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

readOnlyHint=true already declares the safety profile, so the description only needs to add context beyond that. It adds the dependency fact (most tools need a phoneNumberId) and the REST backing endpoint, but says nothing about pagination, result size, or permissions.

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 compact sentences with the resource and return shape front-loaded, followed by the usage hint and endpoint mapping. The REST line is minor but useful for tracing behavior; nothing is padded.

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 zero-required-parameter, read-only list tool with no output schema, the description covers purpose, return fields, and call ordering. Only pagination/volume behavior is unaddressed, a minor gap for a single-object list.

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?

With one parameter at 100% schema coverage, the schema already documents userId fully, including its source tool. The description mentions phoneNumberId but never explains the userId scoping parameter, so it adds no meaning beyond the schema. Baseline 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 and resource ('List the OpenPhone numbers in the workspace') and enumerates the returned fields (id, number, name, formatted number, assigned users, restrictions). This is enough for an agent to distinguish it from sibling list tools such as openphone_list_calls or openphone_list_users.

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?

'Usually the first call — most other tools need a phoneNumberId' gives clear context for when to reach for this tool. It stops short of naming specific alternatives or stating exclusions, but the ordering guidance is actionable.

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