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

Annotations already declare the mutation profile (readOnlyHint=false, destructiveHint=true, idempotentHint=true, openWorldHint=true), so the description only needs to add context. It does so meaningfully: the signature is persisted as a message in the 'mailmcp-signature' folder, photo/logo signatures take a different path, and two capabilities must be enabled first. It stops short of explaining what the destructive hint actually overwrites.

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?

Three sentences, front-loaded with the core action before the edge-case path and prerequisites. It is slightly diluted by implementation detail (storing as a message, mailing to self) that is not essential for tool selection.

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 mutation tool the safety profile is covered by annotations and all parameters are documented, so the essentials are present. The notable gap is that destructiveHint=true is never explained – the agent is not told that an existing signature is replaced rather than appended.

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 100% and the schema already documents maxLength, script stripping, and the html-derived default for text. The description only restates 'HTML and/or text' and adds nothing about the required 'account' parameter, so baseline 3 is correct.

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 gives a concrete verb (stores) and resource (the mailbox signature) and even names the storage location, so the agent knows this writes a signature. It does not explicitly contrast with the sibling get_signature, which would have earned a 5.

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 states real preconditions ('Needs the mailbox signature enabled on the setup page and the draft capability') and describes a photo/logo branch, which is useful context for when the call will succeed. However it never explicitly routes the agent against alternatives such as get_signature, leaving the read-vs-write choice implied.

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