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

Annotations already declare readOnlyHint and idempotentHint, and the trailing 'Read-only' merely restates them. Beyond that, the description adds genuine value: the exact metadata fields returned, the extra detail fields for single-layer lookups, and the UNKNOWN_PART_LAYER sentinel for bad input, which is useful operational behavior.

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 purpose is front-loaded in the first clause, which is good. But the long enumeration of output fields (part_layer, label, description, country_codes, admin_levels, part_count, id_format, ...) largely duplicates what the output schema already provides and bloats an otherwise single-sentence definition.

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, the description need not explain return values, and it correctly focuses on parameters, the read-only nature, and the unknown-layer sentinel. It is complete enough to invoke the tool correctly, though it leaves the sibling-boundary question unanswered.

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 0%, so the description must carry the load, and it largely does: part_layer is described as an optional single-layer lookup with a defined fallback value (UNKNOWN_PART_LAYER), and country_code is specified as a 2-letter ISO code. It stops short of documenting name casing or format rules for part_layer ids.

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 gives a specific verb and resource ('Returns the available canonical part layers for territory construction') and even names concrete layer examples (us_zips, us_counties, ca_fsa). However, it never distinguishes itself from plausible siblings like query_parts or get_part_selection, so an agent must infer the boundary itself.

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 through parameter behavior: call bare to get the list, pass part_layer for deeper metadata, pass country_code to filter. There is no explicit statement of when to choose this tool over query_parts or get_part_selection, nor any stated prerequisites.

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