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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint. The description adds value by confirming 'Every call is read-only and idempotent' and detailing the error behavior ('never raises a protocol error — it returns {"error": ...}'). This provides practical context beyond the 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 efficiently structured: purpose first, then example, usage guidance, error behavior, and retry advice. Every sentence serves a purpose, with no redundancy or filler. It is concise yet comprehensive.

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 tool is relatively simple, has a full output schema, 100% schema coverage, and comprehensive annotations, the description still adds essential context: usage boundaries, error format, retry safety, and a sibling comparison. Nothing obvious is missing for an agent to use 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?

Schema description coverage is 100% with clear parameter descriptions (e.g., 'First numeric series; at least 3 values, same length as y.'). The description adds a concrete usage example ('{"x": [1, 2, 3, 4], "y": [2.1, 3.9, 6.2, 8.1]}') and reinforces the requirement for equal-length series of 3+ values, improving agent understanding beyond the schema alone.

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 starts with a clear verb+resource: 'Compute the Pearson correlation between two numeric series.' It distinguishes itself from sibling 'growth_rates' by explicitly stating 'Not for a single series over time (growth_rates).' The typical input/output example further solidifies what the tool does.

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 provides explicit when-to-use ('Use when two equal-length numeric series may move together'), when-not-to-use with an alternative ('Not for a single series over time (growth_rates)'), and a critical caveat ('Reports association only, never causation'). It also explains error handling, guiding the agent on retry behavior.

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