start_change_number

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

With no annotations, the description carries the full burden of behavioral disclosure. It reveals that the tool fails on linked devices, triggers SMS/voice code delivery, may require a captcha, and does not switch the number until finish_change_number succeeds. These are non-obvious behavioral traits, and all are disclosed.

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?

Each sentence earns its place: purpose, device restriction, code delivery, next-step call, rate-limit handling, and the ordering constraint. The structure is logical and front-loaded with the core action, making it easy to scan.

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 multi-step migration tool with no output schema, the description covers all necessary operational details: prerequisites, error conditions, parameter usage, the follow-up step, and the captcha workaround. An agent can correctly invoke this tool and know what to expect next.

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 baseline is 3. The description adds value by clarifying the voice parameter (requests voice call instead of SMS), the captcha's conditional need (only if Signal demands it), and the E.164 format for number. This goes beyond the schema's brief descriptions, though the schema already covers the basics.

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 ('Begin migrating') and resource ('your Signal account to a new phone number'), and distinguishes itself from the sibling finish_change_number by framing itself as the first step. The device-requirement clause adds clarity about when it applies.

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 context: only as the primary device, and explicitly routes the agent to finish_change_number immediately after receiving the code. It also warns against calling finish without this tool, and explains the captcha fallback for rate limits. This fully covers usage scenarios and exclusions.

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