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

Annotations already declare readOnlyHint=true and idempotentHint=true, covering safety. The description adds valuable behavior: the conditional return source (newest message in a specific folder with HTML inline images vs. plain text from token), which goes beyond the annotations. It does not mention error cases, but for a read-only tool this is adequate.

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 a single sentence that is front-loaded with the primary purpose, but it is a bit dense with a conditional structure. It could be split into two sentences for clarity, but it remains efficient and contains 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 simple read-only tool with one parameter and no output schema, the description explains exactly what is returned in both scenarios and references the relevant sources (folder and token). It does not elaborate on return format details beyond the content type, but that is sufficient given the simplicity.

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?

The schema has 100% coverage for the single 'account' parameter, described as 'Account id from list_accounts'. The description adds no additional parameter semantics, so the baseline 3 is appropriate.

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 ('Returns') and resource ('the signature mailmcp appends under replies'), and distinguishes two retrieval paths (folder-based HTML vs. token-based plain text). It clearly separates this from the sibling set_signature, which is the write counterpart.

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 — call it to read the current signature — and the sibling set_signature is obviously the inverse. However, it does not explicitly state when to use it vs. alternatives or any exclusions, so it falls short of a 5.

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