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

No annotations are provided, so the description carries the full burden, and it does meaningful work: it discloses the auto-resolve/roll-forward lifecycle behavior and the extra response fields (current_slug, rolls_to_next_at), plus the HTTP verb and x402 price. It stops short of noting auth/permission specifics or latency, but the evergreen mechanics are well covered.

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-loaded with the question and the evergreen semantics, and the bracketed footer packs endpoint/price/docs efficiently. Minor redundancy: the question 'Will the S&P 500 finish the year positive?' is stated twice in immediate succession, which costs a little space.

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 simple zero-parameter read tool with no output schema, this is close to complete: it defines the question, the URL stability guarantee, the rolling behavior, and the fields added to the response. The main residual vagueness is the reference to a 'standard probability shape' whose fields are never enumerated, but with no output schema required this is acceptable.

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?

The tool takes zero parameters, so the schema baseline is 4. There is nothing for the description to disambiguate, and additionalProperties:false is consistent with the no-argument framing.

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, self-contained verb+resource: 'Live probability: Will the S&P 500 finish the year positive?'. An agent can instantly separate it from the many sibling evergreen questions (btc, eth, superbowl, etc.) without opening any 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?

Explains the evergreen contract clearly — stable URL for a recurring question, auto-resolves to the current market instance, rolls forward on resolution — which tells the agent when this tool is the right choice versus a point-in-time lookup. It does not, however, name explicit alternatives (e.g. probability_get or probability_search) for non-recurring or historical queries.

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