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

Goes well beyond readOnlyHint=true by disclosing that the message is NOT marked as read — a side-effect expectation an agent cannot infer from annotations. It also clarifies attachments return names and sizes only, not content, and that the body is returned as plain text.

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 tight sentences: return payload first, then the read-state caveat, then the environment prerequisite. No filler and nothing buried.

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?

Covers payload shape, non-mutation of read state, attachment limitation, and the Bridge/plan prerequisite — everything needed before calling, especially given an output schema already defines the return structure.

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%; both the id provenance and the max_bytes default/max/truncation behaviour are already spelled out in the schema property descriptions. The description restates rather than extends that information, 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 and resource (reads one Proton Mail message by id) and enumerates exactly what comes back: sender, recipients, subject, date, plain-text body, attachment names and sizes. The 'by the id proton_list_emails or proton_search_emails returned' clause separates it from the list/search siblings.

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?

Names the two source tools that produce the id and states the prerequisite environment (Proton Mail Bridge running on this computer with a plan that includes Bridge). It doesn't explicitly say when not to use it (e.g. batching multiple messages), but the routing is otherwise unambiguous.

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