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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely new behavioral context beyond them: it enumerates the specific data classes NOT served (quotes, candles, MOEX instrument directory, index levels, commodities, coin prices, CBR rates), mentions rate ceilings and contract editions, and warns it returns no market data. It does not describe pagination or output shape, but that is largely covered by the output schema.

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?

Front-loads the identity of the tool, then the negative scope, then the usage rule. It is slightly heavy — a long nested parenthetical list of unsupported data types — but every clause is informative rather than filler, so the density is justified.

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 an output schema present, return values need no explanation, and the description covers the remaining essentials: what the tool answers, what it excludes, when to invoke it, and the caveat that it supplies no market data itself. An agent has everything needed to call it correctly and to interpret an empty or negative answer.

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% for the single 'lang' parameter (en/ru, defaults to server language), so the schema already carries the semantics. The description never mentions the language parameter or how it affects the returned texts, adding nothing on top of the structured field.

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, unusual purpose: the API describing its own coverage, both what it serves and what it explicitly does not. The closing line 'It returns no market data itself' cleanly separates it from data-bearing siblings such as market_overview, screener or world_benchmarks.

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?

Gives explicit triggers: 'when the user asks what the service can do' and 'when a question needs data you are not sure the service has', with concrete examples (share price, intraday candles, foreign company, coin price). It even specifies the decision point — call this 'before answering that it is not available' — which is the precise failure mode it prevents.

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