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

Annotations already declare read-only, idempotent, closed-world, non-destructive behavior, so the safety profile is covered. The description adds genuine context beyond that: a miss returns urn null together with a coverage indicator of whether the county is loaded, which tells the agent how to interpret negative results rather than treating them as errors. It does not mention authorization or rate constraints, but for a read lookup those are minor.

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?

Three tight sentences, front-loaded with the core purpose and followed by miss behavior and the registry prerequisite; nothing is redundant. The trailing parenthetical defining the FactLot Registry is slightly heavy but justifies its length by establishing a data-availability precondition.

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 no output schema, the description carries the return burden and does so: it names the FactLot URN on a hit and specifies urn null plus coverage on a miss. The only gap is a lack of detail on the full hit response shape, but for a single-lookup tool the essential contract is conveyed.

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% and all three parameters are documented with examples (2-digit state FIPS, 3-digit county FIPS, APN as the county prints it). The description only restates the same three field names, adding no extra semantics such as APN normalization or formatting tolerance, so the baseline of 3 applies.

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 (Find/Resolve) and a precise resource (a parcel's FactLot URN) and names the three identifiers that key the lookup, so an agent immediately knows this is a key-to-URN mapping. It does not explicitly contrast itself with near siblings like registry_resolve_external, registry_search_address, or registry_coverage, which keeps it out of the top band.

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 rather than stated: an agent infers it should call this when it already has state FIPS, county FIPS, and APN in hand. There is no explicit when-not guidance (e.g., 'if you only have a street address, use registry_search_address', or 'call registry_coverage first if you don't know whether the county is loaded'), so routing between the several registry_* siblings is left to inference.

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