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

Annotations already declare readOnly, idempotent, non-destructive, closed-world behavior, so the safety profile is covered. The description adds genuine value beyond that: a data-provenance guarantee ('promo codes are never invented: only those the host publishes') and a warning that the site promotion is Russian-language text, which tells the agent how to interpret the payload.

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 tight sentences, front-loaded with the primary mode, then the no-host mode, then the trust caveat. No filler, though the fragment-style clauses ('Without a host: ...') read slightly telegraphic.

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 carries the burden of describing returns, and it does list the four signal categories returned. Combined with the already-documented read-only, closed-world annotations, an agent has enough to call it correctly; only the response shape/ordering of the ranking mode is left unspecified.

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 coverage is 67%, and the description adds the key semantic the schema cannot: omitting 'host' turns this into a top-hosts ranking rather than an error. The locale→currency mapping and the host slug/domain/name forms are already documented in the schema, and 'limit' is neither explained nor needed beyond its default and bounds.

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 names the concrete resource — discount signals for a VPS host (site promotion text, prepayment discount computed from period prices, trial length, money-back days) — and states the second mode (ranking hosts by discount strength when no host is given). That is specific enough to tell it apart from price-oriented siblings like list_offers or compare_hosts, though no sibling is named explicitly.

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 clearly splits the two invocation modes: supply a host for that host's discounts, omit it for the strongest-discount leaderboard. There are no explicit exclusions or pointers to alternative tools (e.g. 'use list_offers for raw pricing'), so it stops short of a 5.

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