Call an API from the apifare catalog by slug (find slugs with search_apis). The cost in prepaid credits is deducted automatically and returned with the result.
| Name | Required | Description | Default |
|---|---|---|---|
| slug | Yes | API slug from search_apis, e.g. "web-search" | |
| params | No | Params object per the API's manifest |
Changes observed during successful MCP inspections.
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations provided, the description carries the full behavioral burden, and it does disclose something important: the cost is deducted automatically in prepaid credits and returned with the result. However, it says nothing about failure modes, idempotency/retry behavior, or whether a failed call still incurs cost — meaningful gaps for a paid side-effecting operation.
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?
Two sentences with zero filler; the core action and its prerequisite are front-loaded and the billing caveat is appended where it belongs. Every clause earns its place.
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 wrapper with no output schema, the description covers the operation, the required lookup step, and the credit-cost side effect — enough to call it correctly. It could go further on how the nested params object maps to each API's manifest and what a failure response looks like.
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%, so both parameters are already documented (including the example slug 'web-search' and the manifest-driven params object), establishing the baseline of 3. The description's 'by slug' adds no syntax or format detail beyond what the schema supplies.
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 ('Call an API from the apifare catalog') qualified by the lookup key ('by slug'), which is enough to distinguish it from the sibling search_apis that only finds slugs. An agent can identify the tool's job without reading the 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?
The parenthetical '(find slugs with search_apis)' explicitly routes the agent to the prerequisite sibling tool, which is a genuine usage condition. It stops short of stating when not to use this tool or what happens on unknown slugs, so it is strong but not exhaustive.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.