set_pin

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

Goes well beyond the annotations: states the effect is immediate on the real account, that re-calling replaces the old PIN, that it fails on linked devices, and the serious consequence of forgetting it (re-registration blocked until the lock lapses after 7 days). It also discloses the return payload {status: 'PIN set'}. This is exactly the additional context annotations cannot carry.

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?

Front-loaded with purpose, then constraints, consequences, return value, and sibling routing — each sentence earns its place. Slightly long and the PIN-format clause duplicates the schema, but no filler or padding.

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 single-parameter mutation with no output schema, the description covers device requirements, idempotent replacement behavior, irreversibility/forgetting consequences, the return shape, and related tools. Nothing an agent needs to call it safely 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 coverage is 100% and the schema already documents the parameter as '4–20 digit numeric PIN (e.g. 123456)'. The description's 'numeric, e.g. 123456' adds nothing new and actually omits the 4–20 length range the schema provides. Baseline 3 for a fully-covered single parameter.

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+resource ('Set or replace the Registration Lock PIN on your Signal account') and disambiguates from the similarly named pin_message/unpin_message siblings by specifying the account-level Registration Lock. An agent can distinguish it from remove_pin and pin_message without opening any schema.

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 routes to alternatives: 'Use remove_pin to turn the lock off' and notes finish_change_number requires this PIN when changing numbers. It also gives a hard usage constraint ('Primary device only: on a linked signal-cli setup it fails'), which is exactly the when/when-not guidance an agent needs.

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