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

With no annotations, the description must carry the behavioral burden, and it delivers the non-obvious traits: patch semantics (unsent fields are kept), link stability ('Links keep working'), and the critical replace-vs-append distinction between `guests` and `addGuests` with the id-then-name matching rule. It stops short of stating auth requirements (editKey) or failure/idempotency behavior, so it is strong but not complete.

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?

Four compact sentences, the core action front-loaded ('Change a letter'), then the highest-risk behaviors. Every sentence conveys a distinct, decision-relevant fact with no filler.

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 13-parameter nested tool with no output schema, the description covers the highest-confusion areas: partial-update behavior, link persistence, and guest replace-vs-append. The block/envelope/custom/font details are left to the rich schema, which is reasonable given 77% schema coverage, though a note on required `id`/`editKey` context would round it out.

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 77%, so the schema largely documents parameters already, and the description's guest-matching and addGuests details closely echo the schema descriptions. It reinforces the patch semantics implied by the optional 'lang, style' fields but adds little syntax or constraint information beyond the schema, warranting the baseline 3.

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?

'Change a letter' gives a clear verb+resource, and the following sentences clarify it is a partial (patch) update. It does not explicitly distinguish itself from siblings like create_letter or get_letter, but the update framing against the title makes the 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 implies usage by explaining patch semantics ('Only the fields you send change'), which tells the agent this is an edit-an-existing-letter tool. However there is no explicit when-to-use or when-not-to-use guidance, nor any reference to create_letter/delete_letter as alternatives.

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