactly the behavioral facts an agent needs for a destructiveHint=true operation.

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?

Front-loads the trigger condition and stays focused, with each sentence carrying a distinct instruction. There is minor overlap between 'drafts and paid letters not yet in production' and the schema's restatement of the same rule, but no real waste.

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 mutation with no output schema, the description supplies the missing pieces: refund outcome, irreversibility limits, account requirement, and disambiguation from siblings. Nothing an agent needs to call it correctly is absent.

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 letter_id parameter is fully documented in the schema, including its source tools and example format. The description restates the cancellable-state constraint but adds no syntax or format detail beyond what the schema already provides, so the baseline 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?

States a specific verb+resource (cancel a letter created via this service) and immediately scopes it to drafts and paid-but-unproduced letters. It also distinguishes itself from sibling compose_letter by clarifying that this cancels our letter, not a third-party contract.

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 covers when to use it, when not to (letters already printed or posted cannot be recalled), the alternative tool for a different need (compose_letter for contract cancellation letters), and a prerequisite action (ask the user before cancelling).

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