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

Annotations already declare readOnlyHint/idempotentHint/destructiveHint, so the bar is lower, but the description adds substantial behavioral context beyond them: it returns an error for non-existent ids, documents null semantics ('null when none', 'null when not published', '0 means no minimum'), and exposes the critical caveat that the sign-up bonus is only credited when the user signs up through the provided url. This is exactly the kind of non-obvious behavior an agent needs to know.

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 long but dense with value: selection rule, error behavior, field semantics, and a usage caveat each earn their place, with the core purpose front-loaded. It loses a point for minor redundancy — 'with its payout terms for that country' appears twice — but is otherwise well-structured for its richness.

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 lookup tool with an output schema, this is complete: the description covers when to use it, how to source the id, error behavior, field-by-field semantics for the result, and the special display rule for sign-up bonus urls. Nothing an agent needs to call it correctly and use its results is left unstated.

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 the baseline is 3. The schema already explains portal_id as 'GotCashback's numeric portal id, taken from the 'id' field of an earlier get_portals or get_portals_by_name result.' The description repeats this same sourcing guidance, adding no incremental meaning beyond 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 opens with a specific verb and resource: 'Returns one cashback portal by its numeric id... with its payout terms for that country.' It adds the key distinguishing constraint (lookup by id) and explicitly differentiates from the sibling get_portals_by_name, so an agent can tell them apart without opening schemas.

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 gives an explicit when and when-not: 'Use it only when the id is already known; when the user names a portal, call get_portals_by_name instead.' It also discloses the error case for non-existent ids and tells the agent how to use the returned fields to answer user questions, which is clear selection and invocation guidance.

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