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

Annotations already indicate readOnly/idempotent, so the bar is lower, but the description adds substantial behavioral context: deterministic arithmetic, per-identifier output fields, rate limits (25 req/day) with retry_after_seconds and Retry-After header, empty found[] behavior, and the structural-only limitation. This goes well beyond the annotation safety profile without contradiction.

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?

Organized into labeled sections (modes, behavior, returns, limitations, related) that move from purpose to operational detail. Every sentence contributes essential content—rate limits, output envelope, structural caveat, alternatives—without redundancy. The length is justified by the tool's complexity.

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?

Covers all materially relevant aspects: the two input modes, per-type validation details, output structure, error/rate-limit handling, limitations, and related tools. An output schema exists but the description still adds contextual value, making the tool fully understandable without external documentation.

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%, so baseline is 3. The description adds meaningful mode-level semantics: text is for parse mode, value+type is for single validation, and the type enum maps to specific standards. This clarifies the mutual relationship between parameters that the schema does not express, justifying a small increment above baseline.

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 opens with a specific verb+resource: 'Validate and parse freight identifiers by their public check-digit algorithms', naming the three identifier types. It distinguishes itself from siblings in the Related section, explicitly calling out container_lookup and airline_lookup as different in scope.

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?

It gives explicit when-to-use guidance with two modes (text for scanning arbitrary strings, value+type for single validation). It names alternatives (container_lookup, airline_lookup) and excludes registry/tracking use via 'STRUCTURAL ONLY'. Rate-limit handling instructions (back off/retry or get_subscribe_link) are also actionable.

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