Top up the apifare balance from the owner's standing auto-refill mandate (if the owner enabled one). Charges within the owner's configured limits and returns the new balance. Without a mandate, returns payment instructions to relay to the user.
| Name | Required | Description | Default |
|---|---|---|---|
| amount_usd | No | Optional whole-dollar amount; defaults to the mandate's refill amount and is clamped to it |
Changes observed during successful MCP inspections.
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries the full burden and does disclose the critical trait: this tool spends money, bounded by the owner's configured limits, and it describes both the success return (new balance) and the no-mandate fallback (payment instructions). It omits auth/permission requirements and whether charges are reversible or idempotent.
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 sentences, front-loaded with the action and funding source, then the limits, then the fallback. No filler or repetition of schema content.
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 one-parameter, no-annotation tool with no output schema, the description covers the important unknowns: that a charge occurs, that it is limit-bounded, and what is returned in each branch. It could still say more about permissions and failure behavior, but nothing essential to calling it is missing.
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% and the single optional amount_usd parameter is fully documented there, including its default and clamping behavior. The description adds nothing about the parameter, so the baseline of 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 names a specific verb and resource ('top up the apifare balance') and precisely scopes the funding source (the owner's standing auto-refill mandate). It is clearly a write/action tool distinct from check_balance or call_api, though it never names an alternative to contrast against explicitly.
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 the key precondition ('if the owner enabled one') and explains the two outcome branches depending on whether a mandate exists. It does not state when an agent should reach for this tool versus siblings such as check_balance, so the routing guidance is contextual rather than explicit.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.