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

Annotations already cover mutation/idempotency/open-world flags, and the description adds real context beyond them: that installation state is unchanged ('no new QR, nothing to install') and that payment and polling behave like create_order. It stops short of saying what the returned invoice looks like or how long it stays valid, which keeps it from a 5.

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 short sentences, front-loaded with the effect of the call, then the parameter constraint, then the operational comparison. Nothing is redundant.

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 4-parameter write tool with no output schema, it covers the effect, the key parameter precondition, and where to learn about payment/polling. Minor gap: no mention of how the created invoice is retrieved later (get_order) or its expiry, which an agent may need for follow-up.

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%, so the baseline is 3, but the description adds a real constraint the schema misses: plan_id must come specifically from list_topups for that same ICCID. This resolves an inconsistency with the schema text (which says plan_id comes from list_plans) for the top-up case.

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 ('Create a crypto invoice') plus the exact effect ('adds a data plan to an existing eSIM (no new QR, nothing to install)'). The parenthetical immediately separates it from create_order, which sets up a new eSIM.

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 hard prerequisite ('plan_id must come from list_topups for this ICCID') and points to create_order for the payment/polling flow. It doesn't state an explicit 'don't use this when...' rule, but the existing-eSIM framing makes the boundary with create_order inferable.

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