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 genuine context beyond that with the error contract ('never raises a protocol error; returns {"error": ...}') and retry safety after correction. It does partially restate read-only/idempotency already in annotations, which keeps it from a 5.

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 core action and disambiguating example, with usage and error behavior following logically. Slight waste in the 'FREE.' fragment and a somewhat long error sentence, but overall tight.

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 not be explained, yet the description still gives a sample response, plus the error path and the sibling exclusion. An agent has everything needed to call it correctly and handle failures.

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% and both parameters are documented with length/type constraints, so the schema does the heavy lifting. The description only supplies an illustrative example array pair, adding format intuition but no constraints beyond what the schema already specifies.

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 and resource ('Compute the Pearson correlation between two numeric series') plus a concrete input/output example, so the agent knows exactly what operation is performed. It also names the sibling it is not (growth_rates) for a single series over time.

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 an explicit positive condition ('Use when two equal-length numeric series may move together') and an explicit exclusion with the alternative tool to use instead ('Not for a single series over time (growth_rates)'). It also notes the association-only caveat, which shapes interpretation rather than invocation but reinforces correct use.

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