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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so the safety profile is covered. The description adds genuinely new behavioral context: results are newest-first, the call is free (no credit cost), and it is paged. It does not quantify page size or cursor expiry, so not 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 dense sentences with zero filler; the what/ordering/fields come first, then usage and filters. Every clause carries information the agent needs.

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?

An output schema exists, so return-value documentation is not the description's burden, yet it still names the key fields. Combined with the paging and free-operation notes and the sibling routing, an agent has everything needed to call this correctly.

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 all four parameters are documented in the schema and the baseline is 3. The description only restates that status and provider act as filters, adding intent but no syntax or format detail beyond the schema's enum and default notes.

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 ('List recent billed calls'), gives the ordering ('newest first'), and enumerates the returned fields (call ID, endpoint, status, credits, error summary). It also names the sibling tools that consume the resulting ID, so an agent can distinguish it from get_call_result and report_bug without opening a 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?

Explicitly names two use cases ('audit spend' and 'find an ID for get_call_result or report_bug') and the filter axes (status, provider), which is clear routing guidance. It stops short of stating when NOT to use it or what the alternative is for a full-text or user-scoped search, so it falls just short of the top band.

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