topup_credits

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

With no annotations, the description carries the full behavioral disclosure burden. It reveals several key behaviors: the tool returns a URL rather than processing payment directly, enforces a $1 minimum, and instructs that the bby_live_… key should be placed in FETCHER_API_KEY after topping up. It does not disclose all potential side effects, but for a simple URL-returning tool this is sufficient.

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?

The description is two sentences with no wasted words. It front-loads the crucial constraint ('bought on the web, not over stdio'), then identifies the return value, minimum amount, and follow-up action. Every sentence contributes unique information.

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 single-parameter tool with no output schema, the description provides adequate context to call it: what it returns, the minimum, and the post-payment step. Slight gaps remain around whether the optional amount parameter must be supplied or what the exact URL format looks like, but these are minor given the tool's simplicity.

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?

The input schema already has 100% description coverage for the only parameter, amount, including its USD denomination, minimum value, and purpose ('Used only to confirm the floor'). The tool description repeats the $1 minimum but adds no new meaning beyond the schema, so the baseline of 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?

The description states a clear action: 'Returns the top-up URL' for buying prepaid credits. It distinguishes itself from siblings like check_balance and fetch_data by focusing on purchasing credits rather than querying or retrieving data. However, the phrasing 'Prepaid credits are bought on the web, not over stdio' is slightly indirect and could be more explicitly framed as 'Use this tool to get a top-up link.'

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 description implies the tool is used when prepaid credits are needed and sets an important constraint that payment happens on the web, not through stdio. However, it does not explicitly mention alternatives like check_balance for checking credit status or explain when not to use this tool. The usage context is asserted but not differentiated against sibling tools.

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