Get price on a past date

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

Beyond the annotations (readOnlyHint=true, destructiveHint=false), the description discloses important behavioral traits: the exact time (00:00 UTC), the data fields (price, market cap, volume), and the plan/API key requirement and failure mode. This adds substantial context for the agent, especially the authentication limitation and the exact point-in-time semantics, which are not captured in annotations.

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 exactly two sentences and front-loads the core purpose first, then provides the alternative and caveat. Every sentence earns its place: the first states the primary action, the second gives usage guidance and a warning. There is no wasted wording or redundant repetition of schema information.

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?

Given the presence of a rich output schema (not described here but inferred from structured context) and complete annotations, the description covers all essential operational context. It explains what data is returned, the exact temporal scope, the access restrictions, and the alternative for broader ranges. No significant gaps remain for an agent to invoke this tool correctly.

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 input schema has 100% coverage with descriptions for all parameters, so the baseline is 3. The description adds meaningful semantic detail for the 'date' parameter ('00:00 UTC' and 'past calendar date') and clarifies the output fields, which aligns with the schema's parameter meanings. While most parameter documentation comes from the schema, the description enriches the date's temporal interpretation.

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 clearly states the specific action: 'Get a coin's price, market cap, and volume as of 00:00 UTC on a past calendar date.' It distinguishes from the sibling get_market_chart by explicitly noting the difference between a single date and a range of dates. The verb 'get' and the resource (coin's price, market cap, volume) are specific and unambiguous.

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 explicit guidance on when to use this tool ('For a range of dates use get_market_chart instead') and provides a critical prerequisite: 'CoinGecko restricts this endpoint to Demo and Pro plans — without an API key it fails.' It also notes that get_market_chart covers history on the public tier, offering a clear alternative for public-tier users.

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