start_change_number

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

With no annotations, the description fully discloses behavior: SMS code delivery (or voice call), account remains on old number until finish_change_number succeeds, and rate limit resolution via captcha. No contradictions or omissions.

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?

Description is moderately long but each sentence serves a purpose (purpose, code delivery, next step, rate limiting, sequencing note). Slightly verbose but well-structured and front-loaded with the main action.

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?

Covers the core process, rate limiting, and next steps. Lacks return value details (e.g., success indicator), but the tool part of a two-step migration where the agent can infer completion from next call. Sibling tools list reinforces the context.

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 has 100% coverage with descriptions, but the description adds functional context: number format (E.164), voice default (false), and captcha condition (only if required). This adds meaningful guidance 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?

The description clearly states the tool's purpose: 'Begin migrating your Signal account to a new phone number.' It specifies the action (begin migration), the resource (phone number), and distinguishes from sibling tool 'finish_change_number' by outlining the two-step process.

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?

Provides explicit when-to-use: when migrating to a new number. Includes crucial guidance: do not call finish_change_number without first calling this tool, and to use finish_change_number immediately after receiving the SMS code. Also covers rate limit handling with captcha.

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