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

Annotations already cover the safety profile (readOnlyHint, openWorldHint), but the description adds genuinely useful behavior: the authoritative data source (Census ACS 5-year) and a paid access model ($0.003 USDC per call via x402), which an agent must know before invoking. It stops short of noting failure behavior for invalid or non-US ZIPs.

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?

Two sentences, zero filler. Resource and scope come first, followed by the metric enumeration and pricing; the metric list earns its place because there is no output schema.

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 one-parameter read lookup with no output schema, the description covers the resource, source, returned fields, and payment requirement. Remaining gaps are minor: response shape/units and error behavior on invalid ZIPs.

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 coverage is 100% and the single 'zip' parameter is already documented as a 5-digit ZIP, so the baseline is 3. The description restates the ZIP keying but adds no format edge cases (non-US, leading zeros) beyond 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?

States a specific verb-resource pair ('US demographics by ZIP code') plus the underlying source (Census ACS 5-year) and an enumerated payload. The ZIP scoping implicitly separates it from demographics_area, but the sibling is never named, so differentiation is left to inference.

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?

Usage is only implied: the ZIP-keyed input and metric list tell an agent this is the tool for ZIP-level US demographic lookups. There is no explicit when-to-use, when-not-to-use, or pointer to demographics_area for non-ZIP granularity.

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