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

Annotations already declare readOnly, idempotent, non-destructive and openWorld=false, so the safety bar is largely covered. The description adds a genuine behavioral fact beyond them: hard tenant isolation ("Never shows another organisation's lists") and the shape of what comes back (Companies House numbers with links), rather than only list metadata.

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 sentences with no filler: scope and tenant boundary first, then the mode switch. The most decision-relevant clause (what `list_id` changes) is placed before any detail about return contents.

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 3-param read tool with no output schema and rich annotations, the description covers scope, isolation, both modes, and what the per-company records contain. It stops short of describing the list-level fields (names, ids, counts) an agent would need to pick a `list_id` in the first place.

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 per/page/list_id are already documented and the baseline is 3. The description does add one thing the schema only hints at: that `list_id` switches the response from a list catalogue to a company page, which is meaningful but is only one parameter's worth of extra meaning.

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 saved company lists of the organisation that owns this API key (shared lists and the key creator's own)" names the resource and its exact scope, and the second sentence states the two modes (all lists vs. one list's companies). Nothing in the sibling set touches saved lists, so an agent can route here without ambiguity.

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 a clear conditional: omit `list_id` to enumerate lists, supply it to open one list and get a page of its companies, which implies the discover-then-open workflow. It does not explicitly say when *not* to use this tool or name a sibling alternative, but no sibling overlaps this resource.

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