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

Goes well beyond the annotations by disclosing the key destructive behavior — arguments fully replace mailbox, recipients, subject and body — plus that attachments survive, nothing is sent, and a signature is injected unless include_signature is false. This tells the agent exactly what data will be lost on an idempotent-but-destructive write.

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?

Six short sentences, front-loaded with the core action and the most costly gotcha (replacement semantics) first. No filler; every sentence carries either scope, side effect, or format information.

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 nine parameters, a 100%-covered schema, an output schema, and destructive annotations, the description supplies the remaining critical context: replace-vs-merge behavior, attachment preservation, no-send guarantee, and signature default. Nothing needed to invoke it safely is missing.

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% so the baseline is 3, but the description adds real meaning: that omitted fields are wiped rather than preserved, that body may be plain text or markdown, and the signature side effect of include_signature. These are behavioral constraints the schema alone does not convey.

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 ('Rewrite one of the current user's saved drafts') and immediately distinguishes it from sending via 'Nothing is sent.' An agent can tell this apart from create-email-draft-tool and send-email-draft-tool without inspecting schemas.

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 explicit replace-semantics warning ('pass everything the draft should keep') is a strong usage instruction that governs how to call it. It does not name sibling alternatives (create/send/delete draft tools), so routing guidance is slightly incomplete, but the when-to-use context is clear.

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