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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and openWorldHint=false, and the description is consistent with them by stating the tool "opens nothing by itself." It adds genuinely useful context about the return value (step-by-step screens, buttons, fields) rather than just a status. However, the closing sentence about Mailjet's REST API and read-only official MCP server is background trivia rather than behavior of this tool, and it never says whether the guide is generic or tied to a specific campaign.

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 core behavior and the hand-off to cotlet_browser_* tools are front-loaded in the first two clauses, which is good structure. The long trailing sentence about Mailjet's REST API, official MCP server and a verification date does not help an agent decide or call, and weakens an otherwise tight definition.

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?

With no output schema, the description does its job of telling the agent what comes back (step-by-step instructions). But a zero-parameter tool for a per-campaign operation leaves a real gap: nothing explains how the target campaign is identified or what the agent must supply verbally. That ambiguity matters for correct invocation of an otherwise simple guide tool.

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 tool takes zero parameters, so there is no parameter semantics for the description to explain; baseline 4 applies. The description does not introduce any conflicting expectations about inputs.

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 names a specific verb and resource ("Update campaign subject") and then clarifies the surprising twist: it "Returns how to do it in Mailjet step by step" and "opens nothing by itself." That clarification is what makes the purpose legible, though the leading verb still implies an action the tool never performs. It does not explicitly distinguish itself from siblings such as mailjet_send_campaign or mailjet_duplicate_campaign.

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 gives a concrete workflow: call this first to get screens/buttons/fields, then execute with the cotlet_browser_* tools in the Mailjet Cloud Browser. That is clear when-to-use guidance and names the follow-up tool family. It stops short of explicit exclusions (e.g., when to prefer the official Mailjet API/MCP path it mentions) or prerequisites such as needing an open browser session.

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