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

Annotations already cover read-only, idempotent, non-destructive and closed-world, but the description adds substantially more: watermarking, as-of stamping, edge caching, no API key required, and — most valuably — that sector-gated or empty histories return an honest explanation card instead of an error. That failure-mode disclosure is exactly what an agent needs and cannot get from annotations or 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?

Three tight sentences, front-loaded with the usage condition, then the artifact, then the delivery guarantee and failure mode. Every clause carries information an agent would otherwise have to guess.

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 read-only image-URL tool with a full output schema, the description covers what the agent must know: what is returned (canonical URL), how to consume it (<img src>), that no key is needed, and how degenerate inputs are handled. Nothing material 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 all four parameters (including the metric enum and the compare ticker) are already documented structurally. The description adds only a passing reference to 'chart-tier metric' and says nothing about years, metric slugs, or the compare-only-for-bar-metrics constraint, so the baseline 3 applies.

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 verb (render/return) and resource (a metric's history as a chart image URL), including the concrete artifact: a 1600×900 PNG in house style. The 'picture of a metric's history' framing implicitly separates it from data-returning siblings like get_metric_series, so an agent can distinguish them without opening either 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?

Opens with an explicit usage trigger ('Use when the answer should show a picture of a metric's history'), which is a real selection criterion. It never names an alternative or states when-not to use it, so routing versus get_metric_series is left to inference.

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