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

readOnlyHint already declares the safety profile, yet the description still adds meaningful non-obvious behavior: row ordering (newest first), the granularity of usage rows (one per month), and the unit convention (signed euro cents, credits positive). That unit detail materially prevents sign misinterpretation. It doesn't mention pagination totals or response shape, which is expected given the output schema exists.

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?

One dense sentence for content and one short access block, both front-loaded with the highest-value facts (what rows, ordering, units, auth precondition). No filler and nothing repeated from the schema.

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 two-parameter read tool with an output schema, this covers everything an agent needs to call it correctly: authorization path, what the data represents, ordering, and the sign/unit convention. Remaining details (return shape, page limits) are carried by structured fields.

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% — page and limit both carry defaults and bounds in the schema. The description adds no parameter-level meaning beyond that, 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?

Specific verb+resource (list the prepaid balance history) plus a precise enumeration of what rows appear: top-ups, refunds, adjustments, and one monthly usage row. The 'newest first' ordering is stated up front, so an agent knows exactly what it gets without opening the output schema.

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?

Gives a clear when-to-call condition (you need transaction-level history, not a snapshot) and explicitly rules out a wrong path: 'start_login does NOT open this tool,' with the /mcp Authenticate alternative named. It does not explicitly contrast with the closely named siblings get_balance and get_usage, which would be the remaining useful routing hint.

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