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

Annotations already cover read-only and open-world semantics, and the description adds real value beyond them: a concrete credit cost, the fact that a failed read is free, and a hard source-eligibility constraint. What it omits is any sense of the shape or size of the returned 'full record', which matters for a fetch-by-URL tool with no output schema.

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 short, front-loaded sentences: what it returns, how it's paid for, and where it works. The parenthetical '(Craigslist and Redfin rentals among them)' restates the list it just gave and is the one dispensable fragment.

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 two-parameter read with no output schema, the description covers cost, failure behavior, parameter provenance, and eligibility — everything needed to call it correctly. The remaining gap is that 'full record' is never characterized, so the agent cannot anticipate the returned fields.

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 both parameters are already documented, including the enum for `source`. The description's note that `url` comes from a search_listings answer largely restates the schema, and the source list mirrors the enum, so it adds little beyond the structured fields — baseline 3.

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 ('The full record of one listing') and anchors the retrieval key to `url` as returned by search_listings, so the agent immediately knows this is the detail-fetch counterpart to the search tool. The three eligible sources are named, distinguishing it from list_sources and search_listings without opening any schema.

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?

Explicitly names the upstream tool that produces the required `url` (search_listings) and the gating condition ('Works only for sources with details=true in list_sources'), which functions as a clear when-not rule. The eligible portals are enumerated, so an agent can decide before calling whether the listing is retrievable.

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