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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so safety is covered. The description adds genuinely useful non-schema behavior: the pricing model (free within a daily allowance, then $0.001 USDC per call via x402), which matters for call budgeting. It does not, however, mention rate-limit exhaustion behavior or result limits.

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 short sentences, zero waste, with the core action front-loaded and the pricing caveat second. Nothing redundant or padded.

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?

An output schema exists, so return values need not be explained, and annotations cover the safety profile. The description supplies the pricing context an agent needs to call it responsibly. Minor gap: no stated behavior for ambiguous or invalid partials, which would matter for a completion tool.

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?

Only one parameter, documented in the schema, with 100% schema description coverage — so the schema carries the burden. The description restates the 'partial postcode' concept but adds no syntax or format detail (minimum characters, outcode vs full-prefix handling) beyond what the schema already says. Baseline 3 is correct.

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: 'Complete a partial UK postcode,' which is clear and distinct from siblings like validate (full postcode) or lookup. However, it never explicitly names or contrasts those siblings, leaving the agent to infer the routing.

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 implied by 'partial postcode' — the agent can infer this is for autocomplete-style input rather than exact lookup. But there is no explicit when-to-use/when-not, no named alternative (e.g., use lookup for a full postcode), and no note on what a partial input must look like.

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