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

readOnlyHint=true already establishes the safe-read profile, so the description's burden is lighter. It adds useful context about the returned record content (outcome, direction, duration), but says nothing about pagination limits, ordering, default page size, or auth scope beyond what the schema already carries.

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 short clauses, front-loaded with the resource, then filters, then the API anchor. Nothing is padded, though the trailing "FUB: GET /v1/calls" is metadata rather than agent guidance.

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?

With no output schema, the description partially compensates by naming the fields a call carries, and all six parameters are covered by the schema. It stops short of describing paging behavior or result ordering, but nothing essential is missing for a read-only list call.

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 every parameter is already documented in the schema with defaults, ranges and the deep-paging hint. The description's "filter by person or number" merely summarizes the personId/phone/toNumber/fromNumber set without adding format or semantics beyond the schema. Baseline 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?

Names the resource concretely (logged phone calls) and characterizes the record shape (outcome, direction, duration), and the API route anchors it. The verb is only implied by the name/title rather than stated in the description, but an agent can still tell this is a read/list tool distinct from the sibling fub_log_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?

"Filter by person or number" gives implied usage context for the filter parameters, but there is no explicit when-to-use statement, no exclusion of the write-side sibling fub_log_call, and no guidance on when to prefer this over fub_search_people or fub_get_person for call history.

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