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

Annotations already declare non-destructive, idempotent, non-read-only behavior, so the safety profile is covered. The description adds the precondition that the number must already be owned, but says nothing about what happens to the previously default number or what is returned.

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?

A single front-loaded sentence with no filler. It is arguably too terse for a mutation that changes account state, but nothing is wasted.

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-parameter idempotent toggle with annotations and no output schema, the definition gives enough to call it correctly. The only real gap is the effect on the prior default, which an agent might want to know.

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% and number_id is fully documented in the schema (including its source endpoint). The description's phrase 'the user's bought numbers' adds a mild ownership constraint on the value, but no format or lookup detail beyond the schema.

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 ('Make ... the default sender') and resource ('one of the user's bought numbers') for texting. It is clearly distinct from siblings like spur_buy_phone_number, spur_release_phone_number, and spur_get_texting_setup, though it doesn't name those siblings.

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?

Implies the use case (choosing which owned number sends texts) and implicitly excludes unowned numbers via 'the user's bought numbers'. No explicit when-not guidance or named alternatives are given; the only routing hint, 'number_id from spur_get_texting_setup', lives in the schema rather than the description.

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