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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds valuable behavior beyond annotations: output shape (three parallel arrays), timestamp unit (milliseconds), auto-granularity based on `days`, and the optional `interval` override. It does not mention rate limits or error cases, but those are not critical given the strong annotation coverage.

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?

Four sentences with no filler. The most important output-shape and scope information is front-loaded, followed by sibling routing. Every sentence earns its place and the description is compact relative to the tool's complexity.

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?

Combined with a fully described input schema, rich annotations, and an output schema, the description provides everything an agent needs to call this tool correctly: scope, output structure, timestamp units, granularity behavior, and explicit alternatives. No critical call prerequisite or expected-return detail is missing.

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 input schema already documents every parameter. The description adds meaning beyond the schema by clarifying that `days` defines a trailing window, that granularity follows `days` automatically, and that `interval` can force `daily` or `hourly`. It doesn't add detail for `precision` or `vs_currency`, but those are fully described in 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 states a specific verb and resource: 'Historical series for one coin over a trailing window set by `days`.' It also distinguishes itself from siblings by emphasizing the trailing-window scope and by naming what it is not (range, OHLC, history). An agent can tell it apart without opening another 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?

The description explicitly says when to use this tool: 'Use it to chart a trend ending now.' It then names the alternatives for explicit start/end (market_chart_range), candlesticks (ohlc), and a specific past day (history). This is clear, direct routing guidance.

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