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

readOnlyHint=true already covers the safety profile, and the description adds useful non-obvious context: the returned field set, the dependency relationship with other tools, and the underlying REST endpoint (GET /phone-numbers). No pagination or rate-limit detail, but for a simple read-only list this is solid additional context.

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 compact sentences, zero filler, with the resource, its role in the workflow, and the API mapping front-loaded in that order. Every clause carries information an agent can act on.

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 one-optional-parameter, read-only list tool with no output schema, the description inlines the returned fields and explains its place in the call sequence. Nothing an agent needs to invoke it correctly is missing.

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 the single userId parameter is fully documented in the schema, including its 'US' prefix format and cross-reference to openphone_list_users. The description adds nothing about the parameter, so the 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, so an agent knows exactly what it gets back. It also distinguishes itself from siblings by noting it is 'usually the first call' whose phoneNumberId output feeds most other tools.

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 positions usage ('Usually the first call — most other tools need a phoneNumberId'), which is genuine routing guidance relative to siblings like get_call or send_message. It stops short of stating exclusions or a full when-not-to-use condition, so it falls just short of a 5.

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