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

Annotations already declare openWorldHint=true, readOnlyHint=false, idempotentHint=false and destructiveHint=false, so the safety profile is covered. The description adds genuinely new behavior: prepaid credits are deducted automatically and the cost is returned alongside the result, which an agent needs to know before spending. It omits what happens to credits on failure or error handling, keeping 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?

Two tightly written sentences with zero filler; the action and its key constraint (credit deduction) are front-loaded and the slug-discovery pointer follows as supporting detail.

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 2-param, open-world tool with no output schema, the description covers the essentials: what to pass, where to get a slug, and that a credit cost is returned with the result. The remaining gap is that the dynamic, manifest-defined params object is only explained by the schema, not by the description.

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%, with the slug and the per-manifest params object documented in the schema itself, so baseline is 3. The description only echoes 'by slug' and adds no detail on how the nested params object should be populated from the target API's manifest, so it neither compensates for nor extends the schema.

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 states a specific verb and resource ('Call an API from the apifare catalog by slug') and clarifies the lookup key, so an agent immediately understands what is invoked. It differentiates itself from search_apis by routing slug discovery there, though it gives no signal distinguishing it from the sibling vendor_call.

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 supplies the precondition for use ('find slugs with search_apis') and surfaces the cost consequence, giving clear context for when this tool is the right next step. It stops short of explicit when-not-to-use guidance or naming alternatives such as vendor_call.

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